GM 3400 mystery misfire

[Glenn]

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

Topic

[Jeff Ketchem]

Since you were pretty far into it I’d double check all vacuum hose connections and intake gasket making sure no small leaks exist. Try that first and let us know what you find.

[valde]

Check the fuel trims with scanner. Long term and short term at idle. Fuel related problems may cause this “rhythmic” idle. Maybe also IAC, MAF, or ECT. So check scan tool engine temp.

[Christian]

Good post lots of places to start. I have a few questions this might lead to some answers.

1. any history of overheat?
2. when you changed the plugs and wires did you check the distributor?
3. are we sure the timing is right since we changed the timing belt?
4. Pressure checked the cooling system we might still have a leak?

These are a few ideas. I find it helps to double check the work done before jumping to a new possible issue specially since these 4 things are only some of the possible things leading to missing, I would also just to be sure, perform a compression check on the missing cylinder.

Let me know if these are things already done i don’t doubt skill I just know we are human 🙂

[fredsmythson]

I had most of the same symptoms on my 1999 Chevrolet Lumina that has a 3.1 engine.

The problem was a dirty or bad crankshaft positioning sensor.

[Glenn]

Valde, thanks for the tip. This is a bit beyond my skill level. My scanner is a cheapo that simply reads and clears OBD 2 trouble codes. Next Tuesday, I plan to take the van to a friend’s shop who will have a real scanner and proficient skill-knowledge. I’ll certainly take your suggestions with me. I’ll keep you posted…

[Glenn]

OneDTwenty Thanks for your reply.

There is no history of overheat. I went into the engine due to the presence of coolant in the oil. The intake gaskets on these 3.4 engines are notorious for leaking.

This vehicle does not use a distributor. It’s equipped with a DIS system utilizing 3 coils.

I am sure the timing is right…

I have not pressure checked the cooling system. That is a good suggestion.

We’re in the Southeast storm. So it may be a few days… I’ll keep you posted..thanks again..

[Glenn]

Thanks Fredsmythson or your reply. When your crankshaft position sensor malfunctioned did it generate a trouble code? My car hasn’t, which is part of the challenge. The set up on this vehicle actually uses two crank sensors. I did replace the harmonic balancer and seal. Also, I had to remove the sensor near the balancer when I replaced the timing chain. I thoroughly cleaned it before reinstall. None of these symptoms were present before the repair. I’ll keep you posted on this. I’m leaning away from this a bit, as the problem wasn’t present before repair. Thanks again…

[EricTheCarGuy]

If you don’t have any codes, start with the basics. Check for vacuum leaks and perhaps do a power balance test to see if you have a problem cylinder. Also, since you did all that work, it’s a great idea to go back over all of it to be sure everything is hooked up correctly. If it wasn’t doing this before you did all that work, then it’s likely to be something you did in the process. That’s a lot of work, it’s not hard to miss one little connector or vacuum line. Believe me, I’ve done it. You might also check for air in the cooling system. Anytime you open the cooling system, you must purge the air when you’re done. If not, air pockets can get into the system and effect all kinds of systems, including the idle. Here’s a video on how to do that.

Also, here’s an article that I wrote on solving performance issues that you might find helpful.

http://www.ericthecarguy.com/faq/solving-automotive-performance-issues

Keep us posted on your progress.

[Glenn]

Gents, I didn’t get super far along with trouble shooting today. However, I did try a couple things. Per EricThe Car guys suggestion, I bought a spill free funnel and bled the system…made a mess first time I used it..lol…nothing like reading instructions. After purging air, I also cleaned the MAF sensor and reinstalled it. The engine finally threw a code PO 440. That code would come on periodically before the engine work, so may not tell me anything… I’ll do a proper power balance test soon. I did unplug cylinders 2, 4 and 6 at the spark plug. Also, I was unable to find any vacuum leaks. Will smoke test it first part of the week. Thanks Eric for your helpful post…Will keep you guys apprised as I continue the good fight 🙂 thanks to all for help…

[fredsmythson]

The car didn’t show a check engine light when the problem first started. I spent a lot of time looking for vacuum leaks, but I couldn’t find any leaks. Then about six months later the check engine light came on. The code showed it had something to do with the knock sensor circuit, but it ended up being a problem with the crankshaft positioning sensor.

[Glenn]

Fredsmythson, I’m thinking again about your reply. Does any of this sound familiar?

* Sometimes the car will run very smooth initially at idle, until it warms up a bit.
* Other times, the car will skip some and idle rough when first cranked.
* The ‘loping’ previously described doesn’t always occur, but happens occasionally, usually when engine is fully warm.
* I can sometimes make the problem temporarily disappear by raising the rpm’s.
* The car generally hesitates upon take off, but not always.
* The car misses throughout the driving range, but less often at interstate speeds.
* The problem is more apparent at lower speeds and in traffic.

It does act like a vacuum leak. However, I’ve been unable to find it.I did change the harmonic balancer during my repair work, as the old one was bad. The crank sensor was taken off, cleaned and replaced. Anyways, I’m curious as to whether any of the symptoms above we’re experienced when you had the problem with the Lumina. As Eric advises, I don’t like to throw parts at a car. However, nothing is making sense right now. Btw, I once drove a 90 and 92 Lumina years ago as company cars..I really enjoyed them. Thanks should you have time to respond.

[fredsmythson]

Here is a comparison of what happened with my Lumina:

* Sometimes the car will run very smooth initially at idle, until it warms up a bit. – YES
* Other times, the car will skip some and idle rough when first cranked. – YES
* The ‘loping’ previously described doesn’t always occur, but happens occasionally, usually when engine is fully warm. – ENGINE WOULD SOMETIMES “ALMOST” STALL WHEN COLD
* I can sometimes make the problem temporarily disappear by raising the rpm’s. – YES
* The car generally hesitates upon take off, but not always. – YES
* The car misses throughout the driving range, but less often at interstate speeds. – ENGINE WOULD MISS SLIGHTLY IN CITY DRIVING; RAN NORMALLY AT HIGHWAY SPEEDS
* The problem is more apparent at lower speeds and in traffic. – YES

Did you double check your coil pack, sp wires and spark plugs?

Too bad the Lumina has a bad reputation. The car drives great and gets decent gas mileage. I think the GM 3.1 would have been a great engine if they hadn’t went cheapo on the engine gaskets.

[Glenn]

To anyone with more technical knowledge or who has serviced GM’s, are you familiar with the crank position relearning procedure for vehicles equipped with a 24X crank sensor? I did replace the harmonic balancer. The 24X sensor was simply cleaned and reinstalled. Would this possibly cause an issue requiring a relearning procedure as described below? I’ve posted part of an article. Thanks for your time…

Partial Text: DTC P1336 CKP System Variation Not Learned from GM Factory Service Manual

The following text is quoted from the 1999 GM F-Body Factory Service Manual:

“
Circuit Description

The Crankshaft Position (CKP) sensor sends pulses to the PCM as the reluctor wheel teeth rotate past the CKP sensor. The PCM uses the CKP pulses to synchronize the ignition and fuel injector operation, and to time the interval between each CKP pulse. The PCM determines when an excessive change in crankshaft speed occurs by comparing each new time interval with the previous interval. A misfire causes an unexpected change in the crankshaft speed. A certain amount of acceleration/deceleration is expected between each firing stroke, but if the crankshaft speed changes more then an expected amount, the PCM interprets this as a misfire. The interval between the CKP sensor pulses is extremely small. At high engine speeds, slight variations in the following components make misfire detection difficult: Crankshaft, Reluctor wheel, CKP sensor. The PCM learns variations during the Crankshaft Position System Variation Learning Procedure. The PCM compensates for these variations when performing detect misfire calculations. Only a scan tool can command the PCM to perform the Crankshaft Position Variation Learning Procedure again.

Perform the learning procedure after the following actions: A PCM replacement, Any operation or repair involving the crankshaft, the CKP sensor, or the CKP to reluctor wheel gap relationship, An engine replacement, The ignition switch is in the ON position until the battery is drained.

Important: A PCM power-disconnect with the ignition ON may erase the stored pulse value and set the DTC P1336. Disconnecting the PCM will not erase the learned Crankshaft Position System Variation as long as the ignition switch is in the OFF position.

Important: Reprogramming the PCM does not require running the Crankshaft Position System Variation Learn Procedure unless the PCM is new or from another vehicle. A DTC P1336 sets if the Crankshaft Position System Variation is not within an acceptable range, or can not be learned.

[Jeff Ketchem]

Yes sir! If you replaced the harmonic balancer you will have to do the relearn. The only way to do it is with a scan tool capable of doing it. I know mine gives the option with a list of steps that it checks off as the pcm detects each action.

[Glenn]

Thanks DF Ranger…I was hoping to avoid the dealer… 🙂 might I ask just a few follow up questions?…I see you are busy, so know it is much appreciated…

* Would this relearn be necessary any time the new balancer comes off, say for a repair (if not a different balancer)
* Would the same be true anytime in the future I unplug the crank sensor, or clean it?
* If sensor itself is replaced, is the relearn procedure necessary?
* Finally, would you replace sensor before relearn? The Car has 224k at this point. Trying to get at least one more year out of her…
Thanks very much again.

[Author]

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