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Please help with rough running Chevy 1500 5.7

Tiny
J02WS6TRANSAM
  • MEMBER
  • 1998 CHEVROLET TRUCK
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 150,000 MILES
Hello. I have a 98 chevy 1500 5.7. The truck doesnt run right at all. Msifres bad under acceleration. Idles ok but misses when reving and accelerating. I did a full tune up including plugs, wires, cap rotor and a new distributor. With key on I have about 60psi fuel pressure but idleing I only had 50psi. I put a complete new fuel pump unit in and changed fuel filter. Same problem. No different fuel pressure. I changed and replaced the spider injector with the new updated system. Changed all intake gaskets. That didnt make a difference either. Replaced tps sensor and map sensor. Removed egr valve and it isnt sticking or anything. I cleaned it and used new gasket and put back on. Still same. I know these trucks need 60psi when idling. Cant figure out why I only have 50. I replaced everything in the system. I removed the fuel line where id goes into the spider system and plugged it and have 80psi when I turn the key. Then I disconnected the return line by the distributor and plugged it and had 90psi there when idling but engine ran the same. I dont know where to look next. Please help
Thank you
Jon
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Tuesday, February 3rd, 2015 AT 10:53 AM

8 Replies

Tiny
J02WS6TRANSAM
  • MEMBER
  • 34 POSTS
Also to add, I was getting a p300 misfire and a p122 tps sensor code. Thought it had a bad sensor thats why I replaced it. Sensor code hasnt come back yet but just did that this morning and still messing around with it. Could I have a bad spider injector causing the low fuel pressure?
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Tuesday, February 3rd, 2015 AT 10:59 AM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Might be fuel pressure regulator. Also scan for codes and check for vacuum leak may aslo be a plugged fuel filter
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Tuesday, February 3rd, 2015 AT 12:35 PM
Tiny
J02WS6TRANSAM
  • MEMBER
  • 34 POSTS
The new spider injection system comes with a regulator in it. Do you think the new regulator could be bad? If I leave the fuel pressure gauge on the truck and shut off the key it does leak down. Goes from 50psi down to 20 in maybe 10 minutes or less. I have no leaks. Checked for that. And when I plugged the feed line where it plugs into the spider injection and got 80psi, it took 2 hours to drop down to 60psi. So I know everything up to the spider injection is good. I measured the vacuum off the big hose for the brake booster, I have 20 inches of vacuum. Tried to spray everything with brake clean to find a leak and didnt seem to find anything.
Thank you for all your help
Jon
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Tuesday, February 3rd, 2015 AT 12:40 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
If fuel pressure goes higher than 75 psi regulator is or
maybe was damaged. You said it jumped to80 was the fuel filter replaced? It should have been was the pump a delco? A lot of the others aren't that good like airtex for one also because of year of truck a foot of new harness should have come with new pump because of changed connector as well as harness ia bad and gives low voltage
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Tuesday, February 3rd, 2015 AT 3:15 PM
Tiny
J02WS6TRANSAM
  • MEMBER
  • 34 POSTS
When I disconnected the feed line from the spider injector and plugged it I got 80psi on the gauge. I used a aftermarket spider injector system bdw. But I have a delphi system coming today that I will try to install to see if that makes a difference. I did changed the fuel filter. I did also use a airtex fuel pump but I have used them on many gm vehicle before with no issues and was told most airtex are oem pump in them. It did come with the new harness that I soldered into the factory harness. Seems like the pump doesnt have a problem and supplies plenty of fuel if either the return is blocked or the feed line. Also the furl pressure only bleeds down the the lines are installed into the spider injector system with all the lines hooked up. Seems really weird.
I hope the new spider injector fixes the issue. Otherwise I am out of ideas! Thanks for your help
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Wednesday, February 4th, 2015 AT 4:29 AM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Th a 300 code those are hard to diagnose, have a pro put his scanner onit to see if there is a sensor out of whack as that can cause it as well assuming all your compression is good
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Wednesday, February 4th, 2015 AT 5:59 AM
Tiny
J02WS6TRANSAM
  • MEMBER
  • 34 POSTS
Ok I will do that thank you. Is it weird that idling it only have 50psi of pressure? Shouldn't I have 60psi?
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Wednesday, February 4th, 2015 AT 6:06 AM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
It should be 60si minimum all thetime
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Wednesday, February 4th, 2015 AT 6:55 AM

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