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6.5L Diesel Engine Discuss the 6.5 GM diesel engine & associated components. Automatic transmission questions & problems belong in the 4L80/85 - 4L60E - 6L90 Transmission Forum

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Old 10-31-2017, 12:36 AM   #31 (permalink)
bhoz93
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Is this a lift pump symptom?

As stated by many new member, most definitely to include myself, i have found this very helpful. I am interested in purchasing a 1994 6.5 k3500 this week after test driving it today. But after reading this forum, i had a question regarding minor symptoms possibly pertaining to the lift pump. If the current owner states that once his fuel level has decreased to about half a tank it seams to hesitate and sputter, could this be the lift pump never working and the injection pump just being gravity fed? And I am very new to this forum, so if I have asked this in the wrong place just let me know where I should be. Thank you

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Old 10-31-2017, 07:22 AM   #32 (permalink)
guybb3
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Could be a hole in his pick-up tube in the tank. Could be a few things, none of which are hard to fix with everyone's help.

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Old 06-17-2019, 03:24 PM   #33 (permalink)
bilmac36
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Awesome Pix! Appreciate the explanation. I'm thinking my OPS diaphragm is weak. Anyway, pix is worth a thousand words and these are a gem!
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Old 02-11-2020, 11:07 PM   #34 (permalink)
Kgw51gmc
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Lots of ideas on here. My 94 has a fuse on the firewall, hanging up in a socket type holder. I just pull the clip w/fuse in it down and then touch it firmly onto a 12v hot stud on the firewall, touching the top of the fuse activates my lift pump. It's loud and strong pump.

LP will NOT come on without being jumped in this way. 94 OBD1, AUTO trans, put in P/N/D/R and crank the key and no LP. Key forward in any gear and no LP. Replaced LP relay with known relay from another truck and a new one, still no operation.

When I got truck running last week, I poked a small wire into the fuse in the wire clip and put an alligator clip on other end of wire, clamped onto a firewall stud, started truck that way.

NEW ac Delco OPS.

It makes no sense, maybe a glitch in the PCM?


The LP works great when I jump it, I used it to prime the sytem after IP replacement, and to clean fuel lines by flushing with clean fuel.
Would like it to work properly.
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1997 Chevy c3500 6.5L 273k
5-spd NV4500, 4.10, 5T-elecDump flatbed Pensacola Diesel IP '18 , PMD relocate (duh), stock truck.

1994 Chevy C3500 HD 6.5L 175K, 4L80E, 4.63 Dana 80, 34'ish bucket (sunk in wet ground for 10+ yrs, dead due to PO attempts to repair , still hope). PO actually removed optic sensor and intake manifold to (I assume) spray starting fluid in, and after failing to start, gave up and left IP and engine OPEN for 10+ yrs. Walnuts and mouse doody down in around the valve stems in every port! Lots of cleaning. Be funny if it runs worth a darn.

Waynesburg, PA (near Morgantown, WV+ WVU)
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Old 02-11-2020, 11:15 PM   #35 (permalink)
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Quote:
Originally Posted by Kgw51gmc View Post
Lots of ideas on here. My 94 has a fuse on the firewall, hanging up in a socket type holder. I just pull the clip w/fuse in it down and then touch it firmly onto a 12v hot stud on the firewall, touching the top of the fuse activates my lift pump. It's loud and strong pump.

LP will NOT come on without being jumped in this way. 94 OBD1, AUTO trans, put in P/N/D/R and crank the key and no LP. Key forward in any gear and no LP. Replaced LP relay with known relay from another truck and a new one, still no operation.

When I got truck running last week, I poked a small wire into the fuse in the wire clip and put an alligator clip on other end of wire, clamped onto a firewall stud, started truck that way.

NEW ac Delco OPS.

It makes no sense, maybe a glitch in the PCM?


The LP works great when I jump it, I used it to prime the sytem after IP replacement, and to clean fuel lines by flushing with clean fuel.
Would like it to work properly.
When the OPS detects engine oil pressure it will send power to the Lift Pump..
No engine oil pressure= no lift pump power
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Old 02-11-2020, 11:30 PM   #36 (permalink)
Kgw51gmc
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I'm going to start it and then grab the lift pump. If it's working then I'll feel it purring for sure. I had thought it was still supposed to come on while in gear, not running with key fwd, but with no oil pressure=no LP then I understand. Prob works fine, just didn't understand it. I PREFER my 97 where it comes on with key forward.

I can most likely reach the LP by hand directly through the huge rust hole in the floor instead of laying in the gravel driveway. The rust hole doubles as the only brakes the truck has too (flintstones type).
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1997 Chevy c3500 6.5L 273k
5-spd NV4500, 4.10, 5T-elecDump flatbed Pensacola Diesel IP '18 , PMD relocate (duh), stock truck.

1994 Chevy C3500 HD 6.5L 175K, 4L80E, 4.63 Dana 80, 34'ish bucket (sunk in wet ground for 10+ yrs, dead due to PO attempts to repair , still hope). PO actually removed optic sensor and intake manifold to (I assume) spray starting fluid in, and after failing to start, gave up and left IP and engine OPEN for 10+ yrs. Walnuts and mouse doody down in around the valve stems in every port! Lots of cleaning. Be funny if it runs worth a darn.

Waynesburg, PA (near Morgantown, WV+ WVU)
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Old 02-11-2020, 11:38 PM   #37 (permalink)
Glagulator
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On a 94 the LP only runs while cranking the engine over, reason for the relay, and when you have oil pressure.
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Old 02-14-2020, 07:54 PM   #38 (permalink)
bigred1957
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Just my 2 cents. I had a similar problem (among others). I had bought a new fuel cap and had hesitation after about half a tank of fuel, or more. Come to find out I had a vacuum problem. New fuel cap wasn't vented. Not sure if originally it was supposed to be vented but I drilled a small hole in the bottom side if the cap and no more vacuum, no more hesitation. Really simple fix to a problem I over thought and hunted for weeks. Just a thought. Oh yeah mine it's a 94 as well.
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