Manager Forum

I have a 1996 Cadillac Deville 4.6L 150,000 miles with a P0300. At idle the car runs smooth and you'd never know it had a problem. Drive it and it pops out the intake and misses. On the scan tool cyl #7 shows history of misfires. While driving the scan tool rarely picks up the misfires like it doesn't see them.

I have checked everything that you normally would. Coil, wire, plug, fuel pressure, injector balance, back pressure, compression. I have came to the conclusion that it might be cam related. The fact the scan tool isn't seeing the misfires most the time and the popping out the intaking makes me feel this way.

Am I missing something or does anyone have any thoughts I might be overlooking?

What is compression? I would also be thinking about an issue with the intake valve for that #7 cylinder.

120 psi

120 psi is not too bad (mileage depending) if all cylinders are that or within 10% of that.

Is it popping out the intake and the PCM is not picking up on a misfire while driving it?

Tim Martin wrote:120 psi is not too bad (mileage depending) if all cylinders are that or within 10% of that.

Is it popping out the intake and the PCM is not picking up on a misfire while driving it?

Yes it's popping out the intake and the PCM rarely sees the misfire. When it does see it it's on #7 cyl. The customer has drove it this way for a month so #7 does show a history of misfires but I assume not nearly as many as it should.

take a good look at rocker travel on that cylinder.

Have you done a running compression test? And how's the vacuum....any fluctuations?

ricmorin wrote:Have you done a running compression test? And how's the vacuum....any fluctuations?

Good point. The vacuum.

The vacuum did not fluctuate and I did not do a running compression test.

I wasn't able to wave the magic wand (aka plug our computer into it and have it tell me whats wrong) and the customer was in a big hurry to pick the car up so that's all I have at the moment.

Generally a running compression test is warranted if you had a bouncing vac needle, but I might do it here just because of ther nature of the symptom.

Another thing I would do in a case like this would be a cylinder leakdown test. This would show if there is a sealing issue in the combustion chamber.

just a warning here. i still would look at the rocker travel but and it's a big but. if you've never torn into these mills take a good long look at it and remember this, "yes, it's as difficult as it looks". i've done many and i still will but i always treat these like i'm in a laboratory. i've have valve covers that wouldn't seal because they were warped or corroded, intakes warped because the engine wasn't stone cold when it was taken off, etc. i really mean it when i say these are a real head banger if you're not ready for them. there is a lot of light aluminum in them. it's not uncommon to take accessory brackets off in pieces. warn the customer of this or you may be forced into the dreaded "i've never seen this before" position. as a rule i will only r&r if there is any engine issues at high mileage. i've seen there with 300k on them running as well as the day they rolled off the lot and seen then go into a tail spin once they've been opened up. just a warning. it's not impossible to repair these but the sword of damaclies will be hanging over your head. again, and then i'll stop my banter here, i've found that once anything goes wrong internally they all become anchors.

I've found most of the time popping from intake turns out to be fuel starvation of some kind, injector?

could be a lean pop, is the popping consistent with somewhat of a pattern? what are the fuel trims? what is the fuel pressure? does it seem like it's starving?

Guy was in a big hurry so the car is no longer here to answer those questions.