1996 gmc high idle

I have a 96' GMC c1500 2wd with a 305Vortec engine and its got an excessivly high idle.. It idles around 2200rpm.. Using a lot of fuel, and its hard on the tranny..
I used a cheap scanner and im getting codes:

0102 : mass air flow circuit low input
0122 : tps/switch a circuit low input
0123 : tps/switch a circuit high input
0507 : idle control system idle higher than expected

1406:diffrential pressure feedback EGR (DPFE) sensor downstream hose off or plugged

Ive replaced the TPS, cleaned the MAF and EGR valve, and nothing has changed.. Looked all over for cracked or loose hoses but cant find any.. Anyone know whats goin on, or an idea where to start??

Do you have throttle body injection?The gaskets tend to get sucked in and leak causing high idle. Just a guess.

no its got SFI.. I checked the throttlebody, and its just got a ring pressed into the manifold, not really a gasket..

The TPS codes and MAF codes havent come back.. So I think it has something to do with the code 1406.. But I dont really know anything about that EGR system.. it doesnt appear to have any hoses that I can see.. Just an electric EGR valve bolted the the engine..

OK so I cleared all the codes and took it for a test drive today.. it ran fine til' it warmed up, thats when it started acting up

When I put it into drive, it shifts really hard because the rpm is high, than when I put it into Park or nuetral, it races up to 1500rpm for a second, than back down to about 1200..

The only code that come back was P0507 (idle rpm higher than expected)
Does anyone know anything about this code?

Could it be the IAC motor?

If it is running fine until it warmed up, it could be the Engine Coolant Temp sensor (in the intake manifold). The PCM uses this sensor. The colder it reads, the more fuel is delivered by the injectors.

Some scanners will show you the temp that is being reported.

//2000CAYukon

I just have a shitty little scanner.. it only reads out the stored codes, if any..
I tested the temp sensor with an ohm meter and its within spec but maybe i'll replace it anyway

I just have a shitty little scanner.. it only reads out the stored codes, if any..
I tested the temp sensor with an ohm meter and its within spec but maybe i'll replace it anyway

If it is within spec, then it is probably OK. Since you had codes for the TPS, check that with a meter both hot and cold and see if it changes when hot.

I would also check all the vacuum lines and intake for any leaks. Make sure the PCV is working properly, too.

Good luck and keep us posted on any progress.

//2000CAYukon

If it is within spec, then it is probably OK. Since you had codes for the TPS, check that with a meter both hot and cold and see if it changes when hot.
I would also check all the vacuum lines and intake for any leaks. Make sure the PCV is working properly, too.
Good luck and keep us posted on any progress.


Ive replaced the TPS, and nothings changed.. The TPS code has never come back..

Ive also made my own "vacumn leak finder".. a 3" PVC endcap with a valve stem pressed into the center.. I put it inplace of the MAF and pressurize the system, then I can hear any air leaks.. Works very well, and it only takes 5 minutes
The only thing I found was a broken PCV valve that leaking badly.. I replaced it but it hasnt changed anything..

The only thing I can think of is that I screwed up the MAF last time I cleaned it?? (spray with TB cleaner) is ther any way to test it?

take a can of chemtool and spray around base of intake and any vacuum hoses while running, if it bogs the engine down there is you problem. it is most likely a vaccuum leak, could be intake gasket.

I had the same exact symptoms on my 87 T.B.I., idled at 2200, shifted hard, the computer kept trying to richen it up, horrible gas mileage, etc. I checked the T.B.I. gasket looked fine on the outside 4 edges were all there, even sprayed starting fluid to check for leak nothing! Took the T.B.I. off and low and behold the GASKET WAS BAD. Replaced for 7.00 and runs like new!

I too have a 96 CFI with the high idle problem. I have worked for GMC as a tech in the past during the years this truck platform was sold. There is a history of ERG pintle proformance problems and many were replaced with low miles on the vehicle. The 1406 really means the pintle is sticking shut or sticking open. Mine only does it in cold weather. I am going to replace my EGR because of miles. This one is the second one and has been on for about 80,000. I clean it but have not found anything that will break up carbon that is hardened. Sea foam doesn't seem to work for the EGR unless you have the gm scan tool and can command the ERG to 100%. Try finding something you can soak the base of the EGR in to break up the carbon and then see what happens.