Engine cranks but does not start, weak injector pulse

1990 CHEVROLET 1500

150,000 MILES • 5.7L • V8 • AUTOMATIC

MARIUSJTR

MEMBER
2 POSTS

C1500 5.7L TBI. Crank no start. The Noid light doesn’t light. But if you look closely, you can see the wire within glow orange in time with the injector pulse that should be there. We’ve cleaned and checked grounds a thousand times (engine and ECM grounds). We have power to both injectors and fuses check out to confirm this. Coworker recently discovered some wires were shorted together but no improvement after fixing them. “New” Refurbished ECM. Could the shorted wires have fried this one?

Fuel pressure is great. New IACV, new fuel pump, new fuel filter, checked the continuity of wires from ECM to injectors and it was good, new distributor and cap, new ignition module, timing checked, new EGR valve (old one was stuck open. Thought I had solved the darn thing). Don’t have a code scanner that can do OBD1.

ECM is supposed to send the pulse by internally supplying ground to the injector circuit. Even tried replacing PROM chip to no avail. Could the new ECM be shorted? I don’t know what else it could be. Thinking of running wires straight from ECM to fuel injectors just to see. Also is there a way to check for pulse right at the ECM? I can’t be sure if my multimeter isn’t sensitive enough to detect the quick switching to ground or if it’s just not present. Thanks for any ideas!

Mar 21, 2022 at 7:05 PM

KASEKENNY

✓ASE Certified Mechanic
18,907 POSTS

You are correct that the ECM should be supplying and grounding the voltage for the injectors so if they are not lighting a light then the ECM would be the issue.

https://www.2carpros.com/articles/how-to-test-a-fuel-injector

However, we need to verify we are doing have a wiring issue going to the injectors. Make sure there is not high resistance in the wiring by ohming out the wiring.

Here is a guide that will help but basically, we should have less than a half ohm on the wiring to and from the injectors:

https://www.2carpros.com/articles/how-to-use-a-voltmeter

As for the short that you are finding, what circuits were shorted together? That could have burned the driver in the ECM, but it would only make sense if these wires were found after the ECM replacement.

Basically, the ECM was shorted due to the wiring issue, you replace the ECM. It was damaged and then you found the wiring issue. If you then repaired, it you would need to replace the ECM again.

Let me know if this is what happened and what you find with the wiring.

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Mar 22, 2022 at 7:18 PM

MARIUSJTR

MEMBER
2 POSTS

I figured it out today! We did end up trying a new ECM and still had the no start issue. Engine coolant temperature sensor was busted, and the MAP sensor was faulty. I assume it was the ECT sensor that caused the no start. Put in a new pigtail and ECT and new MAP sensor and she started right up. All that headache. In all of the literature I read on this truck there wasn’t a single thing that indicated the coolant temperature sensor could cause a crank but no start condition. Oh well. Glad I figured it out.

And yeah, I had already ohmed out all the injector wires and all the wires to ignition module as well and checked powers and grounds. I appreciate the help! Hopefully someone sees this in the future, and it helps them out.

Apr 12, 2022 at 10:38 AM

KASEKENNY

✓ASE Certified Mechanic
18,907 POSTS

This is great info, and I am sure this will help someone in the future.

Please come back in the future and thanks again for keeping us updated.

Thanks

Apr 13, 2022 at 9:51 AM

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