idle speed gone nuts
- Thread starter dbrub2
- Start date
graewulf
New member
Can't find Squeel
you might want to try and seee if the TPS came loose.. it is adjustable to some degree and may have slipped if it came loose. also see if you can slow the idle down by moving the throttle at the throttle body itself. This can also be caused by a bad mixture setting on the carb.... too much fuel and it will idle up... but since is was all of a sudden i'm more inclined to think its the TPS....(did they have one in 88?) another thing to look at (though its pretty remote) is the mass airflow sensor - this can also affect idle. are there any other symptoms? if you put it in gear does it still idle high? do you still have good pickup?
you might want to try and seee if the TPS came loose.. it is adjustable to some degree and may have slipped if it came loose. also see if you can slow the idle down by moving the throttle at the throttle body itself. This can also be caused by a bad mixture setting on the carb.... too much fuel and it will idle up... but since is was all of a sudden i'm more inclined to think its the TPS....(did they have one in 88?) another thing to look at (though its pretty remote) is the mass airflow sensor - this can also affect idle. are there any other symptoms? if you put it in gear does it still idle high? do you still have good pickup?
Thanks for the replies...let me try and answer all. First I need to say I screwed up the motor description. I have a 4.0L fuel injected motor not a 4.2L...sorry.
Since the last post I have tried unplugging and re-plugging all the sensor connections (MAP, TPS, etc.) to see if there was just a bad connection but no change in the idle speed. I checked for obvious vacuum leaks but didn't find any or hear anything suspicious. I removed the MAP vacuum hose from the manifold and the idle went even higher. The lever attached to the throttle plate (butterfly) is against the idle speed adjustment screw so I think the throttle plate is fully closed but I am not sure if there is an air bypass that is still allowing too much air. The TPS was not loose so unless it has a bad section in its motion I don't think it is the problem but that is why I am looking for your expertise.
Unless you folks have a better recommendation I'll revove the EGR and see if it is stuck open. If the EGR is OK I'll remove the throttle body and do a good cleaning. recommendations?
thx Dan
Since the last post I have tried unplugging and re-plugging all the sensor connections (MAP, TPS, etc.) to see if there was just a bad connection but no change in the idle speed. I checked for obvious vacuum leaks but didn't find any or hear anything suspicious. I removed the MAP vacuum hose from the manifold and the idle went even higher. The lever attached to the throttle plate (butterfly) is against the idle speed adjustment screw so I think the throttle plate is fully closed but I am not sure if there is an air bypass that is still allowing too much air. The TPS was not loose so unless it has a bad section in its motion I don't think it is the problem but that is why I am looking for your expertise.
Unless you folks have a better recommendation I'll revove the EGR and see if it is stuck open. If the EGR is OK I'll remove the throttle body and do a good cleaning. recommendations?
thx Dan
Ok, now I can help ya out 
.
What you need to do is get a carb or throttle body cleaner, and clean out the throttle body paying especial attention to the Idle air control (IAC) motor.
The IAC motor IS your idle speed control, and if the passages around it get dirty, it can become stuck and/or won't return to fully closed, causing high idle.
My Jeep would be racing one minute and trying to die (>400 RPMs) the next during idle. A good TB cleaning did the trick
-Nick :!:
.What you need to do is get a carb or throttle body cleaner, and clean out the throttle body paying especial attention to the Idle air control (IAC) motor.
The IAC motor IS your idle speed control, and if the passages around it get dirty, it can become stuck and/or won't return to fully closed, causing high idle.
My Jeep would be racing one minute and trying to die (>400 RPMs) the next during idle. A good TB cleaning did the trick
-Nick :!:
Thanks. Do you recommend removing the TB to do this or attempt it on the manifold (I'm worried about getting crud into the manifold etc.) Also searching the web I found reference to Chrysler IAC motor that could get 180 degrees out of phase and the harder the computer tried to close the IAC the more it opened because of the out of pahse condition. You ever here of this?
Stilll having problems. Removed the TB and cleaned...it was real dirty (240000 miles worth). Pintle on the Idle Speed Stepper motor was really gunked up so I thought for sure that was my problem but when I reassembled the thing the sympton is the same. If I cover the bypass hole the idle drops to normal so the idle speed motor seems suspect. The problem is that the manual says I got to have a special "exerciser" in order to make the stepper motor move and see if it is working correctly. Well I don't have this device. I know how a stepper motor works but I don't have a way of pulsing the winding in the correct sequence to make the pintle move to verify it works.
With great intrepidation I turned the sleeve causing the pintle to move outward and reinserted it in the TB. I expected to feel the pintle contact the brass plug where it seats and cuts off the bypass air but it did not even after extending the pintle a good 1/4 inch. I know the ECU resets the pintle when the car is turned off so after starting the car and then shutting it off I pulled the stepper motor and the pintle was again retracted so I know it moves one direction ok. Is there anything that would make the ECU think the pintle should open fully even though the throttle is closed?
I put a meter on the TPS with the key on and the book says I should read 0.8 volts with throttle fully closed. Voltage was about 1.7 so I thought Oh this must be telling the ECU to open the pintle. I could not get the TPS down to the 0.8 volts reliably. I think the potentiometer is dirty so to verify the TPS was not causing the problem I removed it and got it to finally ready 0.8 volts but no effect on the stepper motor pintle position.
Looking for more expertise...help
Dan
With great intrepidation I turned the sleeve causing the pintle to move outward and reinserted it in the TB. I expected to feel the pintle contact the brass plug where it seats and cuts off the bypass air but it did not even after extending the pintle a good 1/4 inch. I know the ECU resets the pintle when the car is turned off so after starting the car and then shutting it off I pulled the stepper motor and the pintle was again retracted so I know it moves one direction ok. Is there anything that would make the ECU think the pintle should open fully even though the throttle is closed?
I put a meter on the TPS with the key on and the book says I should read 0.8 volts with throttle fully closed. Voltage was about 1.7 so I thought Oh this must be telling the ECU to open the pintle. I could not get the TPS down to the 0.8 volts reliably. I think the potentiometer is dirty so to verify the TPS was not causing the problem I removed it and got it to finally ready 0.8 volts but no effect on the stepper motor pintle position.
Looking for more expertise...help
Dan
graewulf
New member
Transmission
it sounds to me like the 2 of them are problemmatic.. you may end up replacing both to fully correct your problem. unfortunately, at this point, without the computer reader to get the error codes, you have a 50/50 shot of fixing it the first time... i'd start with the TPS..
it sounds to me like the 2 of them are problemmatic.. you may end up replacing both to fully correct your problem. unfortunately, at this point, without the computer reader to get the error codes, you have a 50/50 shot of fixing it the first time... i'd start with the TPS..
Okay:
Here are the sensors that will effect the position of the IAC motor:
> Coolant Temp sender (the one in the block)
> Knock Sensor
> Throttle Position Sensor
> A/C request signal to ECU
Although the TPS sounds suspect, if you did sucessfully get it to output .8 volts, with no change on the IAC, I'd check the other sensors listed above for proper operation first.
-Nick :!:
Here are the sensors that will effect the position of the IAC motor:
> Coolant Temp sender (the one in the block)
> Knock Sensor
> Throttle Position Sensor
> A/C request signal to ECU
Although the TPS sounds suspect, if you did sucessfully get it to output .8 volts, with no change on the IAC, I'd check the other sensors listed above for proper operation first.
-Nick :!:
You folks have been great in responding to my questions so I wanted to let you know the outcome. I measured the coolant and manifold temp sensors with ohm meter and they were within spec. Measured MAP sensor voltage...it too was in spec. Didn't measure knock sensor and am not sure how to measure AC request signal.
I just happened to have a 4.0L engine in my shed (had picked it up from boneyard two years ago with thoughts of doing a rebuild at my leisure and then swapping next time clutch needed changing) Anyway I took the TPS and IAC from the spare and installed them on the car. I had removed the battery cable two days ago just in case there was ECU memory that might need to be erased. I installed the parts, attached the battery cable, adjusted the TPS for 0.8 volts and fire it up. Much to my dismay the idle was still real high. I turned off the car and checked to see if I left something disconnected. Restarted the car and idle fell to normal like nothing was wrong??? Now I really was confused and worried that I had not fixed the problem and that it would show up again.
Since I had changed two parts (IAC and TPS) I wondered if putting the old IAC motor back on would show anything. I swapped them but still worked fine. I knew the original TPS was flaky so I left it but bottom line is that I'm not sure what the fix was.
I wonder if the original TPS had confused the ECU and the IAC position was trying to adjust the IAC to match what the TPS was wrongly indicating. The new TPS fixed that but since the IAC (during the swap) was moved from its "home" position the ECU didn't know where it was. The first start had the IAC position in this unknown position so the ECU didn't know what to do. When the car was shut off the IAC was set to "zero" so the next time it was started the ECU knew how many pulses to give the IAC to put it in the right position?? Just a wild guess and not from anything in the manual so I could be way off base.
Any thoughts? Thanks again for pointing me in the right direction. I still have the spare motor complete with manifolds, alternator, harness, etc. so if anybody needs it let me know. I have 240000 on my motor but oil pressure is still 60-lbs at cruise so I don't plan on rebuild. Got a leaking real seal that I'll fix when clutch needs changed but other than that motor now runs like a champ.
Dan
I just happened to have a 4.0L engine in my shed (had picked it up from boneyard two years ago with thoughts of doing a rebuild at my leisure and then swapping next time clutch needed changing) Anyway I took the TPS and IAC from the spare and installed them on the car. I had removed the battery cable two days ago just in case there was ECU memory that might need to be erased. I installed the parts, attached the battery cable, adjusted the TPS for 0.8 volts and fire it up. Much to my dismay the idle was still real high. I turned off the car and checked to see if I left something disconnected. Restarted the car and idle fell to normal like nothing was wrong??? Now I really was confused and worried that I had not fixed the problem and that it would show up again.
Since I had changed two parts (IAC and TPS) I wondered if putting the old IAC motor back on would show anything. I swapped them but still worked fine. I knew the original TPS was flaky so I left it but bottom line is that I'm not sure what the fix was.
I wonder if the original TPS had confused the ECU and the IAC position was trying to adjust the IAC to match what the TPS was wrongly indicating. The new TPS fixed that but since the IAC (during the swap) was moved from its "home" position the ECU didn't know where it was. The first start had the IAC position in this unknown position so the ECU didn't know what to do. When the car was shut off the IAC was set to "zero" so the next time it was started the ECU knew how many pulses to give the IAC to put it in the right position?? Just a wild guess and not from anything in the manual so I could be way off base.
Any thoughts? Thanks again for pointing me in the right direction. I still have the spare motor complete with manifolds, alternator, harness, etc. so if anybody needs it let me know. I have 240000 on my motor but oil pressure is still 60-lbs at cruise so I don't plan on rebuild. Got a leaking real seal that I'll fix when clutch needs changed but other than that motor now runs like a champ.
Dan
I need a motor Dan. Send me an email letting me know whay you want for it and where you are. Sound_Man@pcisys.net Thanks and I am glad the problem is fixed.
superfiveo
New member
Well I just signed up to post this problem I have with my 88 4.0L, I will try to see if any of this information works for me. Good info
superfiveo
New member
Thanks,
My dad gave the jeep to me since I helped him put he motor in it when he got it a couple of years ago. My first jeep and I like it alot.
My dad gave the jeep to me since I helped him put he motor in it when he got it a couple of years ago. My first jeep and I like it alot.
