Loping under load at low rpm

[Justardnck]

Hello all I am new to this board and have a question about a loping concern I have with my Sierra. First let me tell you it has almost 300,000 miles on it's original 5.7 liter and has never been taken apart for anything internal, NOT EVEN A TIMING CHAIN! The concern happens at low rpm under a load (like when you are backing a trailer up a driveway) I have replaced the following items... plugs, cap, rotor, wires, fuel filter and air filter. I have an identical truck with a siezed engine parked right next to it in the garage and have swaped every sensor on the engine with the exception of the 2 fuel injectors and the emissions valve, and nothing has changed.I did a compression check and all cylinders have around 140 psi. I have also set the timing. There are no vacume leaks either. The problem is only there when under load and only at a certain point in the gas pedel. I am at a loss and don't know where to turn and I am afraid to take it to a mechanic because when ever I have had a hard to find drivability concern I have had no luck finding a competant mechanic, it seems all they want to do is throw parts at it and never fix the problem while charging me all the while.
CAN ANYBODY HELP ME?
THANK YOU

[Justardnck]

Problem is fixed!!! As a last ditch effort I started unplugging things till I a response from the engine and found that when I pulled and pluged the vacume line on the EGR valve and found the problem went away... I replaced the valve and all is perfect again. It must have had some carbon or something in it holding it open alittle. I would have never imagined an EGR valve would cause that but its fixed.

Good to know, thanks for the post.

[DEEPEE]

I have a '99 5.7 and it's doing the same exact thing. Annoying when I pull up to a red light. I've thought all along it was the egr valve but hate to go spend $150 and hope it was the problem. Mine is a linear valve, no vacuum lines, all electrical. Does anyone know any way to check the valve to trouble shoot? Thanks

[PK55Z85]

justardnck...i'm guessing your trk is pre 95..those egr valve's are vaccum operated...did you check vaccum readings while backing up drive....might have seen erratic reading on gauge then ??
deepee....using the right diagnosistic tool you should be able to get a percentage reading on flow for egr valve...which could tell you if valve is opening all the way or is stuck partially open/closed..you may have dirty throttle body...that seems to be more likely

[Justardnck]

No I didnt try any of those, all I know is when I unpluged the vacume line and plugged it up the problem went away. So I replaced the valve and I have been driving the truck since with no problem. I tow heavy loads on a regular basis so I would have known if it was not fixed. What was the actual problem with the valve was I do not know. When I was a Ford tech drivability concerns was a very weak point for me, I was an electrical specialist. One thing I noticed at the last minute was that when the truck was cold it didn't act up but that only lasted for a few minutes considering I live in South Florida. when the truck was cold I happend to push my finger under the valve and it made the same sputter that it did when it was acting up. So I put 2 and 2 together and put a valve on it. I have an identical truck with a seized engine parked right next to it in the shop so parts swapping was easy.the problem was easy to duplicate all I had to do was put the truck in gear with my foot on the brake and apply light throttle and there it was.

[Crab Spirits]

I think this problem is related to this, though not as severe...
http://www.automotiveforums.com/vbul...1&goto=2169521