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Progressively rough idle but goes away with any RPM, drives fine otherwise

Tiny
ELLIOTTRAV
  • MEMBER
  • 1998 GMC SIERRA
  • 214,000 MILES
I have a 1998 gmc sierra 4x4 with a 5.7l auto. And 214000 km. Truck drives fine except a rough idle, starts fine, dr estate good but sitting at a set of lights you can feel it run smooth and then slowly get rougher and rougher until it feels like it's going to stall out but will not. Spark plugs and wires under a year old with distributor and ignition coil. Possible vacuum leak or wires arcing?
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Wednesday, September 7th, 2011 AT 4:18 AM

8 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,747 POSTS
The first thing I would look for is a vacuum leak. Also, the idle air control valve can be causing the problem too.
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Wednesday, September 7th, 2011 AT 4:26 AM
Tiny
MAINTENANCETECH
  • MEMBER
  • 1 POST
I would almost guarantee it's the Fuel Pressure Regulator, the rubber diaphragm inside has worn out, so it's dumping in fuel at a high rate when your sitting there idling.

http://shop.advanceautoparts.com/webapp/wcs/stores/servlet/product_Fuel-Pressure-Regulator-BWD_19960072-P_484_R%7CGRPFUELAMS_674033751____

You can tell by removing the top hose. If gas runs out, then the rubber diaphragm inside is shot. It's located on the driver side, top middle of the engine. To replace, remove the top hose, pop the clip off from the front side (the new one should come with one). Most of the time the strainer will remain inside when you pull the regulator off, remove it with a small pic, be sure to get the old small rubber O-ring out too. Keep us posted
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Monday, September 12th, 2011 AT 3:06 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,747 POSTS
Before you replace the regulator, go to our homepage under the DIY section. Look up checking fuel pump pressure. It will explaine how to check the regulator too. There is both a video and directions to help you through it.
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Monday, September 12th, 2011 AT 4:15 AM
Tiny
ELLIOTTRAV
  • MEMBER
  • 5 POSTS
Yes that would make sence, I heard about the regulator but never gave it any thought, I had checked for vaccum leaks and found none and none of the wires were arcing so the regulator makes sence, I hope to have time in the next couple days to check it out. Thanks for all your time
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Friday, September 23rd, 2011 AT 2:17 AM
Tiny
ELLIOTTRAV
  • MEMBER
  • 5 POSTS
I checked for fuel regulator issues and founf none, I took off the line to the regulator (I think) the one on the top center of the engine on the drivers side beside the throttlebody? I took off the vacuum line to the regulator and found no presence of gas in the line, it has true dual exhaust and almost seems to only be misfiring out of the drivers side tail pipe
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Thursday, September 29th, 2011 AT 2:21 AM
Tiny
ELLIOTTRAV
  • MEMBER
  • 5 POSTS
Also where is the idle air control valve located?
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Thursday, September 29th, 2011 AT 2:22 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
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It is on the throttle body.
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Thursday, September 29th, 2011 AT 6:11 PM
Tiny
ELLIOTTRAV
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  • 5 POSTS
Beside the tps?
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Friday, September 30th, 2011 AT 2:11 AM

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