Testing injector pulse with noid light

I have two cars with LT1 engines. A 96 Vette and a 79 Malibu wagon with a 94 z28 LT1 engine trans 4L60e swap.

I’m chasing a mis in the 94 setup. I think it’s random misfire of injectors.

I’m comparing tests on both cars to see differences. On the 94, if I pull the flashing noid light out of the injector harness with engine running and plug it right back in, it no longer flashes. It won’t flash again unless I turn the engine off and back on again. On the 96 I can pull a flashing noid light with engine running and plug it back in and it will begin flashing again immediately even without restart of the engine.

I understand the 94 is ODB1 and the 96 is OBD2. Does this explain the difference? Is it normal operation of the 94 PCM to cease providing ground pulse for an injector that has been unplugged? So I’m assuming the 94 (OBD1) requires an ignition off and restart before the pulse from the PCM will be restored, and the 96 (OBD2) will restore pulse as soon as the injector connection is restored, no restart required?

I can't explain the difference between the OBD1 vs. OBD2 Noid light flashing test. The Noid light test is a good method to verify the computer is electrically pulsing the injector on / off.

One method to verify injector function is to measure the exhaust temperatures of each cylinder. If an injector is misfiring, the exhaust temperature of the cylinder misfiring will be lower, because it's not burring as much fuel (generating heat) like the other cylinders.

dave w

Nice suggestion. I have an IR camera and Ill test the header tube temps

I can only provide indirect info, but ...

95 F-body FSM shows if injector voltage is in error (e.g., unplugged injector) for more than 4 sec, that injector driver is disabled for some unspecified period of time.
96 B-body FSM shows the above but the driver is disabled for the ignition cycle; plus has a check for intermittent voltage error (less than 0.5 sec) which apparently allows continued driver operation.

Perhaps someone has more direct info for your platforms, but in the meantime I hope this helps.

Jim

Thanks for that info

Quote:

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Originally Posted by JimCT_9C1

I can only provide indirect info, but ...

95 F-body FSM shows if injector voltage is in error (e.g., unplugged injector) for more than 4 sec, that injector driver is disabled for some unspecified period of time.
96 B-body FSM shows the above but the driver is disabled for the ignition cycle; plus has a check for intermittent voltage error (less than 0.5 sec) which apparently allows continued driver operation.

Perhaps someone has more direct info for your platforms, but in the meantime I hope this helps.

Jim

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it's true that 94-95 LT1 will kill an injector until reset if you disconnect them individually for a while. i seem to remember clearing trouble codes also re-enables the injectors live but you'd have to test that. for testing i prefer disabling the injectors in software individually, and to see if an injector is firing i use the ol' funnel and tube stethoscope, or a pin probe and oscilloscope.

i've never seen injector drivers of that type have an intermittent failure, it's really unlikely, if it's on the output side it would have to be in the wiring harness or something wrong with the injector itself.

a random injector miss on an LT1 is almost certainly an input fault rather than an output fault..in other words your optispark high res pulse has gone flunky, of course that means you'd have an accompanying spark miss as well..