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post #1 of (permalink) Old 11-09-2015, 11:43 AM Thread Starter
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strange idle surge

hello old friends.
I have a tough question.
I swapped in a 2011 hemi from a grand Cherokee into my 2011 JK.
the computer and harness for the swap kit only work with 2010 electronics so I had to swap the transmission solenoid and valve body out for a 2010 unit. the trans is a 545 RFE also from the grand Cherokee. The engine and trans only had 6 miles on them when I got them (brand new GC went under an underpass that was too low and tore the roof off on the car carrier).
The drivetrain sat for 4 years. I cleaned some gunk off the injectors and it fired right up. The computer is supposed to have a stock tune but I have no idea whats actually in there.

Question 1:
when the jeep stops it idles fine for 1-2 seconds. Then the RPM's dip to about 300. The dash lights dim a bit and then the engine surges then it idles fine. since it's still in gear the jeep tries to lurch forward a bit. it's not bad and the brakes hold fine but I have no idea whats happening.
Theories I have:
1. the valve body I bought was a reman. perhaps one of the valves sticks a little and makes the trans lock a bit causing all the symptoms.
2. The intake throttle body has a small dent. it seals well but I suppose it's possible it could affect the idle

Engine and trans have been in and running 8000 miles now. Lights up 38" tires.

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post #2 of (permalink) Old 11-10-2015, 08:56 AM
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There's one of two problems:

A mechanical lock up, or

Electronics can't follow the load/unload of the engine quickly enough.

I doubt it's a lock up problem...that would likely kill the engine. Take a look first at the TPS sensor and see it's voltage as it falls and raises. Also, what happens when you slow down, in gear, as opposed to slow down and throw it into neutral?

Also, is the IAC pintle able to move freely? It may be sticking.

Last edited by LEVE; 11-12-2015 at 05:41 AM.
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post #3 of (permalink) Old 11-12-2015, 09:25 AM Thread Starter
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Good questions.
if I slow it doesn't do it, but if I'm going slowly enough it will surge. this is especially annoying when in low range. I have 4-1 t case and 5:13 gears so in first gear it will surge when I'm idling along. this is especially concerning when riding the brakes on a downhill in low range.

I have developed another symptom now that it cooled down a bit. Sometimes when I first fire it up and get to that first stop sign the engine runs really rough. As a hunch I popped it in neutral to see what happened and it immediately ran perfect. When I put it back in gear it still ran fine. This would lead me towards a sticky solenoid but I'm no automatic tranny wizard.

Thanks for the reply Larry.
I will look at the tps sensor voltage. The sensor is near the top of the throttle body so it is possible it was damaged.
The IAC... Idle Air...circuit? not sure where that would be

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post #4 of (permalink) Old 11-12-2015, 12:14 PM
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Runs rough when cold? Any noises, like a leaking/cracked exhaust manifold? That could foul up the readings to the O2 sensors.
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post #5 of (permalink) Old 11-12-2015, 01:01 PM Thread Starter
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Quote:
Originally Posted by LEVE View Post
Runs rough when cold? Any noises, like a leaking/cracked exhaust manifold? That could foul up the readings to the O2 sensors.
no, it runs fine when it's cold, but when I get to that first stop sign it feels like the trans is still engaged and the engine lugs a bit. I kick it into neutral and back into drive and it runs fine. Exhaust and all that is brand new. less than 8000 miles on all new parts.
All this isn't keeping me from driving it but I'm trying to figure out why it's doing it.
I still suspect a bad solenoid valve. It's the only part that isn't new (it was a reman) and I got it from rock auto... who isn't known for their high quality parts.
I'm thinking of changing it when I change the trans fluid but I hate just throwing parts at it... especially $375 parts.

The only other thing out of the ordinary has happened a couple times. if I am driving at 35 mph and the trans is in 5th gear the engine is turning very low RPM. if I very lightly touch the gas pedal, the rpms surge and drop repeatedly. it has only happened a couple times and pressing the gas pedal any more or less will make it stop but this leads more toward the throttle sensor theory.

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post #6 of (permalink) Old 11-12-2015, 02:16 PM
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Pull and clean the speed sensors on the transmission. There should be two of them...IIRC. These tell the ECM what gear the transmission is in, and what the speed the gearing is rotating. They can cause havoc if they're marginal.
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