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I got a 99 Olds Silhouette mini van with a 3.4L engine. Starts fine, runs good with the exception of an intake manifold backfire that at times will blow off a vacuum line. No codes, fuel pressure 42psi KOEO, 39 KOER. MAF is 4.6 grams at idle, tps is .64 at idle. IAC is 9 at hot idle, it has a new Jasper engine with all new parts kit at the same time. I am perfectly stumped on this one. Anybody with ideas?

Forgot to mention that the intake backfire is only upon acceleration, especially off of idle is the most noticeable.

lean "POP" check for vac leak especially around injector seals. power brake the vehicle and see if the air tube has a split in it or other oissues. when the engine shifts position and problem arises it should be fairly easy to find.

Yea, I've thought of that also. If I spray 2+2 GumCutter around any joint it makes no rpm difference. Or isn't that a reliable method of trying to locate an air leak?

Also, it only does it after the PCM goes into closed loop.

it'll work on any external vac leak and you may get a little "extra" from time to time. the vac leak or intake leak may be internal. i'm not sure enough to bet on it but i think that this intake had an issue of deteriorating inside the egr port. i've also seen issues where the intake was replaced and the pcv valve wasn't inside the port that houses it.

Ok. Thanks steve for the reminder. I forgot to mention that the intake manifold was replaced with a new one when I installed the Jasper engine because the owner does his own work (cause the shop charges too much) and had broken the original one. And the PCV valve is located in the front (left) rocker cover on this vehicle.

Owner says that the problem only started after engine was replaced.

Could O2 sensor be at fault in tricking the pcm to thinking the engine is running rich? even though the pattern on my lab scope is good for an O2 sensor?

Vacuum gauge shows 19" vacuum at idle.

usually a pop like that is a lean pop. i'de look into a possibility of an egr problem.

Does anybody know if the canister purge should be operating at idle? The scan tool shows 17% canister purge duty cycle at idle and I am wondering about that.

Tim Martin wrote:Does anybody know if the canister purge should be operating at idle? The scan tool shows 17% canister purge duty cycle at idle and I am wondering about that.

Should be ok.

What are the fuel trims?

At idle long term fuel trim is -25% and short term fuel trim is -10%. Off idle long term is -10% and short term is 0%. Otherwise runs fine.

Tim Martin wrote:At idle long term fuel trim is -25% and short term fuel trim is -10%. Off idle long term is -10% and short term is 0%. Otherwise runs fine.

Well there you go. Fuel trims always tell the story. Your car is on the verge of setting a rich code. -25 is maxed out. The PCM is taking fuel away. Find the cause of the trim problem and you'll no doubt fix your backfire.

Yea well, that is what I'm thinking and that is why I was wondering if the 17% Purge Duty Cycle was causing the problem. Also, I am kind a suspecting a possible issue with a fuel injector but I did a fuel injector balance test on each one and they all came out with the same test results, so far as rpm engine drop, vacuum drop, and fuel pressure change. So, in my mind I am running out of options. But I would certainly appreciate any input others would have as well.

Thanks for your assistance - it's nice to have others that know more than I do.

Tim Martin wrote:Yea well, that is what I'm thinking and that is why I was wondering if the 17% Purge Duty Cycle was causing the problem. Also, I am kind a suspecting a possible issue with a fuel injector but I did a fuel injector balance test on each one and they all came out with the same test results, so far as rpm engine drop, vacuum drop, and fuel pressure change. So, in my mind I am running out of options. But I would certainly appreciate any input others would have as well.

Thanks for your assistance - it's nice to have others that know more than I do.

You can easily check if the purge is relative by either disconnecting it electrically or mechanically. If short term starts to rise markedly then you may be on to something. Typically, if the PCM is commanding the purge, it's ok. 17% is not alot and certainly not enough to max out long term. But you should eliminate it as a cause. ( I had a hyundai that the purge leaked mechanically, but that caused a lean code)

Fuel pressure regulator leaking? I'm sure you checked....

How about that manifold? Was it OE or one of the Dorman units?

A skewed MAF could cause this too.

Just some thoughts out loud....

Well, MAF is within parameters however that does not garantee its ok. The intake manifold is OEM. Admit, I had not thought of fuel pressure regulator.

Well, as the saying goes: Two heads are better than one, especially when the one is a cabbage head. (me)

Thanks 4 thinking out loud ric! It reached me down here.

fuel pressure? i've seen the diaphrams leak into the vac line.

Haven't we all. Yea, they are problem childs but this one does not appear to be leaking at the present

Bummer.

I'd be looking for anything that'll cause too much fuel. (sounds easy, doesn't it?)

Yea, I just had this thing at the zone and they hung it on their machine and the guy said it was just that little gizmo down under there that is bad! You know they have a machine that tells ya what is wrong with the car.

Haven't we heard that already?

did he have his little plastic "head diagnostician" badge on? did his eyes squint and was there a few white heads on his forehead and a drool trail next to the toothpaste stain on the side of his chin? if yes, you got the big guy himself. ya know the one that flys from store to store solving all the difficult problems. he use to work at walmart as the front end expert untill they found him putting the lug nuts on backwards. before that he headed the study for the impact resistant oil filters for jiffy lube. thats why they're still in perfect shape on the side of the road from blowing off of cars....................... have a great weekend

well steve, you are pretty good at describing the guys at these places, at least what I tend to think they look like. But I didn't have it there myself, the vehicle owner did though. Ya know, it's absolutely amazing how those fellows can't fix cars and can still hold a job. If my failure rate equalled theirs, I'd be mash long ago but these guys........

Back to the intake backfire. I brought the van into the shop late this afternoon and the thing hardly ran. Idle was ok but off idle was bad. The pipe out of the cat was glowing cherry red and missing, sputtering, backfiring, you name it, it did it but not one single dtc. CEL never came on. Totally floored me. I think the devil must be in this thing. Once it got over this fit, it couldnta run better.

Strange indeed.