94 5.7L TBI intermittently dies at idle

chevrolet
3500

#1

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


#2

The backfires you heard just before this all started certainly would make a person think the problem is lack of a good consistent spark, rather than the fuel system. It sounds like you are doing the right sequence of things to try. Without any diagnostic codes, you are sort of flying blind.

What came to my mind reading your post is that the ignition module could be ok, but it isn’t getting its trigger consistently. It has to know which cylinder to fire as the engine rotates. That trigger signal either comes from a crank position or cam position sensor. My guess, one of those sensors are failing.

A bad tank of gasoline, for example water contaminated, could cause this symptom too. At some point if you can’t get to the bottom of it, it might make sense to do a drain and refill of the gas tank. I’d probably do a fuel pressure test too, if the ignition system theory isn’t getting you anywhere. Best of luck. Let us know what it turns out to be.


#3

A 5.7 TBI is fired off the star wheel inside the distributor, I’m pretty sure it has no cam or crank sensor. This engine has a distrubutor and self contained pickup.

I’d be checking to make sure the module is getting a consistent voltage at the module. Wiggle wires and look for a drop out in voltage. Same for the ground. I’d ohm out the coil but bang it around a bit as well looking for opens.

I’d agree with George, suspect this, the pickup coil;


#4

The Ignition Module inside the distributor would be suspect number ONE… It also does have a pickup as well…but that backfire is telltale for the ignition module… Its inside the distributor…and held in by two screws… Costs between 50-80 bucks… I would be looking at that item first. In fact it seems to be Ignition Control Module Week for me over here… I just replaced 2 of them this week and it fixed the problem in a Jiffy.

Blackbird


#5

When you change ignition modules, are you reapplying the thermal compound each time? These are particularly sensitive to overheating. If you are omitting the “grease” then they will act erratically and fail in short order…