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500 RPM idle & hesitations

M

mattbred

NAXJA Forum User
Location
Canada
I have a 1989 Cherokee 4.0L Manual transmission.

When my jeep is cold and I start it up, the idle sets down to 500 RPM's. If I barely even touch the accelerator, the thing hesitates, then speeds up a bit. If I get it up to, say, 1000 RPM's, then let off the throttle, the RPM's will dip below the 500 mark, almost stall, then pick back up. Obviously this aint good.

I've done all the common tune up jobs (filters, plugs, etc). I cleaned all my grounds, and resistance from all parts is 0. I have two TPS's, and checked them both on it. Hell, I even unplugged the TPS and started it up - exact same thing happend.

The problem goes away at normal temperature, yet if I really hammer on the accelerator, it still hesitates.

Any idea what this could be? I disconnected the line that goes from the EGR solenoid to the EGR-Transducer, no effect. I plugged it, no effect. Disconnected the TPS and started it, no effect.

I did search the forums but I can't find symptoms exactly to what mine are, and most threads end without any conclusion as to how the problem was fixed.

I appreciate any help. Thanks!
 
Have your battery tested. Sounds stupid, but a battery going down hill or one with a weak cell will make the truck idle slow and run crappy.
 
EGR valve itself may be stuck and / or leaking even though you disconnected the vacuum actuator lines!

Haynes manual says low fuel pressure can do this!!!!

You may have a bad wiring connection between the TPS and the ECU. Mine does the same thing, but only when I disconnect the TPS.

Lastly there is small chance the air or coolant temperature sensor is giving bad values to the ECU at cold temperatures. They can also be easily tested with an ohm meter and rough temperature estimate (ambient for instance).
 
Ecomike said:
EGR valve itself may be stuck and / or leaking even though you disconnected the vacuum actuator lines!

Haynes manual says low fuel pressure can do this!!!!

You may have a bad wiring connection between the TPS and the ECU. Mine does the same thing, but only when I disconnect the TPS.

Lastly there is small chance the air or coolant temperature sensor is giving bad values to the ECU at cold temperatures. They can also be easily tested with an ohm meter and rough temperature estimate (ambient for instance).
Click to expand...
How do I test the EGR valve itself then? I've always had the sinking feeling that it's the EGR. At a higher RPM, it's way less prone to hesitation issues.

Also, fuel pressure was 30 with it running. (he didnt test it with regulator vacuum hose disconnected).

I put a ground right to the engine block with the TPS, and I've disconnected it, changed it etc.. I dont think its the TPS. not sure tho.

I'm not too sure where the CTS is? I think I found the knock sensor. It was sort of beside the O2 sensor. I didn't see any other connectors going into the wiring connection though. Mind taking me a picture? I would be most grateful. Thanks!

(also, my nephew wanted me to click this, hehe)
:geek:
 
Try cleaning the throttle body with some sensor safe cleaner, mine had some hesitiation when it was off idle and that cured it. Also try some seafoam in the brake booster vacuum line. Both are very cheap to do and they help the engine in the long run anyways.
 
mattbred said:
How do I test the EGR valve itself then? I've always had the sinking feeling that it's the EGR. At a higher RPM, it's way less prone to hesitation issues.
Based on age, desperation, etc, I just finally changed mine, it was about $50 to $70 at Autozone. Other than removing it and inspecting it, I don't know a way to test it that is fool proof. There is a test as I recall, but it does not prove that it is good, only proves one of the ways it can be bad (as I recall, but I could be wrong on the EGR testing part).

Also, fuel pressure was 30 with it running. (he didnt test it with regulator vacuum hose disconnected).

Sounds OK for now.

I put a ground right to the engine block with the TPS, and I've disconnected it, changed it etc.. I dont think its the TPS. not sure tho.

Your TPS is good, I was refering to the wiring harness connections between the TPS and the ECU itself. There is C101 bulkhead fitting on the firewall for instance.

I'm not too sure where the CTS is? I think I found the knock sensor. It was sort of beside the O2 sensor. I didn't see any other connectors going into the wiring connection though. Mind taking me a picture? I would be most grateful. Thanks!

CTS is hiding up under the intake/exhaust manifolds, driver side about the center of the engine block. You can only see it from underneath the jeep.

http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm has a Renix sensor review with pictures in a PDF file you can download. If its not madxj, there is a link to it in one of the "high idle speed" (search words to use) threads in this forum. Those threads have a lot of sensor info you could probaly use for this problem too.


 (also, my nephew wanted me to click this, hehe)
:geek:
Click to expand...
 
Last edited:
This sounds like a head slapper, but recently I made a couple of high speed runs up the autobahn, idle got really low and the motor got sluggish off idle when first accelerating. I blew my air cleaner full of oil, the filter sucks air very poorly when it's oil soaked.
Time for a new air filter and a valve cover to intake vacuum line cleaning.
I clean the connectors and spray a little oil behind the plunger on my IAC periodically. They do get sticky. The IAC also needs a fair amount of juice to do it's job correctly, the ECU ground at the dipstick needs to be clean, the connectors clean and the charging system/battery working well.
When I disconnect my TPS with the motor running, as I recall my idle most always drops, though sometimes very little.
A typical start up for mine, with a cold motor, is it will momentarily rev to a bit above a thousand, them settle down to 700 or so (my tach isn't real accurate at those RPM's), as the motor warms up and gets above about a quarter of the gage the idle will settle in a little lower, 600 or so. The idle sometimes goes even a little lower, if I've been idling for an extended period and the motor get above 210-215 or so. I've often wondered if this is because my O2 sensor is getting carboned up, my engine temp. sensor is out of spec. or it's part of the programing. It works well most of the time, so I leave well enough alone.
Until the motor gets above 140-160 the sensor inputs to the idle program are minimal.
IMO the Renix is designed to idle a little low. To prevent creep with the auto tranny possibly. I've adjusted my idle up and the tranny creep got excessive. I had to push on the brake fairly hard to keep it from creeping forward. I keep it below 600 RPM or so.
You can get a pair of needle nose pliers into the side of the EGR (most times) and twist and apply a little pressure towards the manifold. If it moves any towards the manifold, it's likely sticky. It isn't that big of a deal to remove the EGR, though finding a replacement gasket may be hard, I smear mine up with a little muffler puddy. I've removed a few and never really had any trouble with frozen bolts or other serious pitfalls.
The ECU sends a signal to the EGR solenoid when idling (likely from the TPS) which closes the solenoid and cuts off the vacuum to the EGR. The default for the EGR solenoid is open (passes vacuum) with no power. If the electical plug falls out of the solenoid, you have EGR vacuum at idle which can cause a very poor idle and even stalling.
 
Thanks for all the suggestions. I remember a site now that enables you to bypass the transducer or some such and go to the EGR valve which opens it, and makes it run like garbage. I will test this, and if it runs no different, then its EGR I guess.

Also, I rememberd that my knock sensor is completley destroyed/unplugged. Could the computer be retarding timing on me?
 
Update:

I unplugged the EGR solenoid and put it's output vacuum right to the EGR. Engine almost stalled. So I guess EGR is out of the question?
I tested both temperature sensors and they seemed to be in par with the charts. Also, how the hell are you supposed to replace the coolant temperature sensor? It's right behind the engine mount and I can't find any feasible way to even get a look at it..

And I also found this lil doohicky with two contact points at the very front of the engine, beside the CTS. What is it?
oddsensorah1.jpg
 
Looks like a block heater with the cord removed.
 
Somebody in another thread here was having the same problem as you (except yours behaves when it is heated up), and they just figured it out. The stealership that installed their new TPS had installed it improperly. The lever on the TPS was on the wrong side of the accelerator lever (or something similar) and pushing the accelerator was not moving the TPS sensor! It was as if there was no TPS sensor, accept for the idle setting.

I would still bet most of the problem is one of the temperature sensors is stuck reading a fixed, warmed up engine value to the ECU.
 
Last edited:
hmm unfortunatley my TPS is all hooked up perfectly :p
Still though, it is only set at 2.5 volts.. any higher and it revs at 3000.


I checked both temp sensors and they were giving out a resistance similiar to being at cold engine temperature.. so that can't be it, unless it's different at the ECU. This is really gettin me down..
 
mattbred said:
hmm unfortunatley my TPS is all hooked up perfectly :p
Still though, it is only set at 2.5 volts.. any higher and it revs at 3000.


I checked both temp sensors and they were giving out a resistance similiar to being at cold engine temperature.. so that can't be it, unless it's different at the ECU. This is really gettin me down..
Click to expand...
Where do you have 2.5 volts?

TPS is 0.82 volts from pin B to pin C at idle, and near 5 volts input (pin A to pin B). That is for the three pin connector.
 
B & C is 2.5 Volts. If I reset it to 0.8 volts, my engine will start up and immedietley rev at 3000 RPM's, and the IAC motor will be fully retracted.

A & B is 4.94 Volts.
 
mattbred said:
B & C is 2.5 Volts. If I reset it to 0.8 volts, my engine will start up and immedietley rev at 3000 RPM's, and the IAC motor will be fully retracted.

A & B is 4.94 Volts.
Click to expand...

But that is definately your problem!!!!!!!!!

It should be 0.82 volts, pin B to C.

Do you have the 3 pin connector attached backwards? (don't ask me why I am asking, LOL!).

If not try reseting to it 0.82 volts. Start the engine, let it run a few seconds (even at 3000 rpm), turn it off. Cycle it on (restart it, let it run at least a few seconds, more if the idle dropped) and then off, do this at least 3 times. Be sure you leave it off for at least 10 seconds before restarting each time, that gives it time to reset the IAC through the latch relay during the power down.

If that does not work, then check the ECU to see if it is seeing the same voltage as the TPS at idle. My guess is, if the proceudre above fails, there is bad connection and voltage drop between the TPS pin C and the signal wire reaching the ECU from there.
 
Alright. Well I did set it to 0.8 volts and then let it idle for a couple seconds, shut it off, restarted it, etc about 3 times. Will letting it sit for 10 seconds actually fix it? I sure hope so. Will have to do this when it's warm. Thanks for your advice.
 
Ecomike said:
Somebody in another thread here was having the same problem as you (except yours behaves when it is heated up), and they just figured it out. The stealership that installed their new TPS had installed it improperly. The lever on the TPS was on the wrong side of the accelerator lever (or something similar) and pushing the accelerator was not moving the TPS sensor! It was as if there was no TPS sensor, accept for the idle setting.

I would still bet most of the problem is one of the temperature sensors is stuck reading a fixed, warmed up engine value to the ECU.
Click to expand...

This was my problem ^^^^ :viking:

Believe it or not, I could not get my idle to come down below 3500 RPM, even after doing the turn it on, turn it off 3 times routine. I disconnected neg. battery cable for 10-15 minutes while cleaning up the shop, doing other maintance.....Hooked up the neg. battery cable, then started. Idle came down to ~750 (a bit higher than normal), and only then could I do the start/shutdown routine to allow the computer to learn the new voltages coming from the TPS. Mattbred....This may be something that you need to try


*edit* While my Jeep runs 100% better now, I still have a extremely low idle. She's not stalling out or anything now, so I'm just going to leave it as-is. Probably some issue with my IAC.
 
Well thanks to your advice it seems better. I set the TPS to 0.82, disconnected the negative battery terminal for 40 mins, hooked it back and started it up. Idled at 700ish. Did the shut it off & cycle routine like 4 times, and it likes to idle at 700. Hesitation is lot less noticable too. However a problem still remains. If I let it idle, then rev it up to 1500, slowly or fastly, doesnt matter, then let off the gas, the RPM's will dip below the 500 mark, kinda go rough for a bit, then recover. Is that signs of a bad IAC? I will drive it around a bit and see if it gets better. Much thanks!
 
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