1999 suburban strange idle/driving issue

[travis]

Of course this happens when I’m about to put this thing up for sale...

My 1999 C2500 suburban developed a strange issue suddenly. Cold starting (cold being relative as it is hotter than hell in Texas right now) it immediately jumps to 1800-1900 rpms and stays there for a minute or 2 before settling down to a normal 750-800 rpm idle. Unless you put it in gear then it settles down immediately. It does it on hot starts too to a lesser degree...usually about 1200-1300 rpms. When driving it unlocks the converter at the slightest change in throttle opening whether more or less throttle is used...it is really noticeable when backing off the throttle a bit. It still starts at the touch of the key, and power seems just like it has always been. It doesn’t run hot or surge either.

The only thing that has changed any time recently was I replaced the fuel tank. I did not change the fuel pump. I have driven it several times after replacing the tank and it ran just as it always has.

However...in the last 3-4 weeks the truck has only been started and run long enough to move it around the property. Eventually the battery got low enough that it wouldn’t turn the engine over, so I charged it enough to start and then drove it long enough to get the battery fully charged. Once I charged the battery back up is when the issue started.

I put my scan tool on it but there are no codes, everything shows to be operating correctly. A little google searching came up with a couple of vague references to being stuck in a cold operating mode, which sounds like it could be the case.

Do any of you guys know anything about this (or any other ideas) and how do you clear it?

[donforeman]

Sounds like a vacuum leak or a sticking IAC. Look at the IAC counts on a scan tool for more clues.

[travis]

My first thought was a vacuum leak as well, but couldn’t find one.

Well, as I was closing the hood something caught my eye. There is a multi port water manifold located down by the passenger side exhaust manifold the directs hot water to the front and rear heaters. There is a vacuum operated flapper valve on that manifold, and the hoses looked a little odd. The little 1/8” hose was broke...

[BLstangin]

Also if you jump started it and it had a dead battery and you gave it any gas as you cranked it, the ecm will record that as TPS zero and it will make it do some really odd things, including not idle period on occasion, there was never any codes but once you got the battery charged and unhooked it for 10 minutes and started it back up without touching the pedal it ran fine again.

[TBART1970]

Disconnect battery and touch cables together for 10 minutes. Fully charge battery and test it before hooking it up. How many time has the battery gone dead? How old is the battery?

[travis]

Replacing the vacuum hose fixed it.

The battery is only about 1 1/2 years old. I just don’t drive it anymore so the battery goes dead on occasion