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Did a motor swap on an '02 Bonneville 3800, got it all done, and have a misfire on cyl#3, with the occassional miss on #4, not too worried about 4, but it may be related. good compression, running and not, replaced coils and module and CMP, and CKP, from old motor, also wires and double platinum plugs. replaced injector, has good fuel pressure... miss goes away completely @1500-2000 RPM.. thinking about removing upper manifold to check gaskets (replaced on new motor with old intake, and new gaskets) maybe not seated right? any help appreciated! Also have a 3.4 Chevy Venture with a similar problem, but I need the 3.8 done right away..

Any help or suggestions appreciated!

welcome to the forum. Did the old 3.8 have a miss? was the donor car same ymm? did you transfer any modules too?


3.4 engine famous for intake leaks. most common mistake is not replacing fuel injector o rings and fuel rail seal. i don't understand why some people wouldn't replace them. they are not expensive, and you are already there.
whats expensive is spending hours trying to fix stupid. D.I.R.T.F.ooT.

replaced the iggy module, the motor was from an '01 lesabre i think, was told its the same.. also did the crank sensor relearn procedure, no help there.... the car did not have any running issues before hand, but the bottom end bearings were shot. checked timing with a light on all cylinders, and it seems to be the same as its companion cylinder..

look at the harness where it was unplugged or moved out of the way. quickest thing is to see if there is injector pulse. if yes and spark i'd lean towards a plugged injector. if no impulse or ground look for a broken wire.

there is injector pulse, verified with lab scope and i can hear it rattle, to be sure, I replaced the injector with one from the donor engine (twice just to be sure).

So far it appears that the focus is on electronics, and I can understand that. But there is also a mechanical aspect that I have seen already. Sounds like a valve that is slow in the guide and needs to be loosened up. Has this engine sat for awhile before you installed it? That will do it. Had that already and it nearly drove me up a wall trying to figure it out. Connect a vacuum gauge and watch the needle. That will tell ya alot.

It shows 17-20 " of vacuum @ idle, and seems steady, can't find any vacuum leaks, had the intake back off again, and watched the valve moving while cranking over.. seems the same as any other valve.

been a few years but i think i remember pcm's randomly shutting down cylinders. the only thing that puzzles me is the skip going away. is it actually or just the rpm overcoming it. what's the running compression? did replace the chain?

The clue here is.....

miss goes away completely @1500-2000 RPM

Tell us the ST fuel trim on the bank with the miss at idle, then at 2000 rpm.

The misfire counts on my OTC genesys at low RPM, it stops counting @ about 1500-2000 RPM's swapped coils with another from same motor and no change in cyl#3, also changed injector with #5, still a problem with #3.

There's only one bank on this car, but STFT is between 0-4% at both idle and 2000 rpm and fluctuates rapidly

OK.

Misfires that go away at higher rpm's are generally caused by volumetric efficiency issues, like vacuum leaks or valves. Usually the STFT will be adding at idle if it's a vacuum leak, and not adding at higher rpm. I'd suspect air flow like a valve not seating/rotating or not enough lift or seal like a spring or lifter. Often a vacuum gauge will show that but not always. How about a leakdown test on the affected cylinder and then maybe on on the other bank for comparison?

compression between that cylinder and #5 are very similar at around 175-190, leakdown seems to be non-existent, that is the gauge holds pressure for quite awhile.

orangedoggarage wrote:compression between that cylinder and #5 are very similar at around 175-190, leakdown seems to be non-existent, that is the gauge holds pressure for quite awhile.

Thats what makes me think its a valve that is sticking or at least valve related. A sticking valve will have good compression. Show near zero in a leakdown test but at idle create a misfire and at higher rpms it appears to go away.

wouldn't it make noise though? what's the best way to un-stick it? I can see the rocker arm through the oil fill, and its moving well, and when i had the manifold off it seemed to move well and comparable to the other intake valves while cranking.

I've seen worn camshafts that didn't make noise because the lifter was taking up the difference. Based on everything so far, I agree with Tim.

orangedoggarage wrote:wouldn't it make noise though? what's the best way to un-stick it? I can see the rocker arm through the oil fill, and its moving well, and when i had the manifold off it seemed to move well and comparable to the other intake valves while cranking.

It would take a high speed camera to see it happening ( unless it was an extreme case of it). I have had good luck using seafoam on engines that have sat on the shelf for a while....

I had a misfire like this on a GM V6 a while back. Only did it at idle. New it was volumetric efficiency but couldn't see it at first. I used a borescope and found it. Car was in an accident and a small piece of the airbox made it's way into the intake and was living on the backside if the valve. It disrupted the airflow enough to cause a misfire. Occasionally it would jam into valve and hold it open a little. Compression was good, no rough vacuum gauge. Reached down the intake runner with two Catspaw grabbers and the borescope and got it out. Problem solved.

I have a Castle oil cleaner I'll try- any other rec's are welcome though, and thanx to all for the help so far.....

Just thought of another possibility. I have seen a lot of carbon sticking to a valve which soaked up the fuel being sprayed in the intake runner. This is another tough one to pinpoint.