95 6.5L Bucks, feels like Fuel Starved

[xtrempickup]

I drive my 6.5L everyday, just replaced the lift pump and fixed a borken wastegate actuator. IF i get on it, It will studder sorta feels like a buck or backfire like its Fuel starved. I got this fuel starved idea from a mechanic i took it to that was checking the trans that I told him help like it studdered, but he felt this other studder buck sort of and said thats defiantely not the trans, thats its a performance issue. any ideas??

[94 at Large]

I had similar issues with mine. Heavy on the throttle and I would get severe miss as you would on a gasser when a plug was out. My problem was solved with a new OPS until it went out again, recently. Used a bypass harness to take OPS out of the loop and once again it runs smoothly. Crack the drain petcock and if the truck dies after 15-20 seconds and you know the lift pump is functional then my guess would be that OPS is not. At least this is what the problem with mine was.

[xtrempickup]

lift pump was out until about a week ago when i changed it . its functional now because it dies when i open the pet cock up front in the bay. I'm gonna try changing the fuel fliter see if that makes it any better. if not i'l look into that OPS. anybody else

[Mark Rinker]

Will also do this if you are drawing air into the line between tank and lift pump. If you are in a high corrosion area (salty roads) I'd drop the tank and replace the sending unit all the way to the lift pump if the OPS doesn't fix it.

My 1994 just got through this scenario earlier this fall.

[fastcat800]

If your lift pump is working, opening the water bleed valve should not kill the engine (it doesn't on my truck, fuel comes out the hose). I had exactly the same problem as you and changed the lift pump, then the filter before finally changing the OPS to solve the problem. Until I did the OPS it would die when I opened the water bleed valve. A month later I added the OPS relay and have had no problems for 2 years.

[xtrempickup]

well before i changed the lift pump, it would run with the pet cock open and nothing would come out but it never stopped running either I was told by a Chevy Diesel Tech that it should die as opening the pet cock releases pressure and that the motor needs that few lbs of pressure. The lift was definately bad that was replaced as it didnt run and the harness had power at the lift pump. I have the OPS, just need the socket to change it, its special, not a standard one. il try that as i already bought it because the Diesel tech said the pump not running could be two things, the OPS is bad or the Lift pump is bad.

[d350diesel]

Change that OPS, I'll bet it's your problem.

A Craftsman 1 1/16" 12 point 1/2" drive deep socket is deep enough for the OPS. Craftsman 6 point requires some grinding down in the well. None of the auto parts stores in my area had an OPS socket deep enough for our OPS, go to Sears.

[Squatch]

I agree with Mark Rinker. I had the same problem with my 94 suburban and after replacing the PMD, OPS, lift pump, and lift pump relay only to find out there was a pin hole in the metal lines off of the sending unit. I was able to see the top of the tank with a mirror and flashlight. Good luck.

[65TD]

What about codes? SES light come on at all?

[moondoggie]

Good Day!

Your symptom is CLASSIS failed LP system. Boy, are you blessed. It's REAL easy to prime the fuel lines and/or check LP system function on this truck; too bad the 1st part of this test only works on 95's (I asked some time ago, it really only works on 95's).


Your LP is powered by two circuits: relay during cranking, & the OPS during running.
Crack the fuel filter nut (black plastic round thingy on top of the fuel filter housing) a little. Connect a piece of tubing to the port on the top of this nut; drop the other end in a container.[INDENT]
Set the parking brake or block the wheels. (Maybe better to not use the parking brake on a vehicle you don't know - maybe it never releases, eh? ;))

Put the tranny in any gear EXCEPT park or neutral.

Turn the key to the crank position - the LP will run, but the engine will NOT crank.

[FONT=Verdana]Wait for a solid stream of fuel to be pumped out of your piece of tubing. If so, your LP & relay are working; if not, one of those has failed (most likely the LP, but read on

[xtrempickup]

I think i said this in the beginning, lift pump was out a while, truck ran on IP for a while pulling fuel and ran fine, just was hard to start cold. Lift pump was replaced, tested for voltage, thats fine. truck starts a lot better now. The condition with starvation and studdering was before I did both and is still after I did the lift pump. Just did the glow plugs too, that didnt help they were all out. The truck will die if i crack the pet cock up front and leave it a few seconds. I'm gonna change the OPS and see if that changes the overall condition.

[Iain]

I had a similar problem on my 1994, Lift pump failed but the truck would still run actually fairly weel, just low on power up the hills and harder starting, especially when it got cold. It still did this after I had changed the OPS. Kenneday Diesel has a nice kit which you may want to look at, it bypasses (sort of) the factory OPS to power the lift pump and has a separate control box to power the lift pump once the key is turned on. There is a manual button on it so that you can bleed the system cleanly after you change filters. Check out www.kennedydiesel.com, it is a good system. Truck starts easier and seems to run better now that the lift pump is working like it should be.

My advice, if you are changing the OPS anyway, install this kit while you are back there.

Iain