Low rpm idle / stalling on cold

[SnowyOwl]

Hi all. For the whole time I've owned this car I've experienced this interesting phenomena where I get very low idle RPM (~600rpm) during the first 3-5 minutes after I start the engine. After a few minutes, the rpm gets to around ~800. No misfiring as far as I can hear.

Additionally, during this phase, if I rev the engine up and let the gas pedal off "at once", the RPM will rapidly drop to zero and the engine stalls. Stalling occurs also if I change to D or R. If I let the gas pedal off "smoothly", the engine will barely stay on. I can rev the engine no problem as long as I don't drop the pedal too rapidly.

This happens whenever the engine is cold, summer and winter. This doesn't happen once the engine has warmed up.

I've replaced the T-shaped PCV hose behind the throttle body, and some other parts of the same line as well. To my understanding, another piece that could perhaps cause symptoms like this is the idle air control valve, but is there anything else I should check before replacing that? It seems it's not that fun of a part to change on this 1994 edition of the 4.6 engine.

 

[theterminator93]

It sounds like an IAC to me. Other common culprits for odd idle issues include vacuum leaks, TPS or MAF issues.

TPS is relatively easy to test.

MAF is less easy to definitively isolate, but you can unplug it to force the car to use a different set of tables for computing fuel delivery (if it runs better with it unplugged, it's bad).

 

[KevinVarnes]

Sounds like the IAC to me as well. I'm not as familiar with the 94/95 4.6. What makes it so difficult to change? You could try cleaning it first, but results can be hit or miss on that.

 

[SnowyOwl]

Sounds like the IAC to me as well. I'm not as familiar with the 94/95 4.6. What makes it so difficult to change? You could try cleaning it first, but results can be hit or miss on that.

It appears to be positioned somewhat awkward, at least if compared to the 96/97 models. Iirc I found it to reside in a narrow place somewhere behind the throttle body or so. Although, maybe I should try before speculating further.

 

[White Lincoln]

Also check for vacuum leaks while your at it. Could be a combination of things, especially with our older cars.

 

[Grog6]

Sounds like the IAC to me as well. I'm not as familiar with the 94/95 4.6. What makes it so difficult to change? You could try cleaning it first, but results can be hit or miss on that.

The 94/95 cars have a few issues that make them a drag, mostly fixed in 96-7 cars:
The "guitar case" inlet tube, TB on back of manifold, valve guide probs,egr passages from hell, eec's hang injectors, transmissions suck, and harder to upgrade. Among other issues.