Ok mechanics I need some help. I have a 1998 gmc 1500 5.7l 350 that I had to replace the heads on. I replaced the heads and every gasket/o ring from the heads up. After conpleting the install the truck runs very smooth and sounds good. Howeer it will not drop below 2500rpms at idle. After doing some research I have gone as far as to tear it back down to check the intake gaskets again to make sure they set good and re siliconed the ends. Other things replaced are all 4 O2 sensors, IAC sensor, pcm hose, all new plugs and wires. After the second time in I cranked it and it does the exact same thing. Fires right up but sits a contamt 2500 rpm. Pleae let me know if you have any suggestions or knowledge with this issue. Thanks for any help in advance.
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Had something similar one time w a 350 vortec. I didn't give the lifters an oil bath b4 installing em. Took em out, soaked em 're install, ran like a top -
Double check your vacuum lines. You just moved a whole bunch of 20 year old brittle hoses around in the process of removing the intake.Comment
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This^^^^^. And check the throttle cable and make sure it is in the proper spot.Originally posted by 91cavgt View PostComment
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Start with the vacuum lines then replace your idle control sensor and check wires going to it.Originally posted by 91cavgt View Post
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Possible vacuum leakComment
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Ok so idle sensor has been replaced and i have wore myself out searching vacuum lines. A buddy hooked his computer to it and cannot see anything out of the normal on the readings. If it was a vacuum issue would it have some variences in the idle? It is spot on 2500 rpms and does not fluctuate much at all.Comment
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Is the truck TBI or TPI?
TBI, check the gasket under the Throttle body and make sure the line going to the brake booster is hooked up correctly.....I would plug that line just to rule out a bad brake booster.
TPI, make sure the fuel lines inside the intake aren't leaking, we called that the octopus. I've also seen the fuel pressure regulator on that system leak and cause high idle.
But i'd put my money on a vacuum leak.Comment
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Do you have a plastic intake manifold?Comment
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I dont know if they worked the same in 1998, but when I replaced my wifes 2009 Tahoe Throttle Body it did that. Was crazy. I read that even cleaning it or moving it at all could cause the issue. Most said turning off and on and letting it run would make the computer re-calibrate but it wouldnt. Ended up having to take to dealer and they re flashed it ( whatever that means ) and it went straight down to normal. Was like $99Originally posted by IAFFHunter18 View PostComment
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Sounds like everyone else said, vacuum lines and rule out the brake booster. Take you a can of starting fluid (ether) and simply spray it in areas that may be pulling a vacuum, like the brake booster and large vacuum lines. When spraying, if it idle increases and increases the idle, you have pinpointed the issue/location. Good luck...Comment
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Totally agree with the vac leak possibility easy and cheap to eliminate that option first before anything else.Comment
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Yes sir, checked it for leaks, sprayed all around it and found nothing. Also replaced the gaskets there as well.Originally posted by Radar View PostComment
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Does anyone know how many vacuum lines there are? Just want to make sure I am checking them all. I may also add that it did not do this prior to replacing the cracked head and gaskets. This started after all this.Comment
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Did you try the ole starting fluid or whatever spray in various spots and see if the idle fluctuates.... helps you find your leak. That's what I always do to find leaks.
RyanComment
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