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4.3l express backfires & no acceleration when hot

Tiny
DGHAHN
  • MEMBER
  • CHEVROLET VAN
My 2000 chevy express 1500 with 110,000 miles, started hesitating on acceleration 4 days ago. In the past 4 days it steadily got worse until now when it won't go above 30mph. It runs fine at idle, and will run normal when cold. But after 5 minutes of running it will backfire and sputter if I press the pedal more than a fraction of an inch.

The fuel pump was replaced about 4 months ago, and the transmission was replaced about 3 weeks ago. I have replaced the spark plugs, distributor cap and rotor.
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Friday, July 13th, 2007 AT 9:10 AM

7 Replies

Tiny
2CARPRO JACK
  • MECHANIC
  • 11,533 POSTS
First verify fuel pressure just to be sure. Could be a few different things. Is or has the check engine light been on? Try loosening the exhaust flanges and driving it, noisy but will eliminate the possibility of plugged exhaust. Have the PCM scanned to see if any trouble codes have been stored.
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Friday, July 13th, 2007 AT 10:23 AM
Tiny
DGHAHN
  • MEMBER
  • 6 POSTS
The check engine light is not on, but I will check it with a code reader.

I also noticed that there is a high pitched whine that could be coming from the fuel tank. I don't like the thought of dropping the tank again to replace the $250 fuel pump, but I will pick up pressure gauge and check it. Any idea what the pressure should be?

Thanks. Dan
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Friday, July 13th, 2007 AT 3:06 PM
Tiny
2CARPRO JACK
  • MECHANIC
  • 11,533 POSTS
Should be between 41 and 47 psi.
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Saturday, July 14th, 2007 AT 7:49 AM
Tiny
DGHAHN
  • MEMBER
  • 6 POSTS
Well, I got further today. Based on a list of things to try I unplugged the MAF and Air temperature sensor, and all of a sudden it ran fine. When I plug either one back in it sputters and backfires on acceleration again. Is it possible that both of these sensors went at once(doesn't seem likely)? Or does this point to a different problem. I also cleaned both sensors with MAF cleaning spray.

I am planning on replacing both sensors, but I'm wondering if this could be a sign of engine control module failure or something else.
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Sunday, July 15th, 2007 AT 2:31 PM
Tiny
DGHAHN
  • MEMBER
  • 6 POSTS
I've replaced the MAF sensor and air temperature sensor and it still backfires when accelerating. It idles fine and there are no check engine codes. I'm not sure what the next step may be.

If I disconnect the maf and air temp. Sensor it runs much better, but not perfect. Any ideas?
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Wednesday, July 18th, 2007 AT 8:14 PM
Tiny
DGHAHN
  • MEMBER
  • 6 POSTS
I finally picked up a fuel pressure gauge and checked the pressure. With the key on, but not started the pressure is exactly 50lbs. Once I start the van it fluctuates when I rev the engine between 47 and 52.
You said it should be between 41-47, I was expecting to see lower numbers if there was a problem, but is it a problem if the numbers are higher?
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Sunday, July 22nd, 2007 AT 1:32 PM
Tiny
DGHAHN
  • MEMBER
  • 6 POSTS
It turned out to be the fuel pump. It took me a while, checking everything but the fuel pump, because I thought I was ok with the comment about 41-47 psi.

But I found another site where someone told me that it should be 58-65. My pump was putting out about 50psi, and I didn't think the slight difference would matter. But it did. It's running great now.
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Saturday, July 28th, 2007 AT 3:29 PM

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