Hello everyone,
I’m trying to help my dad fix his 86 gmc van with 6.2 and 4 speed manual tranny.

He says it drives fine around town in stop and go condition with plenty of power. when he gets on the highway around 60 mph ,after about 7 miles, it slows with no power then when he gets off the exit it tries to stall out. Itll run fine after that but then when it gets back on the highway after 6 or 7 miles it does it again.

He changed the fuel filter and ran diesel kleen threw the system.

I’ve spent a few hours searching the forum but was hoping someone has experienced this exact problem. He says it’s very consistent.

Thanks a lot!

I don't think your fuel pump is providing enough pressure.

Leave the gas tank cap loose and see if it happens. Vent could be stuck or dirty.

ill try that with the gas cap.

Whats the best way to test the pump? and your saying the lift pump right, not the i.p.?

Yeah Its Easier & Cheaper To Start With The Lift Pump First.

only way to really check the lift pump is to T a gauge in the line so you can keep an eye on it in the cab. if the low power situation happens when your pressure drops down you've narrowed it down to the lift pump or a plugged sock or pickup tube in the tank

That sounds good, thanks.

Here is an update on the situation:
I was told that you can check the I.P. by pushing in the shaft on bottom right of injection pump (idle set screw maybe) and if it does nothing it means bad pump. Well I pushed it all the way in and it did change the tone, though only slightly.
In the process of doing this I noticed there is a fuel leak from the filer area while the van was running. Looks like maybe the top of the filter housing. I realize that this needs to be fixed regardless but would it be causing this trouble?

Thanks again.

Yeah It Would Because When Air Gets In The Fuel Systems On a Diesel It Will Stall Out Loss Power & Not Run Right. Fix The Leak & Bleed The System & you Should Be Okay. That Will Contribute or be The Cause Of The Problem.

I realize that this needs to be fixed regardless but would it be causing this trouble?

Thanks again.


yes it will. air is easier for pump to suck then fuel and it will starve.

Great, thanks alot.

Take off the top of the ip and see if your metering valve is stuck or sluggish,pull it out and clean it.Seems alot of people here have had the same problem with their motors being sluggish not due to lift pump or air or filter/fuel proplems.Cleaning the metering valve solves this problem,diesel residue builds up on it causeing it to stick and not letting enough fuel to flow thru the ip.The metering valve works in conjunction with your fuel cut off solinoid,if it stays half open then you get half the fuel the ip is supposed to get.

That sounds good.
One thing though, I’ve done that before and when I put it back together and started it the engine ran away on me. Pulled out the key and the thing was still going full rev. lucky I pulled the fuel line off and was able to choke it out but my buddy a few months later wasn’t so lucky. It ran away from him and he wasn’t able to get it to cut out. A mechanic later told him that when it gets like that it gets so hot its starts pulling oil from the rings so that even after he cut the fuel line the thing kept running, ending in one of his rods shooting out of the engine. Needles to say he didn’t have to worry about his i.p. any more.
Any tips on making sure that dosnt happen?

That sounds good.
One thing though, I’ve done that before and when I put it back together and started it the engine ran away on me. Pulled out the key and the thing was still going full rev. lucky I pulled the fuel line off and was able to choke it out but my buddy a few months later wasn’t so lucky. It ran away from him and he wasn’t able to get it to cut out. A mechanic later told him that when it gets like that it gets so hot its starts pulling oil from the rings so that even after he cut the fuel line the thing kept running, ending in one of his rods shooting out of the engine. Needles to say he didn’t have to worry about his i.p. any more.
Any tips on making sure that dosnt happen?

Don't try to start it until you've made sure the shutoff solenoid mechanism is free to move. Turn the ignition switch on and disconnect and reconnect the shut off connector and make sure you hear a click...... If no click, pull the cover off and try again.... When it clicks when you plug it in, then you can try to start it....

well i fixed the leak i found and that was it, runs great now.
the problem was the o-ring around the heater that goes into the top of the filter body.
good to know about the i.p. though, im sure thatll come in handy sooner or later.
Thanks alot