258 with mopar fuel injection fluctuating idle

[kauaiman]

I have a 258 with the mopar conversion kit. I am having up and down idle when it first starts. after the warm up cycle, the idle seems fine! sometimes the idle will go up and then almost stall then it does this cycle again and again til warm up. I have replaced IAC, checked the MAP, IAT and TPS and they all test good. The only sensor left is the coolant sensor. I did check resistance and it seemed ok, but did not replace it. I am leaning towards a vacuum leak, but have not found any. any help would be appreciated. As a side note, after running jeep for 45 minutes, it had a small backfire when slowing down near my house. Seems like engine may be running lean. 

[Moses Ludel]

kauaiman...Does act lean and pecking for an idle.  If you have a decent meter that will read fuel trim, check that out with the engine cold and warmed.  Also look for check engine codes (DTCs), these Mopar conversion kits have a diagnostics function and leads. 

Since this primarily occurs in cold engine mode, it could be the oxygen sensor or the CTS (coolant sensor).  A heated O2 sensor requires a warm-up lead wire, make sure the Mopar system is wired properly and that all connections, especially to the O2 sensor, are intact and functional.

The backfire could be a fuel mixture issue or a vacuum leak as you hinted.  The EFI may be unable to perform the cold start functions and unable to provide fuel enrichment when the engine is cold.  This could be a faulty coolant temp sensor, wiring or related parts. 

Check for a vacuum leak, including a leak at your brake booster check valve or booster diaphragm if you have power brakes.

Moses

[kauaiman]

I have a check engine light installed, does it work with the key on/key off 3 times? I did that and no light. I have a homemade smoke tester, will test for vacuum leaks,dont have power brakes so that's not the problem. I replaced the o2 sensor thinking it was the likely culprit, but no change. The EFI kit doesn't have a connector for my ob1/ob2 tester, so how can I check fuel trim? is there a way to find pin at ECM for fuel trim and backfeed to test. Thanks

[Moses Ludel]

kauaiman...Which Mopar kit do you have, the single rail or return (dual) rail?  There is a provision in the wiring harness for a check code, but you're right, there is no OBD-II plug.  This is because both kits use the 64-way (1991-1995 era) PCM, which predates OBD-II.  These kits were both emissions legal/California O.E. number for a 1994-95 YJ Wrangler 4.0L prototype.

I like your idea of creating a diagnostic plug.  You'd need to follow the wiring schematic for an OBD-type diagnostic plug (prototype 1994-95 YJ Wrangler 4.0L).  Note where the plug picked up a signal(s).  Your idea of the PCM makes sense, as there's no other source for processing this information.  The 64-way PCM pin-outs would be the source for a diagnostic plug.

As for using the Check Light leads to find stored codes, the Key On/Key Off method should work.  Attached is a PDF with factory steps for doing this check in a 1994 YJ Wrangler, which should work for your 64-way PCM.  Try it:

1994 YJ Wrangler 4.0L DTC Retrieval.pdf

Moses

[kauaiman]

Thanks for the info, I have the return rail system(dual)

[Moses Ludel]

kauaiman...You're definitely patterned off the 1994-95 Jeep YJ Wrangler 4.0L system.  With the exception of the inline fuel pump and conversion kit's wiring harness, every MPI component and ignition piece on your Mopar EFI Conversion was off the shelf OEM 1994/95 4.0L.  Simpler.

Moses