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Old August 18th, 2013, 04:48 PM   #1
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Default 98 gmc

When I start my truck 98 gmc k1500 350 vortec

It revs up to 2000 then goes down to 12-1300 Rpms

But when I'm driveing I can let my foot odd the peddle and it will cruse at 50 because of the rpms staying at 1500.

I'm thinking the tps but I'm not sure and don't want to blindly throw money at it

I need your wisdom people of Great Lakes
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Old August 19th, 2013, 11:49 AM   #2
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Check the intake gasket for leaks. I had one that the gasket between the upper and lower plenumn was bad and it did the same thing. The loss of vacuum made the computer think the throttle was open, even though the TPS was fine. It automatically added extra fuel and the rpm's went up. Changed the gasket and the problem was solved.
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Old August 19th, 2013, 04:53 PM   #3
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Like between the plastic part and the intake?
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Old August 19th, 2013, 07:04 PM   #4
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Well that wasent it so any more ideas?

The tps is only 34$ plus tax but that could be money waisted if its not it
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Old August 19th, 2013, 07:19 PM   #5
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Check engine light? You can test a tps with a volt meter. I would guess a vacuum leak. Do the old carb cleaner/starting fluid trick. Spray around the engine with it running and see if the rpm goes up.
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Old August 19th, 2013, 08:38 PM   #6
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Like Wheelman said, check the TPS and check other areas for vacuum leaks - that is how I found my plenumn gasket was bad. Like I mentioned earlier, the low vacuum from a leak tricks the computer into thinking the engine needs more fuel.
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Old August 19th, 2013, 09:17 PM   #7
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Check engine light? You can test a tps with a volt meter. I would guess a vacuum leak. Do the old carb cleaner/starting fluid trick. Spray around the engine with it running and see if the rpm goes up.
I did this with no luck. And no check engine light I had it scanned and it said tps
Air idle control valve and one other thing but they can't tell if one thing is causing another thing but I know for sure it's not the air idle control valve
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Old August 19th, 2013, 09:42 PM   #8
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I did this with no luck. And no check engine light I had it scanned and it said tps
Air idle control valve and one other thing but they can't tell if one thing is causing another thing but I know for sure it's not the air idle control valve
Did you test the tps for 5 volts at the harness with the positive probe on the gray wire and the negative on the black. Then back probe the dark blue with the positive probe hook the connector up and turn the key on engine off. Should be 0.6 volts with the throttle closed. Then gradually go up as the throttle opens to 4.5 volts at wide open throttle. If that all test good then the tps should be good.

I am going with vacuum leak or iac motor.
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Old August 19th, 2013, 09:57 PM   #9
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Iac motor?
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Old August 19th, 2013, 09:58 PM   #10
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The Idle air control motor.
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Old August 19th, 2013, 10:01 PM   #11
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O haha I thought that to but I checked it and seams to be fine and its 94 bucks lol
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Old August 19th, 2013, 10:02 PM   #12
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How did you check it?
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Old August 19th, 2013, 10:10 PM   #13
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Unplugged it motor revved up.

Unplugged tps nothing happens plus my truck seams to be a little sluggish.
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Old August 19th, 2013, 10:11 PM   #14
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Did you test it?
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Old August 19th, 2013, 10:15 PM   #15
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No. The only thing I did was pull it off clean it up put it back in.
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Old August 19th, 2013, 10:17 PM   #16
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test it with a volt meter. Super easy to do. No way to really clean it.
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Old August 19th, 2013, 10:21 PM   #17
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Idk how to set the volt meter to test that or the tps.
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Old August 19th, 2013, 10:23 PM   #18
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For what it is worth, your vacuum leak could be anywhere.

If that truck has the same vacuum system as all the other vehicles I've owned, you should be able to turn it to defrost after letting it sit all night and hear the vacuum "bleed" off. If it doesn't do that you have a vacuum leak. Not sure where check valves were.

On the old lady's xj, i randomly discovered a vacuum leak on a hard line, so I took some copper tubing and fixed that, added some bling to it too! The hose would have had to have two very drastically different ends, so I took the rubber adapters of the hard line and used copper tubing in it's place. The difference in plugging the hole 1/16" or less in diameter resulted in ~450 rpm difference.

Long story short, don't ever rule out vacuum
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Old August 19th, 2013, 10:26 PM   #19
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I sprayed starting fluid everywhere and on every thing with no change
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Old August 21st, 2013, 04:07 PM   #20
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