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Engine has as a rough idle?

Tiny
DEMINGSJ
  • MEMBER
  • 1999 CHEVROLET SILVERADO
  • 5.3L
  • V8
  • 4WD
  • AUTOMATIC
  • 140,000 MILES
A few days ago I was accelerating to merge onto the freeway and my truck suddenly decelerated while my foot was on the gas. It then returned to normal running. Temperature was fine, oil pressure was fine. Two day later it was still running great in the morning, but by lunch it was a rough idle as soon as it started and then it would shut off. Hitting the accelerator made it bog down even worse. Sometimes it would turn over strong, but would not start at all. There are no OBD codes and no lights on the dash. I replaced the pump relay and it did not help. Do you think it is fuel related or am I barking up the wrong tree?
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Tuesday, July 11th, 2017 AT 1:38 PM

12 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,643 POSTS
Hello,

Yes the fuel pumps go out in these trucks all the time. I have put two in my truck.

Here are some guides and some diagrams (below) to help you get the job done:

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

and

https://www.2carpros.com/articles/how-to-replace-an-electric-fuel-pump

and

https://youtu.be/Q5MH2gnyFH0

Please let us know what happens.

Cheers, Ken
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Wednesday, July 12th, 2017 AT 11:04 AM
Tiny
DEMINGSJ
  • MEMBER
  • 2 POSTS
I changed the fuel filter and it started right up. It ran and drove good for about two miles and then it died and wouldn't start again.
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Wednesday, July 12th, 2017 AT 4:48 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,643 POSTS
Yep, it sounds like the fuel pump has gone bad.

Let me know what happens after you replace it.

Best, Ken
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Thursday, July 13th, 2017 AT 11:37 AM
Tiny
JPSOONER
  • MEMBER
  • 1 POST
  • 1999 CHEVROLET SILVERADO
  • V8
  • 2WD
  • AUTOMATIC
  • 180,000 MILES
Truck runs fine at low & highway speeds. Changed plugs & wires & 2 coils code said were bad. Code now says misfiring, but doesnt say which or how many cyls. Truck just idles really rough.
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-1
Monday, November 16th, 2020 AT 11:13 AM (Merged)
Tiny
RASMATAZ
  • MECHANIC
  • 75,992 POSTS
Misfires can be caused by worn or fouled spark plugs, a weak spark (weak coil, bad spark plug wire), loss of compression, vacuum leaks, anything that causes an unusually lean fuel mixture (lean misfire), an EGR valve that is stuck open, dirty fuel injectors, low fuel pressure, or even bad fuel.
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Monday, November 16th, 2020 AT 11:13 AM (Merged)
Tiny
JOEL MONTGOMERY
  • MEMBER
  • 1 POST
  • 1999 CHEVROLET SILVERADO
  • V8
  • 2WD
  • AUTOMATIC
  • 135,000 MILES
I have a 1999, 5.3 Chevrolet Silverado 1500, 135,000 mi. The "check engine soon light" is on and the code is P0303, #3 cylinder miss. Idles rough at about 400 rpm, seems to run fine at normal operating speeds. I checked the coil pack, changed plugs and wires, checked compression, and fuel injectors all good. Checked power to the injector to #3 and it was good. Swapped the coil pack and injector with the #5 cylinder and the code is still the same, #3 cyl miss.

Could this be a computer problem or possibly a lifter problem. I really don't want to get into the engine if I don't have to.

Dumbfounded in NJ
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Monday, November 16th, 2020 AT 11:13 AM (Merged)
Tiny
BRIAN 1
  • MECHANIC
  • 1,030 POSTS
Check the lower intake manifold for vacuum leaks, this a very common problem on these vehicles. You have all the simptoms of a vacuum leak at the intake. Post back with any other questions.
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Monday, November 16th, 2020 AT 11:13 AM (Merged)
Tiny
RONNIEFRANKIN
  • MEMBER
  • 7 POSTS
  • 1998 CHEVROLET SILVERADO
  • 5.7L
  • V8
  • 300,000 MILES
Went to start it today. It backfired and some smoke came from under the hood. Runs good at a high rpm but fluctuates at an idle. Sometimes it almost shuts off. Thanks for your help.
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Monday, November 16th, 2020 AT 11:13 AM (Merged)
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,643 POSTS
Hello,

It sounds like you need a tune up have you installed plugs and wires AC delco? Also is there is check engine light on if so lets run down these guides to see whats going on.

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing

and

https://www.2carpros.com/articles/engine-misfires-or-runs-rough

Please run down these guides and report back.
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Monday, November 16th, 2020 AT 11:13 AM (Merged)
Tiny
MIKETAY
  • MEMBER
  • 16 POSTS
  • 1998 CHEVROLET SILVERADO
  • V8
  • 4WD
  • AUTOMATIC
  • 146,000 MILES
I have changed dist cap, rotor, sparkplugs(@ 100,000), wires, fuel filter, and cleaned maf sensor. I didn't have codes a first but, then service engine light came on and code said cylinder 5 misfire. I pulled the sparkplug and the electrode was built up with black crusty gunk on one side and white and clean on other side. Plug in cylinder 3 was also built up. I changed the plug (5) and added seafoam cleaner to gas. Local Autozone guys said it could be a clogged injector, bad plug, or I may need to change injector spider. I have run a tank of gas with seafoam and Idle seems to be better at times but not always. It is worse when cold. Acceleration was rough but seems smoother after I cleaned MAF sensor Is there anything I missed? Thanks, Mike
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Monday, November 16th, 2020 AT 11:13 AM (Merged)
Tiny
RASMATAZ
  • MECHANIC
  • 75,992 POSTS
Misfire @ 5 could be the plug/compresssion/injector on that cylinder. Try cleaning out the idle air control valve

Misfires can be caused by worn or fouled spark plugs, a weak spark (weak coil, bad spark plug wire), loss of compression, vacuum leaks, anything that causes an unusually lean fuel mixture (lean misfire), an EGR valve that is stuck open, dirty fuel injectors, low fuel pressure, or even bad fuel.
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Monday, November 16th, 2020 AT 11:13 AM (Merged)
Tiny
MIKETAY
  • MEMBER
  • 16 POSTS
I found the problem. After checking all sensors. I decided it had to be the intake or headgasket. On teardown I found the plastic intake gasket I replaced at 100,000 miles had warped into a slight arc. After removing the gasket, I could see the o-ring seal had dropped below the #3 & #5 ports which created a vacuum leak and sucked oil into the cylinders. I replaced the intake gaskets with metal gaskets and reassembled. She runs great and does not use any oil. Problem Solved! Thanks for the help! Mike
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Monday, November 16th, 2020 AT 11:13 AM (Merged)

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