HELP! Problem with throttle response.

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Jun 1, 2011 | 01:26 PM

scram-man

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HELP! Problem with throttle response.

I have a 5.3 vortec that I put into a jeep. It is still a work in progress, but has come far enough to be a daily driver. The motor was complete with harness and ECM. The ECM was flashed to remove VAS, and rear O2 sensors. Other than that it is a stock 2002 5.3L vortec from a Tahoe that has been running just fine.

Enter problem-- The other day the truck would not start. Turns over and over but no go. Started checking usual stuff, power to relays, power to fuel pump, etc. Seemed to be the fuel pump. Pulled the pump and check connections, turned out to be a loose ground wire to the fuel pump. Great! Fixed that and go to start 'er up. Truck starts, but not on the first turn of the key like it usually does. Finally gets started and idles rough for a little while, then smoothes out. However, the throttle response isn't right. If I'm real easy with the gas pedal and can slowly bring it to WOT. But is I try to just push it and go, I get a stumble and sputter. I then let off and it will idle ok. I can then "feather" it and get going as long as I'm careful on the pedal. If I try to just stomp on it, I get a big enpty sucking sound at the intake and it will die unless I back off. It will also backfire some.
I cleaned to MAF, which didn't look bad at all. What else could be going on??
It was fine until the ground/fuel pump issue..... What can I check? I do not yet have the ck engine light hooked up, so don't know if it's going off. Also, do not yet have my OBD port wired up, so can't scan it.

Any ideas??

Thanks for any and all help.

Jun 1, 2011 | 02:04 PM

Texas_WS6

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From: Odessa, Texas

Put a pressure gauge on the fuel system and see what the fuel pressure is. Watch it at idle, easy driving, and when you stomp it to WOT as well as threw WOT. It should hold steady at 58 psi. If not then you have a fuel system issue.

Sounds like it is lean when you stab it, hence the lean manifold backfire.

Jun 1, 2011 | 09:45 PM

zayne0

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From: no mans land

see how it runs with the mass airflow sensor disconnected

Jun 1, 2011 | 09:50 PM

zayne0

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From: no mans land

disconnect the mass air sensor and see what happens, if it straightens out it could be a vaccum leak just from my ls swap experience.

Jun 3, 2011 | 11:33 AM

scram-man

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I had a friend with a gauge ck the fuel pressure. 18psi. lookin gliek fuel pump but haven't had a chance to pull it.
Will start checking sensors if that doesn't pan out.

Thanks

Jun 3, 2011 | 11:50 AM

Jim_PA

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From: Elizabethtown, PA

Yeah, that's just a little on the low side

Jun 9, 2011 | 03:45 PM

scram-man

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Well this is the rest of the story. Low fuel pressure. Put on a new fuel pump.... same low pressure, like 20psi. Scratch head. Could there be something restricting flow?? Pull pump/sender assembly ( for liek the 4th time!) Take a air gun and go to blow out the feedline. There a short length of rubber tube that goes from the pump to the feed line. Hit the airgun and there you have it.....! a small 1/8-1/4" split in the tube. 3" of new tubing later and all is good in the world. What a PITA!
But all back to normal.... Thanks for everyones help and suggestions.