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I recently did an extensive repair on my 2002 Chevy Venture with a 3.4L engine.I replaced head gaskets, intake manifold gaskets, timing chain and sprockets, including the timing gasket. All ignition components, coils plug wires, plugs, etc., are AC Delco original and new. I went into the engine due to the common intake manifold gasket failure and coolant in the oil.
Here are some symptoms of the “soft” misfire that vehicle experiences. I define “soft” as compared to a harder electrical ignition type misfire. I have included some other information that is not symptomatic, but might be pertinent to trouble shooting the issue.
* The vehicle cranks right away and usually runs smooth at first. Sometimes it will miss a bit right after it starts. At other times it runs very well until warmed up.
* After the vehicle warms up, a rhythmic, ‘loping’ idol sometimes occurs. The car never totally dies. The headlights will slightly dim when the loping occurs.
* The vehicle misses intermittently throughout the driving range.
* It seems to run best at sustained speeds/ interstate, etc.
* The vehicle occasionally stutters at take off in traffic.
* no check engine light has come on.
* vehicle has not totally stalled out.
Any ideas/ trouble shooing pointers would be helpful. I have checked all ground wires. They seems to be tight and in good order.
Thanks much
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Actually something as simple as wiring harness repair would warrant a relearn. A far as just removing the balancer I honestly can’t say. The need for a relearn can also not even set a code on these vehicles. To be cautious I would perform a relearn anytime you move anything or replace anything as you could change the air gap to the sensor causing inaccurate readings. I would also go ahead and change the sensor prior to relearn. If your luck is anything like mine it would go out as soon as I had the relearn performed lol
Looks like we’re on track with this one. Please give us an update after you have the relearn procedure done.
I want to say a sincere thank you to Eric and the rest for your help and interest with this troubleshooting issue . This has been my first time on an automotive forum. The experience has been positive and encouraging.
Today, I met with a friend who is a professional mechanic and one of the most knowledgeable persons I know locally. Unfortunately the problem persists after these services/ tests were performed.
* He did the crank position relearn procedure successfully with his scanner. However, the misfire persisted afterwards.
* Secondly, he did a smoke test to check for a vacuum leak. No leaks were detected during this test.
* Next, he checked the fuel trims and injectors, etc., with the scanner. He said it all checked out within specs and fine.
* He did find an intermittent misfire in cylinders number 3 and 4 with the scanner.
Here is why this is confusing….cylinders 3 and 4 are on opposite sides of the engine. They are controlled by separate coils. Again, the coils are relatively new as are the plugs and wires (all original AC Delco).I checked the plugs, wires and connections to the coils. All seems to be fine. The plugs look healthy, dry and are a good color. No signs or carbon tracking or wires crossing, etc.
So I am still scratching my head here. I did find this post on the International Automotive technicians network. I am unable to read the solution and answers. Also, I am unable to join as one has to be an ASE certified mechanic. Maybe one of you guys would be kind enough to look. It sounds like it may be describing the same issue. Here is the link: http://www.iatn.net/techhelp/69/28017/2003-chevrolet-venture-ls-3-4l-misfire-mil-lamp-on
Pressing forward…thanks again
really sounds like the intake gaskets leaking try spraying water around the gaskets and see if the rpm changes also look for cracks around the plenem and spray that down good too. Try ohming out the injectors to see if there in spec. Whats it run like when the maf sensor is unplugged? any leaks at the fuel rail? wiring harness got pinched while replacing the gaskets?
It sounds like you’ve done your due diligence. That said, there’s still mechanical to consider. If a valve isn’t sealing right, or there is a gasket failure somewhere where you can’t see like under the intake, this can cause the misfires you’re talking about. Also, you didn’t state what the fuel trim numbers were. They should be as close to 0 as possible. Something out of range here might indicate an injector problem or something like that.
Don’t rule out mechanical. Myself, and many others, have been burned by a simple mechanical failure that ended up being the cause.
Please keep us posted.
You know maybe a quick health check with a vacuum gauge may help possibly point to an area we haven’t thought of. You can generally pick one up for $20 and up. Take a look at this link may help you decide whether you have an internal issue http://www.motor.com/article.asp?article_ID=457 . Maybe a power balance test may help as well. The vacuum test would point to whether it would be a good idea to pursue a power balance test.
Thanks Eric and DF Ranger…When performing the work, I had both heads off the vehicle. The machine shop checked the valves and pressure tested the heads. They checked out fine. I did replace the valve seals before reinstalling the heads. The vacuum gauge test is a wonderful idea. It would be a good learning experience for me. Eric I don’t know the fuel trim numbers. My mechanic friend was looking at them and simply said they looked fine.
I wholeheartedly agree with the philosophy of not “throwing parts” at the car. I am tempted to change out the crank position sensors to see what happens. Fredsmythson’s post from a couple days back answering my questions almost describes the symptoms exactly. He said the PCM did not throw a DTC for about 6 months. Even then, it was a knock sensor code rather than the crank sensor itself. I found a really good price on Ebay. Don’t throw a wrench at me.. 🙂 bad idea? I’m tempted. Again, know my thanks for your time and knowledge is sincere..thanks very muchHere is some information about testing the 24x circuit and CPS on GM engines:
http://troubleshootmyvehicle.com/gm/3.1L-3.4L/how-to-test-the-24x-crank-sensor-1
Update:
The GM 3400 misfire issue is greatly improved, but not yet completely resolved. Last week I found the fuel rail line had a leak at the top (back side). Also, the number 5 injector seemed to not be seated correctly in the plenum. It’s hard to even see this area with a mirror. There was no raw fuel odor noticeable which was unusual. I happened to hear the fuel rail releasing pressure after the car had been off for a couple minutes. I was lucky to hear it. After replacing an o ring in the rail and reseating the injectors last weekend, things improved quite a bit. However the misfire was still present. Idle conditions improved and the misfires were not as prevalent.
This might get me kicked off the forum, but I have sinned… Yes, I threw a few bucks at the car and replaced the 7x and 24x crank position sensors this weekend. Things improved dramatically. For the first time, the vehicle produced no hard misfires at all during a test drive. Also, it performed wonderful at idle in traffic and performed well over all.
I am still experiencing some hesitation at certain speeds, 15 to 25 mph mostly. Here is my plan forward. I will again take the car to my professional mechanic friend and do the CPK relearn procedure. Also, I’m going to ask him to really check out the fuel trims, pressure, etc.,
I appreciate all of the collective wisdom on the forum. Many of you suspected a vacuum and/ or fuel issue which were true. Special thanks to Eric the car guy, DF Ranger and Fredsmythson for expertise and experience. Thanks to all who offered help.
The car will sit this week as the power steering reservoir is leaking…will repair Friday. I’ll send another update after I get the car to my friend’s shop.
tomatofarmer1
It’s been a while since giving an update. The vehicle was back on the road for a while, with the misfire. After discovering that the lower intake was beginning to leak coolant again, I discovered something else using a cheap cigar and heater hose smoke test…this was after two high tech smoke tests at a friend’s shop. I’ve included a couple video links below. I tried to load the videos here but wasn’t successful…Check it out; it shows what I discovered and an inexpensive fix… Will keep you posted when I get car back together…
Great video, thanks for sharing.
Did you set the plenum down on the intake without the gasket on and do a wobble test for possible warpage? If the plenum wobbles you have to find out what is warped… the lower intake or the plenum.
I’ve cracked my fare share of those plenums in my day.
Thanks JTF,… I did use a straight edge on the plenum to see if any warpage was present. I didn’t see any. However, I did not check the intake manifold. I will do as you suggested. If minimal warpage is present, would you suggest a layer of rtv in addition to the gaskets? It did seat well with the gaskets in place while on the bench.
tomatofarmer1
I would say no, but being in the Dealer world we always replaced with new parts when something was out of specs. In the real world (real repair shops) there are many fixes(that I don’t know about) that are just as good,even better than new and most importantly more cost effective.
Sounds like you continue to make progress. Interesting fix you have there. I think it should work fine. Let us know if it doesn’t and also how things go afterward.
Please keep us updated.
I thought I would give a brief update. The plenum repair shown in my videos worked out fine. The misfire however still persisted. The car finally threw a DTC code at me last week while on a road trip. The code was p0336. Both CPK sensors are new, as well as the camshaft sensor. I thought I finally had the “money shot” today; I discovered insulation on the wiring missing leading from the 24x crank sensor wiring harness under the vehicle. Wires appeared to be contacting each other in this spot. Here is a picture of it.
After repairing the wires, the problem is still there. I may just be stuck until I can get the money together to go to the dealership and pay for them to diagnose the problem. I’ve never had something whip me like this 🙂 Thanks to all for the helpful advise.
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