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

[JLeather]

Still fighting with this. Although I had already switched the IAC out with one from another running truck I decided to replace it anyway since after all my digging it still seemed like the most obvious solution. New AC Delco IAC made no difference. I also unplugged the EGR valve when the truck was warmed up but still idling properly (just warmed up). This made no difference either. The issue still showed up when the truck got hot and went away when it cooled off a little. I removed the EVAP line from the intake and plugged it since according to my scanner the purge solenoid isn't working properly; no difference to the surging issue.

I tried to rule out a transmission issue by jacking the truck up and putting it in gear. The idea was to see if it would idle in reverse with no load (wheels spinning) but this didn't work. With no load at all the engine never 'idled', it ran at like 1,200-1,500 in forward or reverse. I can say that according to the MAP sensor the truck is seeing the same load in forward and reverse.

Still stumped here. I have purchased a new MAP as well because it's fairly inexpensive. I'll try that out tomorrow.

Seems like there are two likely scenarios here. Either the IAC counts are getting messed up by a voltage change when the truck is hot, or the truck is getting extra air somewhere. I checked the voltage at the battery and it showed no drop when the truck is shifted into reverse (14.1v solid). Also made no difference when the lights were turned on. I couldn't figure out a way to get a read on voltage at the IAC, maybe someone can suggest something? Is one IAC pin supposed to be a steady voltage or is the voltage only on the pulsed pin?

As for the extra air, I've capped off whatever I can think of. Still a possibility with the CCV system. I re-read east302's troubleshooting section on it but it wasn't clear. Maybe I'll figure out how to check that tomorrow too.

 

[JLeather]

Side note, a friend suggested a possible test. What if the truck didn't know it was in reverse? It's not like it needs to control shifts or anything. If the signal from the trans range sensor was interrupted in reverse only would the transmission still function properly? If so, does anyone know which range signal is used for reverse? The wiring diagram lists Pink (pin 32) as Range Signal A, Dark Blue (pin 22) as Range Signal B, and Red (pin 11) as Range Signal C. Is one of those the 'reverse' signal to the computer, and if I interrupt it will the trans still function properly in reverse?

Edit - Nevermind, I found a chart showing the range signals at each shifter position and there's no way to fool the truck out of reverse without messing up other ranges.

 

[redfishsc]

As for the extra air, I've capped off whatever I can think of. Still a possibility with the CCV system. I re-read east302's troubleshooting section on it but it wasn't clear. Maybe I'll figure out how to check that tomorrow too.

I tend to agree, I think you have a sensor going haywire when it's hot, or something. Can you read the engine temp with a scanner? If so, that might rule out a faulty OBD temp sending unit (the one in front of your thermostat). Alternatively, you probably should be able just unplug that sensor when the idle starts loping. I'm not sure exactly how the PCM would interpret "no data" from that sensor but at least you should be able to eliminate it as a source of the "oscillating bad data".

 

[JLeather]

Today's results:
Capped off PCV at intake tube and manifold, removed PCV from valve cover, no change. New MAP sensor didn't fix it. Cleaned MAF and swapped one from another truck, didn't change anything. Cleaned battery and ground connections, replaced battery bolts; no help. Tried a supplemental ground wire, also no help.

I ordered a gauge to check transmission line pressure to see if something weird is happening inside the transmission, which is seeming more and more plausible all the time. I also need to find an analog voltmeter or one that responds faster than mine. There is some voltage fluctuation when measured at the battery during the engine surging. It appears to swing down to ~13v at the low side. I can't tell if this is a symptom or a cause for the surging. I tend to believe it's a symptom as the voltage is steady in reverse when it's not surging.

Anyone got another idea?

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[JLeather]

I tend to agree, I think you have a sensor going haywire when it's hot, or something. Can you read the engine temp with a scanner? If so, that might rule out a faulty OBD temp sending unit (the one in front of your thermostat). Alternatively, you probably should be able just unplug that sensor when the idle starts loping. I'm not sure exactly how the PCM would interpret "no data" from that sensor but at least you should be able to eliminate it as a source of the "oscillating bad data".

I can read engine temp and it's 192-194. It isn't changing when the truck starts to act up, it'll hit 192+ in the first 5 minutes Utes of driving but the truck won't start surging until a drive of at least 20 minutes. I'm really leaning toward a trans problem at this point.

The problem with unplugging sensors is that usually sends the engine back into open loop which would stop the issue even if you weren't unplugging the sensor at fault (assuming one is at fault...)

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[JLeather]

Might have found something. Apparently it's more common on '99+, but it seems that if the torque converter isn't supplied enough fluid the TCC will drag, and the stock PR valve in the trans is set for too much line pressure stock. Also a restriction in the cooler lines will starve the converter. Haven't figured out 100% how to ID the issue with line pressures, and it's half as much work to test the PR valve as it is to replace it so I may just give it a shot. Sure sounds like my issue. I will of course check the trans cooler lines first for a restriction. It's be nice if I had a way to test them for real flow, but the PR valve kit is only $40 (have to drop the pan tho).

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[east302]

Thanks for the updates and let us know if you track this thing down. Your theory on the transmission seems reasonable.

Just throwing out ideas...Could the transmission position switch (not sure what it's really called--range position switch?) have anything to do with this? If it's a mechanical linkage, could it be pinching a wire and causing a short? Or could the switch itself be flaky?

A trouble code sure would be nice.


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[JLeather]

I found one trick to diagnosing it is to loop the cooler lines to eliminate the cooler in the circuit. If the problem stops it's a cooler restriction. I don't think this is my issue since I just changed the radiator with the cooler inside, but something else to consider.

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[JLeather]

Thanks for the updates and let us know if you track this thing down. Your theory on the transmission seems reasonable.

The only way you guys don't find the answer is if I get sick of this thing and junk it before it's fixed...

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[JLeather]

I borrowed my uncle's lift and got the line pressure gauge installed. Idle pressures were all in-spec (60 psi in park/neutral, 120 psi in drive, and 150 psi in reverse). 2,500 rpm in drive was about 280 psi and stable. Reverse the pressure gets really unstable as the rpm's go up and pegged the 400 psi gauge before I got to 1,800 rpm (see video). As far as I can tell this is cause by leakage at the boost valve in the pump. There's a kit for a boost valve with OD o-rings specifically for this issue (Sonnax 34200-01K). I also now understand what they mean when they say fixes transmissions with a 'buzzing' sound. I couldn't hear it over the truck, but with the gauge hooked up you can feel it buzz when the truck is in reverse. Parts are on order, and I also found a TSB on this issue that recommends drilling out a passage in the transmission to vent excess waste oil in reverse. I'll post a copy of the TSB I found also.

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