Now we move into desperation territory (seeking advice)...

[YakkoWarner]

I still have not found my phantom (or demon more accurately) electronics problem causing random shutoffs and/or extreme running issues. Without some sort of extremely elaborate test rig, I can't truly test the wiring under all conditions....sitting at home in the driveway with basic DVM confirms I have proper connectivity "at that moment in time", I suppose some sort of fancy impedance tester could potentially flag issues at various frequencies. That is beyond my resources.

The one constant throughout the past several months has been the fact that merely turing it off and back on makes the problems go away (for a while). The only machine that I know which can be consistantly "fixed" temporarily by power-cycling is a computer. So now I have to face the absolutely terrifying elephant/rhino/T-rex...is the ECU itself malfunctioning? Thats the one peice that I havn't replaced, probably not replacable, and how would one even consider it since no one is making them?

If that is the case, should I attempt a carb retrofit, or spend more than the truck is worth to attempt something like a Holley aftermarket injection system. This is so overwhelming because no one even knows everything that the ECU actually does - I know it does the spark timing, idle control and injector pulsing on the output side, and reads 5 or 6 sensors on the input side. It also apparently controls the fuel pump relay. But that only accounts maybe a couple dozen of the hundreds of wires going into it...so what ELSE would quit working if you removed it and went to a different system?

Is there even such a thing as a drop-in replacement? The ideal would something that directly replaces it, but based on more currect technology and able to offer some sort of I/O interface capable of real time monitoring and interaction. I don't think any such thing exists.

Carb refit seems more doable simply because the fuel delivery change is simpler - put in a low pressure pump in place of the 15PSI pump currently in the tank. Running 3-5 PSI through lines rated for 15 is no problem. A Holley-type system wants 60PSI, running 60PSI through lines rated for 15PSI most certainly IS a problem. Both cases will require a distributor swap, which means the brand new distributor I put in has to go, along with all the new sensors I've replaced. I just have to eat the loss on those. I don't even know whats involved up on the intake side - I'm assuming some sort of adaptor plate on the intake and getting creative with the fuel lines at the very least. Is it even a viable option?

I have owned 4 OBD-1 vehicles - This is the 3rd one I have had fail on me electronically (the 4th just literally was falling apart round me physically but it kept running right up until the water pump siezed and shredded the timing belt) - the other 2 electronic failures also had other issues that made saving them impractical. I really like this Suburban and I'd like to keep it alive but at this point I'm afraid to even venture the 15 miles up the road to the grocery store in it, much less do a job or take a road trip in it.

 

[Doppleganger]

Did GM use relays more on the later year trucks? Could be an ECU - also a sticking or failing relay. I've had simple grounds (with stripped bolts) wreak havoc with runability issues. With electronic gremlins it can be a case of what is it or what isn't it.

 

[Catbox]

If it were me, I would make new grounds.
Just for peace of mind.

Take off each ground and grind on the metal of both sides and put it back.
If you are missing any of them, put new ones in place.
You cannot be over grounded.

 

[Frankenchevy]

Good point, start with the grounds. Free to check, cheap to fix. No codes I’m guessing? Battery good?

What transmission do you have?

 

[YakkoWarner]

Good point, start with the grounds. Free to check, cheap to fix. No codes I’m guessing? Battery good?

What transmission do you have?

Transmission is a TH400. Engine is a 454 all factory (no headers/uprated heads/hot cam or whatever).

I get an intermittant 44-Lean error which should not make it randomly stop running. I have reworked all the grounds I can find, including adding an extra one to go between the coil bracket on the intake to the firewall (done to solve a 32-SparkControl error which disappeared for over a year). Pulling a plug doesn't seem to show evidence of lean running (and again that wouldn't make it lose all spark and injector pulse until power-cycle, at which time they cheerfully work again). The lean code comes and goes, sometimes on applying power and sometimes on throttle lift. I'm pretty sure its a seperate independant issue. I've already gone through months of troubleshooting/replacing sensors/checking wires/etc....swapped ignition modules, distributor (did have a bad pickup coil), fuel pump relay. What is really making suspect the ECU itself is are the facts that A:I have replaced every sensor and module on this thing, and B:If it was wiring or a sensor or grounding, merely powering it off an back on wouldn't "fix" it - but it does for a while. Sometimes the power cycle is good for 5 minutes, sometimes an hour. Doesn't seem to be heat related (have had failures within minutes of cold start, or after driving for an hour) but switching the key all the way off and back on temporarily clears the problem. Everything I have read says do not ground the ECU itself because of possible stray currents. Fuses all good, get solid voltage at the ECM fuses but again can only check while sitting in the driveway, no way to monitor voltage at all these various points while driving. I'm tempted to jumper a switch into the ECM fuse so the next time it fails I can reset just the ECU (and not power-cycle the entire truck), that could possibly give me a data point.

Battery is good (1 year old) as are terminal connections, alternator puts out a solid upper 13 to low 14 volts depending on load, have no problems with any of the other electrical systems (lights locks radio hvac etc).

What I'm really trying to determine is the viability of replacing the ECU, or just removing it and going with a carb or aftermarket throttle body setup (no real interest in pulling the entire top of the engine apart to put on a multi-port intake). I just don't know what else would stop working - obviously the fuel pump relay would have to be triggered manually somehow. An aftermarket throttle body setup would have its own sensors and distributor to install, a carb would mean installing a vacuum advance HEI distributor. Does the computer have any input of what the transmission does (I know there's a vacuum modulator down on the transmission body because I had to replace it).

Replacing the ECU would mean finding one (and finding a working one at that) which came from the exact same truck of the exact same year with the exact same build sheet (except for things like color or interior trim), otherwise the programming would be all wrong. And even then its still another 35+ year old electronic device that may or may not work for long.

I'll admit to being rather surprised that there are no modern drop-in replacements for these things, you'd think there would be a large market and even something like a RasperryPi level device would be more than capable of emulating the slow, probably 8 bit computer while also being able to offer things like a user interface and onboard logging. Unfortunately paying someone to design something like that is WAY beyond my income level.

 

[gmbellew]

You should be able to source a new ECM and swap over your PROM chip. Check Ebay. I've bought a spare for my 350TBI from there. Make sure you get the correct ECM for your year/engine.

Also you could look at EBL flash II if you are looking to do more tuning and/or want a faster computer read out.

 

[YakkoWarner]

You should be able to source a new ECM and swap over your PROM chip. Check Ebay. I've bought a spare for my 350TBI from there. Make sure you get the correct ECM for your year/engine.

Also you could look at EBL flash II if you are looking to do more tuning and/or want a faster computer read out.

I'm not looking to do any kind of performance tuning or anything - its a stock engine with no upgrades or mods, but the idea of having something with actual readout ability would certainly be nice. Can you tell me any more about this?

 

[gmbellew]

I'm not looking to do any kind of performance tuning or anything - its a stock engine with no upgrades or mods, but the idea of having something with actual readout ability would certainly be nice. Can you tell me any more about this?

I don't know any more than what I read on their website from doing some research. But if you just want to see what the ECM sees at the sensors, how it is adjusting fuel, RPM, etc, I've been pretty darn pleased with ALDLDroid app on my phone and a Bluetooth ALDL cable from 1320 electronics.

 

[Frankenchevy]

With a th400, I don’t believe a swap to carb would be super difficult. It only needs a vacuum source for the modulator and a kick down switch at the skinny pedal. The kick down isn’t even a must have sort of deal, but you may have to manually downshift at the column, if pulling a grade or passing.

You could keep the in tank pump and run an inline pressure regulator to get it down to 5-7psi. Only electronics for the carb would be electric choke. HEI just wants a clean 12v.

Never done this myself and others will chime in. I’ve only gone the other direction. With as much digging as it sounds like you’ve done on the wiring, you can find the appropriate wires in your existing harness for the few connections that you’d need to make.

 

[mpatton]

Hello,
My problem on a to that had shutoffs and erratic idles, sucking air at times was caused by the tps, the arm was bent and once it was replaced everything was corrected. It was visibly noticeable when you took it off and looked at it. It was a mysterious problem for a while on mine.

 

[YakkoWarner]

Hello,
My problem on a to that had shutoffs and erratic idles, sucking air at times was caused by the tps, the arm was bent and once it was replaced everything was corrected. It was visibly noticeable when you took it off and looked at it. It was a mysterious problem for a while on mine.

Good call to check on that, but I have already replaced the TPS thinking it might have been related to the lean run condition that sometimes pops up. Did not make any change so I'm keeping the old one as a "probably good" spare...

 

[mrdap]

Admittedly I haven't read every word of every post on this thread.
But the first thought that comes to my mind is the ignition / HEI module inside the distributor.
Have you checked or swapped that?

 

[YakkoWarner]

Admittedly I haven't read every word of every post on this thread.
But the first thought that comes to my mind is the ignition / HEI module inside the distributor.
Have you checked or swapped that?

Yes - the distributor itself is brand new (the pickup coil on the orig one did go bad), and it came with a brand new module/cap/rotor installed. When the problem started I changed the module again, also with no change in results.

When the pickup coil went bad, the whole fuel and ignition system was dead no matter what I did. The problems I am having now are temporarily resolved just by powering off the system with the key and immediately turning back on. A bad pickup coil or module won't magically "fix" itself with a power cycle. Thats the one thing that makes me suspect the ECU - a computer is the only machine I know of that can fixed by power cycling it.

 

[gmbellew]

Might try not cycling the key, but instead disconnecting and reconnecting the relay for the fuel pump without touching the key. Maybe it is some fuel pump weirdness going on? That would just take the power off the fuel pump and only reset it, without resetting the ECM?

I don't remember if you tried replacing the fuel pump relay?

 

[YakkoWarner]

Fuel pump relay has been replaced. I swapped it as an outside chance it could be causing an excess electrical draw/voltage sag. I'm not losing the fuel pump itself (I even jumpered the oil pressure switch to force an always-on state and still had it shut down).

When it went into the second failure mode of spitting back through the air cleaner at any amount of throttle, it again cleared itself after a power cycle.