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

[bigj-dog]

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.

[old_master]

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?

[bigj-dog]

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.

[bigj-dog]

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.

[bigj-dog]

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]

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:

http://www.automotiveforums.com/vbul...d.php?t=975186

[old_master]

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.

[bigj-dog]

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.

[bigj-dog]

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.

[old_master]

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?

[bigj-dog]

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]

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.

[bigj-dog]

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?

[old_master]

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.

[bigj-dog]

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.