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1998 Chevrolet Tahoe Idle and high revs

Tiny
BOBTHESUPER
  • MEMBER
  • 1998 CHEVROLET TAHOE
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 115,000 MILES
Hello, I am kind of a "back yard" mechanic, which is a polite way of saying I don't crap. Be that as it may, my 350 Vortec was stalling at idle so I replaced the IAC. That solved my stalling issue but now to revs high while driving (2200 RPM) most of the time. When I come to a stop it will idle down to about 800 rpm, but most of the time it goes back up. I was driving at 40 MPHD today and did not have my foot on the accelerator. I can't really afford to take it to a professional right now so any help you can give me would be greatly appreciated. I also did try to clean various things like the tip of the throttle body but it did not help.

Thanks again for any suggestions

Bob Smith
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Tuesday, February 17th, 2015 AT 2:33 PM

14 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,885 POSTS
Bob:

You made my day. Re read your first sentence. If that is really the problem, you may want to see a doctor. LOL Anyway, I would be glad to help. First, I need to better understand what is happening. You mentioned that the RPM's are at 800 at an idle. When in neutral or park, do they climb up to 2200? If that is the case, have you checked the following:

EGR
Engine Vacuum Leaks
Intake Gasket Vacuum leaks

If the RPM's climb high when at an idle and remain steady, usually it is intake related. Let me know what you have done and if I understand what is happening correctly.
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Tuesday, February 17th, 2015 AT 6:49 PM
Tiny
BOBTHESUPER
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HI and thanks for your response. I actually did see a doctor! LOL. Anyway, the problem I have is intermittent. Usually when I start up my Tahoe, it idles normally. Then as I get going the engine revs up to about 2200 RPMs. When I stop at a light or stop sign, if I wait a second or two with the car in gear the idle comes down. However when I accelerate again the revs go back up. As I drive, if I put the car in neutral it revs up to the 2200 rpms. Occasionally it behaves completely normal. The check engine light has also been coming on for a day or two then going off. When I first installed the IAC, I wasn't sure it seated properly, so I removed it again and replaced the O ring. I also cleaned everything I could think of with some spray cleaner. I tried checking for vacuum leaks with some water spray. I didn't notice any. I have not checked the other item you mentioned. Honestly, I don't know how.

Thanks again for your help, I am very frustrated.

Bob
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Thursday, February 19th, 2015 AT 11:02 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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Bob:

Glad you're feeling better. Lol I see some crazy posts, but never any with the problem you listed.

Anyway, it almost seems like the problem is temp related if it idles normal at start. As far as the vacuum leaks, it can be dangerous, but I use carb cleaner to check. If there is a leak, it sucks the carb cleaner in and the idle speed will change.

As far as the IAC, what happens if you disconnect it? Do the RPMs go really high? If they do, then it is working.

That leads me to the EGR. The egr on your vehicle should be vacuum operated. If it isn't holding vacuum, the RPMs can increase as the engine warms. To check it, you should do two things. First, confirm there is vacuum to it. Using a vacuum gauge, vacuum should be around 15. If that is the case, then you will need a vacuum pump to check if the EGR is holding vacuum. Most parts stores will lend you both the pump and the gauge. If you want to try it, let me know when and I'll try to make sure I'm online to help guide you. It really is simple to do.

One last thought. Double check all of the intake bolts and throttle body bolts to make sure they are tight. I just dealt with an idle issue similar to yours. After eliminating the things I mentioned to you, I found two bolts loose on the intake. I tightened them and the idle dropped to 800. Also, recheck to make sure none of the vacuum hoses are damaged or disconnected. Often times, the manufacturers will use a 90 degree turn in a hose. Over time, they always seem to leak there.

Sorry, this is the last thought. Since the check engine light is going on then off, try to get to a parts store to have the computer scanned when the light is on to see what code is showing up. Even if the light is off, there could be "Pending Codes".

Let me know.

Joe
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Thursday, February 19th, 2015 AT 4:14 PM
Tiny
BOBTHESUPER
  • MEMBER
  • 7 POSTS
Thanks Joe

Your offer to help online is unexpected and amazing. Thank you. I will let you know but it'll be a few days before I can try your suggestions. Another thing I thought of, when my Tahoe is idling normally it occasionally will surge up to about 1000 RPM then back down (this is all according to the gauge in the instrument cluster.

BTW can you give me an idea of what the EGR looks like? I can look it up online but I never know if I can trust most postings.

Bob
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Sunday, February 22nd, 2015 AT 11:47 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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The EGR is located on the intake manifold. I attached a pic for you to see. Actually, this one is electric and not vacuum. However, check to make sure it is tight and there are no vacuum leaks.

Your idle issue sounds like the IAC, but that has already been addressed. That leads me to believe it is vacuum related.
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Sunday, February 22nd, 2015 AT 8:09 PM
Tiny
JACOBANDNICKOLAS
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Sorry. I forgot the pic.
Image (Click to make bigger)
Thumb
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Sunday, February 22nd, 2015 AT 8:10 PM
Tiny
BOBTHESUPER
  • MEMBER
  • 7 POSTS
Hi Joe

I didn't mean to leave you hanging. I got so busy with my car (and my backyard, as in mechanic) I actually forgot this site address. I also had issues with my email. Anyway I thought I would give you an update- my engine light started FLASHING! I know you know that is not good So I had to take it in. I broke one valve push rod and bent the other on number 3 cyl. Anyway after it was fixed it ran good for a few days but I think my vacuum leak came back. At start up it idles about 500 rpm or les and sometime stalls. What is a good place to start looking?

Thanks!

P.S. I book marked you this time :-) As soon as I get some money, I'll make a donation.
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Monday, May 4th, 2015 AT 5:07 PM
Tiny
JACOBANDNICKOLAS
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Nice to hear from you again. However, sorry it is under these conditions. Anyway, since they had the engine apart, start by looking for vacuum hoses that may have come disconnected. Let me know if you see anything.

Joe
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Wednesday, May 6th, 2015 AT 7:00 PM
Tiny
BOBTHESUPER
  • MEMBER
  • 7 POSTS
Hi Joe

My mechanic founda a code that meant my IAC was unresponsive. So I have come full circle. I replaced the IAC again and in ran great for about 5 days. Now the idle is fine (about 700) but it doesn't idle up (like 1000 to 1200) in a cold start, or any start for that matter. I'm going to check for vacuum leaks, again, but took 5 days for the issue to arise. Is vacuum the most likely cause?
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Monday, May 11th, 2015 AT 7:42 AM
Tiny
JACOBANDNICKOLAS
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Does the idle increase if you turn the AC on or turn the steering wheel while sitting still?
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Monday, May 11th, 2015 AT 8:34 AM
Tiny
BOBTHESUPER
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I think it increases a little with A/C but its negligible. I have not motive a change when I turn the steering wheel. Besides this, it's actually running pretty good!
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Tuesday, May 12th, 2015 AT 4:05 PM
Tiny
JACOBANDNICKOLAS
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If you notice a slight change when the AC is turned on, it sounds like it is working. Recheck connections to make sure there isn't something causing resistance.
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Thursday, May 14th, 2015 AT 6:08 AM
Tiny
BOBTHESUPER
  • MEMBER
  • 7 POSTS
Thanks Joe, will do. May I also ask about the heater. I replaced the heater core because it was leaking a little, but I still don't have any heat. When the control is on heat both hoses feel hot and after several hours of running I can feel some heat in the vents but not a great deal even with the blower fan on. The rotary switch that controls the fan on the dash malfunctions sometimes. I have to move it back and forth to get it to work. However that doesn't help the heat.

BTW. I've re read my first email to you several times and didn't see my mistake until just recently. Now I know why you asked me about the doctor! LOL. I'm sure you figured out that I don't KNOW crap.?
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Thursday, May 14th, 2015 AT 7:15 AM
Tiny
JACOBANDNICKOLAS
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LOL I thought you realized right away. As far as the heat, if both heater core hoses are getting hot, then 99% of the time, the blend air door isn't opening to allow the heat to enter the vehicle.
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Friday, May 15th, 2015 AT 8:48 AM

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