91 Chevy 1500 - shuts off while driving on hot days - need help

All stock except for aftermarket cat back exhaust, 350 TBI w/700R4. Starts and drives just fine, no surge on idle or under load sitting at a light. Does not run hot, but when the ambient temperature gets over 80 it randomly shuts off while driving, almost like someone switched the ignition switch off. It usually cranks right back up, but not the best thing to have happen in rush hour traffic. So far I've replaced the ignition module, plugs, wires and distributor cap, IAC valve, O2 sensor, MAP sensor and that other module that sits next to the throttle body whose name I can't remember right now. So, the only parts left for me to throw at it are the pick-up coil in the distributor (might as well get a new distributor) the fuel pump and the fuel pressure regulator. Last time I checked the fuel pressure it tested fine for a TBI (13 pounds at the fuel filter). Am I overlooking anything? Anybody got any suggestions or experience with this sort of issue. I love this truck but it's gonna get me killed in traffic one day if I can't figure this problem out.

My old C2500 did something similar. It was a bad battery cable. I don't remember it being temperature sensitive though, but something to look into.

That may be the one thing I haven't changed, I'll check it out thanks. Was also thinking about doing the "Big 3 Mod" using the heavy gauge cable on the engine, chassis and alternator grounds. Not going to put a giant stereo of big off road lights on this thing, not sure if it's worth it otherwise.

My guess would be to check all grounds and the fuel pump relay.

Now that you mention it, my last fuel pump relay came from Auto Zone about 2 years ago. That's usually old age for one of their electrical parts...

06HHR wrote: So far I've replaced the ignition module, plugs, wires and distributor cap, IAC valve, O2 sensor, MAP sensor and that other module that sits next to the throttle body whose name I can't remember right now. So, the only parts left for me to throw at it are the pick-up coil in the distributor (might as well get a new distributor) the fuel pump and the fuel pressure regulator.

What about the actual ignition coil? When they fail, it's almost always when they get hot.

Bryce

I had similar problems last summer with my 305-equipped '88 1500. Turned out to be the pickup coil (although GM doesn't call it that and I can't remember their proprietary lingo after a few bourbons). Drove me nuts, but it was definitely the part failing when it heat-soaked. And if yours isn't failing now, it's likely to do so relatively soon, so I'd have a spare in the glovebox. To that end, I've heard only the OEM Delcos are reliable, and the aftermarket stuff is a crapshoot. I popped for the Delco and I've had no further problems.

Silly question, did you use heat-sink paste when you installed the new module?

Could be the trigger coil in the distributor like Alex mentioned.

I've also seen a magnet ring in one of those distributors break apart and cause all kinds of goofy ignition hiccups.

Who made said ignition module?

The ignition module is a BWD (Standard Motor Products) bought from o'reillys when the auto zone one I got after a similiar episode towing through Ocala Florida died in the wilds of South Georgia. Next replacement will be a AC Delco or Delphi. I did use the heat-sink paste, but the distributor base had some corrosion on it. It was a field-expedient fix that got me back on the road and has worked well enough with the occasional hiccup in hot weather. For now the game plan is to check and clean all the grounds, test the coil and ignition module and probably replace anyway even if they test good, check the TPS to make sure it's not wonky and if those don't help follow the chain to the ECM and the fuel delivery system (TBI rebuild kit and fuel pump replacement). Drove it to work this morning with no problems, we'll see what it does on the way home.

Two questions. When it dies and you cycle the key, does the fuel pump run? If not, I'd check the ign module first. If it does, crank and see if you have injector pulse when cranking. If it does, check the spray. If it looks like it's dribbling, the fuel pump may be weak. If it's not spraying but the pump cycles then the pickup coil is likely bad. If it doesn't cycle the pump, the ign module is toast.

If all that passes, check the spark at the plug. I'd guess the coil is dying.

It usually starts right back up after it dies, so i'm assuming the fuel pump continues to run. Last summer when it was doing this I was able to replicate the condition in my driveway, the injectors always cycled when I turned the key, i checked the spray and one of the injectors was dribbling, checked the fuel pressure and it was at 13 pounds so I didn't replace the pump. It's always taken 10-20 seconds to crank before firing off when it's cold. Once it warms up it cranks as soon as you turn the key, except when it dies, then it takes 5-10 seconds of cranking to start up again. It's almost like it's starved for fuel when it shuts down. I haven't checked the fuel pressure since then so it could be that my pump has gotten weaker with time. Good thing old Chevy parts are relatively cheap, i'd hate buy stuff it really doesn't need but it's 23 years old so it may need everything i throw at it.

The pump may be leaking down, losing your pressure which is what takes it so long to refire. MAybe.

Have you changed your fuel filter recently? I had a battery cable go as well on my '94 K1500, but that one was pretty obvious because the gauges went all wonky as well (speedo/tach would jump all around).

Last year was the last time I changed it i think, around the end of 2012/beginning of 2013. With some of the stuff around here that passes for gasoline (up to 10% ethanol, sux) it's probably due.

A quick update. Last Thursday it started running rough, picked up a slight misfire at idle under load once up to temperature. Got home and checked under the hood while the engine was running, fuel spray from the injectors looked good. No dribbling of fuel, and the injectors responded with increased flow when opening the throttle so the TPS appears to be ok, and the misfire went away at higher revs. So I ruled out a fuel system issue and started checking the ignition system. Pulled of the distributor cap and notice the coil wire was laying on the bracket that holds the kickdown and throttle cables, and seems to have been arcing through the insulation. I put a piece of rubber fuel line over the wire for protection and additional insulation and routed it away from that bracket. Then I checked the ignition module, the heat-sink paste was still there but seemed a mite thin so cleaned up the distributor base and applied a new coat of past to the Ignition Module. Checked the pick up coil according to the test procedure in the Chilton manual (which may or may not be correct) and it tested within spec for resistance between the terminals, but tested out of spec for resistance between ground and a terminal (supposed to be infinity at the highest scale, tested around 1600 on the highest scale on my multimeter) so now i'm thinking it could be the problem. Cap and rotor looked good with just normal wear. All the grounds are present and solidly connected. Since i've done this stuff it does seem to run much better, but it hasn't been really hot out lately. So, now the game plan is either get new AC Delco or Delphi Ignition Module and Pick-up Coil components, or just get a new or lightly used distributor assembly. I'm a little leery of getting an aftermarket distributor so it would be a factory replacement unless someone has a suggestion for a suitable aftermarket replacement. Any suggestions?

UPDATE: After i did the tweaks in my last post, the truck ran much better and the intermittent stalling was gone, or so I thought. I figured if the problem didn't happen in the hotter temps of the summer then i must have found the cause. So, i didn't get the new ignition components and played the waiting game. Summer (consistent daytime temps in the 90's) didn't arrive in N. Florida until Mid-May this year, so I had a couple of months of trouble free service, until one day waiting at a stop light, the idle surge came back and the truck shut off at a stop light. Started right back up, just like always. Well this went on for about a month, but nothing consistent, it would happen while coasting down a hill, or while sitting at a light. It would happen if the truck was dead cold after sitting overnight or hot from a highway drive, always staring with an idle surge, then a misfire that would go away after I hit the gas or released the brakes. And just as quickly as the condition would start, it would go away. I checked all the connections under the hood and nothing was loose. So, i was at my wits end when one day during this random idle surge and misfire routine the Check engine light came on. Didn't think anything of it, but finally got around to reading the code with my trusty paperclip (OBD1 FTW) CODE 33, MAP sensor voltage too low. Remember I said I replaced the MAP sensor in the OP? Turns out the no-name brand MAP sensor I bought from pep-o-zone (Autozone) was worth slightly less than the money I paid for it. Pulled the original GM sensor out of my toolbox and put it in after I put the grandkid to bed. Purrs like a kitten, no more idle surge and phantom misfire. TL:DR, the moral of this story is, don't use poor quality (read cheap) parts in a 90's era GM ignition system.

This is why if I need it NOW I'll roll the dice on a OE sensor off a pick and pull engine that been rained on for who knows how long before wasting time/money on lowest bidder parts store stuff.

The connection for the fuel pump and sender, are week points on that truck. You will have to remove the sender and check to see if the connection shows sines of over heating. and replace it anyway. Or you can check it using a lab scope. If it is still good replace it anyway it will cause the same problem later.