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GM Tech (Cam)
GM Tech (Cam), GMC Mechanic
Category: GM
Satisfied Customers: 29428
Experience:  GM Grand Master Technician 2007. 14 years experience.
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1996 GMC Sierra 1500: 2wd..5-speed manual transmission..engine warms

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I have a 1996 GMC Sierra 1500 2WD 305 with a 5-speed manual transmission. When I start my truck the rpm's are around 1500. As the engine warms a few, the rpm's will run at 2000 to 2500 and stay there. As I am driving down the road the rpm's continue to do this, most noticeable when shifting gears (neutral). I have changed the tps, idle air control sensor, and I have changed the bank 1 sensor 2 (oxygen sensor). Still doing this. What is the problem?

GM Tech (Cam) :

Hello, I'm Cam. Please include all the details about your issue so I can accurately advise you. I cannot see your vehicle so please be detailed.

GM Tech (Cam) :

Was there codes stored or is the check engine light on?

Customer:

Check engline light is on. It keeps telling me the bank 1 sensor 2 is bad. I just put a brand new sensor on and the truck is continuing to idle super high. Does not run erratic. Rpms race.

GM Tech (Cam) :

OK lets deal with the sensor after. The RPM is of concern.

GM Tech (Cam) :

When you replaced the IAC, did you clean out the throttle body?

Customer:

yes.

Customer:

I checked the linkage and it is moving freely.

GM Tech (Cam) :

OK do you have the means to check that the IAC is being commanded with the scan tool?

Customer:

no.

GM Tech (Cam) :

Ok for the RPMs to go high requires air to be let into the engine, either past the IAC, throttle body plates or somewhere else such as a vacuum leak.

Customer:

I even checked for vaccum leak by spraying carb cleaner over engine looking for sucking.

Customer:

lol

Customer:

maybe I just missed it.

Customer:

were would a vaccum leak most likely occur.

GM Tech (Cam) :

Inside the engine and the booster.

GM Tech (Cam) :

This could be an intake gasket leaking internally. See if there is lots of vacuum in the crankcase. Take the Vent out of the passengers valve cover and plug it. Take the PCV out of the drivers and plug it. Then with the engine running, take the oil cap off and spray some starting fluid in there, just a bit and see if there is a reaction from the engine. If so then the intake gaskets are leaking internally and will need to be replaced.

Customer:

Can you give me a few minutes to go try this?

GM Tech (Cam) :

sure

Customer:

no reaction continued to run high rpms 2000.

GM Tech (Cam) :

From either the booster or engine?

GM Tech (Cam) :

Then something is commanding the IAC open

Customer:

no reaction from either.

Customer:

is there a spring on the throttle body that could be causing the butterfly to stick? and not close all the way.

Customer:

what is the yellow looking thing between the linkage and the throttle boday.

Customer:

MAP Sensor?

GM Tech (Cam) :

it should not.

GM Tech (Cam) :

Does it have cruise?

Customer:

Yes but I never use it. Just work truck.

GM Tech (Cam) :

Take the cable off for the cruise and see if it stops.

Customer:

hold on>

GM Tech (Cam) :

ok

Customer:

Took it lose. Started truck still racing. You can probably guess this is driving me nuts.

GM Tech (Cam) :

I can imagine. now we will need to see what is happening in the VCM and it will require a scan tool.

Customer:

What do you think could be causing the ecm not to be recieving the info from the new oxygen sensor. I checked the voltage and it is correct.

GM Tech (Cam) :

THe oxygen sensor I am positive is not the issue here.

Customer:

What is the VCM. Just picking at your brain. Like to work on cars for a hobby. Rebuilding a 1928 Model A Tudor.

GM Tech (Cam) :

The VCM is the engine computer on the right inner fender.

Customer:

Dealership?

GM Tech (Cam) :

no, any shop should be able to.

Customer:

Anything else to check for before I let you get back to your business?

GM Tech (Cam) :

Other than a good visual inspection looking for anything that is chaffed, there is nothing I can think of at this moment.

Customer:

Thanks a whole bunch cam. Have a great weekend.

GM Tech (Cam) :

You are most welcome. Please click accept to complete this.

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