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

[JLeather]

My '98 K2500 with the 5.7 Vortec surges very badly at idle. Park idle is 700 rpm. In gear idle is 650 when it stabilizes, but drops as far as 350 in a regular hunt that oscillates every 3-4 seconds. Sometimes after 10 or 20 seconds the idle will stabilize. Sometimes when you put it in drive or reverse (especially reverse) it will stall out. The truck only does it when warmed up.

I've checked the usual. No vacuum leaks I can hear or find with propane, vacuum booster has been tested. IAC was removed and cleaned, and it does move when cycled. TB was also cleaned at that time. TPS was checked, no flat spots and it seems to read properly at idle. Truck has no codes. Coolant temp sensor is reporting the correct temp.

I've put a scanner on it and everything seems ok. The IAC actual and commanded positions match, short and long term fuel trim is decent (around -1.6%), and both banks are equal. O2 sensors are around 0.5-1.0v and both match also. IAC counts do seem a bit high during in-gear idle. They get up to ~130, some places I've read say max count should only reach 80-90? What we also found was with a good scanner we could manually set the IAC positions. An IAC count of 0 was equal to ~525rpm in-gear idle and the truck had no problem idling when we were manually adjusting it with the scanner. Pull the scanner off and put the truck in gear and it's back to hunting.

Any suggestions? I wanted to bump the throttle blad up to a min idle of over 550 rpms at 0-count IAC so the truck couldn't possibly idle low enough to hunt, but when we open the TB blade up that far it's past 0.67v on the TPS and the truck stops using the IAC's because it doesn't think it's at idle anymore. The TPS isn't adjustable, although I'm tempted to slot the holes and bump it up to see what I can accomplish. Beyond that I don't know what else to do. The truck is capable of idling at 550 in gear, but the computer makes it hunt badly.

 

[redfishsc]

How old is the fuel filter, and what is the fuel pressure?

Also, how old are the injectors and/or fuel pressure regulator? It's possible the fuel pressure regulator is pissing into the intake enough to mess up the idle but not enough to notice it at higher rpm.

 

[JLeather]

I just bought this truck recently. At the time I didn't run it long because of the intake coolant leak, so I didn't find out about the surging until later. I don't have any of the history of replacement parts. I should do a fuel filter though.

That being said, I've had these trucks before and my Dad also has a '97 I keep on the road so I know about the issues with the I mention. Typically a leaking FPR or spider will show up as an anomaly in the fuel trim or O2 readings, which are fine on this one. Also I had it emissions tested Friday which in my state is a tailpipe sniffer at idle and 2,500 rpms and it passed fine including hydrocarbons.

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[east302]

Here are some things to look at for unstable idle. Diagnostics are for a 1998 350. Maybe it'll give you some additional things to check.

I'd hesitate to modify the TPS as it would just mask the problem...there's got to be some underlying issue.

There are a lot of hyperlinks in the table, let me know if you'd like copies of any of those. I've add the IAC and EGR diagnostics at the bottom.

I would second the suggestion for checking fuel pressure (and an injector leak down test).

Also, how do the battery cables and grounds look?

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[JLeather]

Key on fuel pressure was 60 psi. It does slowly decay, about 1-2 psi per minute down to about 50 psi, before I moved on. Not sure if that counts as a "leak" or I'd that's pretty standard? I only left it drop to 50 psi before I went on to starting the truck.

Running in park idle the fuel pressure is 55 psi. In gear idle the fuel pressure bumps up to 56 psi and holds steady. If I slowly rev up the truck in park the fuel pressure stays at 55 psi, but if I punch the gas the fuel pressure shoots up to like 65 psi and drops off for a second before stabilizing at 55 psi again.

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[JLeather]

Ok, 1 psi/min was a rough estimate. I re-ran a leak down test and the results were as follows:

Start 60 psi
11 min 53 psi
21 min 52 psi
31 min 50 psi
42 min 48 psi
52 min 45 psi
71 min 42 psi
84 min 40 psi
96 min 38 psi
108 min 34 psi
138 min 30 psi
161 min 28 psi



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[JLeather]

east302, there may be something to the EGR checks noted above. The fact that it only does it when fully warmed up could point to EGR since that is essentially disabled when the engine is cold. Also, I'm interested in the "Trans Range" switch. The truck is much worse with this issue in reverse vs. drive, which I always thought was strange. I'll try and get by my buddy's place later this week and see what the scanner shows the TR switch positions are in drive vs. neutral and also run the EGR pintle position checks.

 

[Eskimomann209]

Lotta tech here and I didn’t read everyone’s response
But I had an issue a lot like this one and it was a bad wire it looked good until I hooked it up to the DMM and bent the wire around to show the resistance changing from the broken insulation.
If you have them all under warranty swap em all of you don’t wanna OHM them

 

[knucklehead]

I recently had this problem also. Ended up being a vacuum leak where the intake cap attaches to the throttle body. Check the o-ring in there. Mine did not respond to propane either

 

[JLeather]

I recently had this problem also. Ended up being a vacuum leak where the intake cap attaches to the throttle body. Check the o-ring in there. Mine did not respond to propane either

Are you talking about the plastic upper elbow that goes to the airfilter? Not a leak between the TB and the upper half of the manifold? How did you determine that was the issue, was it visibly damaged or something?