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  #1  
Old 05-27-16, 07:35 AM
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Stumped

Okay, only getting into initial troubleshooting on my beast but I'm already scratching my head.

1994 Chevy W-4 (Isuzu NPR chassis)
5.7L TBI gas

Had a run of about 100 miles round trip on Wednesday. On the way back running at 65 on the interstate, got a couple of quick muffler-end back fires and engine died. Coasted to shoulder. Had memory of almost exact same incident last year and it was dead ignition module. Fortunately had a spare (used) module in glovebox. Swapped it out and engine started right up. Remainder of trip didn't quit, but ran very rough, lots of stumbling and getting it up to 60 mph was a struggle. Also when I got on surface roads it died twice when I was sitting stationary. It would restart, BUT only after cycling key; if I just turned to the key to "start" from "on" it would crank but not fire.

Limped it to my tow buddy;s hop where I freeload told him the tale. He said, "throw those old spare modules away and get a new". Called the local parts house and had them send two, one for installation and one for glovebox. Took for test drive and appeared to be running okay except idle was a bit low. Couple of hours later went on a call and first stop sign I came to, went "blip" and I'm dead in the watrer again. Same story as earlier, must turn key off and back on to get it to fire. Drove rest of day without incident. Yesterday, no issues all day, drove total of about 120 miles both highway speed and surface. Gassed up this morning and mpg was above average (yay!), actually about .5 above average which is significant improvement (about 6%). Success?

Today, has now died four times while sitting still. Same having to cycle key for it to fire. Did some quicky ts to satisfy my curiosity about the key cycle thing. With truck running, pulled fuel pump fuse to kill truck. Then reinserted and it started up without having to turn key to off. Tried same thing by pulling one of the module plugs to simulate loss of signal to ignition. Reconnected and again fired up without key cycling.

Got a feeling this is going to be fun to track down. Any and all ideas gladly accepted.
 
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  #2  
Old 05-27-16, 08:17 AM
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I've had similar behavior with a failing crankshaft position sensor.

The first time it happened, it seemed like it would fail when it got warm and had to cool down again to work so that it was fine for a short trip but if it failed on a long trip, I would have to wait a while before the car would start back up.

The second car where I had this issue was a little less clear, it sometimes failed while still relatively cold but I suspected what it was that time and had it replaced much more quickly.

FWIW, the shop to which I took the first car replaced the ignition module on their first attempt and that obviously did not resolve the problem. Additionally, the first car did give me a check engine light but the second one did not.
 

Last edited by stickshift; 05-27-16 at 08:33 AM.
  #3  
Old 05-27-16, 10:23 AM
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No crank or cam sensor on this one; plain jane 90's 5.7L GM.

No codes at any time since this came up.

Swapped out the module since I had bought a spare, no change.

Ohmed the ignition coil and no problems noted. So far, only dies sitting still at idle, has not died driving since Wednesday.

Not going to think about it again until Monday.
 
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Old 05-27-16, 11:02 AM
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Ah, dying at speed/only at idle would have knocked out my suggestion anyway.
 
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Old 05-27-16, 01:08 PM
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After pouring over shop manual and kicking around, going to try ignition relay. As logical as anything else we've come up. Thinking being that off-idle more juice in electrical system that could mask weak relay. Also discovered this relay also powers circuit to wiper relay/motor, so if it manages to crap again at an opportune moment, I'm going to switch wipers on. Might not happen today and won't get relay until after holiday. Stay tuned.
 
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Old 05-27-16, 01:33 PM
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sounds like it could be ignition related besides the module the pickup coil could also cause erratic ignition, have also seen distributers so worn that they would hit the pickup and cause erratic ignition.
check the cap and rotor and distributer shaft for play maybe hook it up to a scope if you know of a shop that has one is about the quickest way to check the secondary ignition pattern.
 
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Old 05-28-16, 07:57 AM
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Pickup coil checked and right in the middle of the range on resistance, no short;. that was replaced couple of years ago due to failure of previous one. Also would not account for having to cycle key to off to recover. Distributor only a few years old and play in shaft is negligible. As with the coil, can't figure a link between having to recycle the key to recover.

Plug wires, cap, and rotor have <1000 miles on them. As with all the other suspects, should restart immediately on turning the key without having to cycle to off first.

FWIW, it dies while I was sitting still in a parking lot late yesterday and I tried the wipers before cycling the key and they worked. Will have to look closely at the schematic again to see if I missed something; looked to me like if the ignition relay de-energizes I should also be losing wipers.

I'm done with it until Monday when I'll replace ignition relay even though now it's questionable whether that is the circuit at fault.

 
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Old 05-28-16, 10:44 AM
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if it wont restart until you turn the key off would be checking for power at module and coil before attempting to restart would also check for power at the fuse boxes see if you find any that don't have power that should have.
might be worth a try to give the ecm the tap test also to see if the engine dies.
 
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Old 05-29-16, 02:49 PM
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Already have ecm in the back of my mind, but haven't attacked it yet. Will be pouring over the schematics on Tuesday looking at things like power to components as you mention. Have already done the wire wiggle test on module wires to try to get it to die but without success. Keep hoping one of my mechanical gurus will say, "Oh, yeah, I've seen that before it's the ___________." Will be a long day with the meter and test light Tuesday.

 
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Old 05-31-16, 09:12 AM
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Still no resolution and now getting worse (good news?). Now also quitting when I am driving and more frequently standing still. Stopped in at one my client garages this morning and they did a quicky voltage check at coil, appeared still had 12v when dead. Now staring at schematic for any clues. One suggestion was ignition switch issue, not out of the realm of possibilities. I also asked if this could be pcm issue and they said possibilty. On another forum, two different posters suggested distributor pickup coil; don't think that's going to be it; checks good resistance and open-wise and wouldn't necessarily explain hainvg to cycle key to restart. Really trying to avoid throwing parts at it willy-nilly.
 
  #11  
Old 05-31-16, 10:34 AM
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maybe you can at least determine if your losing spark or injector pulse or both and do a power check at the injectors while its dead.
while an ohmmeter is commonly used to check components with, it also isn't uncommon to get a good resistance reading out of a failing part under no load bad connections or high resistance isn't always evident.
 
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Old 05-31-16, 01:47 PM
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Those trucks were problematic and have the controls and sensors from the K car era.. I have had a few.

So I used to jump the code reader molex connector which is under the steering wheel .. But you may have a real code reader...lol.

You would count how many times the light blinks on the dash to lead you to the issue. Do that with the key on but not running. Two pins on the top either left or right I jumped.

Well anyway I believe you issue is related to the throttle position sensor or the idle air control sensor.

When you stated the idle was low it reminded me of my 1986 reliant K car I had for many years...

Look there and let us know..

Oh edit.....

I found the code list...

GM – OBD Trouble Codes
 
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Old 06-01-16, 01:43 PM
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I carry a paper clip for emergency code reads but also have a small scanner. No codes at any time on this one. And the answer is [drum roll, please] it apparently just needed an oem AC-Delco ignition module.

So, yesterday, replaced the ignition relay because I had one to do that with. No change. Finally decided it was time to start throwing parts at it, since nothing in particular was suspect (other than that when it died it wouldn't spark). Replaced the coil (AC-Delco) and thought I had it licked when it ran for almost 30 minutes without a hitch. Just getting ready to go road test and it went BLIP and it was dead. Restarted and it took it out for spin to parts place. Bought AC-Delco module, AC-Delco dist rotor (possible burn-through issue?) and a pickup coil (several people suggesting that). Started easy with module and rotor. Voila! Sat running on lot for 50 minutes, then took for about 12 mile loop on surface and interstate. No problems. Drove it another 5 miles to my overnight parking location and never missed a beat. Couple of the pro mechs I consult with have opined it was a resistance problem in the non-oem modules causing the pcm to kick out the driver for the ignition circuit. I also mentioned heat as a possbile contributing factor as the non-oem had some thin vaseline-looking thermal paste, whereas the AC-Delco had a thicker white computer-type thermal paste (better heat transfer). Saving the pickup coil for future (I hope not) use.

My sanity has returned.
 

Last edited by the_tow_guy; 06-02-16 at 04:22 AM.
  #14  
Old 06-01-16, 01:44 PM
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You wouldn't think I would have so much trouble on a truck with only 689,000 miles on it.
 
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