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TBI noinjector spray, failed noid test

J

Josue

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1976 k5 with 1995 suburban tbi, harness, electronic transmission

tore apart the stock harness and modified to fit the engine and transmission. printed all pcm wiring harness, and followed it. truck ran, and broke in the crate engine.

truck cranks, but does not start. inspected the injectors for fuel spray, niether spray. removed and checked 1 side for 12v, both have 12v. installed noid light tester. no light on either of the injectors harness. flipped tetser around, and nothing. checked fuses for intermittent contact, all good. so i checkedharness from injector to the pcm, not open, very low ohms 0.something. what controls injector ground pulse?i know the pcm controls the inj, but what does the pcm need to see in order to fire the injectors

also fuel pump is good, have fuel gauge on it with 16 psi if i remember.
 
PCM is looking for DST, distributor reference pulse.

Do you have spark? Either with spark or no spark, if you aren't getting an injector pulse, it's likely something in the ignition circuit...pulse isn't getting to PCM for some reason.
 
i do have spark, fire some ether and it fires, runs for couple seconds but while it is runing for those couple seconds, the injectors are still not firing

and on the 4 pin connector to the dizzy your talking about dizzy reference ic reference low wire 453 red with black between dizzy and the computer is lost?

or ic reference high ppl/wht between the dizzy to the pcm?
 
and i noticed my fuel pump primes for 2 seconds, which didnt always do that, i read that other trucks do it for 5 seconds, normal?

Starting Mode:
When the key is first turned ON, the control module turns ON the fuel pump relay for two seconds, and the fuel pump builds pressure up to the TBI unit. The control module checks the Engine Coolant Temperature (ECT ) sensor, Throttle Position (TP ) sensor, Manifold Absolute Pressure (MAP ) sensor, and the Ignition signal, then determines the proper air/fuel ratio for starting. This ranges from 1.5:1 at -36°C (-33°F) to 14.7:1 at 94°C (201°F) running temperature.
 
Don't have my manual handy (plus yours is newer) but yes, there will be a wire from the distributor module to the PCM in some form or fashion.

NORMALLY with the GM ignition modules it simply fails, which is easy. No spark=no pulse.

I wouldn't say the signal is lost, it could be (broken wire, bad connector) but I'd be more expecting the signal isn't getting to the PCM because the signal is not coming from the module.

Do you have the manual for the truck? Flow chart for crank no start should be in there. You already know it's probably not fuel, and you have spark, so should be able to "skip ahead" a bit.

Note: followed flow chart when I had a MAF error on my truck. Got all the way to the bottom, "faulty MAF sensor". Did the testing at least 5 times. Got good spare MAF, same issue. Turned out one of the terminals on the ECM connector was broken, and wasn't making good contact.
 
Just a thought. Did that year have the low oil pressure shut off? If no oil pressure, the injectors are shut down. If so, you may have a bad switch or wire issues in that area.
 
I don't know about the later stuff, but the earlier stuff never used the oil pressure switch as a cutoff...it was a failsafe for the fuel pump relay. Thus why one of the symptoms of a bad fuel pump relay is significantly increased engine cranking before it starts.
 
I know up to 91 di not have it. Not sure what year they added that fail safe. I did run into it on a 98 heep. That's why I brought it up.
 
But you know what? Good point. People when retrofitting (which this is) at least the '87-91 stuff often remove the oil pressure switch bypass. If something goes wrong with the relay, then no-start.

Unfortunately since the fuel pump runs (2 seconds is fine, it's PCM controlled), and he has fuel pressure, we should be able to rule out that as a cause.
 
Is the switch by passed or did he put one on the engine? The PCM's are designed to run the pump to build pressure for a couple of seconds then shut off. Then the pump will restart if oil pressure builds. That's how it was explained to me. But like I said, not sure on the 95 stuff.
 
Fuel Cut-off Mode:
No fuel is delivered by the injector when the ignition is OFF. This prevents dieseling. Also, fuel is not delivered if no reference pulses are seen from the Distributor Ignition (DI ), which means the engine is not running

The oil pressure switch is a back up to fuel pump relay, it supplies voltage to the fuel pump positive after building 4psi of oil pressure. I did install the oil pressure switch under the dizzy

I've have energized the fuel pump and kept it on with the fuel primer wire, just to make sure that my fuel pump wasn't cutting out at start up.

Today after work I will check the reference signal to the PCM. I just gotta figure out what the. Voltage is soppiuse to be.
 
ktmoutfront said:
Just a thought. Did that year have the low oil pressure shut off? If no oil pressure, the injectors are shut down. If so, you may have a bad switch or wire issues in that area.
Click to expand...

Actually that is incorrect. MANY people misunderstand what the oil pressure switch actually does. The oil pressure switch is a secondary fail safe in case the fuel pump relay goes bad, as long as there is 5psi of oil pressure that switch will allow the fuel pump to continue to run. Also if you have a hard starting issue (cranks longer before starting) this is usually a sign that the fuel pump relay is bad and it takes a little longer for the engine to crank and build up that 5 psi of oil pressure to allow the fuel pump to work.

To the OP your ignition module is probably the culprit, they can fail by not sending the trigger signal to the ECM but still have spark. The modules are cheap and I keep a spare in the truck since they so often go bad.
 
Replaced the icm... Nothing still no injectors at all. Looked up ohm spec for the injectors and soppose to be
All with TBI 1.3 ohms min

One was at 1.3 and the other at .3

According to the flow chart the injector is toast and PCM :/
 
I'd be VERY leery of blaming the PCM at this point.

You mention the flow chart, is the DRP getting to the PCM?

I'm not intimately familiar with your PCM, I know on my setup (TPI) bad injectors made it appear the ECM was bad...not sure if the bad injectors messed with the injector drivers in the ECM, or if the injectors simply stopped firing, but the vehicle would start when cold, and run until I turned it off. But once it had been started, it would not start again until it had sat for hours.

However, in your case, removing both leads from the injectors when testing should eliminate any potential injector driver issues. But without knowing if DRP is getting to the PCM, then its just guessing.
 
so i want to check for codes with my scan tool, and nothing could connect, said could not connect, so what i did is
ignition off
dicconnect pcm connectors
turn ignition on
probed 440, and 439 with a test light to ground
test light not turn on at all

somehow i blew fuse for the ecm, replaced fused, tested again good. truck still does not start, chart a-1 said to isolate the problem, hooked up the test light to power and tested the grounds on the PCM, A1 and A12, both are fine, even tested the pcm case all good...

since i replaced the icm i decided to check the wires, all 4 wires to the icm to the pcm, checked continuty, highest was .3 ohms, checked continuity to ground, none ohmed out to ground. hooked everything up, and tried to fire, still nothing...\

what else can i check? i am getting a pair of known good injectors tomorrow, another known good icm tomorrow as well
 
An also I must have had my computer chip slightly out, pushed in all the way and now fuel pump primes for about 6-7 seconds.. Which is not good
 
I don't know as the fuel pump prime is a good indicator...could be, I have not spent enough time with the various year TBI setups to know if prime was modified...I know for awhile GM was using the "hot fuel module", which ran the pump for a prolonged period of time due to heat issues.

You are still getting spark? You've had enough issues (fuse, PROM, etc.) you might want to verify you've still got it.

I suppose with a blown fuse ICM could be bad now, but I would do the ICM diagnosis instead of just replacing it.
 
Every fuel injected vehicle I have owned always primed the fuel pressure for a few seconds when you turned the key on and also when you shut it off.

Mothers '95 pick-up w/43. v6, my '88 IROC TPI 350, '89 Burb TBI 350, '87 Burb TBI 350 and even the '05 Avalanche with the 5.3,, all prime when key is turned on and all run for a few seconds after shut down.

I have even gotten in the habit of turning the key on, but not starting the engine right away, to give the fuel pump time to pressurize the system.
 
I think his concern is that the amount of time it is priming has changed. AFAIK all the GM EFI setups run the pump prime based on a time specified in the PROM, so within the same vehicle, the amount of time it primes, should never change.

That's what I was getting at but maybe didn't make clear...whether it primes for 2 seconds or 6 seconds is PROBABLY irrelevant, but again, it should be consistent for the same vehicle, and it COULD point to some other issue. But I wouldn't focus on it.

I know as a for-instance on my setup, if certain codes are triggered and the ECM goes into limp mode, the electric fan is triggered. Perhaps GM did something similar with the fuel pump prime in some setups. Just don't know.

If nothing else, after probing, testing, replacing parts, disconnecting/reconnecting things, it's probably a good idea at this point for Josue to make sure he removes power from the PCM for a second or two before reconnecting and re-testing anything, just to make sure.
 
Replaced the entire throttle body with a known good one off a working truck, truck fires right up

One injector on the old tbi ohmed at .3 minimum is 1.3

New tbi, truck runs again
 
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