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truck loses power after warming up w/ bizarre symptoms

onetonjimmy

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Howdy fellas- I need for ya'll to put your thinking hats on for this one..... I've got a military CUCV 1028 pick up that's my daily & only driver. I've driven it for a year and 10k miles now, with little to no problems. I do regular p.m.'s, and check under the hood regularly for any issues, since I have almost zero knowledge of diesels, but 35 yrs. experience with gas motors. Anyway, here goes: last Thursday, while driving home from work, I was approaching a traffic light, and as the truck geared down, I could tell that it was idling very slow; not shutting off, just very slow. Upon take off, there was so little power, an urban outdoorsman was able to outrun me across the intersection. Power picked up somewhat, after 25-30 mph, but still very weak, I barely made it over the overpass & r/r tracks and limped home. After reading, reading, and more reading, it sounded like either the lift pump was failing, or a clogged fuel filter. I bought and installed both Sat. using arveetechs sticky at the beginning of this forum- worked great!!!! It fired up on the second try, and ran great. I let it idle for probably 15-20 minutes. On Sunday, I drove all over the place, shut it off, started it back up, and did everything to mimic my drive to work. On Monday, (today) I drove it to work (approximately 15 miles), and about 5 miles from work, it started doing the same thing, except at 40-45 mph, where it seemed to run fine. I limped into work, and it stayed parked all day and cooled completely. This afternoon, I took the interstate home, thinking,ok, it seemed okay at 40-45, lets see what happens. Again, after about 10 miles, I could feel the loss of power, it would slow to about 35 on the "hills" and get back up to 45-50 on the "flats, then the gas pedal sucked itself down to the floor, and speed remained at 45ish. I figured I better come up with a plan for exiting the highway, so I threw it in neutral to see where the rpms are. I started to free rev, so I dropped it back in gear, shut it down, and pulled over. The gas pedal slowly raised back to its normal position, but the truck wouldn't run for more than a second, and then died. While waiting for a wrecker, as it cooled off, it would run longer, and still rough. By the time I got it home on the rollback, it had cooled off enough, that I backed it off the wrecker, and drove it under the carport. It's running rough, and I have no doubt that once it warms up again, the problem will reappear. WHAT THE !@#$% is it? I need to keep her going 'til the big block goes in at the end of the year. Any solutions???? suggestions?????????? thanks, Henry
 
The gas pedal took itself to the floor? On a vehicle that doesn't have cruise control?

Bizarre. Your engine does have a governor that will adjust the throttle as needed, but I've never felt mine take control of the pedal like that. :dunno:

It does sound like classic lift pump/plugged filter symptoms. Not sure what else could cause that, but I'll noodle on it for a bit and see if I come up with anything. Maybe a collapsed soft line between the pump and the tank? Or a plugged screen at the bottom of the sending unit? :dunno:

@AgDieseler, @longbedder, @kennyw, @Russell, @tRustyK5, the usual diesel squad. :wink1:
 
Surging and losing power are both signs of air in the lines. I would inspect all fuel lines for leaks. If fuel can leak out, air can leak in. If you don't find anything there, it could be a worn injector pump.
 
If you're certain that you're getting good fuel supply (are you really certain?), and that your return lines aren't leaking or plugged, then that pretty well leaves the injection pump (IP). If you got crap in your tank the pickup sock could be clogged.

Anyway, if you're pretty sure of the above, then I'd pop the IP top cover off and have a look in there for the following:

  • If there's debris like coffee grounds in there, then your plastic governor weight retainer ring has deteriorated and failed. The governor stops working right and the crap will also clog your injection pump up. The factory switched from plastic to a more durable material in mid-1985.

  • If there's no debris, it could very well be a sticky metering valve. They do gum up occasionally, and can be cleaned or replaced.
 
Surging and rough idle sure sound like air in the lines. Low/no power makes me think fuel filter or a failed lift pump.

If the symptoms are acute, I would follow the advise above and trace down air leaks first. I usually smell them before I see them. Lift pump and filter are good places to start. If the symptoms have been building, I would tend more toward injector punk wear.

Start with the simple stuff.

David
 
Is it surging? I interpreted his description as him seeing a loss of power across the board, not a rapidly coming-and-going gasping like air in the lines would cause.

If it is air, and it's leaking in fast enough to cause the engine to cut out, I'd think he'd be unable to start it afterwards. :dunno:
 
pedal was absolutely NOT under my control, and no I don't have cruise, although this would fall under the category of hillybilly cruise right? It wasn't a problem since I was only doing 35-40ish ,and it only raised back up shortly after I shut the truck off. I'm thinking suction of some sort released after shut off, allowing pedal to rise. There is no surging, bucking, or rpm variance while all this is going on- it just loses power and as a result, inclines slow it down, while declines allow it to "catch back up" to about 40ish tops. If it happens to quit raining anytime soon, I want to crank it up and see what happens, motor is cold since I am not driving it until I figure out what the problem is. This all started abruptly Thursday afternoon, with no prior symptoms, sounds, hiccups, etc. Lift pump and fuel filter were bought/installed Sat., and everything seems to be fine until "something"- either pressure or vacuum builds up too much, or something finally warms up to full operating temp. and either contracts or expands too much.
 
pedal was absolutely NOT under my control, and no I don't have cruise, although this would fall under the category of hillybilly cruise right? It wasn't a problem since I was only doing 35-40ish ,and it only raised back up shortly after I shut the truck off. I'm thinking suction of some sort released after shut off, allowing pedal to rise. There is no surging, bucking, or rpm variance while all this is going on- it just loses power and as a result, inclines slow it down, while declines allow it to "catch back up" to about 40ish tops. If it happens to quit raining anytime soon, I want to crank it up and see what happens, motor is cold since I am not driving it until I figure out what the problem is. This all started abruptly Thursday afternoon, with no prior symptoms, sounds, hiccups, etc. Lift pump and fuel filter were bought/installed Sat., and everything seems to be fine until "something"- either pressure or vacuum builds up too much, or something finally warms up to full operating temp. and either contracts or expands too much.

You don't have engine vacuum on a 6.2, it can't cause problems. There is a vacuum pump at the back of the engine, but in the military rigs it is only connected to the TH400 transmission.

Pedal falling like that is exactly what cruise would do, but there should not be anything else attached to the control of the IP. I don't know whether the internal governor can relieve pedal pressure, but it only adjusts idle speed, it should not be doing anything if you are loaded down at cruising speed.
Losing power like that sounds exactly like a fuel delivery problem, as stated before.

Does the problem happen before the temp. gauge comes up to operating range? There is a cold idle advance switch that trips about 1/2-way through warmup (on my rigs this is before the gauge starts climbing). When working, it advances the "throttle" control on the IP and also bumps up the timing a bit. This switch turning off (returning to normal timing and dropping the idle) should be the only thing that changes when the engine warms up.
 
I'm going to agree with air in the fuel lines or a plugged pickup.

Does the engine have any smoke when it is misbehaving?
 

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