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Old January 10th, 2014, 06:01 PM   #1
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Default Engine RPM higher when in gear

1996 5.7L Vortec

I noticed that my truck will rev higher when in gear, around 1100 RPM/1000 RPM, but when in park it will idle at around the right RPM 700/800

What the heck would cause that? Never had it happen nor have I heard of it
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Old January 11th, 2014, 09:40 AM   #2
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Nothing?????
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Old January 11th, 2014, 09:52 AM   #3
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Maybe a vacuum leak? That would be my first check.
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Old January 11th, 2014, 10:44 AM   #4
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Now I honestly have no clue here. However a friend of mine has the same year k2500 with the same motor; and was having the same problem you're having only reversed.... Much higher idle in park. It ended up being the throttle position sensor.
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Old January 12th, 2014, 01:09 PM   #5
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I had a similar problem years ago with a S10 Blazer with Vortec 4.3 - it was the plenum gasket (between the lower and upper) leaking vacuum. I would first check the TPS and MAP sensors to make sure they are working properly.
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Old January 12th, 2014, 07:37 PM   #6
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Originally Posted by Iggy View Post
I had a similar problem years ago with a S10 Blazer with Vortec 4.3 - it was the plenum gasket (between the lower and upper) leaking vacuum. I would first check the TPS and MAP sensors to make sure they are working properly.
Thanks, I will make sure to check those. I haven't messed with it yet; I haven't had to go anywhere beyond walking distance this weekend.

I'm gonna take a can of ether to it first to see if there are any obvious vacuum leaks
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Old January 14th, 2014, 08:21 PM   #7
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Took a can of carb cleaner to it, didn't find any vacuum leaks, kept looking up other peoples' issues, and found out that the engine coolant temp sensor is a good cause of that.

I replaced it; it helped, it stays under 1000 rpm now, but still revs up to 15/1600 when I first start it, but now it does go down faster. It still idles to fast.

I'm going to swap out the ecm for the stock one and see if it's a computer issue, it's quick to do. I will also be check/cleaning the IAC; last resort i'll be replacing that and possibly the TPS
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Old January 26th, 2014, 02:40 AM   #8
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Possibly brake booster? You say idle in gear I'm assuming your not moving with your foot on the brakes.
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Old January 26th, 2014, 08:41 AM   #9
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Possibly brake booster? You say idle in gear I'm assuming your not moving with your foot on the brakes.
Could he use use the E-brake to check? Engage E-brake, put in drive, take foot off brake peddle and then check idle?
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Old January 31st, 2014, 08:48 AM   #10
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Could he use use the E-brake to check? Engage E-brake, put in drive, take foot off brake peddle and then check idle?
I'm actually not sure if my E-brake works lol; I've never used it

The main problem was the engine coolant temp sensor; replaced that and it solved most of the issue. I think it still idles high but I have not checked anything else
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Old January 31st, 2014, 11:32 AM   #11
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my snap on scanner has a "reset iac" feature, has worked for me in the past....
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Old January 31st, 2014, 11:48 AM   #12
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my snap on scanner has a "reset iac" feature, has worked for me in the past....
I looked at mine for that a while ago, but it didn't have anything like that. I didn't think it would because it was only a cheaper $150 scanner
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Old January 31st, 2014, 02:44 PM   #13
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Our genesis at work doesn't either... my snap on solus does
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