98 Trans Sport stalled on highway - now backfires - Page 2

LMP · 04-11-2005, 08:35 PM

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.

divemaster · 04-11-2005, 09:15 PM

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

divemaster · 04-13-2005, 08:39 PM

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

divemaster · 04-14-2005, 09:37 AM

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

cdru · 04-14-2005, 11:54 AM

Quote:

Originally Posted by divemaster

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.

divemaster · 04-14-2005, 12:46 PM

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

cdru · 04-14-2005, 01:53 PM

Quote:

Originally Posted by divemaster

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.

Quote:

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]

cdru · 04-14-2005, 02:55 PM

Quote:

Originally Posted by divemaster

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.

divemaster · 04-14-2005, 05:13 PM

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

LMP · 04-14-2005, 08:46 PM

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

divemaster · 04-14-2005, 09:56 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

cdru · 04-15-2005, 12:25 AM

Quote:

Originally Posted by divemaster

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.

Quote:

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.

LMP · 04-15-2005, 07:25 AM

Quote:

Originally Posted by divemaster

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?

divemaster · 04-16-2005, 08:28 PM

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!)

divemaster · 04-17-2005, 03:08 PM

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

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