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redline at start up

W

wender

NAXJA Forum User
Location
virginia
i have a 99 xj that i just replace the head gasket on. i did everything following a repair manuel for a 99 cherokee. after hooking everything up i started the engine. the motor will rev until it redlines, either shutting off itself or turning the key off. its hard to trouble shoot with an outa control idle. i did disconect the map sensor and the engine idles better (though rough) and the idle dosnt redline the engine. searched the database here and am testing different possibilities, just wondered if there are any owners with the same issues. thanks
 
my real lil $.02 might be little bent throttle wire holder or such near the throttle body putting tension on it. messin with the engine i've yanked too hard and bent little things before
 
wender said:
i have a 99 xj that i just replace the head gasket on. i did everything following a repair manuel for a 99 cherokee. after hooking everything up i started the engine. the motor will rev until it redlines, either shutting off itself or turning the key off. its hard to trouble shoot with an outa control idle. i did disconect the map sensor and the engine idles better (though rough) and the idle dosnt redline the engine. searched the database here and am testing different possibilities, just wondered if there are any owners with the same issues. thanks
Click to expand...
On the older jeeps it would be the TPS sensor, a really bad TPS/computer ground or a huge vacuum leak, or combination of these. I have seen a real poor sensor ground run mine up over 3,000 rpm at idle.

Check for vacuum leaks, disconnect the TPS and start the jeep, then while running reconnect the TPS, that should bring the idle down temporarilly.

A bad fuel regulator will raise the idle rpms some too.

Test the TPS with a meter, should read 0.80- 0.82 volts from pin C (+) to ground at pin B, with igniton power on, but the engine off (don't start it just turn the igniton key power to the on-run position.)

Fix any bad grounds on the engine and firewall first (there are at least 4 of them on the older jeeps, not sure about your year) , then recalibrate the TPS idle voltage (loosen the mounting screws for the TPS and turn it until the voltage reads 0.80-0.82 volts). TPS is the right sensor on the front of the throttle body.
 
adrnalinlovr said:
my real lil $.02 might be little bent throttle wire holder or such near the throttle body putting tension on it. messin with the engine i've yanked too hard and bent little things before
Click to expand...

Yeh, have someone press your gas pedal while you watch the throttle assembly. See if the throttle goes through full range of motion, or moves at all. Could be jammed open? Also, if the throttle position sensor was removed, is it possible to put it on the wrong side of the lever?
 
Check the throttle plate and make sure it's fully closed. If it is and the engine still idles very high, it's almost certain you have a big vacuum leak from the combination manifold gasket between the intake manifold and the head.
 
thanks you all. i think im chassing it down. i noticed that the idle air control seems to be stuck open and might be suckin air. i snugged up the intake bolts, checked all the sensors, and am buying a throttle body gasket. the old tb gasket showed signs of gas/air creeping in. the tb butterfly remains closed during start up and stays closed even thru its high revs. i noticed a braided silver ground wire from the fire wall to the cylinder cover, i cant remember if it was mounted(grounded) to the fuel rail or the engine. any suggestions? thanks again and i will let you know how it turns out.
 
If the throttle plate is staying closed you have a large vacuum leak somewhere. IAC should not supply enough air to make it go redline. That means that you most likely have a bad intake gasket. I spray mine with CopperCote before installation.
 
Would a vacuum leak at cold start up alone cause a 99 to redline???

I would think a vacuum leak that large would be obvious, but maybe not with the engine redlining (just too loud).

If oldman is right, (and he probably is) that the IAC can not supply enough air to redline the engine, then one or more large vacuum leaks sounds like the cause.

Of course when I ever I have one of these problems it usually turns out to be everthing is broke and adding to or causing the problem.

Therefore, I would would work on and check for vacuum leaks and problems with the IAC and the grounds and the TPS, after all its a JEEP!:eek: Gotta love em!
 
Try disconnecting the cruise servo, if you have cruise control. After I got mine pulled out of a 4 1/2' water hole, it redlined. I had to shut it down and restart it. It did it again, until a buddy disconnected the c.c. servo by pulling off the electrical connector. We reconnected it later, and it was fine. Done and done.
 
we had this very same experience on a 2000 tj after a head swap. check the intake and it's gasket VERY closely for unseated spots or cracks. the guy that put the tj back together didn't seat the intake properly before tightening it down; it had a hairline crack around the bottom of the 3rd or 4th runner, and would scream to redline immediately upon starting.
 
Ecomike said:
Would a vacuum leak at cold start up alone cause a 99 to redline???

I would think a vacuum leak that large would be obvious, but maybe not with the engine redlining (just too loud).

If oldman is right, (and he probably is) that the IAC can not supply enough air to redline the engine, then one or more large vacuum leaks sounds like the cause.
Click to expand...

I would bet a sixpack that you have a leak where the manifold mounts to the head. You are using Bellville washers.....right?
 
On my 95 there's only one correct way to put in the throttle position sensor, no adjusting. Once I put it in backwards and it revved really high at start because it was holding the throttle plate open slightly......but if you put it in correctly there's only one way.

Make sure it isn't upside down.
 
wow what a response. very appreciative. i combed thru the engine today and i mi ght have to agree its an intake/exhaust gasket problem. i installed a gasket froma 97 model 4.0 because it looked the best and had thin metal gromets around the intake ports. now i think about it, those ports might be giving me trouble as far as seating right. i will reinstall the manifolds with a new gasket in the morning and will let you know if it helps. i will most likley try to disconect the cruise first. thanks. that poor jeep will run again.

ps i did use the washers(domed shaped?) on the install. i replaced the head bolts, is ther any need to replace the manifold bolts too?
 
havnt been around a pc in a while, but i thought i would still follow up.
it end up being a massive intake leak. i was sold a gasket for a 97, not a 99. i didnt think it would make much difference, and the 97 seemed to be a nicer/cleaner fit. but indead the intake didnt seat until i replaced the intake gasket. thanks again for the help.
 
wender said:
havnt been around a pc in a while, but i thought i would still follow up.
it end up being a massive intake leak. i was sold a gasket for a 97, not a 99. i didnt think it would make much difference, and the 97 seemed to be a nicer/cleaner fit. but indead the intake didnt seat until i replaced the intake gasket. thanks again for the help.
Click to expand...

Chalk another one up in the win column. :D
 
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