High Idle

[rcaw]

Hello,

I have a 99 Tahoe, 5.7L, no mods.

I have a problem with a high idle.

The idle will increase to about 1500-1800 RPM when

the truck is in Drive going about 30 MPH.

If I shift into Neutral the idle indicates 1500-1800 RPM

The idle will not come down until the truck

speedo indicates 2-3 MPH.

When in Park, I cannot duplicate the problem.

My question is why does the idle jump up at 30MPH when in Drive?

Why does it come back down at 2-3 MPH?

I have checked for vacuum leaks and have found none.

The Check engine light is on and code 0507 is indicated.

It just says idle control system RPM High.

I have cleaned the throttle body, MAF, IAC valve, changed the TPS sensor,

changed from CPI injection to MFI injection. Nothing has helped.

Any info or advice would be appreciated.

Where on the forum can I find the Idle Relearn Procedure?

Thanks, Ron

[rcaw]

Never mind, I fixed it.

I replaced the lower intake manifold gaskets.

Passenger side gasket had failed, causing a vacuum leak

that I couldn't detect with conventional means.

Ron

[69BOWTIE]

So how did u end up finding it?I have the same symptoms and have cleaned everything under there and it is not consistant ,just does it once in awhile.

[rcaw]

I read on another thread that if you have vacuum in your

oil dipstick tube that you probably have an internal vacuum leak.

I disconnected the PCV valve and plugged the PCV valve end of the

line that goes to the manifold. Then I plugged the PCV valve valve cover hole.

I also disconnected the fresh air tube on the passenger side valve

cover and plugged its hole in the valve cover. Plastic wine corks fit well

in the holes. I didn't need a vacuum gage to tell me I had vacuum

in the dipstick tube. It was a pretty bad leak. Previously I did spray carb cleaner

around the top of the upper and lower intake manifolds but there was no

change in the engine idle. So it was an internal leak.

This is the second time I changed the lower intake manifold gaskets.

This time I got the more expensive Fel-Pro gaskets. FPG MS98000T at NAPA.

I'm sure other auto part stores carry them also.

Ron

[1999 Tahoe 4x4]

I read on another thread that if you have vacuum in your

oil dipstick tube that you probably have an internal vacuum leak.

I disconnected the PCV valve and plugged the PCV valve end of the

line that goes to the manifold. Then I plugged the PCV valve valve cover hole.

I also disconnected the fresh air tube on the passenger side valve

cover and plugged its hole in the valve cover. Plastic wine corks fit well

in the holes. I didn't need a vacuum gage to tell me I had vacuum

in the dipstick tube. It was a pretty bad leak. Previously I did spray carb cleaner

around the top of the upper and lower intake manifolds but there was no

change in the engine idle. So it was an internal leak.

This is the second time I changed the lower intake manifold gaskets.

This time I got the more expensive Fel-Pro gaskets. FPG MS98000T at NAPA.

I'm sure other auto part stores carry them also.

Ron

Thanks for sharing the whole experience!

 

How about any internal coolant leaks at the manifold? I hear the old OEM style gasket usually fails around the coolant ports. So actually it is quite a mystery to me why yours failed around the intake air area. Seems like bad luck.

[rcaw]

Fortunately I didn't have any internal coolant leaks.

I only had a small external leak.

When I replaced the gaskets I smeared a very thin

layer of RTV around the coolant holes in the gasket.

I think the first replacement gasket failed because of

an amatuer installation job. (Me). I may have

improperly torqued the intake manifold bolts the first time.

On the second job, I was able to borrow a pretty sophisticated

Snap-On torque wrench that buzzes when you reach the

proper torque. I thought the click type was cool.

This wrench was really cool.

Hopefully the second gasket replacement will be the last.

I still have a slight stumble at idle. An independent tech

who also owns a Tahoe scanned my vehicle and said all

my switches and sensors are working properly. He said

I need 60-66 PSI at the fuel pump. Turns out I have 50 PSI.

He said that may be the cause of the stumble because of a possible

lean burn.

Last year I dropped the fuel tank to repair a fuel line leak and

a friend advised me to change the fuel pump. Good advice.

So I changed it. I had 85,000 miles on it. At 50 PSI, looks

like I'll be replacing the fuel pump again.

I'm always doing things twice, oh well live and learn.

It's a pain but I figure I am saving a ton of money doing it myself.

Ron