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TBI not firing injectors.

79rustyk10

79rustyk10

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My 79 K10/20 has a swapped in TBI 350 from a 1988 K5. This pile has been sitting for almost a decade, until recently. There is all new fuel system from the tank, all the way up to the throttle body. As of last night it would start and run.

Today I get home to chase some vacuum leaks, and I get a crank, no start. It will start with carb spray, but is not getting fuel. The relay cycles, and the pump runs. It builds oil pressure per the gauge. I can visually see the injectors not firing when cranking.

I have power to the red, and white injector wires, with key on. When cranking the blue and green wires will not fire.

My first thought is the ignition module, as by my understanding, that controls the ECM firing the injectors. I will swap that tomorrow, as I have one as a spare. Where else would be a good place to start if not an ignition module? Seems my searches online mainly bring up guys shotgunning every part possible at it. I'd rather diagnose this system, and not spend any more than necessary, as it is only temporary at this point. This is a stone cold stock, TBI 350 as well. Short of headers, nothing has been modded.

I'm already regretting not going back to a Qjet instead of fixing the TBI.
 
Since it runs it would be somewhat unusual to have the distributor be the issue since the ecm uses the fact that it's firing the plugs to ground the injectors and fire them. Not impossible I'm sure, some electrical magic could happen with the module.

I would *think* even with a couple psi of fuel pressure you'd see something out of the injectors, but have you used a test light on one of the injector connectors just to make sure they aren't being pulsed and it's a no fuel issue?

It's relatively simple if you aren't aware of how it works...Injectors are fused ignition 12v. You know you have power to them. When the module "sees" the distributor firing, which you know it does because it will run, the ecm gets this info and then grounds the injectors to fire.

Your check engine light works properly? Turn key to run, flashes once then stays on?

If you want to do the research, the service manuals (not electrical) at the link in my signature have the flow chart for diagnosing a crank no start condition. You are expected to know the wiring is good and not assume it is.

When it runs, you'll forget all about your frustration and temptation to put a carb on it. The more you learn about the TBI, the easier it will be moving forward. The qjet Imo is no simpler, if you really start digging into them.
 
I agree with everything @dyeager535 said. Just confirm there is fuel pressure at the injector. A clog, pinched line or plugged filter will cause the same. Sometimes moving around new components moves the dirt
 
dyeager535 said:
Since it runs it would be somewhat unusual to have the distributor be the issue since the ecm uses the fact that it's firing the plugs to ground the injectors and fire them. Not impossible I'm sure, some electrical magic could happen with the module.

I would *think* even with a couple psi of fuel pressure you'd see something out of the injectors, but have you used a test light on one of the injector connectors just to make sure they aren't being pulsed and it's a no fuel issue?

It's relatively simple if you aren't aware of how it works...Injectors are fused ignition 12v. You know you have power to them. When the module "sees" the distributor firing, which you know it does because it will run, the ecm gets this info and then grounds the injectors to fire.

Your check engine light works properly? Turn key to run, flashes once then stays on?

If you want to do the research, the service manuals (not electrical) at the link in my signature have the flow chart for diagnosing a crank no start condition. You are expected to know the wiring is good and not assume it is.

When it runs, you'll forget all about your frustration and temptation to put a carb on it. The more you learn about the TBI, the easier it will be moving forward. The qjet Imo is no simpler, if you really start digging into them.
Click to expand...

I had a test light on the wires for firing the injectors, and there was nothing there at all.

I am 95% positive the CE light works, the entire harness from the K5 was transferred into the truck. Will try pulling codes tonite, but it does come on.

Will add more later, I'm at work now.
 
Yeah, I'm not concerned with codes as much as I am that the light works. That flash/steady on tells you the ECM passed an internal test. If that doesn't work right, it's better to figure that out before dealing with any other issues.

And while this was a swap, it HAS run after the swap, right? It sounds like it did, just making sure.

If you had a test light on the injectors, and it didn't blink, then the ECM ground wasn't happening. You had the test light legs connected to each pin of a single TBI connector, correct?

If the ECM isn't grounding the injectors to fire them, the ECM is not getting the signal from the module that it's cranking, the ECM is bad, or there is a wiring issue. Which is unlikely if it just stopped working right without any other work going on, but not impossible. The ECM being bad is unlikely as well, but anything is possible.

Bad connections from swaps are a reality, I had a terminal lock that broke and one wire at the ECM would randomly lose connection because of it. Took me awhile to find that issue.
 
The distributors that I've seen from the TBI era seem to wear out faster than other era's of GM distributors.

For the cost of a new one from a parts house, I'd be checking into that instead of just a module swap...
 
That is definitely correct. If the pickup coil is bad (which is another common failure at this age, which will leave you stranded) you have to pull the distributor to replace it. Edit: Not saying the pickup is bad, it runs, just saying if you are inclined to replace any ignition components, replace the whole distributor at one time.

You can get a complete distributor for less than $100 nowadays.

Also not a random parts swapper guy, but I happen to have spares of everything, and that eliminates the cost part of that method lol.
 
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dyeager535 said:
That is definitely correct. If the pickup coil is bad (which is another common failure at this age, which will leave you stranded) you have to pull the distributor to replace it.

You can get a complete distributor for less than $100 nowadays.

Also not a random parts swapper guy, but I happen to have spares of everything, and that eliminates the cost part of that method lol.
Click to expand...

I would have to go with your first assessment. If the engine will run with a fuel source other that the TBI injectors, then the pick-up coil and control module must be working, which create the signal to the ECM to fire the injectors, or the spark plugs would not be firing at all. Need to find out if there is any fuel getting up to the TBI at all. If there is, then the ECM is not getting the signal to fire the injectors from the pick-up and control module. You should also check to see if the ECMB1/ATO fuse is burned out.
 
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I'm not sure how the signal for the module works, is it impossible for the module to fire the coil but not signal the ECM? I don't know if there are two circuits that handle that, or the same signal for both. I don't have a wiring diagram handy I can view.
 
I believe the module controls both,I have seen a bad module have no spark,but pulses the injectors OK,and others that had good spark but no injector pulse--evidently one circuit can fail and not the other,or "come and go" intermittently too..

My friend has had to replace quite a few of the TBI "small" distributors,they seem to have caps and rotors fail often,and the shaft gets loose in the housing--some had severe rusting going on inside too,they were all pasty white on the die cast and all the steel parts were rusty..
 
Somebody posted one time that the initial injector pulse for a TBI starts at the pick-up coil, goes through the control module, and then to the ECM.
 
Ok so...

This truck ran, and drove after the swap. Both before being parked in 09, and last weekend.

None of the nearby parts stores can test my spare ignition module. Tried it anyway. No change.

The check engine light does blink once, then will stay on, turning key to on position.

All fuses check out good.

Fuel is slightly dripping from the injectors today, pump still runs, relay cycles, and the injectors still wont pulse. At this point, fuel pressure is not the issue anyway.

The wiring harness doesn't appear to be damaged, or chewed up anywhere. From ECM, all the way out. I opened up the little cover on the ECM, nothing appeared out of the ordinary.

I'm at a loss of where to go from here, other than double, double checking all the plugs. Logic would have me thinking that the issue is somewhere between the ECM, and the engine seeing as it passed it's self test, correct?
 
you were testing for a pulsing ground on the injector wires right? From your first post this is unclear. Power to one side of the injector, pulsing ground to the other. Also, have seen the pickup cause no injector pulse but have spark.....many times.
 
black dawg said:
you were testing for a pulsing ground on the injector wires right? From your first post this is unclear. Power to one side of the injector, pulsing ground to the other. Also, have seen the pickup cause no injector pulse but have spark.....many times.
Click to expand...

I was testing for the pulsing ground. Both injectors have power to them with the key on.
 
I have personally had this work on an engine that sat for less than a year.
Just tap on the injectors with a screwdriver handle or something. I even had the engine cranking while tapping on them. Since you have done all of the other stuff, smack them!
 
79rustyk10 said:
I was testing for the pulsing ground. Both injectors have power to them with the key on.
Click to expand...

I know very little about TBI but can you give it ground manually to see if the injectors open?

For what its worth, my dads 89 454 TBI had a similar no start situation. Replacing the ignition module also didn't help but replacing the distributor fixed it.
 
Carcinogen said:
I know very little about TBI but can you give it ground manually to see if the injectors open?
Click to expand...

You could, but since the ECM grounds them, that unfortunately really won't tell you anything beyond what the test light told him. The problem is definitely "upstream" of the injector, but that includes the ECM and the distributor, plus all run of wire between those. At worst the injectors could ALSO have failed, but that isn't related to the ground issue he is seeing, and would of course be incredibly unlikely.
 
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