Perplexing no-start issue (Page 3/3)
buddycraigg NOV 02, 10:13 PM
That would depend on how you are testing it.
WalkerTexan NOV 03, 12:45 AM
The red wire should be a solid 12V when ignition is on. The blue wire is the actual ECM signal which is active "low" meaning it pulls to virtual ground in the ECM when pulsing the injector. When the blue wire is "low" (near ground) current flows through the injector coil because of the 12V differential across the two wires which causes it to open (spray). When the ECM needs to turn off the injector, the ECM places the blue wire in the default state of "high" which is 12V. With 12V on both wires there is no current flow through the injector because the differential is now 0V which keeps it closed. I don't remember the exact number of pulses at warm idle but you can easily see that it isn't a constant spray.

You should be able to see the fuel spraying if the engine is running by removing the lid of the air cleaner. Also, if it won't start then have someone else crank it while you watch and see if it sprays while cranking. If it doesn't then you may have a problem between the ignition module and the ECM. I would expect you to see a code ( 42 IIRC) but who knows. The module produces a "cranking signal" to the ECM which tells it to spray fuel to get it to start. Once it starts the module signal is a higher frequency which tells the ECM to assert the "bypass" signal so that the ECM takes over the spark advance timing (EST) for the system. Maybe the cranking signal is low or flaky causing this. I seem to recall the cranking signal is equivalent to 400 RPM, or around 14 Hz IIRC. Could possibly be the module, the pickup coil or poor wiring between the distributor and ECM. Just my two cents since I haven't worked on the distributor-based system in a long time.
WalkerTexan NOV 03, 12:52 AM
Some of what you said makes sense that this is the problem. The ECM controls the spark once the engine starts, thereby taking away the control from the ignition module. If that "hand off" works then that explains why it runs when you "snort start it" with carb spray. The ECM detects the increase in speed from the module and takes over. If the signal from the module isn't quite up to the 400 RPM (again, IIRC) then the ECM may not spray enough (or at all) because it doesn't realize the engine is cranking.

I know you changed the module so maybe it is a wiring issue? Maybe at the ECM end?
Yorgle JAN 25, 05:31 PM
For posterity, I thought I'd update this thread as I finally found the source of my no-start issue. Turns out that for the past year I was wrongly assuming the problem to be that the injector wasn't firing, when in fact, it was the TPS- damn thing periodically go it into its mind to send a steady 4.5v the ECM for no apparent reason, causing the Duke to hopelessly flood itself. I figured it out only after hooking a digital multimeter to the TPS signal wire inside the car so I could see what was going on while driving. Sure enough, after a few days of trouble free driving, enjoying watching the TPS voltage vary with the gas pedal, it suddenly jumped to 4.5, the engine light immediately came on. I stopped and shut the car off, let it sit for a while and then tried to restart. Sure enough, it wouldn't start. Pulled the plug off the TPS and it started right up. Reconnected the TPS and it still read 4.5 with the ignition on (engine not running). Manually moved the lever on the TPS a few times and it dropped back down to .9v. Car then started perfectly. The lever on the TPS wasn't actually sticking, but there must be some sort of short inside that every now and then causes the wiper to contact the 5v feed. Anyway, I hope this info helps someone down the road.
Patrick JAN 25, 06:36 PM

quote
Originally posted by Yorgle:

The lever on the TPS wasn't actually sticking, but there must be some sort of short inside that every now and then causes the wiper to contact the 5v feed. Anyway, I hope this info helps someone down the road.



Interesting. Thanks for posting that!
cvxjet JAN 25, 10:52 PM
We need a symptom/cure list for troubleshooting our Fieros.....
zkhennings JAN 26, 11:10 AM
Good you have this solved, but these shady 80s electronics always seem to benefit from extra grounding. I would definitely consider running at least one extra ground wire direct from battery to the block or heads. If there is any resistance dropping voltages at the sensor or ECM, especially when the cars been sitting a bit and the batt is a little low, then the electronics go wild. Once running and the alt is upping the power it may be less of an issue or masking the issue.

I highly recommend this, you may find the car runs significantly smoother. I did this with my 2.8 and it was extremely noticeable.

I did have an issue with my 2.5 where one of the pins on the ECM was getting intermittent current, it had 12V at the ECM pin, but checking with a test light only lit it up dimly. I ran a new power line to it and my stalling and no start problems were solved. It is worth going through the harness at some point and ensuring that you can illuminate a test light at all the power pins going to the ECM, and ensuring your ground pins are low resistance.

A good quality ICM is also a must if you ever find it needs to be replaced. Try to find NOS so you can get a US made Delco one vs the Mexican made ones of today.

I am going standalone ECU for my new swap, and a big reason for this is to get a brand new wiring harness and much higher quality electronics. I love my Fiero but it has given me all the electrical issues in the time I have owned it.

[This message has been edited by zkhennings (edited 01-26-2023).]