Rough idle Renix 87

[Ecomike]

Time to start a new thread.

Here is the old one with all the blow by blow details of recent work, problems and multiple fixes.

http://www.naxja.org/forum/showthread.php?p=246509037#post246509037

Now down to just a rough idle, worse cold, and everything seems to be working and has passed all device (6-8 of them) tests we could come up with.

Since the problems I have found and fixed did not start with a rough idle, it seems to be a new problem but the recent tests, repairs and data may be useful:

http://www.naxja.org/forum/showthread.php?p=246509037#post246509037

Fuel pressure normal, except for a 1 psi vibration at idle while the engine is shaking, and cold (IIRC it got steady when good and hot last night, or at least stopped the original 2 psi shake yesterday after waterboarding it and spraying Gumout in the intake for a good while to remove carbon)

Vacuum gauge tests all passed.

Injectors all passed multiple tests.

Swapped the HV coil, and the ICM with known working ones (no change in the original problems), the TPS is Mopar new working flawlessly, the CPS is new performing like a champ. None of them solved any problems.

New indexed dizzy, new MAP and new Vacuum line to the MAP, fixed all the original problems. It was backfiring at higher rpms, then got worse and then died....see the old post. Dizzy shaft gear came loose, sheer pin died and engine died at that point.

Tools used, Volt-ohm meter
Snap-on MT-2500
Matco Ferret engine analyzer
Vacuum gauge
Fuel pressure gauge
Compression gauge (compression is 135-140-150-on all the others, so good) Runs like a bat out of hell now at up to 4500 rpm in park.

O2 sensor works, switches to and stays in closed loop in about 30 second cold start, 5-10 second hot start. Fuel trims are almost text book near 128 now. Was way low on STFT before multiple fixes. Just a little under 128 on both STFT and it is converging on 128 nicely as it runs more and I as I run more cleaning operations to remove carbon fouling...

Grounds all perfect, battery and alternator perfect, exhaust is not blocked, cooling is all working fine, but if I stand on the gas pedal at idle it will backfire out the intake if press it hard enough. If I give it gas carefully at idle it stumbles some till it hits 1200-1500 rpm (takes 2-3 seconds) then its fine. At 1500 to 4500 I can punch the gas, let off hard and hit hard again without it giving me any grief now (so the old original problem is fixed). EGR system passed the EGR tests too.

So the basic hardware must be OK???

What am I missing?

 

[cruiser54]

Waterboard it first.

 

[Bent]

Just thinking out loud here.

Harmonic balancer could contribute to a rough idle and not affect higher rpms. It won't cause a backfire, though.

Timing chain could be suspect except that the idle problem is new and was immediate.

A bad/barely broken wire at or near the MAP sensor plug can cause a temperature dependent problem. The wire will contract when cold causing a loss or reduction in the signal.

Don't know that any of this helps but it's what I got at the moment.

 

[cruiser54]

Harmonic balancer only takes out the harmonics of the crankshaft undetectable by human senses.

 

[Ecomike]

Seems even a Master Jedi Jeep mechanic can make a rookie mistake :doh: and cross two spark plug wires, LMAO. Gotta love these EASY fixes that do not require pulling the timing chain cover, or a head job, or waterboarding the jeep

Had the #3 and #4 plug wires criss-crossed (probably when I index the dizzy, and swapped caps twice in the process, they got crossed). And it still ran like a beast above idle even with the 2 SPwires crisscrossed. These engines are beasts!!!! My first real clue was that it ran like another jeep I fixed 10 years ago that had the head gasket blown between cyls #3 and #4, so I decided to verify I had the plug wires right from the multiple cap swaps. Sure enough #3 and #4 wires were crossed.

Runs like a new engine for the first time in 13 years now even with serious carbon fouling, only 290,000 miles on it. The 87 Wagoneer XJ.

Thanks for the input folks.

Just thinking out loud here.

Harmonic balancer could contribute to a rough idle and not affect higher rpms. It won't cause a backfire, though.

Timing chain could be suspect except that the idle problem is new and was immediate.

A bad/barely broken wire at or near the MAP sensor plug can cause a temperature dependent problem. The wire will contract when cold causing a loss or reduction in the signal.

Don't know that any of this helps but it's what I got at the moment.