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Old 10-01-2006, 12:58 PM   #1 (permalink)
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Repriming the high pressure fuel pump on GXPs


I've not seen this addressed. Since the '60s I've owned and used many Diesels. One of the things I learned well was don't run them out of fuel. If you did repriming the high pressure pump was a bitch. This is especially true with my dozer - a Fiat Allis 16B with (eat your hearts out engine aficionados) a true BUDA --- IIRC with 4 valves per cylinder and 988 CI. Repriming this one takes a minimum of an hour and pray that the batteries don't run down before you finish. This may not be a problem with more modern Diesels - I just don't know cause I just don't let one run out of fuel anymore.

So has anyone seen any warning or even worse run out of fuel and experienced a reprime problem. Or mirabile dictu the problem doesn't exist with the GXP.
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Old 10-01-2006, 01:30 PM   #2 (permalink)
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I'm not an expert here but I think the GXP fuel pump is cam driven so I don't know if this will make a difference or not.
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Old 10-01-2006, 05:02 PM   #3 (permalink)
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i ran out of gas a couple weeks ago, but mine is not a gxp.

as soon as i felt the loss of power, i punched the clutch, took her out of gear, and coasted about half a mile.

pontiac said maximum 60 minutes to bring the gas. ninety-eight minutes later the guy shows up, puts a couple of gallons in, and i go to start the engine.

started right up on the first try. the first time i ran out, it took quite a while to get it started again.
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Old 10-01-2006, 05:42 PM   #4 (permalink)
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Clarification!!!

It appears my question isn't as clear to others as it should be. Below is a direct quote from GM describing the fuel system.

================================================== ============
A high-pressure, returnless fuel system is employed. It features a high-strength stainless steel fuel line that feeds a variable-pressure fuel rail. Direct injection requires higher fuel pressure than conventional fuel injected engines and an engine-driven high-pressure fuel pump is used to supply up to 1,276 psi (150 bar) of pressure. The system regulates lower fuel pressure at idle approximately 752 psi (50 bar) and higher pressure at wide-open throttle. The cam-driven high-pressure pumps works in conjunction with a conventional fuel tank-mounted supply pump.
================================================== ============
Obviously this is not a Diesel engine but there are many similarities. The fuel pressure is very high compared to other gasoline engines but somewhat lower than most Diesels. Two reasons come to mind. One - the higher the pressure the easier to atomise the fuel charge and two - the pump is having to work against some pressure already in the cylinder both from the turbo and the compression stroke of the engine. The second situation is the one I'm concerned with. On Tiny (my dozer) to reprime itself the high pressure pump has to push the air inside itself out into the cylinders when the pistons are at top dead center and therefore the pressure inside the cylinder is at maximum. Since air is compressible this just doesn't work all that well. To reprime the pump on Tiny I have to loosen at least every other fuel line going from the pump to the cylinder injectors and then turn the engine over till those lines are pumping Diesel again. Then I can expect him to fire up and run.

Hope this does a better job of explaining my question.
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Old 10-01-2006, 06:51 PM   #5 (permalink)
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I thought I read some time ago that the GXP utilizes a dual fuel pump system; a high volume/low pressure electric pump in the tank and an engine driven high pressure unit to get pressures up to where needed for the direct injection. If this is the case, re-priming might not be a big concern. If there is only a cam driven high pressure pump, the starter could fry before the battery runs out of juice. My knowledge of turbo systems, admittedly not extensive, makes me think a larger problem w/ running out of fuel is the excessive lean-burn condition just prior to empty and the internal damage that this could do. Make sure the guage is working, keep an eye on it, track your mileage per tank, & make sure you don't run out of fuel.
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Old 10-02-2006, 07:04 AM   #6 (permalink)
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I thought I read some time ago that the GXP utilizes a dual fuel pump system; a high volume/low pressure electric pump in the tank and an engine driven high pressure unit to get pressures up to where needed for the direct injection. If this is the case, re-priming might not be a big concern. If there is only a cam driven high pressure pump, the starter could fry before the battery runs out of juice. My knowledge of turbo systems, admittedly not extensive, makes me think a larger problem w/ running out of fuel is the excessive lean-burn condition just prior to empty and the internal damage that this could do. Make sure the guage is working, keep an eye on it, track your mileage per tank, & make sure you don't run out of fuel.

With the first pump (low pressure/high volume (lift pump)) there shouldnt be much issue with the GXP if you ran it out of gas. Give the electric pump a few seconds to reprime the high pressure pump (cam driven injector pump) and it should start shortly. It may take one or two 15 second runs on the starter. This is minimal compared to running "tiny" out of fuel. The modern diesel trucks without lift pumps have a hand pump and a bleed port to facilitate priming the system. It isnt as big a deal as priming "Tiny" however it is much more work than those of us that have added the lift pump. I happen to know that my dmax truck will restart in 20 seconds after having been run out of fuel. I also happen to know that my 34 gallon tank only has 32 gallons of usable fuel. That is the important part.
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Old 10-02-2006, 06:01 PM   #7 (permalink)
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[QUOTE=b_faster;367514]With the first pump (low pressure/high volume (lift pump)) there shouldnt be much issue with the GXP if you ran it out of gas. Give the electric pump a few seconds to reprime the high pressure pump (cam driven injector pump) and it should start shortly. It may take one or two 15 second runs on the starter. This is minimal compared to running "tiny" out of fuel.

I know of one case where the cam-driven GXP fuel pump failed from
lack of lubrication because at the time the intank pump had been
disconnected for some reason and then the engine was cranked over.
Power was found to be down and later was traced to bad HP pump and
the lack of lub from the fuel supplied by the intank pump.

That should not be a problem unless a user takes out the intank fuse
for some reason.

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Old 10-02-2006, 10:38 PM   #8 (permalink)
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Good point about the fuse though.
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Old 10-03-2006, 10:42 AM   #9 (permalink)
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Originally Posted by turbofreak View Post
I know of one case where the cam-driven GXP fuel pump failed from
lack of lubrication because at the time the intank pump had been
disconnected for some reason and then the engine was cranked over.
Power was found to be down and later was traced to bad HP pump and
the lack of lub from the fuel supplied by the intank pump.

That should not be a problem unless a user takes out the intank fuse
for some reason.

turbofreak
If the cam driven (inj.) pump was designed to always have a lift pump, then that scenario is very likely. A lift pump breakdown would show up during hard driving, how long before the inj. pump is damaged I dont know.

I added a lift pump in my diesel truck, it is required to support >700 ft.lbs. Without the lift pump, the truck ran out of fuel (guts).
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Old 10-03-2006, 11:38 AM   #10 (permalink)
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Originally Posted by dlowie View Post
It appears my question isn't as clear to others as it should be. Below is a direct quote from GM describing the fuel system.


Hope this does a better job of explaining my question.
The key difference in the two systems, the hi-p common rail system supplies the high-psi fuel down the railo but has no effect on timing or opening of teh injectors. These injectors are electrically opened. The diesel systems other than the most modern smog certified over the road diesels still rely on the actual hi-psi pulse of fuel from the distribution pump to open the injectors. These systems also employ a return line off the top of the injectors to th e tank. In this situation since air will not open the injector all teh air has to be removed. Some tractors are actually self-bleeding to an extent and do not require bleeding after changing filters.
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Old 10-03-2006, 11:55 AM   #11 (permalink)
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The key difference in the two systems, the hi-p common rail system supplies the high-psi fuel down the railo but has no effect on timing or opening of teh injectors. These injectors are electrically opened. The diesel systems other than the most modern smog certified over the road diesels still rely on the actual hi-psi pulse of fuel from the distribution pump to open the injectors. These systems also employ a return line off the top of the injectors to th e tank. In this situation since air will not open the injector all teh air has to be removed. Some tractors are actually self-bleeding to an extent and do not require bleeding after changing filters.
That analogy died over 5 years ago in the light truck (3/4-1 ton) diesel business. The common rail system has been here for that long.

I agree "tiny" more than likely has the PSI driven injectors. Most of those engines also happen to have a manual pump to bleed the air from the system, however it still requires loosening the injector feed lines (cracking) to bleed the air. If I never have to do it again on an industrial engine, I will still be too familiar with the process.
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Old 10-03-2006, 12:02 PM   #12 (permalink)
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Quote:
Originally Posted by achieftain View Post
The key difference in the two systems, the hi-p common rail system supplies the high-psi fuel down the railo but has no effect on timing or opening of teh injectors. These injectors are electrically opened. The diesel systems other than the most modern smog certified over the road diesels still rely on the actual hi-psi pulse of fuel from the distribution pump to open the injectors. These systems also employ a return line off the top of the injectors to th e tank. In this situation since air will not open the injector all teh air has to be removed. Some tractors are actually self-bleeding to an extent and do not require bleeding after changing filters.
If I've got this right the hi pressure pump supplies a relatively constant supply of fuel to the rail which in turn supplies it to the injectors. The injectors open electronically both whenever the ECM calls and also for whatever amount of time the ECM demands - longer for hard acceleration, etc. This begs the question of the ECM knowing the pressure in the fuel system.

At any rate since GM has stated this is returnless unit any air that gets into it will be pumped into the cylinders or bled away by some other and unknown to me system.
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