89 Cherokee Erratic Idle

Hi,
89 Cherokee automatic. Starts like a dream as usual, but begins to drop rpms periodicaly. Sometimes its fine and others I will put it in drive and unless I am quick and give it a lot of gas it stalls. Ive done searches but cannot find any simalair cases.
Any input?
Quinny

TPS- Throttle Position Sensor

This thing being out of adjustment on my '89, gave me issues for months until I got it adjustd right.

TPS, I will go look it up and see what I can do. Let you know how it went in a bit.
Thanks
Quinny

I back probed the TPS as instucted by manual. All readings where fine. I have cleaned the throttle body. This is why Im a bit stuck, Checked O2 sensor, its fine.
Any other ideas?
Quinny

How long since you replaced the plugs/cap/rotor?

Air filter ok?

Fuel pressure good?

Did the problem start recently? On my 2000, when I have the defrost on I'll get a surge in rpm's as it turns on and off. Not enough to kill the engine though.

Hi,
Long time no speak, air filter is clean, fuel pressure is good, cap and rotor are fine, gonna go pull the plugs and have a quick check.
Back in 20 mins!
Quinny

I may actually still be on line in 19 minutes.

How about the plug wires? They can be bad even if they look ok...if showing signs of wear that could contribute.

I got to the workshop and my Step Dad already had them out and they look good. I rechecked the distributer roter etc and the rotor looks pretty bad! I only replaced it 1 yr ago. I will get a new one this afternoon. The wires were replaced at the same time and they look good too. Any ideas as to why the rotor will degrade so quickly?
If there is a reason why I would love to fix it.
Quinny

Couple of thoughts on the rotor:

1st, it's the miles, not the time. The rotor only degrades when used. I've always replaced mine each 20000-30000 miles, and never had a problem. Make sure to replace the rotor at the same time.

2nd, what cap & rotor did you use? Copper (penny colored contacts) is the best and longest lasting, and worth any extra you might pay. You can also get aluminum (silver contacts inside the cap) which is usually cheaper but doesn't last as long. Also I've found some vehicles run much worse with the aluminum.


What I've found in older vehicles with higher mileage is that each little electrical issue adds up and can make bad things happen. I've seen a vehicle that would hardly run start to run perfectly with new plugs, cap, rotor and wires.

Aluminum is what I used. Will buy a copper and see what happens. Will also buy a rotor cap. Well only time will tell now but I will let you know tonight what worked and what didnt:)
Quinny

I think you'll be pleasantly surprised - let us know.

Shoot. I'm interested to find out what happens next. *bites nails*

Hope citizen@large has some nails left! Still happening.
Maybe I can describe it a bit better. I will be driving and come to a light and stop. 1 out of every 5 times it tries to stall when I give it gas to leave intersection or sometimes idles eraticly when stopped and I need to put it in neutral and give it gas quick before it stalls.
Quinny:o

Oh and how would you adjust the TPS? Might try fiddling with it and see what happens.

Well now, that could get you into more issues than you'd expect...

Just make sure to clearly mark the origional position of the TPS before you go and adjust it.

Anyway, it's located on the throttle body and has two torx-style screws that keep it in position. Just loosen them and barely adjust it one way or the other. Drive it, repeat.

Like I said, it took me a long while by this method to find the 'sweet spot'.

Also look at you IAC- (idle air control / idle stepper motor) It may need cleaning or replacing. Forgot about that one...

-----------------------------------------------------
From- http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

Throttle Position Sensor (TPS)


TPS and Idle Stepper Motor.
http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics/Sensor_TPS.jpg
Located on throttle body facing toward radiator. Located on the right side of the front of the throttle body. To test, disconnect the connector at the TPS. Check the voltage at TPS connector pins B (Ground) and A (5.0 volt supply) with the ignition on. If you see about 5 volts, hook up the connector again. and check the voltage at TPS terminals C (positive) and B (Ground) with the ignition on and the throttle plate closed. You should notice approximately 0.8 volts. If you see that voltage, your TPS and wiring should be ok. If not, you'll have to adjust the TPS to get that voltage. If you cannot adjust the TPS to get that voltage, you'll have to replace the TPS and adjust the new one properly. Also, with the voltmeter hooked (terminal C) up and the ignition on, move the TPS arm down and back up slowly and watch the voltage. The change should be increasing and smooth. If there are any dead spots or it is not smooth, it probably needs to be replaced.

> What does it do? The voltage is interpreted by the ECU to determine current engine operating conditions. Just as a note to you, the automatic transmission versions have 2 integral connectors (1 for ECU and 1 for tranny). If you have the auto, make sure you're checking the right connector (I'm not exactly sure what it looks like as I have the manual tranny).
Update: Suggestion: If you are having intermittant idle / surge issues and can't quite get the voltages to set right - before going out to buy a new TPS try checking your ground connection. Renix engines are notorious for having bad / faulty ground connections as a result of age and not so great design in the connectors. A common "fix" is to add a jumper ground wire from the TPS ground that bypasses the connector and harness entirely.
First - Check ALL of your engine ground connections.
Second - Check the ground wire from your TPS to the firewall to ensure that there is not a poor circuit to ground. If the circuit resistance is high then the voltage reading will be incorrect to the ECU and the computer will not know what the throttle opening is actually at & the engine will not run or shift properly.

Citizen@large, that is without doubt the best and most detailed message I have ever recieved from any forum on any subject!
Thank You very much!!
Thanks to everyone so far and I will let you know what I find.
I liked this site before but now Im Impressed!
Quinny

hmm... guess my 'style' is finally 'paying off' around here.

I aim to resolve. That's about it Quinn. I see no need in getting tired of reposting the most frequent issues people face with this vehicle.

Thank you for your appreciation.

All I ask in return is that you, and anyone else that gets any help from this forum, please state the resolution of their issue(s) in order to further educate the many masses that frequent the XJ section of AF.com

~Good Luck Friend