1990 305 TBI LO3.

Issues first started with car starting & running fine, then after 20 -30 minutes would die. Straight-wired fuel pump at relay (connected orange and grey wires on relay connector) to make fuel pump run constantly to eliminate fuel issue. Decided must be ignition related, that after 20-30 mins ignition module and/or coil getting hot and not making spark. Verified by letting car die, taking coil wire off distributor cap, testing for spark while cranking - no spark from coil wire. So I replaced ignition module and coil, go to start, and no injector firing - they fired fine before ignition module/coil replacement. Tested injectors - each has one wire (I forget what color) that has 12v with key on. Verified gas in TBI unit by removing fuel pressure regulator and seeing fuel in bowls, and watching gas spray out with fuel pump straight wired to run. Soooo, put it all back together, tried to start it one more time, and when I wired the fuel pump to run, it didn't.

My questions:

1) Fuel pump should constantly run with a wire jumped from orange wire to grey wire on the fuel pump's relay connector, right?

2) If above is coorect, this eliminates the ECM and the oil pressure sending unit from the fuel pump circuit right? Allows fuel pump to just run, regardless of anything else, right?

3) I read that injectors get a constant 12v, and that what fires them is a grounding enacted by the ECM - I do have 12v on each injector with key on, so is this correct?

4) If above correct, then how can I verify that the injectors are getting the grounding signal to make them fire from the ECM?

5) What information does the ECM get from where to trigger it into firing the injectors?

6) If wiring the fuel pump at the relay connector the way I have, doesn't bypass the ECM to make the fuel pump run (and that's why the pump now runs and then doesn't), and since the injectors are not firing, and given the intermitent ignition problems, should I be looking at replacing the ECM?

I really hate to even think about ECM - 99% of problems that folks want to attribute to the ECM are actually something else - I rarely see anyone replacing an ECM with any success. In addition, the chip has been custom programmed to accept my mods, including ported/polished heads, LT1 cam, headers, carb intake, 1.6:1 rockers.

So I'm looking really for something else if anyone has any ideas.

Much thanks in advance!

 

I think you're back in distributor land...

It ran but shut off after the module got hot, so you replaced the module and now the injectors don't fire.

The ECM gets its signal from the module. You either got a bad new module or your pickup coil is actually the culprit. Check where the pickup wires plug into the module and see if the connector is damaged. You may be losing your signal to ground or it may be open.

 

Jeff, just so you know the ECM does not run the pump. The oil pressure switch is what keeps the pump running. The ECM however does prime the pump IIRC. Jumping the relay terminal will send constant power to the pump like you said. I doubt you have an ECM problem. You're also correct that injectors are ground side switched. To test that with a meter can be tricky. If you read across the injector connector terminals while cranking the voltage will likely drop from 12V to roughly 6V give or a take a few volts. The BEST way to test the injector circuit would be either with an o-scope or a noid light. Either will be able to show you the rapid voltage changes while a meter is generally too slow.

I would agree that the issue is likely in the ignition system since you were already in there. I have learned in the past that when replacing a faulty ignintion module it really can't hurt to replace the pick-up coil at the same time. I've replaced the module only to have the coil fail within the next few weeks, if not days. Ohm out the pick-up coil and see what you get. It should be between 500 and 1500 ohms generally. And remember if it is on the high end of that range that resistance goes up with temperature, but since you are having a starting issue that is not necessarily a concern.

Let me know if you need any help. I'd be glad to lend a hand, just shoot me a PM.