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revs up to 1500 when starting.

jack SF

NAXJA Forum User
my 91 4.0 revs up to 1500rpm and sometimes as high as 1800. in the morning i can hear a little rod knock. is there a way to make it start with out reving up. when it starts and revs under 1500 than i cant hear any noise.
i only revs up for a second. it does not stay there but its long enough so i can hear the bearings. jack
 
This should be normal. The jeep computer makes it rev up for about a second. If it sticks at 2000 ish RPM, you might need to clean the Air stepper motor on the throttle body. Just pull the two screws and inspect it. Make sure the seal goes in tight.
 
i tried a couple of different motors and its all clean in there. i was hoping there is a way to bypass that reving up. jack
 
When I started having a similar problem, on and off, about 2 years ago, I replaced the TPS and the problem quit. Afterwards, when I had some major work done, including a new exhaust manifold and rear main seal, etc.,a mechanic adjusted the TPS and I haven't had a problem until more recently. Only occasionally do the rpms stay below 2500 on first startup, when warm. I have to start my '87 XJ once, turn it off, wait a few seconds, turn the key on, and back off, wait a few seconds and she'll start fine, always revving up to about 1100-1500 rpms, settling at anywhere from 500 to 750 on the factory tach. I think your problem, as long as the idle settles down, is normal.
I wanted to make a new posting for my problem, but I couldn't figure out how to do it, so I thought I'd offer my two cents.
 
I was having an occasional problem with my 88, occasionally over reving at start up. Cleaned the TB and the IAC, actually got a little worse (tried another IAC, just to compare, with no improvement).
Recently clened the oil off of the MAT and readjusted my TPS (very minor adjustment), now she starts up consistantly to around 1500 and settles in to a cold idle of around 1000, after she is a little warm idle settles in too around 750-800.
Haven´t driven it enough with the clean MAT and a newTPS adjsument, to certify this as a cure, but so far so good.
Related question, the hot air flap in the air box. Does anybody know how it works? Mine doesn´t seem to be doing anything.
 
Yeah it's normal to rev like that. If it really bothers you then you could clean the IAC and run a thicker oil and it would probably make a difference. If you clean the IAC you'll notice a huge difference the next time you start the engine. After I cleaned my 91's it started up at around 3000 rpms and it took 2-3 minutes to finally settle back down to normal. Shows you how gunked up it was. Looked black and after wiping it down good it was shiny brass.
 
Appleby said:
I have to start my '87 XJ once, turn it off, wait a few seconds, turn the key on, and back off, wait a few seconds and she'll start fine, always revving up to about 1100-1500 rpms, settling at anywhere from 500 to 750 on the factory tach.


I do the exact same thing on my '87.

8700xjs
 
TPS, lAC, you guys are killing me with the acronyms. My 87 does the same thing, revs up at start up, then comes down to 750- 800. It was staying at about 1,000 to 1,400, ran there constantly, then the alternator went out, when i replaced it the rpms stayed down.
 
My AFHAF (air filter hot air flap) or is it called the TAC? :roflmao: Isn´t working anybody have a suggestion? Gets even more interesting when you start mixing Jeep model runs (they´ve changed over the years) acronyms, with Chev and Dodge with a little dyslexia thrown in. :variety:
 
8700XJS said:
I do the exact same thing on my '87.

8700xjs

on my 87 it revs up to 2100 for about 2 seconds then drops to around 750. occasionally it will rev to 2100 and stay there...if i give it some gas it revs higher and drops back to 2100 and sometimes wont go down so i end up shutting it down and starting it again. this seems a little high compared to others revving to 1500, any one know whats up?
 
I'll throw something else out here to try. (B+ latch relay) passenger side fender is a bank of 4 relays one is the B+ latch relay,the one with pink wires going to it.This relay has something to do with remebering what the idle was when you shut down for an easier start-up,positioning IAC for start-up,in not so technical terms.Pull it out,might work for you,usually it is the throttle body needing a good cleaning take it off for the best results with a new gasket.Good luck :cheers:
 
i was always under the impression that if it revs high say over 2000 and stays there for any length of time beyond a few seconds that the TPS is going bad, i used to occasionally have to start and shut down my 88 because it would hit close to 3000 rpms when started the second time it would settle to normal rpm, replaced TPS solved the problem.
 
OW! My ovaries!! Damn Acronymns are doing a number on me!! hehe.

Seriously though, Mine usually hits about 1200rpm's when I start up, sometimes a bit less, and then drops to somewhere around 250-350 idle according to my tach. When it's cold enough out though, it goes to about 1500, then stays around 800-1000 until it warms up fully, then drps down to 250-350ish. Is this under 'normal'? or is it too low? Never seems like it's going to stall out, and so far hasn't.
 
WELL MY IDLE WAS AROUND 500 RPMS FOR SOME TIME AND THERE ARE SOME IMPORTANT THINGS THAT AFFECTS, first the mechanicalclutch cooling fan is moving slower at idle and will not cool as much, second at the rpms i was running if i turned any accesory on my voltmeter would drop below 12 volts, i stepped the idle up to about 900 and i have better cooling and stronger current from alternator.
 
Appleby said:
When I started having a similar problem, on and off, about 2 years ago, I replaced the TPS and the problem quit. Afterwards, when I had some major work done, including a new exhaust manifold and rear main seal, etc.,a mechanic adjusted the TPS and I haven't had a problem until more recently. Only occasionally do the rpms stay below 2500 on first startup, when warm. I have to start my '87 XJ once, turn it off, wait a few seconds, turn the key on, and back off, wait a few seconds and she'll start fine, always revving up to about 1100-1500 rpms, settling at anywhere from 500 to 750 on the factory tach. I think your problem, as long as the idle settles down, is normal.
I wanted to make a new posting for my problem, but I couldn't figure out how to do it, so I thought I'd offer my two cents.

I have this SAME EXACT problem..thought I was going nuts. Sometimes it will be ok for a few days. I have replaced the IAC, the TPS checked all intake and exhaust bolts all lines are connected. Funny thing is whe it is raceing I can unplug the TPS everytime and it will idle back down.
 
8Mud said:
Related question, the hot air flap in the air box. Does anybody know how it works? Mine doesn´t seem to be doing anything.

I believe it is controled by the little air temp sensor on the back of the air box. It's supposed to open when it's cold enough to allow vacuum to open the flap. The flap opens the airway from the preheater hose to let in warm air on cold days to help heat up quicker.

Of course I disconnected all that long ago. Mine didn't work anyway and I haven't noticed any problems.
 
Zipdrive007 said:
I have this SAME EXACT problem..thought I was going nuts. Sometimes it will be ok for a few days. I have replaced the IAC, the TPS checked all intake and exhaust bolts all lines are connected. Funny thing is whe it is raceing I can unplug the TPS everytime and it will idle back down.

WELL 007 IT SOUNDS TO ME LIKE YOU FOUND YOUR PROBLEM.
 
joe88xj said:
WELL 007 IT SOUNDS TO ME LIKE YOU FOUND YOUR PROBLEM.

Well Genius...now all ya gotta do is fix yours....
caps-lock.jpg


:rolleyes:
 
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