2.3L ('83-'97) High idle with coolant sensor failure

[corerftech]

1990 dual plug, runs exquisite.

‘cept- coolant temp sensor NOT changed when the truck was worked over for defects. The only sensor not altered/tested/replaced.

Temp gauge used to move 1/8 inch off left stop, slowly to zero motion now. No code for missing sensor so it’s still in loop but it is certainly not working.
Coincident: idle went from 850 to 1000 regardless of warm up cycle, etc. 200 mile drive, goes right for 1000 rpm.

I have assumed the ECT sensor is giving ice cold condition and computer is raising idle to accommodate like it’s stuck in a warm up cycle.

Fuel economy is still 24mpg or better. Else the truck and engine are simply superb.

Is there a flaw in that line of thinking? Both defects occurred simultaneously.

I have a new sender and will attempt install this weekend. Just wanted some confidence —— or rebuke—- on line of diagnosis and thinking.

 

[scotts90ranger]

The gauge sender has nothing to do with the computer, the computer sensor is in the top of the intake manifold near the fuel pressure regulator right by the fuel rail I believe (2 wires in a connector). The gauge sender I'm trying to remember where it is but it just has a single wire like the oil pressure sender wire. My '90 has always lived at the low end of the scale, the only time it's gone to the middle has been when I've broke a radiator or had a hose blow...

The only things that would cause a high idle is pretty much a vacuum leak, and since it runs fine I'm going to guess it's an early '90 with a MAP sensor not a later '90 with a MAF since a MAF system DOES NOT appreciate vacuum leaks, speed density doesn't care... if it's not a vacuum leak the IAC is stuck open some...

 

[corerftech]

The ect in question is the one down below the intake (the sensor I knew I should have replaced but didn’t have one on hand!!). That is the gauge sensor and with your clarification, not part of equation.

I have one now. Will replace to make gauge work correct …. But otherwise sounds like IAC is most likely at fault.
It’s circa 1990! Very OG!
I don’t recall performing a TB cleaning on this truck either. So it’s due for some cleaning and inspection/testing.

Thank you sir for the rebuttal, sincerely. That’s what I was after. I had forgotten that there are two sensors. I’ll check EVTM before work.

 

[scotts90ranger]

Didn't see it was you while posting at work yesterday, I figured you would have checked for vacuum leaks , was just past lunch time so was just working on thought processes...

My '90 does odd things on idle too sometimes but it's a janky mess of random parts thrown together (from random years and random vehicles...), if it was a daily driver I'd care more but it's a fun rig mainly used for offroading... I'll get some kinks worked out someday...

The throttle cable is old too, but they're as rare as hens teeth, I've pondered making a new internal cable using the original sheath as the stock one with who knows how many miles had a fray just off idle which was REALLY annoying in low range, the junkyard one I'm running now has a similar issue but not as bad...

 

[scotts90ranger]

As you probably know, if you change the gauge sender use thread sealant sparingly and probably avoid thread tape since the sender uses the ground of the threads to send to the gauge...

 

[corerftech]

Follow up.I have never touched the Throttle stop screw on my truck.

I went to check oil, coolant, etc just “because”, started the truck and it idled at 1000 rpm after warm up.

I jacked the throttle at the body a couple of times and to 800 rpm it went. I disconnected the IAC wire, no effect. Shut off motor. Started again and immediately high idle kicked in. While idling high, disconnected IAC, stumbled like it should and when reconnected, right to 800 rpm.

The 800 rpm was like it behaved prior to problem. This 800 rpm came from nowhere- like going to dentist, tooth stopped hurting. The cold has arrived and maybe there is some goo or gelled oil or schmutz on the throttle cable that I freed when whacking it manually- unlikely.

Adjusted the Throttle stop after warm up to 700-725, called it good.

Drive car 250 miles of and on, behaved like a new car. Problem disappeared during diagnosis.
Plus I dropped idle 100 rpm from what was prior and now at or slightly below book spec.

no vacuum issues found either.