edit: changed original message title: "Problem: ECM retards ignition"
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Hi,

I have on my hands my friends '88 Firebird with 305 TPI.
The car has been running like crap, and he has spent a load of money trying to fix it. Now i have narrowed the problem to ignition.

EST wire disconnected, initial advance set anywhere, say, from 6 to 15, the car idles nice, steady and sounds like a healthy v8.

EST connector plugged ECM retards! timing 10-15degs from the initial. The dash rpm gauge jumps once a while. And engine barely runs with no spark advance.

The HEI module inside the distributor has recently been replaced.

I tried bypassing the EST Knock Module (in driver side firewall) and feeding the wire EST to ECM regulated +10V so that (regarding to shop manual) ECM should see EST in 'no-knock' state. No help there.

No trouble codes whatsoever.

All ideas and help are greatly appreciated
Thanks.

 

Sounds like the polarity is backwards on the icm to pickup coil.

 

Thanks for reply, you were right about the polarity... i ended up replacing the pickup coil too.

the main problem stil remains: engine running rich at idle and idle is not very good, bad mileage, high on emissions.

Now with new pickup coil, initial advance set to 6*, and the EST cable connected, the ECM controlled advance is around 35 degs at idle and once awhile the timing jumps back and forth. EST unplugged and initial set to 15* it seems to idle pretty nice and does not smell that rich. Timing sticks right where it is set. As soon as (the advance is set to 6 and) the EST is connected the problem begins.

Test drove it, and the car has decent power, no pinging at any pedal position or speed.

new parts on the car: maf, hei module, cap&rotor&wires&plugs, injectors, O2, all intake&plenum gaskets (no vac leaks),... fuel pressure ok

I am running out of ideas...

Thanks in advance (pun intended )

 

The rich condition cold be many things, any check engine light on ?
Are the injector bodies a greyish color ?
Do you have a hand held scanner to see sensor data ?

 

Thanks for your feedback.

I multimetered the Engine and Intake temperature sensors seem to be ok.

Isn't 35* degs advance at idle way too much furthermore, it is not steady 35 but jumping occasionally. I have a hunch this might be a symptom of the root cause. Especially as EST unplugged and static adv at ~15 the engine sounds right and timing is steady.

Any ideas what might root cause this excessive advance condition?

I gotta ask the car owner for the sensor data reader. Will get back here about the results. As far as i know, he has done some data scanning already, no smoking gun found.

Thanks again.