Misfireing at Speed

Hopefully you guys can help me.

I have a 1994 Pontiac Trans Sport 3.8L V6 Automatic.

This is a european spec car as I live in the Netherlands.

When I am doing the highway speed of 120 Kmh after a little while the car starts to kick quite bad as though it is misfireing and the rev counter starts to jump around.

If I slow down to 100 Kmh everything seems to calm down but as soon as I speed back up to 120 it starts it all over again.

City driving it seems to be fine.

I have tried cleaning out and replacing the fuel filter as I thought the engine may be getting fuel starved but it made no difference.

The engine light is on dim most of the time as well.

I don't know what else to check and there are no dealers around here so I can't take it to a dealer for them to look at it.

Any ideas?

...at that age, a number of "conventional" things must be held for sure before going for the unusual..and generally, it avoids indeed searching for the unusual.
Being able to read the codes would.., well, might help, but '94 is the "interim" OBD 1.5 and that does not help. MAy be, an early '94 "might" use that
www.avigex.ca/xport/readingcodes.jpg
but it MUST have a plug like this one
www.avigex.ca/xport/plug.jpg
with connectors in A and B ....otherwise, it is more complicated.
So , the ordinary would be:
- ignition module, 45$ new on e-bay..very easy to change
- crank position sensor ..somewhat more complicated because you need pulling the crank balancer-pulley
and, of course, new ignition wires and spark plugs.

Fact is both these component carry signals to firing, injection and tach...so it makes sense.

Start wiyh the basics,new plugs and plug wires.Sounds like you have a bad plug wire or two.

ok well i have a set of plugs and wires on special order from a Chevrolet dealership, I did this so I could be sure they are originals and not some cheap copy.

170 Euro so hopefully that solves the problem.

ok I just looked and my ODB connector has 3 wires like the one pictured above but they are in different places so I am assuming a need some sort of reader for that.

I checked Ebay but could only find ones for 1996 and newer

ok I just looked and my ODB connector has 3 wires like the one pictured above but they are in different places so I am assuming a need some sort of reader for that...
...alas yes...very few scanners come with the right connector and software for these. THere are several softwares allowing to connect to a PC using a cable and interface and several users have been successful with that... I bought an interface cable but to date got no readings...Nicke66 on this forum has success with his.
..and when you change the front three wires, do not bother to route them through the original path imbedded with the valve cover...cut the old wires and get rid, and pass the new ones "free path". Better for your nerves.

ok well plugs and wires arrived I have not installed them yet as they did not come pre gapped for this engine and I am having trouble finding out what the gap should be. They are original AC Delco replacements tho :)

It's a 1994 3.8L V6 injection with an Automatic Transmision so if anyone could tell me the proper plug gap it would be handy.

But anyway an update on the symptoms.

It missfires even at city speed it just takes longer to do it.

It's almost as tho something is getting too hot and then acting irratic.

Occasionaly as well it will stall as I slow down for a traffic light or a corner but this is rare and probably directly connected to the above symptom.

Could this be caused by a dirty injector?

I'm at a bit of a loss here I did grow up in canada and do know quite a bit about GM vehicles but fuel injection I am not as familiar with.

What else can I check without buying more parts if these plugs and leads don't sort out the issue.

I don't really want to replace parts unless I am pretty sure thats where the fault lies unless I have to because as you can probably figure parts for this van cost a fortune here as they all have to be special ordered from a dealership.

0.060 in, 1.5 mm

thanks for that :)

I have a haynes manual on order for it as well so that should stop me asking silly questions like that again :)

ok 170 Euro later on factory replacement plugs and wires and it makes no difference LOL.

They needed to be change anyway it still had the original ones from new.

But what else can I check without having to actualy buy any parts?

Is there a way I can test the coil packs and ignition module with a multi meter or something like that?

YOur first post said
When I am doing the highway speed of 120 Kmh after a little while the car starts to kick quite bad as though it is misfireing and the rev counter starts to jump around.If I slow down to 100 Kmh everything seems to calm down but as soon as I speed back up to 120 it starts it all over again.City driving it seems to be fine.
THIs would let me think the torque converter clutch (TCC) starts slipping under load, something commonly reported. What happens then if you just kick the throttle a little so it just exits from TCC (will gain like 400 rpm) ? Does it still jerk?
BUt this should not show at low speed, where TCC is not engaged.

kicking the throttle does not solve it so I don't think it is gear box related.

As I said later it has started doing it at city speed as well and if I have to stop when it is doing it acceleration is jerky as well and you can actualy tell it's misfireng from the sound it's making if you get what I mean.

I have just recieved the Haynes manual so I will do the coil and module checks as described in the manual

LMP I have to thank you for you help so far :) you have been very patient with me.

Now I have done the following:

New plugs and plug wires.
I have performed all the resistence checks as laid out in the haynes manual on the coil packs, ignition module and injectors all have passed those tests without issue.

The problem still persists.

I'll try and decribe the problem a bit better.

When i start driving everything is fine the car runs perfect and there is no lack of power or anything like that.

After about 10 or 15 minutes at city speed of 50kmh the engine light comes on and then I notice the rev counter start to dip intermitently and very shortly after that it starts the misfire.

The reason I don't think the problem originates in the gearbox is because if the gearbox was loosing it's grip, for lack of a better term, in gear then I would expect the rev counter to spike as the load on the engine dropped.

Sometimes I can slow down a bit by just taking my foot off the accelerator and it will stop doing it. Other times it will keep it up and if I try to accelerate or hold the speed the car will shudder or kick each time the revs dip and power decreases then comes back up and power increases. Sort of like a learner driver starting off in a manual in first gear.

There were no warning signs of this problem we went out one night and on the way out it was fine and then suddenly it did it on the way back and it has done it ever since.

All shifting of the gearbox is very solid with no slipping and I can be driving along before it starts and drop my foot down and the gearbox will kick down and the car will go as you would expect it to with no hesitation and no hint of slipping.

I have noticed that when the car warms up the oil pressure guage does go down near the bottom of the scale but does not fall in to the red zone.

Now as I said earlier I am at a bit of a loss when it comes to fuel injection so I don't know the factors that can cause this issue.

Could pulling the injectors and cleaning them make any difference and if so whats the best way to clean them? For instance is the in the tank fuel injector cleaner you can buy any good?

Should I buy replacement coils and ignition module anyway even tho they pass the resistence tests?

What other advise can you offer on things I can check?

IN my first reply, I mentioned the crank sensor and ignition module as plausible suspects. What you describe really sounds like an ignition problem and teh components I mentioned are all that is left now .
YOu cannot test the ignition module, that wafer like component with coils on top, with a resistance meter as these are active electronic components. This is a component that, after mine failed and so many reported same in other threads, that I would recommend as mandatory maintenance at that age. I had the chance that if failed just after a rather long trip, and just before another I was planning, otherwise I'd have been stranded in the middle of nowhere needing a repair job at high cost. It is so easy to replace and procure new on e-bay near $50 + shipping Part number is D-1977.
http://www.avigex.ca/xport/d1977a.jpg
OF course, having a reading of the codes would add interesting info ; is a GM dealer in your area who could read .?
Crank Position SEnsor has the same degree of risk as that age, but is more cunbersome to change because you must pull the crank balancer-pulley .
http://www.automotiveforums.com/vbulletin/showthread.php?t=736818

ok well i will certainly look at getting the module then.

I have found a US supplier that sells the coil packs at $15 each and the Ignition Module at $39.99 on ebay and he ships to The Netherlands.

I'm just waiting to hear back from him on the total cost including shipping and we will replace the lot.

As for dealers to read the codes unfortunatly Cheverolet is a new brand here they only started selling them with actual dealers about 5 years ago although there are plenty of older models that were imported before that and the dealers are not equipped for the old models only the new ones.

We have found a scrap yard about an hour away that has 2 or 3 Trans Sports in and I am going there today as I need to replace the wiper motor so i'll have a look at the ones they have and cross my fingers that they have a 3.8L V6 in there. if they do i'll grab the module and coils from that one.

Question tho are they specific to the engine size or do the ones from the 3.4L V6 work as well?

I do not suspect the coils themselves, but OK anyway, better safe than sorry. If the same can procure the crank sensor,www.avigex.ca/xport/cksphoto.jpg go for it, it will add nothing to the shipping fees.
www.avigex.ca/xport/ignition3800.jpg
As for the wiper motor, what is the problem..??? THe erratic wiper problem is a well known fact due to soldering on the control circuit. A salvage unit can have the same problem.
check these out
http://www.automotiveforums.com/vbulletin/showthread.php?t=677547
http://www.automotiveforums.com/vbulletin/showthread.php?t=627829
http://www.automotiveforums.com/vbulletin/showthread.php?t=512424

coils for the 3.4 might be the same but not the ignition module... check for part number at www.rockauto.com ....an excellent information source.

...a common source of CEL lighting up...although not causeing large upsets in relability, is the Exhaust Gaz Recirculation valve... again, reading the codes would tell at once.
www.avigex.ca/xport/egrphotoL.JPG EGR acting up causes variation in engine RPM or torque (depending if TCC is applied or not) specially going uphill...even when it works normally and the computer has problem deciding when it is just on the limit of opening/closing one section....(this is a 3 section valve)
...ooops..my mistake...'94 is the linear model, also leading to some problems
www.avigex.ca/xport/egrlin.jpg

and..and...since you have a resistance tster....test the Throttle Position Sensor TPS: this is a variable resistor ; read between one side and the center pin , let someone depress the throttle slowly, and you shoudl read a continuously increasing/decreasing resistance value, from a few ohms to svera kohms, like 5k or 7k...what counts is that it vcaries without misses or jumps. Easy to test, costs 0. Usually, it becomes damaged at the place where throttle is the most used, the track wears out and the resistance wiper arm looses contact, causing jerks and misses . THrottle up or down a bit and it returns to normal. Not a heat sensitive component though, and a code for this www.avigex.ca/xport/dtc21.jpg would inhibit 4th, cruise and OD.
www.avigex.ca/xport/maflocation.jpg

still waiting to here back from this guy about combined shipping but as soon as I do i'll ask him about the CPS.

I'll check the TPS tommorow as it is pouring with rain now :)

The wiper motor tho I did take it apart to check the connections as mentioned on another thread but it turned out that the large plastic worm gear was stripped totally down one side so it was a definate replacement job.

I got one from a 99 and I don't know if they changed the circuit boards between the years or if the one I got had a naff board but it only worked on high and did not return when switched off.

I just took the cicuit board out of my old one and put it in the replacement and the wipers work perfect now :)

...I got one wiper motor from a 99 and ...I just took the cicuit board out of my old one and put it in the replacement and the wipers work perfect now :)
Good to know: it is interesting to be able to refresh the hardware as sticking with the "then years" might procure no better than what we already have.

Curious to see the outcome of this one... I'm thinking that the coils and module will fix the problem.

Scott

Sure sounds heat-related, and coils/electronics are susceptible.

ok ignition module ordered.

I did not bother with the coil packs as they past their tests and you say you do not suspect them. I can always order those later if needed.

I tested the TPS and it performs fine slow movement through the full throttle range produces and nice even change in the resistance readings with no sudden drops or spike and the voltages when the key is turned on are nicely within range.

I have noticed a couple of other things while doing the testing though.

It appears that the gearbox has too much fluid in it but I am going to go for a drive in a bit and get the car up to operating temp and check it again.

If it's too high i'll have to drain some to get it within range.

I have also noticed a slight exhaust leak that I think it coming from the rear manifold gasket. I am making arrangements at a local hobby club that has a car lift and a couple of pits to take the car there so I can get a clear view from underneath and establish excatly where the leak it so I can replace the appropriate part.

The donut shaped joint at the rear exhaust manifold leaks after a time. I had three already on mine and the fourth is in the glove box waiting to be installed.
THere have been several opinions on this forum that the 4T60 transmission level be kept 3/8".. 8 to 10 mm, above the mark.

Start engine, allow to warm up, shift trans. into both a forward range AND reverse for a few seconds at least, shift back into PARK, keep engine running, THEN check trans. dipstick.

ok update

I have just had to do a lot of running around town looking for something and my impression from the drive is that LMP was right all along and the problem lies in the TCC

when cruising at 70kmh down one of the longer straights the car is rock steady then whatching the rev counter as soon as it drops signifying that the TCC had engaged you start to feel a slight surging in the cruise of the car. if you do nothing eventualy it will start do kick bad but if you just vary your foot on the throttle by taking your foot off and putting it back on every 10 seconds or so it will drive around merrily for as long as you want and never start kicking really bad.

Now in town this is no big deal but it could get to be a pain if I have to do any extended highway driving as it would prevent me from using the cuise control.

Now all that said does this definatly point to the TCC given what has been tested so far?

And if it does am I correct in assuming this is an engine out of the car repair?

Like does the whole tourque convertor have to be changed or is the TCC a seperate component?

And as sort of a bodge it fix is there any way to bypass the TCC without pulling the engine so it does not try and kick in? I realise that this will cause more fuel to be burned but ATM I don't think I could possibly afford to have an engine out repair done on this car.

And lastly a stupid question if there is too much fluid in the gearbox could this cause the TCC to act strange?

When my 1995 3.8 needed new plug wires, I could easily feel it misfiring/bucking when the TCC applied. But with the TCC disengaged, the hydraulic torque converter soaked up most of the feeling, and it was pretty smooth.

I think your TCC is fine, and you have an ignition problem somewhere.

...with the TCC disengaged, the hydraulic torque converter soaked up most of the feeling, and it was pretty smooth..
INdeed this is very hard to pinpoint and when my ignition wires were shot, I would have sworn the TCC was THE culprit, not meaning it could not be, but that some reserve must be used in the conclusions. In fact I still have a bit of shaking of that kind, and I think a CV joint on the right might have some part in that ...but this is very difficult to isolate the problem ..and, I really do not like that, but we are caught in doing with trial and error.
HOwever, CV joint and spark problems would not change the actual RPM of the engine while TCC is engaged....
http://www.avigex.ca/xport/tccshudder.jpg

LMP,
You sure have some excellent resources! The thing that threw me off about my plug wires was that these engines do not behave like older ones, insofar as bad plug wires. Years ago they were suspect if you had misfire at WOT--when the voltage conditions were highest.

But with the 3.8 at least, there is some kind of sensor that apparently measures the voltage developed vs that which is required, and it sends a signal to the computer. My van sometimes wouldn't even START with the old stock wires. That all disappeared when I replaced them.

Now my only problem is an old one. I still have either an intermittent ground problem or a momentary loss of 12 volts to the engine. It never lasts more than a second, but in that second the gauges fly up and down and the trans. clutches release, allowing the engine revs to fly up until the clutches reapply (and I take my foot off the gas pedal).

I've checked a BUNCH of connections, pulled and checked all the fuses in the block, no help. I guess I'll have to replace the computer next.

... there is some kind of sensor that apparently measures the voltage developed vs that which is required, and it sends a signal to the computer. .
Yes..that is indeed how it can trig a code indicating precisely which cylinder is misfiring.
..and you 12V freak "power outage" is something I wish you can find the cause...and post a picture :) A corroded ground tie under the floor carpet has been mentioned at times as related to this kind of incidents...Mind boggling....

ok bear with me let me see if I understand how the TCC operates.

The TCC is essentialy and automatic clutch that engages between the input and output shafts of the tourque convertor thereby directly linking them together bypassing the fluid in the tourque convertor?

And if I understand what LMP has posted above if the TCC engages and disengages smoothly and when it should and also disengages with the application of the brake or by pressesing the throttle smoothly then this indicates there is not a problem with the TCC and that it is fine?

That being the case then I can rule out the TCC as it engages and disengages smothly and quietly as it should.

So I am still at an ignition fault somewhere.

I'll change the module out as soon as it arrives and see where that leaves me.

However as has been said more than once in this thread being able to read the codes would make the fault hunt so much easier.

If I take a picture of the OBD connector under my dash and post it would some kind person have a look on Ebay for me and point me in the direction of the right reader as there are so many on there that I am getting confused as to which is the right one.

Yes about TCC nature, gray zone about "shudder" once applied. As they say, loss of pressure for faulty oil rings and seals could apply less pressure than needed and TCC could slip under load. BUt if that happens really often, computer will detect speed mismatch and pop a code. www.avigex.ca/xport/dtc39.jpg (would be another code nomber for 94)
www.avigex.ca/xport/dtc39test.jpg

About reading codes: This OBD1.5 is really a pain in the....; except GM and some specialty or high price code readers, interface cables are available on the internet to connecet to a PC; one can also build an interace from scratch using this info:
http://www.pontiac-interface.de
Nicke66 (on this forum) from Sweden has great success with this one and could provide mode comments.

WHen I changed my ignition module 2 years ago, I had changed the crank sensor a few days before cause it was part of the chain of suspects. At that time, the CKPS swap proved to no avail but given the rate of failure of these 2 components, I'm glad both are new...so either it saves the day for you, or you simply have a much more reliable machine...with spares ...for not much money.

several on thsi forum have disconnected TCC by pulling connector D or A from the transmission control plug. It means a little more fuel on the highway, but nothing more in the city....but I remember now that your problem occurs also in city driving, so that eliminates TCC and all of the above is "for the record".

Benessero: I found that fromthe Bonneville foru, http://www.automotiveforums.com/vbulletin/showthread.php?t=658078
but the same was read TS forums too.

several on thsi forum have disconnected TCC by pulling connector D or A from the transmission control plug. It means a little more fuel on the highway, but nothing more in the city....but I remember now that your problem occurs also in city driving, so that eliminates TCC and all of the above is "for the record".

Actualy city speed here is 50kmh which is above the 25mph engage limit for the TCC so it would still engage doing the city speeds here.

I'll have a look at the cable thing asn see if I can sort someting like that out.

But ATM I am just waiting for the Ingintion Module to arrive and see if changing that out makes any difference.

One thing you might try is to get a simple timing light. Not the big battery-powered gun type, but a cheap old style small light that emits a weak red flash. Hook it up and tape the thing to the outside of your windshield so you can see the flash tube. Drive around until it misfires and look at the light. If it goes out for a second or two, you'll know your problem is ignition-related.

You may have to run several tests using different coil wires. I've done this test on cars that used only one coil, and attached the light to the coil wire. The non-battery type lights can respond fast enough so that the light looks like it's on continuously, as long as there's no misfiring.

ok problem has progressed quite badly now.

We were coming home today only doing about 40kmh when the van started acting up.

I'll do my best to try and describe the feeling.

it started shaking quite bad and and then the only way I can describe it is as tho it was slamming into gear like the feeling you would expect if you gave it a bit of gas and then shifted into drive.

When I took my foot off the gas pedal to slow down it shuddered quite bad as tho something was dragging badly

I just took it for a 30 second trip to the store as it has cooled down fully and within 10 seconds it started doing it again.

So what I take from that it that it's a gear box problem.

and that becaomes a big issue not only because it's an engine out job to replace the gearbox but also the fact that even a second hand gear box here will run us 700+ Euro and we simply don't have that kind of money.

--- EDIT ---

I just went around the area and tried a few things to try and get a better idea.

once the car passes around 25mph (using that to make it easier) it feels like there is something dragging really bad. the van just lacks acceleration. if I give it more gas it eventualy gives the thump feeling of catching a gear and accelerates but as soon as I take my foot off the dragging feeling comes back.

Best way to describe is is as tho you have left the parking brake partly on. thats the feeling something slowing the car faster than you would expect with coasting.

as soon as the speed drops below 20mph you can actually feel the dragging sensation disapear and if you put your foot down it goes as you would expect until it hits 25mph again then the drag is back.

It also only does it in Drive and ODrive. if I leave it in 2 there is no drag at any speed and I can go as fast as I like with no problems. Just higher revs obviously.