Mechanics

CHEVY IDLE SPEED STAYS HIGH TOO LONG

1997 Chevrolet Tahoe

Engine problem
1997 Chevy Tahoe V8 Four Wheel Drive Automatic 125000 miles

While driving, after releasing accel pedal, IAC counts will stay high for around 30 seconds resulting in rpm's around 2000 (verified by shifing into neutral). Eventually fall to zero and then rpm will be normal. Effect on vehilce is excessive braking force needed to slow vehicle.

For what I can tell, engine control sensors appear to be working within reasonable limits when I monitored using diagnostic tool ( also how I monitored IAC counts).
What could cause engine control module to command IAC for higher counts to cause higher rpm's when accel pedal is released?

Additional problem: cyl 3 gets 2 to 3 digit misfire counts when engine cold. Mostly goes away when engine warms up. Interesting, cyl 5 was getting misfires for some time. Then for no apparent reason cyl 5 became ok and misifres switched to cyl 3.

Recently replaced both pre-cat 02 sensors, PCV valve, rotor, rotor cap, plug wires, plugs.

any help would be appreciated,
thanks,
Paul
Avatar
Ptkaneman
January 12, 2008.



The pental of the IAC valve may be sticking or dirty. Also, your MAF sensor may need to be cleaned.

The misfiring cylinders are a common problem with these vortec engines. There are several reasons your cylinders may be misfiring. One is--a clogged or dirty fuel injector--try running the gas tank to below 1/4 tank and put in a large bottle of Cheveron Tectron fuel system cleaner. Run the tank to almost empty and put in another bottlle. This time fill the tank and see what happens. These injectors also have a design flaw and an upgraded version is avalable. The OEM injectors have the nozzle far away from the injector body, -- the new version has the nozzle and injector body connected as one at the inlet of the port---THE WAY IT'S SUPPOSED TO BE!
Also, the vortec head has awsome power potential, however, right around 125000miles, carbon builds up on the inside of the exaust valve guide because of the design of the valve oil seals. As the carbon builds up and hardens, the valve sticks. I discovered this little known fact on my personal 98 Tahoe.
If the fuel system cleaner doesn't work, try swapping out the spider injectors for the up graded version. If that doesn't work then the misfiring is most likely a sticking valve--in which case just put new heads on. New GM performance vortec heads are cheaper than having your heads rebuilt. Part 12558060.
Another thing about your engine. These engines are famous for leaking lower intake gaskets. If yours haven't already leaked, they will--it's only a matter of time. They may be already--this could be the reason for the misfires also. In any case, in order to change the lower intake gasket, the lower manifold must be removed and you may want to consider changing the heads now because your 3/4's of the way there.
Just one more thing--I have also seen the distributer on these engines wear out. This will absolutley drive you crazy trying to find and it will also cause a random misfire. Let me know what you find.

Tiny
Pear69
Jan 16, 2008.
Thanks for your advice. Have run a couple 100 gallons of fuel with different fuel system cleaners. No improvement has been observed.

I removed the IAC and cleaned. No improvement. Didn't replace since it appears the engine module was " commanding" the IAC to be open when the rpm's were high.

Injector issues, valves, lower intake gaskets and distributor - yes these things you suggested were areas I was afraid may be cause.

Will start looking into them further.

Thanks for help

Tiny
Ptkaneman
Jan 16, 2008.
I own a 1997 chevy tahoe 4 door 4wd and just replaced the intake gaskets and manifold. Its at 175,000 miles and after putting it all back together I had #3 and #5 cylinder missfires. Turned out the oem injectors are not great and stopped working. I also replaced the fuel pressure regulator after replacing the fuel injector module with the upgraded on as spoke of. I now however have an idle speed of 900-1000 and a check engine light with it. It gives me a code of PO507 which states engine control unit rpm is higher than expected.

I'm pulling my hair out now but I would say definitely replace your intake manifold gaskets (lower) as well as upper air plenum (black plastic piece on top of lower aluminum intake manifold) gaskets in one shot because they will fail soon. Use plenty of high temp black or red rtv sealant around the coolant ports and center intake manifold seat. Great proceedure on wiki answers. Good luck and let me know if you figure out the high idle.

Tiny
Fixplanes
Jun 17, 2009.
Have you checked the Fuel pressure PSI?
I had a faulty fuel pressure and it was giving
me too much gas, or not enough. Never exact.
Then I replaced it.

Clean the EGR Valve yet?
And the Air take censer on the hose.

Tiny
BernieM
Jun 25, 2009.