No injector Pulse

Tiny
GANDALF0531
  • MEMBER
  • 1995 CHEVROLET S-10
  • 4.3L
  • 6 CYL
  • 4WD
  • MANUAL
  • 300,000 MILES
Vin Z. Engine cut out for about a minute, shut off, and won't restart. Good spark, ECM fuses good, TBI injectors are not pulsing. I can ground an injector and get a good cone of fuel. Cleaned the ground wire at the back of the engine by the distributor, can't find any ground by the thermostat housing. The ignition module was the original GM, so I replaced it as a shot in the dark, but no change. It will run if I spray fuel into the throttle bores. I've read dozens of post on various sites about the same problem, yet no one seems to have the answer. Any ideas?
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Saturday, June 1st, 2019 AT 2:17 PM

19 Replies

Tiny
KASEKENNY
  • EXPERT
Hi,

Good job in grounding an injector. This basically shows that the fuel system is able to function when it is commanded to. The control module (PCM/VCM) depending on which you have controls the injectors by grounding to pulse them.

It looks like the engine control module is your issue.

Let me know if you have any questions. Thanks
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Saturday, June 1st, 2019 AT 5:58 PM
Tiny
GANDALF0531
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Engine Control Module. Computer on passenger side fender?
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Saturday, June 1st, 2019 AT 6:08 PM
Tiny
KASEKENNY
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It depends. Either you have a PCM is on the dash below the glove box or a VCM which appears to be located on the overflow tank.

Depending on your vehicle you have one or the other.
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Saturday, June 1st, 2019 AT 6:16 PM
Tiny
GANDALF0531
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Replaced the ECM with a Delphi re-manufactured unit, truck ran for about fifteen minutes and shut off, same issue. Any ideas? About ready to find a carburetor, intake and HEI distributor!
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Wednesday, June 5th, 2019 AT 1:54 PM
Tiny
KASEKENNY
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That is odd. The only thing that makes sense is something shorted out the ECM. Usually those units have a warranty. It may be worth to test your wires on that connector for a short to power and ground then see if they will replace it. Or at the very least call them and see if they will rebuild this one and tell you if any circuits are shorted.

The fact that you have no pulse, replaced the module and it ran for a while. Clearly that was your issue. Just not sure why it is shorting other than a wiring issue sending to much voltage to the injector circuit.
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Wednesday, June 5th, 2019 AT 4:03 PM
Tiny
GANDALF0531
  • MEMBER
The ECM grounds the injectors, there is constant voltage to them. I checked and there is 12.5, the same as the battery, to them. I read just now that the ecm receives rpm signal from the ignition module to trigger the injectors. Will check that tomorrow. Many thanks!
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Wednesday, June 5th, 2019 AT 4:33 PM
Tiny
KASEKENNY
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Sounds good. Let me know what you find. Thanks
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Wednesday, June 5th, 2019 AT 6:03 PM
Tiny
GANDALF0531
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Update. Checked continuity on injector wires to ecm, good. With fuel pump on, ground each injector at the ecm connector and each has a good cone of fuel spray. Checked continuity between ign module and ecm, good. VOM shows voltage from ign module at ecm connector when cranked. Rebuilder sent replacement ecm, installed and no change. At this point I'm unsure what to do next.
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Monday, June 10th, 2019 AT 5:05 PM
Tiny
KASEKENNY
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This is all great information. Did we ever check for any codes?

It seems like the ECM is just not firing the injectors. All else seems to be able to fire the injectors but the ECM just doesn't pulse them when cranking.

I found this stuff specific to your engine. Let's go through each test as it states. It will have you check basically the same things you have but in a different way. What I suspect is you are grounding the injector and assuming that everything is okay. However, this doesn't take into account inputs into the PCM so this test should take us to the culprit of preventing it from pulsing.

Just running through this, I assume you either have an issue on Step 4 or step 5. Because of that, I included the A-4 and A-6 tests so that you can test those systems if the result is "No."

Make sure you review each of these documents before starting so that you can follow the diag path. The third page of each set is a description of the test so it is good to read that so you understand what it is having you test and why.

Long story short, I suspect something is telling the ECM that it is not okay to fire the injectors. IF this is the case, I assume we would have a code so if we have not checked for codes then lets start with that.

Keep me updated on what you find. I know if it frustrating but I think you are close.
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Tuesday, June 11th, 2019 AT 11:22 AM
Tiny
GANDALF0531
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Many, many thanks! Will post what I find out.
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Tuesday, June 11th, 2019 AT 11:57 AM
Tiny
KASEKENNY
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Sounds good. I will wait to hear back. Thanks
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Tuesday, June 11th, 2019 AT 12:31 PM
Tiny
GANDALF0531
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Update. I used chart A-4, it seemed the most appropriate for the issue. I don't have a noid light, but I know from prior testing that I have 12 v at injectors, and if I ground the injector ground wires at the ecm plug, I can hear a definite click and if the pump is running a cone of fuel spray. Wire from injector to ecm is therefore good. With LED test light on 12 v. And touched to 430 light glows dim, and no click at injector. Wire from 430 is good because I checked for cranking signal at ecm earlier. All points again to a bad ecm, yet this is the second one. Reman by Delphi in original delphi box. Not impossible to be bad but unlikely.
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Tuesday, June 11th, 2019 AT 12:53 PM
Tiny
KASEKENNY
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Okay. Yeah I agree. I would call the tech support for them and tell them your results of your testing because I don't see anything that I would disagree with based on your findings. I just wonder if they are sending you the incorrect ECM. Reason is, it looks like there is one for your vehicle VIN Z 4.3L TBI or VIN W 4.3L CPI. Plus each one of these either has a PCM or a VCM.

Looks like the VCM and PCM are not interchangeable but the CPI or TBI has separate call outs.

Just a thought.
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Tuesday, June 11th, 2019 AT 1:10 PM
Tiny
GANDALF0531
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From my research I have found in mid 1995 GM switched S10's from OBD1 to OBD2. If my understanding is right, the OBD1 is TBI vin Z, and the OBD2 is CPI vin W. The Z vin models (mine) had the ECM on the passenger fender, and the W is mounted under the dash behind the glove box and are entirely different in appearance. But I've read so much lately that things are starting to run together!
BTW, I've read that replacement ECM's must be programmed. I assumed (never a good idea!) That meant moving the PROM from the old to the new. Am I correct?
Once again, many thanks!

Jim
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Tuesday, June 11th, 2019 AT 2:02 PM
Tiny
KASEKENNY
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Yes. Digging a little deeper I found the following for your ECM. VIN Z in on passenger side fender which makes it a PCM. When replacing the PCM you need to move the PROM over.
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Tuesday, June 11th, 2019 AT 4:03 PM
Tiny
GANDALF0531
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Fixed! On one of the 5 or schematics I've down loaded for a 1995 Vin Z (BTW, no two of them alike LOL) I found one showing a third wire on the oil pressure sending unit, one of them going to the ECM, as well as the fuel pump and gauge wires. $25.00 sending unit and it started and ran!
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Saturday, June 15th, 2019 AT 8:34 AM
Tiny
KASEKENNY
  • EXPERT
Unbelievable. I am going to go in and look at what our diagrams show to try to understand what I missed. However, the third wire must be the signal input to the ECM if it was shared and if the sender was shorted it could take down that input to the ECM which will then prevent the start. Which is probably why you kept getting lead back to the ECM.

The only thing I still can't connect is why after you installed one of the ECM's it started and ran.

Either way, it is one of those ones that I learned a couple things on. It is always the ones that kick my butt that I learn the most. Thanks for keeping us updated. Glad you got it fixed.
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Sunday, June 16th, 2019 AT 8:06 AM
Tiny
GANDALF0531
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Not sure why the brief run happened either, perhaps the sender came back to life briefly, dunno. So far its ran for several hours and 15 or so miles. I'll attach the schematic that shows the circuit. Thanks much for your help!
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Sunday, June 16th, 2019 AT 9:33 AM
Tiny
KASEKENNY
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Got it. By you grounding the injectors, you were taking place of the ECM control. However, that feed (oil pressure input) is shared by the fuel pump, as you stated, so if the sender was shorted, the ECM wasn't getting a signal on that wire and was preventing the start.

Agreed, you must have had an intermittent short that just starting working when you replaced the ECM as a horrible coincidence.

Sounds like you got it. Good work!
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Sunday, June 16th, 2019 AT 10:08 AM

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