KOER test '87 2.3L EFI

[SECESH]

The test codes displayed were numbers 21 and 13. The manual states that these indicate, respectively, the engine coolant sensor out of range, and the idle speed control or air bypass not controlling the idle, with a side note that this usually results in too high an idle.
There is still popping going on in the exhaust. Sounds like a potato banging around in a metal bucket, if one can imagine what that would sound like. I would not characterize the idle as overly high, seems normal to me.
Any thoughts?

[5.0 bird]

Well, when the coolant temp sensor was out on my Dad's '90 model 2.3 it made it run and idle terrible. So that may be your problem.

[SECESH]

Is this sensor separate from the temperature gauge sender? I notice two potential candidates listed among the available parts.

[Haystack]

Yes, there are two sensors, one for the guage and one for the sensor.

If the idle doesn't change when you unplug the IAC, the IAC is bad. Try taking off the IAC and cleaning it, then try to do a base idle reset. Essentially you just unplug the IAC and idle it down until the engine starts lugging or barely runs then plug it back in. If the idle doesn't raise, then either the IAC is bad (likely) or bad wiring which is easily checked with the key on and a volt meter.

[Walking-Tall]

Quotes from Charles O. Probst SAE book, "How to Understand, Service and Modify Ford Fuel Injection & Electronic Engine Control":

KOER 13 - "Cannot control rpm during ER Self-Test low rpm check - idle too high (DCM/BPA)"
KOER 21 - "ECT out of Self-Test range (short or open) (run for 2 minutes and retest)"

Code 21 doesn't outright call out the sensor as bad, but connections or it's wiring circuit. A short circuit or open wiring.
With a fully warmed up vehicle (~195-degrees Fahrenheit) the ECT should output about 0.61v.

"An open between ECT and the control module results in a constant 5v signal at the control module. A short will result in approximately 0 volts in the circuit. Corrosion in the circuit at terminal connections results in higher-than-normal voltage due to the voltage drop at the connection. Extra resistance could cause hard cold starts."

The 2.3 ECT sensor should be at, "... at center of intake manifold". One wire to it should be LG-Y (light green w/yellow stripe) and the other B-W (black w/white stripe)

[SECESH]

Thanks so much. This modern stuff really isn't quite as complicated as I thought. Makes sense now. Different but not much more difficult than the old breaker points and carburetors.

[Walking-Tall]

You're welcome. So, while it could be the sensor, the code makes the call to check the circuit. Checking and/or fixing a short or open connection certainly costs less than replacing sensors. It's all different enough that I don't care to have to mess with it all much longer with my '86, lol

How did you make out?...

[SECESH]

I went ahead and checked prices, and since the stuff is all 29 years old, I bought the two parts and replaced the easy target, the air valve, first. Running as it should now, so the other sensor will either go back to store or get stashed against future need.
All in all still a considerable saving over having a shop work on it.
Really appreciate all your help.

[Walking-Tall]

That's great, glad you got it sorted out