6.2 dies when you let off throttle

[86m1028]

Truck is new to me.
M1031, 11k miles, over all in great shape.
Purchased from an auction, someone stole the starter relay & glowplug card after previewing it (wont get into that) complete otherwise.
I installed a relay & card, truck took a few cranks to start but did.
Runs fine until you let off the throttle, dies like you turned the key off.
I have installed a new fuel filter, cleaned return line & broke out glass ball.
Checked all fuel lines (no leaks).

I have searched & read tons of posts aboout low power, clogged return lines, bad fuel lines, bad ip's ect.

This truck has 11K on it, WHAT AM I MISSING ?

 

[JGBallew]

1- will it start and idle well?
2- if you let the gas pedal up slowly, will it return to a stable idle?

 

[86m1028]

1- will it start and idle well?
2- if you let the gas pedal up slowly, will it return to a stable idle?

1 will start & idle easily, will idle great for awhile then fluctuates & dies, will restart instantly.

2 yes but you must let up EVER so slowly on it or it will die.

 

[southdave]

fuel pump is first place I would check, if it is think about electric pump swap
second I would check the fuel line and the filter housing they leak and suck air

 

[86m1028]

fuel pump is first place I would check, if it is think about electric pump swap
second I would check the fuel line and the filter housing they leak and suck air

Why do you suspect fuel pump ?
Truck will rev up (not starving for fuel).

I have looked over lines (dont see any cracks or leaking).

 

[Barrman]

More than likely, it is your injection pump. Time instead of miles kill them.

 

[86m1028]

More than likely, it is your injection pump. Time instead of miles kill them.

I was afraid of that.
I have been reading alot of your posts.
Where did you have yours rebuilt & would you recommend them ?

 

[ranchhopper]

Before you worry about a new pump try turning the idle screw up just a bit mine did the same thing when I got it. At first I figured bad fuel delivery so I went ahead and put an electric fuel pump on it ( I was going to do that anyways) then added a spin on base and fuel filter and it did the same thing just not as bad. I went and drained all the old bad fuel from the tank added fresh fuel then turned the idle screw up very slowly and its like a new truck now runs and idles great.

 

[southdave]

yeah the IP pump will create enough suction to draw out of the tank at certain RPM ..
hence the stall at idle

 

[chevyCUCV]

Bad tank filter sock,

 

[86m1028]

Before you worry about a new pump try turning the idle screw up just a bit mine did the same thing when I got it. At first I figured bad fuel delivery so I went ahead and put an electric fuel pump on it ( I was going to do that anyways) then added a spin on base and fuel filter and it did the same thing just not as bad. I went and drained all the old bad fuel from the tank added fresh fuel then turned the idle screw up very slowly and its like a new truck now runs and idles great.

Turned the idle up & turned the fuel up, didnt help any.

 

[86m1028]

yeah the IP pump will create enough suction to draw out of the tank at certain RPM ..
hence the stall at idle

but doesnt the pump have to be working for it to start ?

 

[southdave]

no it draw out the fuel filter and the system lines there is bowl in ip pump it will run for about 3 to 5min before it dies also check the rubber connection and hose remeber the fuel system works off pump curve you could be sucking air at higher rpm you over power the leak maybe.. but sounds like your pump is weak 60 at auto zome for diesel pump you can leave the mech pump in place ect..

 

[islandguydon]

I would put my money on the fuel filter. I would also add an electronic lift pump right at the tank. There are many threads on the subject. Good luck.!

 

[CPAPPY]

I had (2) M1028 that ran that way and all I have done is add 16oz. of Diesel 911 and 16oz. of Diesel Kleen plus cetane boost & add 12 oz of Lucas fuel treatment and pump 4 gal. of diesel fuel in a can so it mixes up, and then pour into tank . . And every vehicle I pick up at auction I do the same thing to . It was a cheap fix .

 

[86m1028]

pulled fuel line loose & cranked it.
pump is working fine

 

[86m1028]

I would put my money on the fuel filter. I would also add an electronic lift pump right at the tank. There are many threads on the subject. Good luck.!

Just installed new filter, same issue.

I have been reading them & trying everything.

Thanks

 

[chevyCUCV]

Fill your filter with atf, an old bottle if you have it.

 

[Keith_J]

This is the injection pump. All diesel injection pumps have a governor, otherwise idle would be impossible, the accelerator changes the governed RPM and most have considerable droop.

What happens when you let off the accelerator is the governor shuts off all fuel since the engine RPM is far above the governed injected quantity. The governor controls engine speed by balancing centrifugal force against a variable spring force. As speed changes, centrifugal force changes. Mash the accelerator and the spring force increases, as engine speed builds, centrifugal force eventually matches the spring force.

Now, when the engine is spinning at 2800 RPM and you let off the accelerator, centrifugal force reduces injected quantity to zero and in the case of failing injection pumps, the spring force cannot reopen the quantity adjuster.

You will notice the diesel clatter completely stops when coasting from around 50 MPH. If it doesn't resume at about 35 MPH, this is the problem. There is a check valve which could cause this, most likely it is varnished and worn internals.

 

[Rustygears]

Or air getting sucked into the fuel line at high rpm, causes stumble and die when pulling off the throttle. My '82 k10 with the 6.2 has had this issue. It Is the 12v version of a cucv. Same pumps, etc. the fix is to find the rubber clamp in the many that's loose or the hose end that's swelled and letting air in under high suction. I'd make sure this isn't the culprit before laying out the buck and hassle for an IP replacement.