New engine, won't idle -
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post #1 of 3 Old 07-11-2020, 06:18 PM Thread Starter
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Join Date: Apr 2016
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New engine, won't idle

I blew the engine in my '99 GC. It's a 4.0L. I ordered a new long block and just finished installing it today. It starts if I give it gentle gas pedal, but will not idle. Strangely, the CEL is on, but my OBDII reader says no codes. The battery has been disconnected since mid June when I screwed the engine.
Long block
Crank PS
Cam PS with new synchronizer
Temp sensor
water pump
Oil pressure sending unit
all hoses
air filter
oil pump
oil pan

Probably a few other thing I can't remember.

One thing I have to mention. It has the California exhaust with dual cats off the dual exhaust manifolds. I'm not certain I have the post cat O2 sensors plugged into the correct plugs. In other words, they may be reversed. The pre cat O2's are correct.

Is this just a matter of the ECM learning the new engine? And by new, I mean brand new long block.
I double checked all vac lines and electrical connections. I'll put a vac gauge on it tomorrow.

That CEL with no codes is puzzling. No codes pending, but a few in completes.


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post #2 of 3 Old 07-12-2020, 01:44 PM
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Bigrigr's Avatar
2000 WJ 
Join Date: Jan 2020
Location: Near SLC
Posts: 1,094
I have seen several of these machines with similar issues after they have been parked for an extended period of time. The battery usually runs down dead, and the PCM loses all of its parameters in memory. I have been told that the trick to get them running again is to hold the idle up manually for 6-10mins to get the memory to reset. Give this a try.....

'00LMTD-4.7l,242hd,D30,D44A,3.73's,True-trac front,Spartan rear,4" short arm lift,JK'sW/32's
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post #3 of 3 Old 07-12-2020, 03:36 PM
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2014 JK Wrangler 
Join Date: Oct 2017
Posts: 8,480
the CEL is on so I would suspect codes, might be the code reader is basic, read inexpensive. Have yu tried the key on off trick to check codes?

It will start and run if you feather the throttle, have you tried this for a few minutes to help relearn? Should omly take a few moments to relearn - as mentioned above.

The o2 sensors won't play into start or cold running - we can straighten the wiring out later if just switched bank 1 to bank 2 .

Also with jeep and idle issues the IAC is kneejerk suspect, be sure its clean.

Doyou have a vacuum gauge?

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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