95 Dodge Van 2500 Intake backfire

Hello folks,

This van backfires out the intake (once) under moderate to heavy load. It also misses at idle during cold operation. The miss disappears as it warms up. The backfire remains.

I replaced the worn out plugs, wires, cap, rotor and PCV. This improved the idle quality but didn't cure the miss. Idle vacuum increased from 15" to

17".

The TPS, MAP, MAT, CTS, Cam Sensor, and Crank Sensor all test within spec. No codes.

The O2 was replaced, cross counts were too few and it ran better with the old one disconnected.

Fuel pressure is rock solid all the time and in spec.

EGR tested ok.

The only other problem I can find is the catalytic converter seems to be rattling/broken up. I don't believe its blocking the exhaust because it passes the vacuum test for plugged exhaust.

What am I missing? Could EGR cause the backfire?

Regards, Brent

Reply to
Brent
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burnt vavle you sucking cold air back into the chamber do a compression check wet and dry

Reply to
tudysmuck

You have a classic symptom of a valve train problem. It may be a burned/leaking/sticking valve, a bad/broken spring, a bent push rod or a bad cam lobe. A loose or cracked valve seat may also cause this but, they usually very quickly become obvious in failure mode. You may or may not be able to isolate the problem with a compression test as any of these can result in low compression. I have never seen EGR cause backfire to the intake.

Lugnut

Reply to
lugnut

Well, the compression test resulted in this:

In the order tested:

2: 170, 4: 120, 6: 100, 8: 135, 1: 160, 3: 130, 5: 175, 7: 180

Incidentally plugs 4 and 5 are the one that are burning differently than the others. 5 being much darker and 4 looking stained.

On to the leak down (a *royal* folly)

I brought #6 to TDC (double checked with a straw in the plug hole) Air blew out like I was on valve overlap. So, I tested #1 (thinking I was

360 off) and it didn't leak. Now I figure I will just follow the firing order (18436572), turn the crank 90 and test #8. Air blows out like I'm on overlap. :-\ So I check its companion #5 and it's not leaking. So I turn another 90 to check #4 which also blows air like I'm on overlap. Its companion, #7 is not leaking. :-\ so I turn another 90 to check #3 and its sealing like it should. &#$!!*^$ So I turn 90 more back to the TDC marks to check #6 and it blowing air again. Oh and #1 is sealing again. So I turn the crank 360 back to the TDC marks and pull the distributor cap. The rotor is pointing at #6. I test #6 and it blows air AND #1 IS SEALING AGAIN!!!!

Why can I only seem to test the drivers side (odd) cylinders? I'm sure its very simple and I should have stayed home and watched tv as I'm too tired to complete a leakdown test.

Anyhow, I hope this was good for a laugh.

Regards, Brent

Reply to
Brent

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