'98 K2500 surging at idle, can't figure it out

So the high IAC counts (and poor idle) are only in reverse?

Your line of thinking makes sense--what happens in reverse to increase the engine load (or decrease RPM) that doesn't happen in Drive or neutral/park?

Grasping at straws, but could a bad motor mount cause this? Not sure if the engine tries to shift differently in reverse than in Drive. It seems like that would be more of a vibration than anything, unless some obscure wire is getting pinched.

How about an electrical short in the reverse lights circuit since the IAC counts are based on voltage, among other things.

What is your voltage in reverse?

An issue with the transmission position switch?

The ECT sensor is good, right?








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The scanner shows a reasonable engine temp, usually around 197 F. I assume the scanner sees the same ECT as the computer?

Trans shifter position shows as correct on the scanner when shifted through the gears. Again I assume this is the same signals that would be going to the computer.

Commanded idle is the same in both drive and reverse, 550 rpms, so the truck isn't intentionally trying to idle lower in reverse.

Voltage is an interesting question. Should I check it across the battery, or at the IAC somehow?

I suspect the EVAP failure is a leak. I know the steel EVAP line to the tank is corroded. I don't have the EVAP cart mentioned, but I will try to rig up a way to pressurize the tank and check for a leak. Still, that should affect the truck equally in forward and reverse I'd think?

Is there any way to check actual load in drive vs reverse? The scanner shows a load but I think it's theoretical based on rpm and MAP readings. I could compare MAP readings?

Also there's nothing about the torque converter that should change in reverse is there?

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Yes, the scanner should be seeing the ECT signal which is also what the VCM uses. It sounds fine, assuming that the thermostat has opened by then.

As for voltage--beats me, but it'd be interesting to see if it is dropping when in gear. Measure from the alternator post, I suppose.

Unless the IAC is flaking out only in reverse, then I'd say the valve is probably fine. It's just that the VCM is going nuts trying to compensate for something, and the IAC is only adjusted based on voltage, ECT value and the ambiguous engine load and low rpm.

I suppose that the VCM is basing the rpm on the tach signal from the coil?

I kind of doubt that the evap thing (and I'm not sure why that hasn't tripped a code) is your issue, but, hey you never know. Yes, you'd think that any impact from that would be constant, regardless of gear position.

I'm not sure how the VCM even defines engine load. Maybe, as you suggest, it's the voltage from the MAP sensor as the rpm changes.

Random thoughts--

Is this independent of the air conditioner?

What is the camshaft position sensor (CMP retard) offset reading? Should be -2 to 2 degrees at around 1000-rpm. That would usually toss a code if it were really out and could be indicative of a distributor issue, but I guess it wouldn't explain the Drive/reverse thing. Yep, a random thought.


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Try taking a pair of jumper cables and go from a good ground on the engine directly to the battery. It is possible that you have a bad ground that is making good connection until the drivetrain is loaded differently by being put in reverse.
That is my best theory. If it isn't the ground, I'm betting that you will find if you start moving the harness around that your problem will be replicated in some fashion or another. Either way it's going to be a pain in the donkey balls to get it figured out.

I'll have to borrow the better scanner to see CMP offest. Maybe Friday afternoon.

Thermostat is a new 195 F unit. Radiator is new too (and it's about to get a new water pump). All unrelated to the current issue, the truck had several annoying leaks when I got it.

A/C doesn't work. Needs a charge, can't engage the compressor. I still doubt the engine load in reverse is actually higher than drive or I'd feel it in the brake pedal holding the truck still. That being said, in drive even with 0 counts of IAC the idle doesn't drop down to 400 like it does in reverse so there must be extra load of some kind?

I'll try the ground suggestion.

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See if you're getting a voltage drop when it's put in reverse.

I wouldn't think that reverse would really matter unless the engine is physically moving or shifting when reverse is selected and making some already poor electrical connection worse. Maybe watch the motor mounts as someone shifts it.

Like skylark said...donkey balls.


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Just for grins, here's a video of the issue. It is extremely consistent, 100% of the time it does this after the truck has been driven for more than 15-20 minutes. It does not do it necessarily as soon as the engine reaches operating temperature, that happens in 5 minutes. It does have to be driven for a while. Once it starts doing it, however, it will keep doing it until the truck is shut off and fully cooled down. Shutting it off for a few minutes won't stop it, and letting it idle for a long time won't stop it...

What was that noise that went along with it? Sounded like something vibrating.


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east302 said:

What was that noise that went along with it? Sounded like something vibrating.

Click to expand...

Below 400 rpms the engine shakes at just the right frequency to make the door panel rattle. In the truck it's not that bad, but it must have been like right next to the phone mic because it picked it up way louder than the engine noise itself. I never noticed it before because I never turned the volume up after taking the video.

Ok, it's officially getting weirder. I swung by the shop today to use the scanner and fiddle with the truck some more. It's only about a 10 minute drive if I go straight there from my house, so I took the long way to make sure the truck was warm enough to exhibit the issue. When I get to the shop I try the truck out in reverse and it's surging, so all is good. I leave it idling to stay warm while I go inside and BS with my Uncle and some buddies for a minute. Pull the truck in, hook it up, and it's running perfectly. Still showing above 192 *F operating temperature and running in open loop, so it didn't get cold, but it wasn't having problems. IAC counts were right around 40 for park idle and around 48 for idle in drive and reverse. Both were dead on, no surging, no issues. At this point I'm thinking maybe it's an off and on bad connection somewhere so we start wiggling wires to the IAC, adjacent harnesses, etc. The engine ground strap is not loose but we wiggle it too. Nothing makes any difference, the truck idles great in all gears. I also check the voltage in reverse vs. drive and both are the same (14.1 v). I also try it with and without lights, no change to voltage either.

At that point I ran out of things to try, and I'm cold, so I take the 10 minute drive back to my house. When I get there, sure enough the truck has the issue again. IAC counts are all over the place again (4 in park, 150+ in reverse, etc). I pull the EVAP line and plug it to see if that makes a difference, and it doesn't. So I let the truck idle in park for 10 minutes and the issue is once again gone. IAC counts are good, idle is stable in all gears. WTF?

I'm no longer inclined to believe it's a poor connection somewhere, as that shouldn't get better from letting the truck idle for 10 minutes. I'm now thinking it's a heat-soak issue of some sensor, or possibly even the IAC itself although this is already the second IAC (which was used, not new). The truck was at operating temp and running in open loop the whole time, but it apparently only experiences the issue when it's been driven some distance and something gets hot? Previously it could idle for an hour and the issue would persist without change, but this is the coldest night I've driven the truck. What sensor could be getting heat-soaked and has the power to mess with the IAC counts like this? MAP? MAF? Ignition Coil? IAC?

To answer some previous suggestions, the motor mounts appear solid and the engine does not move much when put in and out of gear. I didn't see a CMP offset reading, but the distributor at idle was showing -10*? Still no codes of any kind from the truck.