83 T Type-,code 12 comes up?

[Guest northmn]

During engine start up & on choke for 2 or 3 min.- the idle suddenly jumps several hundred rpm all by itself & the check engine lite turns on.When I give the gas pedal a boot to take it off choke -the lite goes out & engine idles to normal.I see this doesn't have an ICS.Could the problem be the ECM ,HEI module,or open or grounded reference lead? Am I looking in the right place here?

Bob.

[Guest pfloro]

Hello Bob:

You may already know this but with 1) the ignition on/engine not running and 2) ECM in diagnostic mode (ALCL terminals 'A' & 'B' shorted), a code '12' will <span style="font-weight: bold">always</span> be present. The ECM doesn't see a 'distributor reference pulse' if the distributor is not turning. <span style="font-weight: bold">NOW...</span>, if you can 'flash out' a '12' with the <span style="font-weight: bold">engine running</span>, then something is wrong. <span style="font-weight: bold"> <span style="font-style: italic">Are any other error codes stored?</span></span>

Yes, I believe you're looking in the right area. The idle speed is increasing because the ECM is commanding the HEI module to advance the timing. To verify this, connect a timing light and check the timing as soon as the engine starts. When the idle speed increases, the timing mark should indicate a decent amount of advance. The mark will probably jump past the indicator scale (this is normal).

The fact that that the check engine light goes out & the idle returns to normal when you blip the throttle points me in the direction of a flaky ECM. When my '84 Toronado was about 12 years old, the original ECM would sometimes turn on the check engine light for a few minutes but no error codes were ever set.

I would try to isolate the problem by disconnecting the 4 terminal distributor EST (electronic spark timing) connector & observe what happens. The ECM should set a code '12' and '41' (41 = no distributor reference signal with engine vacuum present). If the idle speed (and the timing) increases, then the HEI module is flaky. The HEI module can't advance the timing by itself. Be aware, however, that the HEI module has some spark advance designed into it. If EST fails, the HEI module will advance the timing as engine RPM increases (sort of 'limp home' protection).

Also..., disconnect & reconnect as many ECM connectors as possible including the ECM itself (<span style="font-weight: bold">disconnect the battery first</span>). A little oxidation on the contacts can send the system into a tizzy. Clean & tighten the ECM system grounds (there are 2 on the intake manifold on my '84 Olds 307 V8).

I would suspect that the OBD '1' spark control systems are 'basically' the same on an '83 T Type and an '84 307 V8. Does your engine have a spark knock sensor?

I can scan & email you the appropriate diagnostic pages from my '84 Olds Chassis Service Manual...

Keep us posted,

Paul

[Guest northmn]

Paul.

Yes I have the knock sensor -it's new.My grounds are all clean.I have the big black 83 dealer service manual.I ran the (trouble Code 12 no reference pulse to ECM) tests and found the terminal 10 voltage on ECM was increasing steadily above 2.8 volts at part throttle -which means the ECM is bad -(according to the manual -I guess).After ordering a new ECM ,I walked back to the car & noticed I left A&B jumper on for some reason -(not good).Code 12 fails to come up. I ran the test for this & so I have the engine light driver on order too.I will move on to your EST isoilation tests if I still have problems.I'll keep you posted.-Thanks-.

[RivNut]

I have an observation that may or may not make a difference. Bob's car is a T-type and has the turbo charged v-6 in it; Paul's Toro would have the 5.0 L V8. Are all the codes/tests for each different engine the same?

[Guest northmn]

Ed

Good point.I haven't seen any of the tests pertaining to code 12 as being specific to a engine,but there may be other tests that are.I'am still waiting for the new ECM to come in from the dealer so i can carry on here.

[Guest pfloro]

During the early '80s when OBD 'I' was born, I know that GM used different ECMs for different powertrains. The replaceable PROM chip was unique for a specific car model & powertrain. I would highly suspect that a given error code identified the <span style="font-weight: bold"><span style="font-style: italic">same</span></span> condition across ALL ECMs. During the life of OBD 'I' (thru 1994), I suspect that GM kept adding error codes as the system evolved.

Keep us posted. We'll help get this problem solved. Hey, the new ECM might just resolve it...

Paul