Chasing down a miss

[8T7K5]

An oscilloscope would be ideal, or at least a set of Noid lights. Both can be used to check the signal from the ECM to each injector to verify that they're firing correctly. It sounds like you may have a wiring or ECM issue, given your symptoms and what you've replaced.

Not really familiar with Noid lights or what they do, I'll have to do some reading.

Looked it up, looks simple enough. Dumb question but given the dual intake system on the Vortec BB, how's it supposed to run without the upper intake?

 

[stutaeng]

Yeah, noid lights are simple and inexpensive.

When I did the injector balance test on my 5.7 Vortec, #6 was stuck closed. Basically, it was dead.

Just to be sure I didn't have an electrical issue I back probed the wiring for this injector and tested with an LED test light. When I cranked the engine the test light blinked, so I knew then the injector was getting pulse.

I know they are different injectors as yours, but you can test this as well. Never hurts to test the basics, and helps you narrow your possible culprits.

If you are not getting pulse, then obviously problem is elsewhere.

But oscilloscope would be better, as it could actually show the waveform of the signal. I don't much more than that. Just seen Eric O from South Main Auto on YT use it. BTW, good channel for electrical diagnostics.

 

[SAATR]

Not really familiar with Noid lights or what they do, I'll have to do some reading.

Looked it up, looks simple enough. Dumb question but given the dual intake system on the Vortec BB, how's it supposed to run without the upper intake?

It won't run without the upper intake. The injectors being under the plenum makes diagnosing a wiring issue a bit more difficult given your lack of access to the wires. You can use T pins to probe at the ECM connector and check signal there, but if the issue is in the harness you'll still have to check at the injector itself. An LED test light connected in parallel will give you some idea of how good your signal is or isn't, as gaps in the flash pattern will be pretty obvious at idle. An oscilloscope would be better, and can be had for less than $50 off Amazon, but I can understand not wanting to buy a tool you may only use once.

Backprobing the injector connectors like this:

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Using T pins:

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Can give you a useful test point without having to damage the wires themselves. You may have to pull the plenum to set this up, but it will allow you to eliminate the ECM and harness as a potential problem, or confirm and have a diagnosis rather than having to replace more parts and hope for a fix.

 

[skylark]

18436572. 4 and 3 are right next to each other in the firing order I would make sure that I was 100% positive that the cap was good.

 

[8T7K5]

It won't run without the upper intake. The injectors being under the plenum makes diagnosing a wiring issue a bit more difficult given your lack of access to the wires. You can use T pins to probe at the ECM connector and check signal there, but if the issue is in the harness you'll still have to check at the injector itself. An LED test light connected in parallel will give you some idea of how good your signal is or isn't, as gaps in the flash pattern will be pretty obvious at idle. An oscilloscope would be better, and can be had for less than $50 off Amazon, but I can understand not wanting to buy a tool you may only use once.

Backprobing the injector connectors like this:

You must be registered for see images attach

Using T pins:

You must be registered for see images attach

Can give you a useful test point without having to damage the wires themselves. You may have to pull the plenum to set this up, but it will allow you to eliminate the ECM and harness as a potential problem, or confirm and have a diagnosis rather than having to replace more parts and hope for a fix.

You went way over my head on that one. Another dumb question but since I've never used noid lights, is it enough to just spin the engine over to see if things are working right or does each injector have to fire like with the engine running?

 

[8T7K5]

It won't run without the upper intake. The injectors being under the plenum makes diagnosing a wiring issue a bit more difficult given your lack of access to the wires. You can use T pins to probe at the ECM connector and check signal there, but if the issue is in the harness you'll still have to check at the injector itself. An LED test light connected in parallel will give you some idea of how good your signal is or isn't, as gaps in the flash pattern will be pretty obvious at idle. An oscilloscope would be better, and can be had for less than $50 off Amazon, but I can understand not wanting to buy a tool you may only use once.

Backprobing the injector connectors like this:

You must be registered for see images attach

Using T pins:

You must be registered for see images attach

Can give you a useful test point without having to damage the wires themselves. You may have to pull the plenum to set this up, but it will allow you to eliminate the ECM and harness as a potential problem, or confirm and have a diagnosis rather than having to replace more parts and hope for a fix.

You went way over my head on that one. Another dumb question but since I've never used noid lights, is it enough to just spin the engine over to see if things are working right or does each injector have to fire like with the engine running?

 

[8T7K5]

18436572. 4 and 3 are right next to each other in the firing order I would make sure that I was 100% positive that the cap was good.

That's gonna be my next step. The one that's on it is an O'Reilly brand. I've read the wires are bad about cross firing or whatever, anything in particular to look for?

 

[SAATR]

You went way over my head on that one. Another dumb question but since I've never used noid lights, is it enough to just spin the engine over to see if things are working right or does each injector have to fire like with the engine running?

Noid lights plug into the injector harness in place of the injector, so you'll have a dead miss while using it. It isn't enough to simply spin the engine over while watching the light, since you're looking for intermittent fire while the engine is operating.

 

[8T7K5]

Noid lights plug into the injector harness in place of the injector, so you'll have a dead miss while using it. It isn't enough to simply spin the engine over while watching the light, since you're looking for intermittent fire while the engine is operating.

That'll be pretty difficult to do since the upper intake has to be on in order for it to run.

 

[SAATR]

That'll be pretty difficult to do since the upper intake has to be on in order for it to run.

Right, so you'll have to make an extension harness. Two pieces of 16 or 18 gauge wire and a couple of male spade terminals like this:

You must be registered for see images attach

You'll have to trim them to fit the injector plugs, but you should be able to run them out of the front side of the intake valley and connect your Noid lights or test light to the wires and watch the injector pulse. Since you have to pull the plenum, I would rig both up like that and secure them with some electrical tape or a zip tie. Then run the engine and watch the pulses on your light to see if you are losing signal. I suggested using T pins at the ECM first because it doesn't require you to pull the plenum and will allow you to rule out the ECM before pulling the intake again. If you want to use Noid lights, you can get the same type of injector connectors off of most GM and some Ford vehicles at a junk yard. Just cut the injector harness with enough wire to splice into the wire extensions that you will make. This is the best way I know to check your injector signal, and would be very straightforward on nearly any other engine. If you aren't sure a out spark, check it again before doing any testing like this. Inline spark checking tools are fairly cheap and easy to get, and the distributor caps aren't expensive either... but if you've already checked all of that and compression is good, then you don't have too many other options if the miss is isolated to cylinders 3 and 4.

 

[Schurkey]

When it was my 7.4L with a dead miss, I verified the ignition system first--it's easier to get to than the fuel system.

Previous owner had done intake gaskets before I bought the truck. #8 didn't run at all.

Turned out to be cracked porcelain on #8 spark plug.