Misfireing at Speed

[Larkspeed]

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?

[LMP]

...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.

[Autopro1962]

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

[Larkspeed]

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.

[Larkspeed]

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

[LMP]

Originally Posted by Larkspeed

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.

[Larkspeed]

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.

[LMP]

0.060 in, 1.5 mm

[Larkspeed]

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

[Larkspeed]

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?

[LMP]

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.

[Larkspeed]

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

[Larkspeed]

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?

[LMP]

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.

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/vbul...d.php?t=736818

[Larkspeed]

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?