1990 k1500 5.7L DD
Recent purchase from off the farm, previous owner has done:
Complete tuneup with all AC Delco parts: plugs, wires, cap & rotor, EGR
New fuel tank and pump, new fuel pump relay

Recently having apparent fuel issues. When truck is warm, no start, have spark.
When cool or after sitting to cool will restart.
With all that has been done, one thing not done yet is the coolant temp sensor.
Will change that out today. Cheap $18 part.

Question is, could this be the ECM going bad?
​​​​​​Before I order the correct AC Delco one for this motor, over $200 with shipping,
I have a 1988 k1500 5.0L that starts every time, even after sitting for weeks.
If I swap the ECM out of the '88 is this compatible from the 5.0 to the 5.7?

Is there something else I should be looking at, something we're missing.

Thanks for any input, I've got a lot of good info from this forum over the years.

 

Does it attempt to start at all (sputtering/etc) or is it just a crank no start?

Have you verified if there’s fuel pressure when the problem is present? If yes, have you verified whether or not the injectors are pulsing?

Try disconnecting the ECM spark control circuit and see if it starts.

ECM compatibility is questionable - get the part #’s off the 88 ECM and I should be able to verify it.

 

Crank no start
Will fire with fuel sprayed into intake
Have spark
Something is interrupting fuel system when engine is warm.
Not hot, temp gauge shows 180, but under hood feels really hot
That's why we're looking at this sending unit, or possibly the ECM failing. It is after all 34 years old.
​​

 

I understand the reasoning of it being 30+ years old but I’ve seen plenty of these trucks with stall/no start/driveability issues and the ECM is rarely at fault. They’re surprisingly durable.

If you want to know whether or not the ECM is to blame, keep a test light at the ready for the next time it stalls. Disconnect an injector and use the test light in its place to see if the ECM is pulsing the injectors while cranking. The test light will either pulse/flash or glow steadily (I can’t remember which at the moment, but basically if it doesn’t light, the ECM isn’t pulsing the injectors).

As for temp….

Verify the correct sensor is being looked at for the gauge - I’m pretty sure there was a genuine game of Russian Roulette Beer Pong being played on the factory floor when these trucks were being built, because some of them used a temperature switch, some used an ECT, and some used dual CHT’s to monitor temp.

They could’ve been mix’n’match style too, because some used one type for the gauge and a different type for the ECM temp input.

To verify what’s feeding the gauge if you have multiple temp sensors, turn the key on, engine off, and just unplug it (or both CHT’s if equipped) and see if the gauge pegs to cold. If so, jump the circuits with a paperclip and see if the gauge pegs to hot. If so, gauge & circuit functionality is confirmed.

The quickest, easiest check on a temp sensor is to read the surface temp right next to and on the sensor base using an infrared thermometer - these things can usually be found for cheap.

And if you could, please clarify the following -

 

Rained all day yesterday so no chance to play with the truck.
Today had my son out to help, new set of eyes. He's no mechanic, but an electrician. Asked if he might help look for something I'm not seeing.
Looking for possible bad grounds, possible old poor wiring,etc.

Again turns over, crank no start. While cranking no fuel at injectors.
Connection to TPS looked iffy, pulled off connector, looked as if not getting good contact, dirt and gunk/corrosion inside. Pull connector on the '88, looked nice and bright. Pulled the connector off of '88 and spliced it in, picked up new TPS. No difference. Still no start.
I guess ill try the temp sensor next.

As for feeling hot. My neighbor who's been helping to try and diagnose this thing, when we pop the hood feel like A LOT of heat under the hood when it was running. Especially when you account for how much room there is in this engine bay. Especially hot on the top end of this motor. Like smokin' hot.

But if heat is the issue, would not explain why wouldn't start today. Much cooler weather today and truck hasn't started since Saturday.
I understand what you said about the age and, the '88 still starts and runs great on what is I'm sure is the original ECM. Is it possible that heat caused a failure of the ECM? This is mounted right adjacent to the intake manifold. Is their any way to test if it is bad?

Any other things that might cause this condition? MAP sensor? Not much else I can change.

Sorry to be so wordy, just getting real frustrated with this truck. Picked it up weekend of July 4th. Ran great for 2 months then all of a sudden has been a pain. Really need to get back on the road. I'm senior citizen, can't keep bummin' rides to work, and need my 4wd when the snow flies. Really like these older trucks, can't stand these newer trucks. Was real happy with the ' 88 til tranny went out, body starting to go, broke a shock mount.

Thanks for reading.

 

I understand it’s frustrating, but in order to figure out what’s going on, a methodical approach is necessary. You can swap parts all day long and still never fix the issue.

I’ll do what I can to help but clarity is important, so please provide the most direct answers you can. It’s too easy for things to get lost in translation on these boards.

The first thing we need to determine is what system you actually have - which of these ECM’s matches yours?


OBD1 ECM


OBD2 ECM

I only ask because you say it’s mounted in the engine bay, but from the factory, the ECM’s in pre-1996 trucks were mounted behind the glove box. It’s certainly possible the PO relocated the ECM, but we need to be certain a newer engine/ECM hasn’t been swapped in.

I know what you mean about the engine bay feeling very hot - if it’s as hot as you say, I’d suspect the fan clutch is worn and it’s not moving enough air - but it also wouldn’t hurt to wash the fins for the radiator and condenser - dirt/debris can collect in the fins and restrict the air flow, resulting in less heat dissipation and higher engine bay temps. Just use a garden hose with sprayer nozzle and spray as much area as you can through the grille and inside the fan shroud.

If it starts again and runs long enough to reach operating temperature, the fan clutch can be checked by shutting it off and seeing if the fan spins freely by hand - if so, the fan clutch needs replacement.

I’d also verify the coolant system is full and the condition of the coolant (is it dark/contaminated or clean?).

When you say no fuel at the injectors - does that mean there’s no fuel pressure or that the injectors aren’t pulsing?

Also, have you been able to verify if the alternator is charging when the engine is running?

Final check for now - if it still has the factory side post battery terminals, try to wiggle the cables side to side - if either one moves (even a little) remove the cable from the battery and check for corrosion - clean as needed and replace the battery bolt if it’s stripping.