Electronics Gurus Please Help - The 5.0Tpi from H*ll

The storey so far - Replaced PROM, CTS and Cold Start Sensor, TPS & IAC. Everthing Set Up correctly as per Tech Articles.

The problem - Start engine from cold, starts to idle @ 1500rpm then drops to just over 1000rpm after 2 minutes.

3 minutes or so later later the SES light comes on, idle goes crazy and engine nearly dies then idles back up to 1500rpm then settles at just under 1000rpm again.

Then the SES light stays on and the Cooling Fan starts to run for 30 seconds or more then stops. During the fan running the SES light is on and off, then engine starts to idle at 750rpm in Park but is definately still cold.

The car then seems to clear itself of these strange cold start running conditions and warms fully to operating temp without any further problems.

The Codes coming back are 51 and 42.

I guess 42 because the idle went under 400rpm when going crazy but 51 is the PROM MEMCAL fault code.

Is my ECU shot? (I haven't got a plug in scanner but might have to resort to taking the car to a workshop for diagnosis)

(Just a thought but would any excess wear in the Throttle Body Cross shaft cause this sort of problem because I have been told that this will effect the idle settings - although the engine idles correctly @ 600/650 in drive)

Anybody throw any light on my problems?

Any information gratefully received before I get the cutting torch on the car and send it to Tpi heaven.

& not necessarily in that order!

 

The code 42 is a problem with the electronic spark timing. Check for a bad connection at the ignition module and make sure that the EST bypass wire is hooked up(will be a single lead wire on the passenger side of the car near the heater blower motor, cant miss it, it sticks up out of the wire harness).(Tan/black wire) Your ignition module might be bad too.----- As for the code 51, you are right about a prom error, make sure you are using the right prom for your year car and engine.(cant put an 89 305 tpi prom in place of an 88 305 prom) Proms are different for each year and will make your car not run right and act funny when running. Make sure it is installed right and make sure there is not any pins bent where it plugs into the computer.(make sure you have the right prom before you go out and get another computer) The wrong prom can cause all of your problems.-----If you have excessive movement in your throttle shaft, I would look into get it fixed or replaced(could be a problem) Hope this helps you out.

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1989 iroc-z 350 TPI, auto trans, leather int. t-tops, edelbrock headers and cat back system, SLP cam, crane double roller chain, 4 wheel disc, and aftermarket hood. Iroc-z's forever, Ford Mustangs NEVER

 

Thanks for your reply jermdm.

The code 42 set because the idle dropped below 400rpm. I know the base timing is okay and that the ignition side of things is working correctly.

The qusetion is my ECM shot? and if the Throttle Body Crossshaft excess wear will cause the problems listed?

Anybody else got any thoughts?

Thanks once again in anticipation.

 

Hi Guys,

Just a note to thank everybody for replying to my previous postings.

I'am pleased to say that my Tpi is now running as it should.

Turned out to be a bad ECM and a bad Cooling Fan Relay!

Got a used ECM from eBay for $42.00! Works perfectly.

Thanks again.