98 Trans Sport stalled on highway - now backfires

Hi there!

Wondering.. I was driving home - nothing unusual when the engine sputtered and stalled. I tried to restart and it would if I stayed on the gas - but it backfires and spits and coughs badly....

I have had a service engine soon light on and off for about 2 weeks - but everything was working just fine.

I guess it was not kidding - I could have sworn that light was just for decoration!! :)

Anyway - my gut tells me timing belt - not even sure if that has one or a chain... Also not sure if there is an inspection cover...

Has anyone else got any other suggestions as to what the problem may be or how to check timing belt / chain and or how to change it??

Thanks in advance.. - DM

the backfiring is from the intake - not the exhaust.

please help!!!

Pulling the codes would provide a hint..but this one is OBD2 and requires a scan tool. It might be the fastest way but remember several codes are often generated as a consequence of other problems, so beware the easy sell.
I think this 3.4 is a rebored or rechanked 3.1, and if so it is chain driven. However, while chains do skip teeth ( and this has occured with 3.1 on that forum), it does not translate into back fires or stall: rather a loss of power (1 or 2 teeth) and eventually with 3 teeth slip, it will not fire up and crank very rapidly.
I rather think a poor fuel supply, but there is more than one cause. First suspected is always fuel pump but supply pressure can be tested easily. DO NOT change the pump unless the pressure loss is DRAMATIC: a few psi under target is no problem.
MAF sensor problem ? firing problem (crankshaft position sensor is a common culprit).
ALso confirm it will do the same trick on cold starts as well as when warmed up. Working OK when cold may indicate a failed O2 sensor because they come in line only after engine has warmed up.

thanks! I will check out in the morning and repost with results.

-DM

how do I check the fuel pump pressure? I don't want to take a chance with having valves hitting pistons....

Is there a way to check the MAF sensor without a scan tool??

I am looking at buying a $250 scan tool - not top of the line but maybe????

Thanks!

I donot have the specifics for the 3.4 about provision for checking fuel pressure, but I have several additional pages here based on the 3800 that I could scan .
www.avigex.ca/xport/fuelpressuretest.jpg
Nominal values are 40-47 psi at KEY ON. At engine idle, vacuum applied to the regulator drops the presure to 3-10 psi. . On KEY ON, the pump will work for may be 2 sec then shut off. Pressure applied to the rail is controlled by the regulator, so a stuck regulator, in my view, is more probable than a pump problem.

NOtice that partial degradation of fuel pressure is automatically compensated by an increase in the injection duration, and that is why, unless pressure is way down low, it is not noticeable in normal operation.
THe MAF is not easily tested. This is an electronic device that outputs a series of pulses of variable duration and that cannot be tested with simple means. Someone with an oscilloscope ...I have one ... would see that easily thouth. Otherwise,here is a procedure available for testing with a simple voltmeter too
www.avigex.ca/xport/maftest1.jpg
www.avigex.ca/xport/maftest2.jpg
(based on 3800....probably very similar with yours).

Thanks for the advice!

I bought a scanner - and came up with cylinder #6 misfire.... I checked the spark plug - does not look so healthy. I would say that cylinder has not been pulling it's weight!
I am going to try changing the front 6 plugs - and check for spark in all . I can't change the rear plugs as I don't have a hoist but I would assume that changing 6 should make a difference - then maybe I could drive it to a shop with a hoiste to have the others changed.
Does it make sense to you that one cylinder not firing would make it ALMOST unstartable and backfire so badly???

Thanks for hanging in there with me!!

Shane

sorry - front 3 plugs not 6...

My opinion is the condition of the plugs are a consequence, not a cause, or it may be purely incidental; but changing the plugs is not expensive and the result will be interesting. The fact that the problem came in a sudden means something gave up, and that is probably not only a spark plug....
coil pack failure and other sensors would normally have provided codes....so a mechanical problem might be a serious option....in fact, what mileage does the engine have?

I am going to try changing the front 6 plugs - and check for spark in all . I can't change the rear plugs as I don't have a hoist but I would assume that changing 6 should make a difference - then maybe I could drive it to a shop with a hoiste to have the others changed.
Does it make sense to you that one cylinder not firing would make it ALMOST unstartable and backfire so badly???Was it an actual backfire, or a misfire? There is a significant difference. A misfire happens when the engine thinks that a explosion should occur and it doesn't. A backfire is when the cylinder ignites either too soon or too significantly too late.

Changing the sparkplugs and/or wires would be the first step. Checking the coil pack would be the next step if the problem doesn't fix it.

The rear spark plugs don't require a hoist. The recommended way to reach them is to rotate the engine forward using a belt strap. You can but a cheap strap at hardware stores or homecenters ment to be used as ratchet tie-down straps with hooks on the end. Hook it through the engine mount then to the framing member at the top of the radiator, or around/through the hook latch hole if you opt for a hookless strap that you make a loop out of. Rotate the engine forward, remove the coil back bracket, then reach behind.

Some people have also said you can change them from the underside through the gap around the exhaust pipe (or even remove the exhaust pipe. This will allow access to all three, or at least cylinders 3 and 5. Cyclinder 1 is also suppose to be able to be changed via the wheel well so I've heard but have never investigated it.

Thanks guys - I have much to consider, test and do! The code was misfire - but it is actually backfiring. I will change plugs and check wires and coils.. I will repost with results - but not until later tonight.

The vehicle has 160,000 kms on it....

Ok - so here is the latest.

I replaced the front 3 plugs for now as they were easy to get at (AC Platinums) and checked the front 3 cylinders for spark.

All good spark. Vehicle still coughs and sputters.

I have an optical scope - and was able to look inside the cylinders at the pistons (all good - no nicks) and the valves (all good). I did notice that the middle cylinder (#4 i think) had a little bit of rust wipe coming from the head gasket down about 1-2" wide... but everything else in the cylinder looks good.

I will be working on checking for fuel pressure next. It is hard for me to get at the vehicle as I have 3 young kids, a working wife and I work an hour away from home.

I appreciate you guys hanging in there and giving advice!!!

Any thoughts?

Shane

WOW...so you can look inside the cylinders.....I would not loose the occasion to look for the position of the valves relative to the position of piston....can you move the borescope in such a way?......yeah you said that.....! ....would like to check that possible timing chain slip......
about that rust at center cylinder.....any loss of coolant recently?...these engines are prone to head gasket failure...., but you would have lots of heater and cooling problems....
A compression test might also be interesting...although if the engine still fires up, the compression loss might not be conclusive...but I'd check anyway;
ALso..does it crank faster than what you are used to?..That would be a definite clue about slipped timing chain.


keep us posted..

WOW...so you can look inside the cylinders.....I would not loose the occasion to look for the position of the valves relative to the position of piston....can you move the borescope in such a way?......yeah you said that.....! ....would like to check that possible timing chain slip......
about that rust at center cylinder.....any loss of coolant recently?...these engines are prone to head gasket failure...., but you would have lots of heater and cooling problems....
A compression test might also be interesting...although if the engine still fires up, the compression loss would not be conclusive...anyway, keep us posted..


Good point - I will have to check for the valve position - will be difficult until I can get someone to hand crank over the engine while I look. Is there much overlap or should all be closed at TDC on compression stroke??

Shane

WOW...so you can look inside the cylinders.....I would not loose the occasion to look for the position of the valves relative to the position of piston....can you move the borescope in such a way?......yeah you said that.....! ....would like to check that possible timing chain slip......
about that rust at center cylinder.....any loss of coolant recently?...these engines are prone to head gasket failure...., but you would have lots of heater and cooling problems....
A compression test might also be interesting...although if the engine still fires up, the compression loss would not be conclusive...anyway, keep us posted..

Yes - i can manipulate almost 360 degrees inside the cylinder!

Good point - I will have to check for the valve position - will be difficult until I can get someone to hand crank over the engine while I look. Is there much overlap or should all be closed at TDC on compression stroke??

Shane

There is no doubt both valves have to be closed on the compression TDC. What is more indicative is where the intake valve starts to close after the intake stroke. It should be closed near to the bottom DC or just after a very few degrees on the up stroke. If it stays open for a significant part of the compession stroke, then we have a problem.........and all that pop and jazz in the intake manifold would be clearly explained.

got it - I will check and repost. Thanks!!!!

Ok - so here it is...

I used the bore scope to check out the valves this evening. What I came accross was this.

#6 cylinder's intake valve (the greyish white valve - as opposed to the black valve - too hard to tell small from big) does not close on the upstroke of the piston - to the point of damaging the scope!!!

I checked cylinder #4 (i think) and it behaves normally as far as valves go - which I figure rules out a broken or skipped timing chain.

I think I have lost some valve keepers - what do you think?? I also think that I am lucky that the valve did not drop and make contact with the piston (which looks perfect).

My concern with this diagnosis (if all agree) is that would one cylinder with the intake valve stuck open cause the engine to stall at highway speed and cough, stutter and backfire through the intake (obvious) when trying to start it?
I can start it but I have to stay on the gas...

With a V8 - you would not even hardly notice a bad cylinder - why would a 6 cylinder be so different? Or do you think that perhaps I have 2 bad cylinders?? This would be too huge of a conincidence.. I cannot easily check the rear cylinders so I have not...

What next?????

Thanks to all! - Shane

any chance I can get away with changing only one head gasket or is this just a dumb idea????

any chance I can get away with changing only one head gasket or is this just a dumb idea????Not a dumb idea...just not very wise. When you replace the one head gasket, you most likely are going to get it resurfaced. Resurfacing one head without the other is a bad idea as it can create a unbalanced engine since one side will have a higher compression ratio (slightly). You need to resurface both when you resurface one. Plus you already are going to have most of the top of the engine torn apart. It's really not that much effort to get the other one off. Do it now and you won't kick yourself if/when you have to replace the other one and have to tear down the engine again. Do you really want to save $45 for the gaskets and $80 to resurface/pressure check the head at the cost of 20 hours of labor again? My time is free...but I'd gladly pay twice that just so I didn't have to tear down my engine again.

I've been doing my head gaskets over the past week and thought about doing just one as well..briefly..as money was tight. I'm glad I did bite the bullet as the other gasket did have signs of problems and was just a matter of time before it became a really bad problem if it wasn't already. In the process, I found several bolts that had losened over the years and two exhaust studs on cylinder 2 that had snapped off.

thanks!

Do you agree with my diagnosis?? Why would one dead cylinder cause such a noticeable problem? Is it the fact that it is the intake valve therefore pushing air etc. back through the intake to be sucked into other cylinders disrupting the flow of air??

Thanks - shane

I think I have lost some valve keepers - what do you think?? I also think that I am lucky that the valve did not drop and make contact with the piston (which looks perfect).Highly unlikely. They get wedged into place and unless something extrodinary happened, they are not just going to break. They are essentially captive with spring pressure pushing them on tighter. You would also have a variety of problems as the spring would have nothing to stop the pushing up of the rocker arm. The valve also likely would eventually fall down far enought to eventually make contact with the piston.

The valve cover comes off the front of the engine without a huge amount of work. Pop it off and do a visual inspection. Take a rocker arm off and if the spring is lose, you have a definite problem.

My concern with this diagnosis (if all agree) is that would one cylinder with the intake valve stuck open cause the engine to stall at highway speed and cough, stutter and backfire through the intake obvious) when trying to start it?A simple compression check would rule out a open valve. A test guage is pretty cheap. A leak down test also can be preformed to see how quickly the cylinder leaks out it's compression. This can also help indicate where the leak is happening at or point to other problems, such as a head gasket. The rust streak would be of concern. There shouldn't be condensation inside the cylinder like that, especially as a streak.[/QUOTE]

Do you agree with my diagnosis?? Why would one dead cylinder cause such a noticeable problem? Is it the fact that it is the intake valve therefore pushing air etc. back through the intake to be sucked into other cylinders disrupting the flow of air??A dead cylinder disrupts the balance of the engine. Basically a 6 cylinder is divided up into 6 different "thirds" over two rotations of the crank. If one of these isn't pulling it's weight, it will stumble on that stroke as there is a lack of power to get it past that point. If the loss of power is signifiant enough, you will feel it in the form of a stumble.

If a valve is opening prematurely or staying open as you hypothised, it can cause problems as well. An exhaust valve sticking open would allow other cylinders' exhaust to be pulled in instead of fresh air. An intake valve open would always suck in fresh air, but it would also allow exhaust discharge to blow back into the intake manifold. If either/both valves were not properly sealing, the cylinder couldn't properly compress, resulting in a weaker or no explosion.

If an intake valve was leaking, the explosion could leak back into the intake manifold. This could cause a rapid compression of other cylinder's air supply...much like a turbo/supercharge would force air in at pressure instead of just sucking in air. This forced air would be unexpected so the air/fuel mixture would be thrown off, resulting in a lean mixure, upsetting the balance of the engine.

I think the first thing you should do is a compresion check on all cylinders, including the back ones. You will want to record the values for the first three rotations watching how the pressure builds up. The first stroke will be less then the other two, but the pressure should build up quickly. I've read that the first reading should be greater then half of the last reading and that no two ports should be more then 10% different then each other. When I did mine last week, they were all somewhere on the order of 90-120-150. This was with a cold engine (just had it torn apart and checked compression just for convienence). It's best to do it with a warm/hot engine.

compression test with all plugs out??? I am concerned that it will start - driving the open valve into the piston...

Remove the valve cover and pull that valve back up to where it belongs.
Check your PM.

Ok - verdict is in. Took off the rockers - valve stayed open - gave it a light - and I do mean light tap and the valve head fell off - into the cylinder - in my shop - damn this horse shoe hurts!!!

What are the chances it happened with the engine off???
Sooo I was thinking of pulling the heads now....

Thanks so much for your support through this. i will continue to keep you posted as to my progress -which i expect will be slow as I have 4 other commitments that will always come before the engine. When they go to bed - I will be working.

Shane

Ok - verdict is in. Took off the rockers - valve stayed open - gave it a light - and I do mean light tap and the valve head fell off - into the cylinder - in my shop - damn this horse shoe hurts!!!Wow. Murphy's luck I guess.

Sooo I was thinking of pulling the heads now....

Thanks so much for your support through this. i will continue to keep you posted as to my progress -which i expect will be slow as I have 4 other commitments that will always come before the engine. When they go to bed - I will be working.Pulling the heads soulds like a good idea. It's a lot easier to pull the heads leaving the block then to pull the block leaving the heads. :)

I just got my van back up and running less then a half hour ago after replacing the head gaskets. After all was said and done, I had probably 24 hours of actual work involved, but that probably could be cut down as I was in no hurry and cleaned up a bunch of things. Total cost for everythign was around $400 after you include bolts and gaskets and o-rings and oil change and ... If you need any pointers, yell. There also is a "more overheating" thread in the Venture forum that has some pointers from others and myself.

Ok - verdict is in. Took off the rockers - valve stayed open - gave it a light - and I do mean light tap and the valve head fell off - into the cylinder - in my shop - damn this horse shoe hurts!!!Shane

So the valve stem was actually BROKEN? !!! Or was it unlocked at spring retainer first, and the valve end protruding in the cylinder was hit and broken by the piston, stayed in the guide then was dislodged by your light tapping.......Or is is possible the valve guide seized , valve did not retract, was hit by piston, broke stem, stayed there....
So.......in fact, where were the valve locking devices when you removed the valve cover?

Hi! Sorry for the delay again in updating - I was up to my neck in engine!

LMP - The valve never made contact with the cylinder - thank someone!! The valve is sitting on the piston - and the stem is still in the guide. The keepers are in place.

I am well into the teardown of the engine -- shocking!! Who the hell were the engineers who thought this one through!!! Anyway - enough whining...

I should have the heads off tomorrow - have not been able to work on it soley as the kids need their daddy's attention as well.

I will update the saga as it develops! Once again - thanks for all of your support!

Shane - Aylmer, Ontario Canada (in case you wanna stop by for a cold one!)

Hi again!

I now have the heads off.

1. Do I have to have the heads machined?

2. Are there any mandatory replacement items besides gaskets? (bolts etc.)

3. Any suggestions for getting broken exhaust studs out?

Thanks! I will also be looking in the forums for these answers - I have not yet looked.

I will keep you posted! Thanks in advance for any suggestions.

Shane - Sunday afternoon - 4:20pm

one more quicky - should I be concerned that the valve broke??? Will the new one do the same?? Are there some checks that I can do to make sure that the new one lives???

Thansk!

1- The heads were OK before, so no fundamental reason to machine them. Clean the surface immaculate for the new gaskets to seat properly and that is OK.
2- I do not know the specs for this engine..I know some manufacturers want old bolts thrown away...but if you have proper torque meter installed in your elbows, I,d do with the original...however there is nothing against installing new ones..but the torque elbow is still required..
3- m....good old way ends up always the same: drill into the broken stud....in the center as much as possible....enlarge the hole one size at a time and when you are just about to start drilling into the threads, rethread the hole.....
other fancy tricks have never worked with me....

Thanks!! I will keep plugging away - tomorrow. Time to take it easy for a bit.

Have a great Sunday night.

Any thoughts about that valve?

Indeed...this one needs second thoughts...or does it?
I understand the remaining part of the stem would move freely in the valve guide...so there is no option other than to think of a flaw that was there from the start...I think forwarding the parts to a metalurgical engineering dept at university they would surely find the type of flaw that existed...they have the means to analyse that. Maybe GM would be interested too.....

Good idea. I will check with GM and the schools.

1. Do I have to have the heads machined?
Do you HAVE to? No. But you would be a fool not to. They might not have been leaking before, but our vans have a history of leaky head gaskets. You want the absolute best surfaces you can get. Even if they take off a 1/1000 of an inch, it will probably be a better surface then what is there now. Plus as part of the process, the rest of the head gets cleaned up. Replace the valve, then have them machined and pressure checked. This may show that there are other problems as well. It would seriously suck to get everything back together just to find out there was a leak (either an air leak around a valve or a coolant leak through a crack) somewhere else. You need to resurface both heads to keep the compression balanced on both sides of the engine. When I had mine done, it was $160 plus change to have them cleaned, planed, pressure checked, and a stud extracted.

2. Are there any mandatory replacement items besides gaskets? (bolts etc.)Obviously the head gaskets (OEM $40/side). Exhaust gaskets as well (OEM $15/side). You can reuse the head bolts as they are NOT stretch bolts and this is likely the first time they have been off. They are TTY thought...38 lb ft plus 90 degrees. Ends up being around 80 lb feet but still TTY. If you had pulled the heads multiple times then I would replace the head bolts. Dealer costs will be $100+ for all 16 of them so they are salty...aftermarket will be ~$40/side. Note that aftermarket kits DON'T include the special bolt for the lower left side of the front head. It's a bolt/stud that fits into the engine mount. While the heads are off, access to the O2 sensor (OEM $65) is a cinch so if it hasn't been replaced in the past 75,000 miles, do it now. Don't forget new spark plugs as well. I use Bosch Platnums (~1.50/plug) but others swear to use A/C Delco ($5/plug). Your call. I'd also suggest getting new studs (OEM >$2.00)for the ones that came out cleanly (and for the ones that broke off once you get them out). You also probably twisted off the 6 studs ($2.50 OEM) on the exhanst cross over pipe. Nuts will run you about $.20 per nut.

This should get your heads back on. For the intake manifold, you MUST get a new one as it's been redesigned (but I bet you already knew that) so it will run you I think around $70 for both the upper and lower gaskets. You will also need to get 4 new long bolts and 4 new diagonal bolts for the lower intake. FOLLOW THE NEW TORQUE SPECIFICATIONS AS THEY HAVE CHANGED. They get torqued in two steps and the outside bolts get tightened down to 18 lb ft now. Put antiseize on all studs. Leave anti-seize off head bolts and upper/lower intake bolts.

Since you have everything off, you will also want to get the oil pump drive shaft o-ring (under the silver "cap" at the right side of the engine near the valley...it just lifts striaght up after you take off the clamp), bypass pipe o-ring (under the bleeder screw near the water pump), heater core pipe (comes out of the rear of the thermostat housing, injector o-rings (optional) (12...top and bottom..aftermarket as dealer will rape you on costs), thermostat, radiator cap, egr gaskets, plug wires (if needed) and dielectric grease for the plugs.

If you are ambitious, you can also clean the lifters, pushrods, and rockers of all their oil varnish. Parts cleaner and a nylon or brass brush work well for this. If you do this, get some assembly lube to lubricate things for when you first start the engine and crank it with the fuel pump fuse unplugged to prevent it from starting until oil pressure can build up.

3. Any suggestions for getting broken exhaust studs out?They can be removed by the machine shop when you have the heads planed. It wil probably be $15-30 depending on how stuck they are. I had two when I did my head, but one just popped out so they didn't charge me.

There are a variety of techniques to getting them out. Sometimes the are just drilled and a screw extractor is used to back them out. Other times progressively larger drill bits are used to get the "meat" of the stud out leaving basically the threads and then the hole is retapped. Other times a threaded rod is welded on and used to back it out. Worst case, it has to be completely drilled out and they use a helicoil to repair the hole.

hello I have the same problems as yours , my car is also running with LPG and when I am on LPG it is perfect and on petrol the engine sputters and stops, so I think it is not a problem of sparks plugs coil pack or wires , I have now to check for the pressure of the gas pump and may be injectors.
If you find an answer tell me

Thanks

Paul

Thanks CDRU - I tried (stupidly) to take out the studs myself - now the machine shop has them!!

Hey again!!

Believe it or not - just got the thing back together. I went to start it and it is running REALLY rough!! Checked wires - and I have a bad wire - must have broken the core taking it off.

THE PROBLEM: I have absolutly NO oil pressure at all. Take the filter off - start it and no oil comes out at all!!!

A buddy was over and asked if he could help- I told him he could change the o-ring on the oil pump drive....
Can anyone think of a way to screw that up??? From what he said it went easy - no pushing or jamming... any thoughts??? PLEASE????

I appreciate any advise you can provide - I don't want to rip this appart again!!

Shane

If its a 3.4 check for a broken cam.
very common

If its a 3.4 check for a broken cam.
very common


You were quite correct - broken cam shaft - in a big way. I replaced the engine...

Now the damn thing stalls once it gets up to temp! It is not tripping any codes - it looks like it is stalling exactly at the point that the high speed fans try to kick in... the low speed works fine...

I have eliminated all fuse block and relays.. one wire to check but it looks like a computer issue but for $400.00 I am not keen on just replacing it to see if it fixes the problem - especially since once it is installed - I can't return it.

Any thoughts??

Shane