94 5.7L TBI intermittently dies at idle

thetowguy
Sat, 28 May 2016 19:36:22 +0000
Maintenance/Repairs

This is in my 94 Chevy W-4 (Isuzu NPR chassis). Bear with me, the explanation is long to provide as much info as possible:

94 Chevy W-4 wheel-lift wrecker
5.7L TBI
4l80e trans

Had a run of about 100 miles round trip on Wednesday. On the way back running at 65 mph 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 shop where I freeload and told him the tale. He said, “throw those old spare modules away and get a new one”. 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 water 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 died four times while sitting still. Same having to cycle key for it to fire. Did some quicky t/s 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.

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.

Additional info:

Plug wires, cap, and rotor <1000 miles. Distributor low miles and negligible play in shaft. Pickup coil checks right in middle of ohm range. None of these, of course, should need to have the ignition key cycled to restart if they caused a stall.

Next up will be replacing ignition relay (had to order), but I’m not very optimistic.

Amy help gratefully accepted.

John

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