Help with diagnosing an issue (no temp no tach on dash) 1991 reatta

[jg2003]

Alright so since the last time I posted about my car the (1991) reatta has had spark plugs replaced,fluids flushed,new fuel tank and fuel pump+filter. On top of that a new iac and maf and a new thermostat and coolant Temperature sensor. She starts at first crank now and idles but whenever it goes into "limp mode" it idles rough and the fans turn on.

 

We're chasing one last gremlin so currently the car will either run fine or run rough and in "limp mode" sometimes it fixes itself occasionally. What's happening is that if temperature isn't registering on the dash we get no rpm signal.

What I noticed is that when the car showed no temperature or maxed out the gauge the car thought it was running at 150 Celsius which is impossible when the car hasn't even been on for 30 seconds and there will be no rpm signal just 0.

(occasionally maxed out temperature engine not actually overheating)

 (No rpm signal)

 

Sometimes all of a sudden temperature comes back and the car reads 80 Celsius and the rpm reads again and the car idles like normal oh and the seat belt chime starts up.

(normal temperature reading and rpm is displayed)

 

I got a related coolent temp too high code. I saw that some people had similar issues with temperature and rpm going out and it would turn off the ac some said a new ecm helped.

 

How should I tackle this issue? any ideas on where to start would be appreciated.I really want to get her running properly to be able to enjoy driving during spring and summer.

 

(Limp mode means car is stuck with low power and won't go above 20 mph ish activated when no temp or rpm is displayed)

 

(Oh and also sometimes if I let the car warm up for 15 to 10 minutes the temperature will slowly tick to the middle and rpm will come back)

Edited December 12, 2023 by jg2003

[DAVES89]

Sounds like a failing Ignition Module/coil pack. Try your previously tested spare and see what happens...

[2seater]

Those symptoms are almost identical to what I experienced with a bad PROM in the ECM. I am not certain what the 91 uses since the transaxle is electronic and I believe it is an integrated unit, a PCM. The lack of a couple dash displays, rough running, cooling fans turned on are identical to what I experienced. When I tried onboard diagnostics, I received an engine data or communication error.

[jg2003]

On 12/12/2023 at 7:31 AM, 2seater said:

Those symptoms are almost identical to what I experienced with a bad PROM in the ECM. I am not certain what the 91 uses since the transaxle is electronic and I believe it is an integrated unit, a PCM. The lack of a couple dash displays, rough running, cooling fans turned on are identical to what I experienced. When I tried onboard diagnostics, I received an engine data or communication error.

Hmm is there a way to check that or would I just need to get a new prom to test and see if that's the issue? Hopefully it isn't the PCM cause Ive heard it's a pain in the ass to get to.

[2seater]

Do your onboard diagnostics work? If they do, do they act strangly when the gremlins show up? You may try unplugging the harness connectors at the pcm and even better, remove and reinstall the prom. You seem to be getting an intermittent. Can you access engine data in diagnostics while it is running?

[jg2003]

2 hours ago, 2seater said:

Do your onboard diagnostics work? If they do, do they act strangly when the gremlins show up? You may try unplugging the harness connectors at the pcm and even better, remove and reinstall the prom. You seem to be getting an intermittent. Can you access engine data in diagnostics while it is running?

 

Yeah, I can access onboard diagnostics and I can get engine data, which is how I know it doesn't register rpm when the temperature gauge goes wonky. I'll look into accessing the Pcm then and unplugging the harness connectors and checking out the prom.