High idle all the time

I have a 94 C2500 suburban with a TBI 454BB. Obviously, I’m not concerned with fuel mileage, but I am currently getting about 4 miles to the gallon because the idle is stuck at 1800RPM. Normal idle for my model according to the index book is 750RPM. it’s usually pretty close to that when operating normally (+/-25RPM). 3 days ago I did my biannual throttle body cleaning. Took it completely off replaced the gasket cleaned the throttle body and put it all back together. This is when the problem started… It now idles at 2300 RPM, it threw the code for bad idle air control valve, and I figured I probably got the plug wet when cleaning the throttle body, so I went ahead and ate the 60 bucks and replaced it. Went through the reset process for the ECM to learn the new one and it the idle has come down to 1800RPM, it also literally DUMPING fuel through the injectors into the top of the throttle body. While I don’t believe it is actually running rich, it seems more like the idle is trying to compensate for the excess fuel that is being dumped into the top end. Now, Before I go any further I want someone else’s input. “I” think it is a also a bad throttle position sensor that probably got wet when cleaning the TB. But I am worried it could also be the PCM has finally gone out, although this is probably not likely because the entire ignition is run by this and if it was bad, it would stand to reason that it wouldn’t run at all. Or it could be a bad vacuums control module. I have checked all my vacuum lines and do not have any leaks, so I know that isn’t the problem. So that’s where I am. I am hoping to check the leads on the TPS today with a ohm meter. I know my resistance levels, but figured I would throw the problem on the board and see if anyone had any other ideas to check.

This is very typical for a vacuum leak. If you damaged the TPS, you would get an immediate code for that and the engine would go into “limp” mode which would result in very low engine power. If your code was P0507, then that is almost always a vacuum leak, most often the intake manifold gasket, but since yours happened in conjunction with the throttle body cleaning, I would look there for the problem.

It is possible that the IAC got stuck during the cleaning process. I would not replace it, just pull it out and check for any gunk on the valve stem that might restrict its movement. GM usually uses a rubber boot over the valve stem so check that it is not torn and restricting the valve movement, and also check the plug for the IAC for a bent pin or a socket that has pushed back.

I am only getting a code when I’m holding the brake while in drive for more than about sixty seconds, then when I let of and start moving, the code clears itself.

I would have to believe the throttle body gasket is not where it belongs, causing a vacuum leak which in turn causes a loss of vacuum to MAP sensor which will make it idle high and rich.

What’s the problem? I’m ‘highly idle’ all the time, too!

My first thought is a possible problem w/the MAF sensor, or the air channel and baffling between the air cleaner and the intake manifold. Is it possible you could have damaged the MAF sensor in the process of cleaning the throttle body?

(The MAF usually looks like a cross-hatched screen or honeycomb looking gadget that sits in the intake airflow stream. It measures the amount of intake air flowing into the engine, and is used by the ECU to decide how much gas the fuel injectors should be delivering. If it measures this airflow rate incorrectly, it could adversely affect the idle speed.)

Other possibilites as other’s above said, are the IAC and various gasket and vacuum hose or device leaks.