95sc automatic randomly stalls when at stop or reverse- potential problem shooting?

1995 Supercharger, v6 for reference:

When at at stop the engine just suddenly gets quiet and usually her rmp drops a bit before and then the engine cuts out. Like the dash and everything still on so you have to 'turn the car off' then on again.

When at a red light, and she starts to do that, if you put her in neutral she's okay and wont stall. (this is more frequent that she stalls in this manner)

When I go to reverse, rpm goes down (normalish from what I understand because heavier load on engine) but sometimes she cuts out (not always but it's happened three times in the last five months).


Her fuel is okay and was above a quarter tank each time. She starts up just fine, and has only puttered I think once and thats because I took my hand off the key too soon I think. I checked transmission fluid a while ago, and it's a reddish pink each time I look at it (after the engines ran a bit ofc).

No check engine lights at any of the events listed. Based on research done so far, o2 sensor is my next suspect, but I wanted to bounce some ideas to make a checklist of sorts.

I find that with my '93 the rpm will do that. Summer problem. Try opening the windows and turning off the ac. When the ac cycles on it loads the engine. Maybe all you need to do is turn up the idle a bit.
Alan

Could be a number of things, from worn plugs, to vacuum leaks, to failing IAC, etc.

Something you can try that won't cost more than a can of spray carb cleaner, is to remove the black rubber air intake tube off the throttle body, and then closely inspect the bore and throttle plate for black carbon-like build up. Use the spray cleaner and a lint free rag to wipe down/scrub the bore and plate, being sure to open the plate so you can get to any buildup on both sides of it and on the bore as far as you can reach/see. Reassemble when clean and monitor symptoms for improvement.

Thanks for noting the MIL, but you still might want to read codes.

As for the O2 sensors, I'm not sure they'd be suspect in this particular example, but...how many miles/years on them? They don't last forever, so this may be a good excuse to make new. Pro tip: if you do replace the O2 sensors, -don't- buy the cheapest ones you can find, and be wary of fake motorcraft parts/packaging.

Good luck.

KMT said:

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Thank you so much for the ideas! And sorry for the late response, been doing lots of work on her to get her running perfect for the inspection.
The reason I thought O2 was because she does misfire at times, seldom but it happens, and like you said, nothing is forever.

For the MIL: I read that some problems don't throw lights so I got my dad to mail me the OBD reader
The codes that ran were 116, 538, 536 and 532.
I will have to decipher them properly, but I have the just of them from the manual.

I do notice it happens when she's nice and toasty like ALS35 mentioned, so another culprit maybe is the radiator?
In any case I bought a new thermostat and its on the side waiting incase that is it. (I mean for a few bucks its a spare part I don't mind having in the garage).

116 - O,R - Coolant Temperature Sensor (CTS) out of range

532 - ?

536 - O,R,C - Brake on/off switch circuit failure

538 - R - Insufficient change in RPM

-=-

Basically, those mean the test wasn't run correctly, starting with 116, where the 'action' is "Normal operating temp not reached", meaning the engine has to be up fully warmed up. Speak up if you need directions, etc.

So, have to ask...was the engine up to full operating temp, or did you just connect the reader and run codes? If it was up to temp, I'd start looking close at the cooling system.

Issues that can cause that code:
- low coolant level
- faulty connector
- faulty sensor

Where is the coolant level in the expansion tank when the engine is hot? Any bubbles in it while the engine is running? Is the coolant in the expansion tank clean, milky, dark color, oily? Remove the oil fill cap and see if it is clean or milky underneath.

How old is the radiator cap? How old is the coolant? When the engine is cold, remove the cap and look inside the radiator to check for gunk, scale, etc. Look inside the nose to see if there is any debris blocking airflow into the radiator and/or A/C condenser.

Is your issue worse/consistent with the A/C on? The computer is supposed to detect when the A/C is on, and increase idle to avoid stalling. If the CTS (aka ECT sensor) has failed, it could be feeding incorrect info. There are ways to test, but low hanging fruit is to check the connector to make sure it is seated and there is no corrosion that can inhibit current flow, etc. The CTS sits on a stand-off pipe, on the intake manifold, right front/pass side, up top, just ahead of the oil filler cap. Note that according to one of the Ford diagnostic manuals there are cases where low idle, A/C on requires a new computer, but I don't think you're there just yet.