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Runs rough when engine is warm

Tiny
BOOMER2782
  • MEMBER
  • CHEVROLET SILVERADO
I have a 99 silverado, 5.3 liter 4x4 137k miles. Runs great when engine is cold. After about 10-15 minutes of driving when engine warms up, engine bogs down, idles rough, and dies when throttle is applied. I ran the codes, had low voltage x4 oxygen sensors, and random misfires. I replaced the 02 sensors because I am well over 100k miles. I also replaced the plugs (gapped to.060), wires, flushed the fuel system, new fuel filter, put in Shell 91 octane, replaced gas cap (the o ring was cracked badly). Still no fix. I have heard it might be the MAF, MAP, TPS, intake gasket leak. Any suggestions? I'm a full time student and broke as hell. I cant afford to have it fixed at a shop. What is wrong? Any ideas will help! This truck is nothing but problems! :X :x :x
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Sunday, February 12th, 2006 AT 6:01 PM

48 Replies

Tiny
CHEVY350GUY
  • MECHANIC
  • 109 POSTS
Look for a vacuum leak first here is a guide to show you how.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Also the MAF sensor can cause the engine to run rough, I would try cleaning it first and if that doesn't help replace it, here are some guides on that.

https://www.2carpros.com/articles/mass-air-flow-service

and

https://www.2carpros.com/articles/how-to-replace-a-mass-air-flow-sensor-maf

Please let us know what happens.

Best, Ken

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Sunday, February 12th, 2006 AT 7:32 PM
Tiny
BOOMER2782
  • MEMBER
  • 22 POSTS
If its the MAF will it set a code?
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Monday, February 13th, 2006 AT 5:48 PM
Tiny
CHEVY350GUY
  • MECHANIC
  • 109 POSTS
It may but it may not. It depends how far the sensor is out of range. But because it happens after it warms up I would start there.
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Monday, February 13th, 2006 AT 5:59 PM
Tiny
BOOMER2782
  • MEMBER
  • 22 POSTS
I replaced the MAF sensor. The "service engine soon" light went out. I let it warm up to about 200 degrees or so, drove it for about 15 minutes and it was driving a lot better than before. The light came back on though, 'system too lean, bank 1.' It only seems to do this when I give it a lot of throttle. I put some fuel system cleaner in the fuel when this whole thign started last week, you think this might be causing it to be sluggish? Or what about another fuel filter, or even fuel pump? Are there more advanced ways of diagnosing this truck sinece there are no codes if I take it into a shop? I thought I would save money by doign it this way, but I have spent about $400 so far, and its still not fixed. Thanks- Eric.
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Tuesday, February 14th, 2006 AT 10:28 AM
Tiny
CHEVY350GUY
  • MECHANIC
  • 109 POSTS
This should be hooked up to a scanner so you can see live data as it happens to better diagnose the problem. Before throwing more money at it.
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Wednesday, February 15th, 2006 AT 6:28 PM
Tiny
BOOMER2782
  • MEMBER
  • 22 POSTS
Do you have any idea how much a shop might charge me for something like that?
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Wednesday, February 15th, 2006 AT 11:04 PM
Tiny
BOOMER2782
  • MEMBER
  • 22 POSTS
I tested the fuel pressure tonight. I get 18-20psi with the key on-off a few times. When running I get between 15-22psi. What next, new fuel pump or regulator? Thanks, Eric.
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Friday, February 17th, 2006 AT 9:49 PM
Tiny
WINDDILLA
  • MEMBER
  • 7 POSTS
I have a 99 Silverado 5.3 as well. I recently had it in the shop to have them check a couple of diagnostic trouble codes and was charged $84. (North Dallas)

Have you cleaned the inside of the intake port on your throttle body?
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Saturday, February 18th, 2006 AT 1:43 PM
Tiny
WINDDILLA
  • MEMBER
  • 7 POSTS
Pressure is definitely low. Should be about 60 psi I think. Low pressure points to fuel pump rather than regulator. Can you hear the pump whine when you turn the key over without starting engine?
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Saturday, February 18th, 2006 AT 1:57 PM
Tiny
BOOMER2782
  • MEMBER
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No, but I will.
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Saturday, February 18th, 2006 AT 3:15 PM
Tiny
BOOMER2782
  • MEMBER
  • 22 POSTS
My book says it should be 55-62 psi for key on-key off. And should drop 3-10 psi with th eengine running. It fluctuates between 15-22 when running. I can hear the fuel pump with key-on for about a second or so. I doesnt seem as powerful as it used to be (could be a mental thing though).
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Saturday, February 18th, 2006 AT 3:17 PM
Tiny
WINDDILLA
  • MEMBER
  • 7 POSTS
How did it run after you cleaned the throttle body? You should hear the fuel pump whine as long as motor is running. It shouldnt stop.
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Monday, February 20th, 2006 AT 5:32 PM
Tiny
BOOMER2782
  • MEMBER
  • 22 POSTS
Its sort of weird, I dont hear anything now. I dont even hear the 'whine' from the fuel pump, even with key on-off. I really think its the fuel pump now. Thanks, Eric.
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Tuesday, February 21st, 2006 AT 10:11 AM
Tiny
WINDDILLA
  • MEMBER
  • 7 POSTS
Thats what it sounds like. Let me know how it goes. I havent changed mine but I have a fuel level sensor thats flakey and I think you have to take out the tank to change it. Either way it looks like a major pain.
Done any mod's to it?
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Tuesday, February 21st, 2006 AT 3:38 PM
Tiny
WINDDILLA
  • MEMBER
  • 7 POSTS
Does it even start anymore? If it does then starts sputtering, check the PCV valve before you start the daunting task of the fuel pump. Their cheap and easy to replace. It'll make your truck cough and hack if it has un-seated itself or gets clogged.
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Tuesday, February 21st, 2006 AT 3:46 PM
Tiny
BOOMER2782
  • MEMBER
  • 22 POSTS
Already replaced the PCV.
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Tuesday, February 21st, 2006 AT 4:03 PM
Tiny
BOOMER2782
  • MEMBER
  • 22 POSTS
I dont have any mods on the truck. It has about 137k miles, so I'll save my money to put into a new 2500 HD Sierra when I get the money in a few years.
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Tuesday, February 21st, 2006 AT 4:05 PM
Tiny
BOOMER2782
  • MEMBER
  • 22 POSTS
Does anyone know how to tell if my fuel pump is a TCU or TCF without pulling the tank or bed, and what the difference is? I think its something to do with emissions. It was originally sold in CA, butI dont have to smog it anymore. Is it going to matter?
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Tuesday, February 21st, 2006 AT 4:07 PM
Tiny
SSGDALEADKINS
  • MEMBER
  • 2 POSTS
It has to do with the VIN or you can do like I did this past weekend. Take a picture of the top of your fuel pump to see how many fuel lines connect to it and what the electrical connector looks like. Also, Autozone and O'Rielly's sell the same type fuel pumps but (I think it was an AVTEX brand), the correct fuel pump may have a new oval connector instead of the factory square connector. The new pump may have an adapter kit that fixes connector problems. The new style is oval in shape and is easy to swapp. Just stagger cut the old wires/connector off and crimp on the new oval style connector. I didn't have a transmissioin jack so I took out the bed bolts and jacked up the bed about a foot, secured it with jack stands, and squeezed in between the bed and tank to replace it. It was a tough way to go about doing it.

Hope this helps.
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Tuesday, June 30th, 2009 AT 11:03 AM
Tiny
BOBWHEELERJR
  • MEMBER
  • 14 POSTS
  • 2003 CHEVROLET SILVERADO
  • 20,300 MILES
Check engine light came on, scan said mass air flow sensor, replaced still ran real bad with misfire's. I changed plugs still runs bad. It will not idle and runs rough?
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Friday, March 10th, 2017 AT 3:29 PM (Merged)

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