1988 4.0L 2000 RPM start-up

[metrictonner]

Other than revving to 2 grand at start-up, hot or cold, before dropping back down to the correct hot or cold idle, it runs fine. Started intermittently a few months ago, now it does it all the time. 4X4 5-speed transmission. 140k miles. Any ideas? I've replace the O2 sensor, MAP, TPS, and idle control unit and checked for vacuum leaks. Thanks for anything you can you can say about this. this is actually mine!!!!

[BLHTAZ]

How long does it stay that high? Could be a sign of the ECU going bad.

[Geonovast]

Both of my Renix 4.0s would do that most of the time, they'd stay at about 2000 rpm for 4-5 seconds. I always thought it was normal.

[metrictonner]

Thanks, I edited the post to reflect that it zooms up and then zooms back down.

[metrictonner]

Correy

 

According to Chilton 91 was the start of a new computer system. Although I must say since it started doing this I get no more lifter ticking.

[metrictonner]

Brent

 

ECU was next. All $270 of it. :redX: I popped all the relays, I'm going to pop the ECU connections tomorrow.

[Geonovast]

Correy

 

According to Chilton 91 was the start of a new computer system. Although I must say since it started doing this I get no more lifter ticking.

 

I know. I had an 88 Cherokee with a 4.0 and an 89 Cherokee with a 4.0.

[Eagle]

Fast idle on start-up is almost always the IAC (idle air controller) being dirty. Second choice is throttle position sensor. ECU is about a probability of 0.00000001%.

 

Mine does it every once in awhile. I dump a can of fuel system cleaner into a half tank of gas and within a day (two at most) it clears up.

[BLHTAZ]

Only reason I said ECU right off is that he has already reaplced the TPS, IAC, o2, MAP etc... ;) . It's definitley not the first thing to go for, but it is looking more likely with all the other stuff already done.

[james750]

Fast idle on start-up is almost always the IAC (idle air controller) being dirty. Second choice is throttle position sensor. ECU is about a probability of 0.00000001%.

 

Mine does it every once in awhile. I dump a can of fuel system cleaner into a half tank of gas and within a day (two at most) it clears up.

 

Yeah, sometimes the injectors will stick open a little bit and cause this problem. as some of you may recall, i recently had this problem and it ended up being that my injectors on cyl 1, 3, and 5 were sticking. had them jet cleaned (?) and it hasnt had the problem since. \

 

Here is a similar problem (don't mean to be hijacking this thread, it just seams relevent)

 

My moms 97 xj (aw4, 4.0) is revving up to 2500 rpms whenever in nuetral (or park) but calms right down when put into gear. any ideas? Also, it only does this when the check engine light goes out (once a day) when the check engine light is on it acts normal. I thought it took 51 key on key off cycles to turn off the check engine light but its going on and off once or twice a day. :nuts:

[Eagle]

Only reason I said ECU right off is that he has already reaplced the TPS, IAC, o2, MAP etc... ;) . It's definitley not the first thing to go for, but it is looking more likely with all the other stuff already done.

Yeah, I saw he replaced the IAC. But he didn't clean the throttle body itself.

[metrictonner]

I cleaned the plunger seat when I put in the IAC and I ran a couple of bottles of injector cleaner. When I cleaned the throttle body I checked for fuel smell, there was none. :wall: