Idle issues

[rumblecloud]

Backstory...
I picked up an ECM from the pull yard awhile back from a matching 94 4.0, 4wd - rare to find one still in the vehicle let alone from a matching Splash model. I had already replaced the 3 capacitors in my stock unit, but the board was still a bit sketchy. It ran very well with the new capacitors until recently and starting running rough. So I replaced the stock unit with the pull yard unit (which was very clean inside) and the performance was noticeably better - still threw a 157 code, but still way better than before.

The idle issue happens when sitting still, in gear or in park, foot off the throttle, idle at about 8-900 rpm. After about 3-4 minutes the idle would drop to 600, then pick back up. then drop again. The cycle repeats, dropping progressively lower until it finally just stalls. It starts right up again and rinse and repeat.
I'm about to go out and go thru the 157 chase, but thought I would ask if there may be another unrelated issue.

Thanks for any input.
R

 

[franklin2]

What is the idle set to when it's warmed up and you unplug the idle air control valve? If it stalls, I would check/clean the throttle body and idle air control valve passages, and if it still stalls when you unplug the idle air control valve, I think I would tweak the idle stop screw. Adjust it so the engine is slowly turning, but does not stall when the wiring is unplugged to the IAC

 

[rumblecloud]

Ahhh...thank you franklin2
I'm poking at it now and will do that first.

 

[rumblecloud]

In answer ti your question, the idle is 900 when warm, and the drops in rpm occur when idling at stop - foot on the brake in drive, in park and in neutral.
So far, It seems to be pointing to the MAF. I pulled the Throttle body and gave it a good spraying with Throttle body cleaner. Upped the stop screw to 1000 rpm. Pulled the IAC and cleaned that.

I thought that when the IAC is unplugged that would cause a stall? And it does that - well the idle drops.

With the IAC plugged in, the idle still drops. Unplgging the MAF, the idle drops a bit but stays running. Voltage at the red and black wires is battery voltage - a little less but not much. Still have to check the other two tomorrow.

In the meantime, I'm thinkin about bringing out the parts cannon

 

[bobbywalter]

what year?

 

[rumblecloud]

Hey Bobby...it's a 94 4by 4.0...

 

[bobbywalter]

i wonder if the modulator has an issue or leak in the vac line....pvc elbow ect.


usually that is high idle though..

 

[bobbywalter]

i wonder if the modulator has an issue or leak in the vac line....pvc elbow ect.


usually that is high idle though..

this is more like converter drag or bump. i have seen the modulator surge though and do wierd stuff.

 

[rumblecloud]

Modulator?
Sorry.

 

[franklin2]

I thought that when the IAC is unplugged that would cause a stall? And it does that - well the idle drops.

It can drop in idle, but it should not stall with the IAC unplugged. Each year can have a little different spec, but it should keep running with it unplugged.

 

[rumblecloud]

I did turn the idle screw up to get 1000 rpm at idle.
MAF voltage checks out. Smoke test revealed two issues: Intake tube has a puncture near the throttle body and the bottom bolt hole in the intake manifold that holds the IAC in is stripped, so I can't tighten the IAC down enough to stop air from getting in.
If correcting those two issues doesn't fix the the problem, I guess I'll move to the oxygen sensor .ugh.


EDIT: Unplugging the MAF does not cause stalling, which would make me think the MAF is the issue. But it doesn't matter if it's plugged in or not, the engine still stalls as described earlier...

My brain hurts....oh well, one thing at a time.

 

[franklin2]

Fix those leaks. Those are uncontrolled air going into the engine, the computer and MAF would not be happy about that. Once you get those fixed, get it warmed up and then unplug the idle air control valve and set the idle to 500-600. 1000 is too high.