coronet charlie

I have a 1988 w100 4x4 with a 318 engine w/TBI this engine idles rough , once-in-a-while it stalls at a red light but off idle runs pretty smooth, any ideas 💡

dodgem880

Being pre-OBDII makes things tricky.

I have no experience with the factory OBDI TBI systems but, since your problem only seems to be at idle I'd turn my attention to the IAC (idle air control) valve. Dodge called this the Idle Speed Control Actuator.

Ideally you would want to find a way to test it before just replacing it. I'll see if I can find a testing procedure when I have more time. (For example an ohm test).

Other than that, do you have a check engine light on? If so, maybe you can get some more information or even confirm a faulty IAC by using flash codes:

https://www.2carpros.com/articles/re...d-light-trucks

coronet charlie

Thanks for your reply, At this point changing that part may be the easiest route to start with, I have looked for vacuum leaks found none , I ran a compression test and wasn't happy with the result but tested cold so not accurate anyway . Thanks I will change that part.

Kuvasz101

EGR valve stuck open?

dodgem880

Good one. I really like the idea of checking the EGR if the truck is equipped with one. I completely forgot about those since my trucks don't have them.

And since the EGR is vacuum operated one should be able to connect a vacuum hand pump to it. With the engine running. If the valve still functions the idle should get worst maybe even stall. If there is no change in idle then the valve is stuck or blocked off with carbon.

I would check this before replacing anything.

coronet charlie

I can certainly check the EGR VALVE but it was new when I bought the truck one year ago .

dodgem880

I understand but, it never hurts to check.

I found some test information for the IAC if the EGR checks out.

The IAC has four wires.

GRAY/RED = IAC motor (to pin 19 of the PCM)
BROWN/WHITE = IAC motor (to pin 17 of the PCM)
(or just brown)
VOILET = Idle Contact Switch (to pin 26 of the PCM)
BLACK = Ground

For some reason the service information wants you to take measurements at the PCM connector with it is unplugged... Personally I wouldn't dare mess with that connector in a vehicle this age for fear of breaking it.
Instead I would recommend checking ohms at the pins of the IAC itself while unplugged. I tried to rewrite the steps to reflect this.

Test 1 - ohm across motor
No ohm range is given for the IAC motor other than it should have some resistance. Shouldn't be shorted but, also shouldn't be open. I believe either condition should cause a code 25 dtc to be set.

Test 2 - ohm across idle switch and ground
From what I can tell this switch should change state (open or close) as you move the throttle. Just make sure it's not stuck. Trouble here may set a Code 35 (idle switch motor fault).
Pin 26 of the PCM is labeled "CLOSED THROTTLE" in the wiring diagram. You would think the TPS sensor would already supply this information... Interesting.

Here are some links that may be useful:
https://www.allpar.com/threads/the-1...-codes.229009/
https://www.allpar.com/threads/chrys...-range.229098/

coronet charlie

Thanks so much for all your leg work! I will try to check this out this week end, I will let ya know what I find .thanks again

Skwerly

i worked at autozone for years; bad parts come in new boxes. trust me.

coronet charlie

Thanks for that insight 👍

I got caught up installing side stepts and have not checked egr yet