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'94 S-10 4.3 Stumbles on acceleration, rough running

12-13-2010, 01:06 PM
1994 S-10 4x4 pickup, 4.3L, VIN W. 215,000 on the truck, 40 or so on a GM crate engine. A week or 2 ago the truck started missing and surging anywhere but idle or wot. Starts fine, even today when it was -15, fuel pressure 52 running, holds after shut down, newer plugs/wires/cap rotor, tested MAP sensor and tests good, tested resistance changes in TPS (with sensor out) and changes were smooth along range, unplugged every sensor I could find individually looking for changes, no luck. After you take it out and drive it, it'll idle fine for a couple minutes, then will drop a couple hundred rpms every 30 seconds or so for just a count, then back to normal. Any ideas? I'm pulling my hair out.

12-13-2010, 04:37 PM
Sounds like the problem occurs when the PCM goes into closed loop, (30 seconds to 2 or 3 minutes after start up). If thats the case, there might be a problem with the Engine Coolant Temperature sensor, (ECT located near the thermostat) not reporting the correct temperature to the PCM, or possibly an oxygen sensor. The PCM ignores oxygen sensors during open loop operation. How many miles on the O2 sensors?

12-13-2010, 05:27 PM
Not sure how old the o2 sensors are, you'd think it'd throw a code if they were the culprit, but then again, it is a 94. I thought I unplugged the coolant sensor, but I'll check it out again, that would make sense.

12-16-2010, 12:10 PM
Update: Put in a new O2 sensor, owner said it was due anyway, no change. The only coolant temp sensor I could find was in the driv side head, gauge works normally, unplug and gauge drops, no noticeable improvement with the issues. I do have a little more info though, it seems to work just fine when it's cold, and as it warms up, the symptoms get worse. I drove it for a few miles a few minutes ago, returned home and left it idleing, the momentary drop in the idle started happening, and got progressively worse until it killed. At that point it would fire now and again, but not restart. I know they put a used ign module in a month or so ago, wondering if that could be the culprit? I'm sure if I wait fifteen min it'll start right back up.

12-16-2010, 01:27 PM
Update Part Deux-
Nabbed another ignition module out a parts truck, and no change. After i tried it with that module, and then gave it a half hour and came back to check fuel pressure again, now it won't start, unless you open up the throttle. I'm wondering about crank sensor? Any other ideas? Or next thing to check? Fuel pressure was 60 non-running, so that's still good.

Rick Norwood
12-16-2010, 04:46 PM
What brand name of Distributor Cap did you use, and how long has it been in the truck? If you used anything but an AC-Delco it might be your culprit.

Read this Thread:

12-16-2010, 07:27 PM
Should be two coolant temp senders on it. The one you found in the cylinder head is only for the gage on the dash. There should be another one for the PCM near the thermostat in the intake manifold, that's the one that could be wacky.

12-16-2010, 10:00 PM
That's what I thought about the coolant sensors, but I didn't see that other one. I'll look again. The cap is a BWD Select from Oreillys. It's been on for a few thousand miles, and it I inspected it again today when I was changing the ign module, looked ok, but I'll switch it out if I run out of other ideas.

12-18-2010, 09:11 PM
Well, the new coolant temp switch didn't make any difference, in fact, it didn't even warm up enough I don't think where it could've. The intake leaks coolant at the pass front, so I'm now leaning towards a possible intake vacuum leak causing the problems. Today I started it after replacing the coolant switch, and it started fine, ran fine for a couple min, then started surging and dropping, like it would if you had the master cylinder vacuum hose unplugged from the intake. I had previously checked the vacuum pressure while running, and it was 18 or so idling, but who knows. I do know it needs intake gaskets, so I'm going to fix that next, and hope it's better after that. It's gotten to the point where it'll stall at idle, after dropping a coupe hundred rpm, surging a couple hundred rpm, and having that become more extreme, then dieing. Sometimes after that it won't even start until it's given time to either cool down or reset. Does that point to anything else? I'm running out of ideas.

12-18-2010, 10:16 PM
Typically if the lower intake gasket leaks vacuum, it only affects one cylinder and causes a noticeable misfire all the time. On your engine, the ECM batch fires the injectors, it can not richen the mixture for individual cylinders. I suppose it's possible that the plenum gasket is leaking vacuum, and that could affect more than one, if not all cylinders. On a cold start, (open loop operation) the ECM commands a rich mixture, which could mask a moderate size vacuum leak. When the ECM switches to closed loop, the mixture leans out substantially which could cause the unstable idle. Carefully inspect both gaskets when you take it apart for the intake gasket replacement.

Another thought... have you checked/adjusted base ignition timing?

12-18-2010, 11:17 PM
I haven't checked timing yet, but it's a friend of mine's truck, and they put the engine in and are very capable, and it's had this engine for a few years, and this problem has only recently shown up. I wish my decent scanner (that has access to engine data) had a connector for pre-OBDII trucks. I'll check it, and when I do the intake, I'll certainly look for trouble spots. It'd be nice if the truck would idle decent enough when warm to spray carb cleaner around the intake and other places to look for leaks. I think I'm going to see if I can isolate all outside vacuum leaks, by blocking off all vacuum ports and see if that helps. They've replaced the EGR valve not that long ago, so i'm assuming that's not causing the problem. It really seems to run like a 3.1L that you forgot to hook the vacuum booster hose back up to the intake after doing head/intake gaskets. Hopefully tomorrow's experiments will lead to either an answer, or more stuff to check off the list. Thanks OM for your time. I appreciate it.

Rick Norwood
01-03-2011, 02:32 PM
This is starting to sound like the old Ignition Module taking a crap issue.

What year did GM go to the external Ignition Module?

My old 93 S-10 blazer had an internal Ignition Module, inside the bottom of the distributor. I know this much, if those little PIA's start winking out, you will see all kinds of issues similar to what you are describing. The good news is that they are testable. You can carefully remove it from the bottom of the distributor by removing the two screws and unplugging the wire clips, and have it bench tested at some of the better Auto Parts stores. Be very careful as the wire clips are brittle and will easily break.

01-03-2011, 03:39 PM
Thanks for the input Rick. I swapped in an ign. module already, and no change. I also checked base timing, and it's good. Anyway, I did come into some new information. I guess the guy's boss accidentally topped off the gas tank with diesel a few weeks ago. The owner says that the guy put about 3 or 4 gallons into it, and he never drained it out. He says he kept topping it off with gas every 50 miles and assumed it's blend in and not cause any problems. I'm guessing that's the problem. Which makes sense, because the longer it's sat at my house, and the more times I've tried to move it, the worse it's gotten, as in, now it barely runs and only does for less than a minute before it kills, and they it's hit or miss if it'll start back up. I'm going to empty the tank and see where it goes from there, hopefully no damage was done to anything besides my schedule. Does anyone know if it'd be a problem if I just take the line off at the filter and run the pump until the tank is empty?

01-03-2011, 03:40 PM
The last year for HEI, (internal module) was early 95. Late 95 4.3's with OBD1.5 have the external module mounted next to the ignition coil.

01-03-2011, 03:49 PM
Did the Vortec have a different module then? I was able to swap the ign module out of this one and into a '96 Blazer. Two screws under the cap, next to and below the rotor, only one plugin I think.

01-03-2011, 03:54 PM
Using the pump to drain it will take FOREVER. The return fuel line directs unused fuel into a well that the fuel pump motor is mounted in. It's part of the fuel pump module, and the circulated fuel helps keep the motor cool. The fuel pump picks up fuel from inside the well. The pump will pump the fuel until the well runs dry, then you have to wait until it seeps back in to fill the well. If you've ever removed one of these pumps, if you tip it after you remove the module, it dumps fuel all over, that's the fuel that was in the well.

I'd say you're on the right track with diesel fuel causing the problem.... probably plugged up the injectors.

01-03-2011, 03:58 PM
94 & 96 ignition modules are not interchangeable.

01-03-2011, 04:03 PM
Ok, that's what I was wondering about the pump. I have just ran the pump to empty the tank, smacking it with a hammer to restart the pump after it seizes, when I've changed the failing pumps in similar trucks before. It didn't seem that they emptied that slowly, but I guess I never paid close attention either. I'll just drop the tank the way it is and use my regular pump to empty it then. How about those injectors? Will they clear themselves, or do I need to do something else?

And you're right about the ign modules, I thought about it some more, I took it out of a '89 K1500 5.7. My bad.

01-03-2011, 04:13 PM
Another thing with the pump, you need to "hot wire" it to keep it running, which poses a problem with cooling it. The PCM will shut it off if it, (the PCM) doesn't receive an ignition pulse.

The injector might clear out by itself, if it will pass gasoline through it. If it's totally plugged, probably not. The 94 "W" engine only has one injector, but it has 6 poppet valves, and those are most likely what's plugging up. Same deal with them, as long as they're passing even a little bit of gasoline, they might clear out.

89 & 94 ignition modules ARE interchangeable ;)

01-03-2011, 04:23 PM
Great info. I appreciate it. When it did run, it seemed to run ok, it just wouldn't speed up/accelerate under any power smoothly. And now it will run, it just has to be revved up to 2k or so, and as it drops to idle, it'll try to die (and succeed most of the time), so it can't be all the way plugged, but have the ambient temp about 0 probably doesn't help the injector pass diesel.
I ususally either hot wire the fuel pump relay, or find the test lead, but in this case, i think I'll just drop the tank to save time. I'll update when she's running, hopefully anyway.

01-03-2011, 04:28 PM
Yup, diesel fuel tends to get kinda thick at zero ;) You just might get lucky on this one.

01-03-2011, 04:40 PM
My buddy might get lucky. I'm already unlucky for chasing around demons without knowing that someone poured hardly refined oil into a gas tank.

01-03-2011, 04:42 PM
Any idea how much they put in there?

01-03-2011, 05:19 PM
Supposedly 3 to 4 gallons. And I guess my friend just kept filling it up whenever he could to try and dilute the diesel. It did run fine for the most part for a couple weeks, just kinda intermittenly acting up, but it's gotten progressively worse now.

01-03-2011, 05:45 PM
Probably be a good idea to replace the fuel filter after you get the diesel fuel out of the tank.

01-17-2011, 10:30 AM
Finally got the fuel drained, it didn't look horrible, or smell too much like diesel. Installed a new fuel filter, and primed the pump and it started right up (it was 25 this morning though, not -10 like last time). It idled perfectly for 10 or fifteen minutes, so I backed it down and into a parking space, barely touching the throttle, and everything went fine, until.... I put it in park and then decided to rev it up a little bit to see if it'd miss/stumble then. Which it did, and then killed. It barely accelerated, almost immediately it died when I gave it more throttle. I got it to start a couple more times, but only ran for a couple seconds. I wandered off for a few minutes and tried to start it again and then it wouldn't even fire. I know it has fuel pressure, and I can hear the pump cycle for a second or so before I try to start it each time. Previously I had checked spark when it wouldn't start and it always had that too. I know when I get home it'll fire right back up and idle fine. Any of that ring any new bells? I'm wondering if there's something wrong with the injector/spider assembly? I know these are problematic, but I'd like to be able to prove the problem before I spend more of his money on hope.

01-17-2011, 04:58 PM
Check fuel pressure and leakdown. 92 to 94 CMFI systems: Key ON, engine OFF, fuel pump running: pressure must be 55psi to 61psi and must remain above 50psi for 3 to 5 minutes after the fuel pump shuts off.

Rick Norwood
01-28-2011, 05:34 PM
Could the injectors be partially plugged? Open enough to idle but not enough to run?

01-30-2011, 11:40 PM
have you changed the plugs? they could have been fouled out. also, make sure you are getting good spark. almost sounds like a cracked distributor, which allows moisture in weakening the spark.

I dunno how easy it is to get the spider out on a 94, but if it is easy enough, i'd pull it, maybe replace the regulator, and give the poppets a good spray down with carb cleaner.

i drive a diesel for work regularly, and i know that when diesel gels, it clumps up. for example last winter, my fuel gelled, but enough was able to pass for the truck to idle, however under load it stumbled and stalled.

09-26-2011, 04:56 PM
I can't believe I forgot to finish this thread when I found the problem. It ended up being the electrical connector on the fuel injector. Unbeknownst to me, one of the tabs was broken off prior to me looking at it, and the heat cycles made it brittle, so it'd lose contact sometimes. I was able to buy a replacement pigtail and soldered it in, now it's been fine. What a waste of time. At least there's a lot of good information in this thread for other people with problematic 4.3s.

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