RPMs dropping and stalling at stoplights

[keel]

1990 Ranger XLT with the 2.9L and auto trans. No CEL, no modifications.

Runs pretty much perfect except that when slowing down to a stop or idling, the RPMs will occasionally drop really low and the engine will either almost stall or stall out completely. Only happens at operating temp. If the engine is able to recover, I notice that all the cluster lights dim and the oil pressure gauge will go to zero like the truck has lost electrical power for a couple seconds. I assume this is just because the alternator isn't turning fast enough to maintain voltage when the RPMs drop but it could be related to something else.

If the truck stalls, I can easily just restart it and drive away like nothing happened.

So far I have tried cleaning the idle air control with some MAF cleaner and I was able to get a decent amount of crud removed from it but it didn't fix my problem.

The tachometer is also wildly inaccurate although I have no clue if this has anything to do with the stalling issue or not. It will bounce between reading normal and reading super high with no rhyme or reason and seems to have zero effect on how the truck performs.

 

[PetroleumJunkie412]

1990 Ranger XLT with the 2.9L and auto trans. No CEL, no modifications.

Runs pretty much perfect except that when slowing down to a stop or idling, the RPMs will occasionally drop really low and the engine will either almost stall or stall out completely. Only happens at operating temp. If the engine is able to recover, I notice that all the cluster lights dim and the oil pressure gauge will go to zero like the truck has lost electrical power for a couple seconds. I assume this is just because the alternator isn't turning fast enough to maintain voltage when the RPMs drop but it could be related to something else.

If the truck stalls, I can easily just restart it and drive away like nothing happened.

So far I have tried cleaning the idle air control with some MAF cleaner and I was able to get a decent amount of crud removed from it but it didn't fix my problem.

The tachometer is also wildly inaccurate although I have no clue if this has anything to do with the stalling issue or not. It will bounce between reading normal and reading super high with no rhyme or reason and seems to have zero effect on how the truck performs.

Did you Pull codes from ECM?

 

[keel]

Did you Pull codes from ECM?

No, can I expect to see codes even if there's no CEL?

 

[PetroleumJunkie412]

No, can I expect to see codes even if there's no CEL?

Possibly, yes.

Tach and stalling are likely related tbh.

 

[keel]

Okay I will try pulling codes then. Any theories as to why the tach would be acting like that and how to fix?

 

[PetroleumJunkie412]

Hall effect sensor, but I'm usually wrong.

Let's see what the codes say

 

[SenorNoob]

That model I'd suspect the TFI too. It's between that pickup and the coil right? I know it's hard to get a good one new these days.

 

[PetroleumJunkie412]

That model I'd suspect the TFI too. It's between that pickup and the coil right? I know it's hard to get a good one new these days.

Could be a few things. IIRC tach is the hall effect in the distributor. Mine used to flutter until I changed the sensor. Leveled out stalling issues as well.

But, not sure of OPs issue. Could be a ton of different stuff, and I'm completely unfamiliar with auto's.

 

[keel]

Okay I pulled codes.

KOEO - 11, system checks ok, nothing stored in the memory

Ran KOER and forgot to press brake pedal/blip throttle - got 41 system lean, 77 no goose test, 74 BOO circuit
Ran KOER again properly - got 13 idle speed control (but no 41 strangely)

Looks like I might need an O2 sensor and an IAC. What do you guys think?

 

[PetroleumJunkie412]

IAC is hit and miss, some are junk out of the box. For stuff like that, I abuse Amazon and order 3 or 4 different ones, use the best, return the rest. YMMV.

O2 lasts 60-80k mi before they get lazy and crappy.

O2 shouldn't cause stalling issues. IAC will.

Tach issue has me scratching my head tho.

I'm trying to channel my inner RonD...

 

[keel]

Tach issue has me scratching my head tho.

I'm trying to channel my inner RonD...

Yeah I would really like to resolve that too but the truck was doing that long before it was having idle issues. It's very strange, sometimes I will be cruising on the highway and the tach will suddenly jump up like 2k RPM and it will look like I'm doing 4k+ RPMs down the freeway even though I'm just chilling in overdrive. Then it will jump back down after some time. Or it will just consistently read 500-1000 too high and then the next time I drive it is normal.