ECM injector driver damage?
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HT4100 4.1, 4.5, 4.9 Discussion, ECM injector driver damage? in Cadillac Engine Technical Discussion; I've read about this problem before, I'm wondering how common it is. Long story short, I'm working on a 4.9L ...
  1. #1
    drewsdeville is offline Cadillac Owners Master
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    ECM injector driver damage?

    I've read about this problem before, I'm wondering how common it is.

    Long story short, I'm working on a 4.9L in a 91-93 Deville, not sure of the year and unknown repair history.

    I have a misfire in cyl #3 according to the balance test. It's very noticable at idle, you can feel the rythmic miss as the cylinder never fires at idle. However, the car feels like it has full power and runs smooth at WOT or any engine speed above 1k rpm's or so.

    It needed an ignition system tuneup anyway, so I slapped on new plugs, wires, cap and rotor.

    Misfire in #3 cyl still there.

    Before going any further, I decide to check compression though I am sure it's good as it only misses at idle.

    Compression good.

    Ok, so it has good spark and good air. Last part of teh equation is fuel.

    Swap out #3 fuel injector. Old injector not shorted

    MISFIRE STILL THERE!!!

    Cycle through the ecm data, integrator and block learn differences between the banks of course due to the misfiring cylinder.

    I'm unsure of how to check for proper ECM signal to the injector, but I suspect the ecm is the problem at this point and almost just want to swap it out. I don't know where else to go.

    I personally have never witnessed this problem before, but the internet makes it seem like it's somewhat of a common problem on early PFI GM ecm's, regardless of engine.

    Any ideas how to confirm this diagnosis without a scope? Anyone else witness something like this?

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  3. #2
    Sevillian273's Avatar
    Sevillian273 is offline Cadillac Owners Connoisseur
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    Re: ECM injector driver damage?

    You need a tool called a 'noid light'. It plugs into the injector connector and flashes as if it IS the injector. Or wire up a 12v test light with a resistor to match the resistance of the injector(12-17ohms). Btw is the injector resistance in spec? When my #4 injector went, it would not fire at idle but it would kick on and off at higher rpms. It was at 1.2 ohms Any injectors that are below spec will overheat and cook the driver circuit. Easiest way to check all the injectors is to use a DVM at the injector harness connector(s). An injector with a good coil can also be mechanically stuck closed or binding but is less likely. Also, to be 100% sure of spark at idle, you can hook up a timing light to the offending cylinder and watch the flashes. There's nothing in between the ECM and injectors besides the wiring, so if injectors and wiring check out ok, replace the ECM. There are quite a few in the $30-50 range on ebay used. Any 4.5 / 4.9 caddy from 90- 95 is compatible.

  4. #3
    ewill3rd is offline Cadillac Technician
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    Re: ECM injector driver damage?

    I have also seen similar symptoms related to a valve issue.
    You need to perform a "running compression test".
    In other words what you did was referred to as "static" compression or basically cranking compression.
    The dynamics in the cylinder change when the engine is running and a problem with a valve guide or spring can cause a loss of compression under the conditions you described.
    You can do some searching for running compression test and get a good idea of how to do it.
    It is a pain but I had a 4.9 a few years back doing something very similar due to a valve guide issue.

  5. #4
    drewsdeville is offline Cadillac Owners Master
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    Re: ECM injector driver damage?

    Quote Originally Posted by Sevillian273 View Post
    You need a tool called a 'noid light'. It plugs into the injector connector and flashes as if it IS the injector. Or wire up a 12v test light with a resistor to match the resistance of the injector(12-17ohms). Btw is the injector resistance in spec? When my #4 injector went, it would not fire at idle but it would kick on and off at higher rpms. It was at 1.2 ohms Any injectors that are below spec will overheat and cook the driver circuit. Easiest way to check all the injectors is to use a DVM at the injector harness connector(s). An injector with a good coil can also be mechanically stuck closed or binding but is less likely. Also, to be 100% sure of spark at idle, you can hook up a timing light to the offending cylinder and watch the flashes. There's nothing in between the ECM and injectors besides the wiring, so if injectors and wiring check out ok, replace the ECM. There are quite a few in the $30-50 range on ebay used. Any 4.5 / 4.9 caddy from 90- 95 is compatible.
    thanks for the info.

    Yes, the injector was in spec and I even swapped it out with an extra I had. No difference.

    this car also had a small electrical fire due to an aftermarket alarm system, but supposedly that was years ago, as in 10+ years ago. You'd think any damaged wiring would have shown itself a long time ago but nothing would surprise me.

    So, I'm sorry to mooch off of everyone else FSM's when I haven't bought my own yet...makes me feel dirty, can anyone post a scan of the pinouts on the ECM harness or ECM itself so I can test the wiring from the ECM to the injectors? I'd rather check all the way back to the ecm rather than just the short length of the injector harness.

    Speaking to the owner, the car does have bad injectors in it's history, but I don't know if they were shorted, dirty, stuck, leaking, etc.

    Thanks for any help guys. I gotta figure this out!!!

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    the recluse is offline Cadillac Owners Connoisseur
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    Re: ECM injector driver damage?

    I have had nothing but problems with the injectors for this motor.

    Have you physically checked the wiring harness all the way to the connector and beyond? Is this the motor you had apart for months?

    I'm wondering if in removing anything you either pulled the wiring harness or pulled a wire. Clean the connector for the injector harness with some electrical spray cleaner, may be you got some crap in the harness that's gumming up the works.

    Trace back all wiring before going the ECM route.

    My .02

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    ewill3rd is offline Cadillac Technician
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    Re: ECM injector driver damage?

    I'm sorry I forgot to mention that most of our injector drivers are pretty well circuit protected.
    I can see one going bad, but we usually have other failures related to PCM's than injector drivers.
    If you had a bad injector driver it would fail under load also, not just at idle.
    In fact it would probably fail worse under load.
    Just some info from my point of view.
    Been a while since I worked on one of those so...

  8. #7
    drewsdeville is offline Cadillac Owners Master
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    Re: ECM injector driver damage?

    Quote Originally Posted by the recluse View Post
    I have had nothing but problems with the injectors for this motor.

    Have you physically checked the wiring harness all the way to the connector and beyond? Is this the motor you had apart for months?

    I'm wondering if in removing anything you either pulled the wiring harness or pulled a wire. Clean the connector for the injector harness with some electrical spray cleaner, may be you got some crap in the harness that's gumming up the works.

    Trace back all wiring before going the ECM route.

    My .02
    This is not the motor I've had apart. It's someone elses and a lot of the history is unknown. My own Eldo is back together and purring like new.

    I'm looking for a diagram of the pinout on the ecm side of the harness so I can check all of the injector wiring at the ecm connector with a VOM instead of wasting time checking it visually, which can also be a source of error.

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    Sevillian273's Avatar
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    Re: ECM injector driver damage?

    Well I'd scan my fsm for you but I know that in '91 they changed from 2 injector harness connectors to one big one so there might be a chance of the wring being different colors. I do know that the ECM pins would be in the same places but again, I cant be sure that the colors are the same.

    EDIT: I already had this one on photo bucket. It has the ECM pin locations. It is likely that the colors are the same but I'd hate to throw you off if not. Note that each lead on the ECM side is a ground.

  10. #9
    drewsdeville is offline Cadillac Owners Master
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    Re: ECM injector driver damage?

    Thanks a lot Sevillian, that should help a lot.

    If the ECM's are interchangeable between '90 and '93, I'm going to assume that the diagram you sent is correct.

  11. #10
    Sevillian273's Avatar
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    Re: ECM injector driver damage?

    Quote Originally Posted by drewsdeville View Post
    If the ECM's are interchangeable between '90 and '93, I'm going to assume that the diagram you sent is correct.
    The reman part # is the same from 90-95 and I'm running one out of a '91 deville 4.9 in my '90 4.5.

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    the recluse is offline Cadillac Owners Connoisseur
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    Re: ECM injector driver damage?

    Quote Originally Posted by drewsdeville View Post
    I'm unsure of how to check for proper ECM signal to the injector, but I suspect the ecm is the problem at this point and almost just want to swap it out. I don't know where else to go.

    Any ideas how to confirm this diagnosis without a scope? Anyone else witness something like this?
    I don't know if you fixed it already, but this video is very informative (if you can get past the artificial voice).


  13. #12
    justin4yu is offline Cadillac Owners Member
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    Re: ECM injector driver damage?

    find a ecm i found one for $10 bucks it fixed the problem im on all eight now make sure you use the prom out of you old ecm

  14. #13
    daniel58 is offline Cadillac Owners Fanatic
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    Re: ECM injector driver damage?

    I had this same problem with a 90 4.5, as previously mentioned using noid lights I was able to see that the ECM was not firing the injectors at idle but was off idle. It makes you think it is ignition related or bad injectors, been there done that. Save all of those old parts, most of them are still good and you can use them later to diagnose future problems!!!

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