89 xj -high idle woes,dammit!!

1989 xj 4.0, auto, Having problems with high idle, first appeared on the trail sunday, driving and stopped shut it off for a few minutes, restarted and stuck at 3000rpm, I thought maybe throttle linkage or the like,but no. Throttle is on the stop, no obstruction, was running great. I unplugged the 3 wire connector to the tps and idle came to normal,enough to get me off the trail and home (power was crap and shifted poorly). I replaced the tps with a new one last night set it up with a multitester and it was good,shifted normal,power was great. Stopped to fuel up today and same thing, stuck at 3000rpm when I started it. unplugged the three pin flat connector and idle is normal, shift poorly again. What the heck is causing this??
a few things i've done in the past little while:
-new motor (reman -new fuel pump and filter
-33's
-thats about it, I will post up if I remember anything.

What am I doing by unplugging the 3 pole connector?I can understand why It shifts funny and runs like a dog when unplugged. help! thanks kindy gents!

Have ou cleaned your throttle body? How about the IAC?

Im thinking the IAC is the culprit here.

Seems like a stupid question also, but you did adjust the TPS correctly right?

Muad'Dib said:

Have ou cleaned your throttle body? How about the IAC?

Im thinking the IAC is the culprit here.

Seems like a stupid question also, but you did adjust the TPS correctly right?

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unplugged and plugged in the iac while runnin,no change, even tapped on it. yes the tps was adjusted correctly as per fsm with a voltmeter and a calculator. Throttle body is spic and span.I'll try a used iac off a spare t.body in the morning.

check vacuum lines. make sure tps isnt sticking. Mine does that everyonce in a while i just rev it up and it comes back down to normal idle.

If I touch the gas it will go up and stay up. I have tried to let it run throught the high idle with no luck. anybody? this blows

bcmaxx said:

If I touch the gas it will go up and stay up. I have tried to let it run throught the high idle with no luck. anybody? this blows

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This happened to me when the IAC was sticking. Make sure the IAC cavity is clean, and try a few different IAC's. I just pick them up at the junk yard for testing purposes.


It also happened when i had some vacuum leaks that were to me hard to find. Check all your vacuum lines carefully.

Before i knew what to look for, and how to fix it.. i could usually get it to idle correctly when i would stomp on the gas till it reached about 5000 RPM's then it would go down to normal idle.

Really weird, The new Iac did not make a difference, I am adding another ground from the motor to the firewall,Ground is new from batt to engine. I also plan on checking the grounds at the tps harness tonight.
If the tps is unplugged it will start and idle good, as soon as I plug it it it raises to 2500-3000rpm. If its is plugged in when I start it it immediatly goes high idle, if I unplug it and crack the throttle it idles down again to normal. I also brough a spare complete throttle body to work today to test out if all else fails.

Just going through the fsm and it stated the three pin connector three pin connector supplies input to the tcu, Why in the world would unplugging it fix anything. If anything unplugging the 4 pin should effect idle right???
I'm so lost right now!

Sounds like a bad or misadjusted TPS . Or a bad ground to the TPS . Make sure when you adj the TPS you do it from the TCU side of the TPS . This will cause all kinds of trans shifting problems with a bad or misadjusted TPS . Have you checked your intake manifold gasket ?? Good Luck .

oh and brand new intake gasket, studs nuts and all, no leaks, good vacuum.

if you unplug the TPS basically the computer runs a pre-planned program so it'll run like shit because it doesn't know whats actually going on, but it will at least run still.

try backprobing the TPS (the signal out and the ground wire) and with the key in the run position engine off open the throttle until it reaches 5v open and close it a few times it should raise voltage smoothly from .8 up to WFO

Oizarod115 said:

if you unplug the TPS basically the computer runs a pre-planned program so it'll run like shit because it doesn't know whats actually going on, but it will at least run still.

try backprobing the TPS (the signal out and the ground wire) and with the key in the run position engine off open the throttle until it reaches 5v open and close it a few times it should raise voltage smoothly from .8 up to WFO

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Sorry my internet terminology sucks what is wfo?

i have an 88 xj too, and it does that to me a lot. the only way that i can get it back down is to gas it a bit and sometimes it will come back down. if not, turn it off and wait a couple mins

update, won't idle down at all now, even with iac, and both tps connectors unplugged. can no longer drive it now :banghead: .

Is the harness for the TPS itself clean, no corrosion? Might want to start moving wires around to see what that does.

Sensor ground by the oil dipstick, is that still attached and relatively clean?

WFO= wide fuckin' open

this is a far reach but check the PCV stuff on top of the valve cover make sure its not popped out,

and try taking the hose off the brake booster and plugging it
another far reach but maybe the brake booster is doing something funky and lettin loose makin your truck rev?

Man im having the same problem, New TPS and IAC. New wire connectors on the Male end of the wire harness going into the TPS, I have Adjusted the TPS from the both connectors and get the same problem, Start the Jeep and it goes to normal rpm then turn off and start agian it goes to 2500 +

Vacuum Leak?

bcmaxx said:

Just going through the fsm and it stated the three pin connector three pin connector supplies input to the tcu, Why in the world would unplugging it fix anything. If anything unplugging the 4 pin should effect idle right???
I'm so lost right now!


update, won't idle down at all now, even with iac, and both tps connectors unplugged. can no longer drive it now

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The IAC is wide open causing the high idle, with it disconnected the ECU can not return it to the normal idle air flow possition even if it wanted to!!!!!:shocked:

The three pin, flat connector is the engine fuel injection computer, AKA ECU. The square four pin connector is the TCU (tranny) computer.

Have you guys tested the TPS ground wires at the TPS on both sides to the battery negative ground, it should be less than one ohm, power off.

I fought this problem for 2 years before i discovered all my ground wire values were changing while the engine was running. When that happens the back voltage in the ground wires builds and the TPS voltage to the ECU builds up telling the ECU to give it gas and air (IAC)!!!!

Also, any, and I mean any small vacuum leaks, even at the valve cover gasket run the idle up!!!!! Leave no leaks untended.

Lastly, my only remaining high idle problem is a loose floor mat, The floor mat slides up against and under the gas peddle randomly!!!!!!:shiver:

i agree with the prior two posts on vac leaks. i had a buddys 89xj over at my house and we got to talking about vac leaks. i have used seafoam in my engine introduced through the vac system (brake booster hose) and i am familiar with the way the engine will rev when the hose is disconnected from the booster assembly- creating a vac leak. he got to looking and found a hose completely missing at the "tower" at the front of intake manafold. im not too sure where the hose was supposed to go to. when the spot on the "tower" (had more than one vac line) was covered, the rig idled beautifully, remove the obstruction (finger) and the idle would jump back up.

i would check for vac leaks. it cheep to diagnose-free- and has a realy cheep to bandaid, tape. the bandaid fix doesnt last long as the heat from the engine will screw with the adhisive. if you find a leak, tape it up if possible and keep looking for more. start at the intake manifold and follow each line out. pay attention to all connections and any oily spots on the plastic lines. after all the vac leaks have been found, check how it idles. replace the broken lines as soon as possible. when i checked my rigs vac system i was able to find 4 vac leaks and i wasnt even having any idle issues yet.

hth
stewie