Chevy 6.5L turbo diesel 1995. My truck every once in a while will hesitate. Just once, like a miss (not sure whether it is or not). Randomly, does not matter temp, load, gear etc.. Any ideas.?

PMD is failing on you most likely. Check air in fuel supply by cracking bleed on top of filter first, but I don't think thats your problem.

http://www.dieselplace.com/forum/smileys/Cry.giffuel pump probablyhttp://www.dieselplace.com/forum/smileys/Cry.gif

Remove a non-vented fuel cap and replace with a vented one.

If it is the PMD, can't I get a relocation kit with new driver. This way I won't have to fuss with the injection pump.

yes, from Heath or kennedy.

Just being curious - does it happen when the engine is at, or near idle, or at any rpm's?

Fuel pump. In the frame, below the drivers seat. If it works you will hear it...

Keep in mind that hearing the fuel pump operating means it does work electrically, but I've seen some units work, but the pump itself can become weak and not capable of maintaining pressure or volume.

mine did the same thing till i changed the lift pump (located on the inside of the drivers side framerail)

How old fuel filter? Open petcock on thermostat housing, see if you have fuel flowing out, or if it dies in 30 seconds. (Be sure to catch that diesel if it's flowing, don't want to kill all the daisies.) If it dies, pin down lift pump or OPS. Check batt connections WELL, and engine grounds. Got any codes?

Lunicy,


Can you offer more details on what your "miss" is? I mean can you describe it any further? The PMD is always going to be a possiblity, but there are other things that will cause issues.


Mine has what I call a "buck" on occasion no matter what the load or other conditions, except it always occurs while driving at normal speed or accelerating. I have a new PMD and have checked numerous things. My fuel pressure is low, however (3psi), but I cannot figure out why.


SnowDrift

Replace the fuel transfer pump, located below the driver on the frame rail.

snowdrift: i would bet money that your lift pump is bad like canadian winter has said. mine did the same thing.


where are you located at in ohio?

Or the OPS is bad, either one same problem.

a problem with the OPS would affect function, but not performance.

Steiner,


outside Marion.


On the lift pump, I know the pump is good and have proven this by taking it off and putting in on another truck. It produced +7 psi at idle where it made 3 psi on mine. I have problems elsewhere and don't know where to look. I'm leaning toward the possibility of collapsed fuel line(s) because nothing else I seem to test proves to be bad.


SnowDrift

With the lift pump you must also replace the Oil Pressure Sensor (or maybe it's Switch, either way an OPS). It is located on the manifold just in front of the Fuel Filter housing, cost is cheap compared to the pump. If you do think it is a fuel line, check the return line from the Injector Pump back to the Tank. On pre-95's there have been problems with it, and you may need to pull apart the Injector Pump itself to check for blown up gaskets that might clog it. 96 and newer have a different system. If you don't have a (Haynes) repair manual for the Diesel engines (GM & Ford) get one, it can be a big help. Also search other threads here for advice on replacing these parts.

Yes, the OPS contacts can weaken over time pulling the 3 amp liftpump on 1 amp rated contacts. It will work but not well. Wire the lift pump off an ignition wire, or bypass the OPS with a relay to run the lift. Also try blowing compressed air back down the fuel line from before the lift pump back to the tank. also check the screen going into the IP for clog.

quantum,


If you're suggesting this in response to my comment, I've done all this. I've dropped the tank, replaced the "sock". I've wired the lift pump direct and got the same PSI reading. I've tested the voltage at the pump (13.6, I believe) while running.


Everything seems to be in order, which is why I am leaning toward fuel line troubles.


SnowDrift

I didn't get that far because my OPS turned out to be the problem after all, but the next step was to check the fuel return line coming out of the Injection Pump. If it is clogged it can back up the pressure so much that the Transfer (Lift) Pump can't send fuel to the engine.


The return line is easy to reach, right at the front of the Intake manifold, but I suggest pulling apart the Pump itself if your engine is a pre-'94 because apparently there is a gasket inside that can break and clog the lines. Post '94 was a different system. The Haynes "Diesel Engine GM/Ford" manual shows details, but is a fairly useless guide to all other problems. Bon Chance!

Did you test the liftpump off the truck to see if it was weak? OR blow air through the lines to see if it resisted? Or make sure the cap is venting and not vacuum locking.

quantum,


again, if that is directed toward me, yes, I tested it off the truck. I also tested it on another truck and it made +7 psi on that truck. I've not blown air through the lines. I don't want to blow it toward the tank because I don't want to blow off the sock.


I have found that with a full tank, upon removing the lift pump, fuel will siphon at a really high rate automatically. That leads me to believe that the rear side of the line is not an issue. Would that be a fair thing to assume?


SnowDrift

7 psi should be enough. It would seem that the backup is after the lift pump. There is a screen in the feul inlet of the IP. Also take the fuel return line off the IP( the return lines come off the injectors) and blow air back to the tank. Your looking for a line that resists.

I think that you can easily reach the return line off the Injection Pump, as it is at the very front of the engine, at an opening through the Intake Manifold - no need to remove the manifold or pump. The return line is a loop of black hose that sticks out like a "nose" in the very center and runs downward.Edited by: CanadianWinter

I don't think the restriction is right at the inj. pump because my low pressure shows up before the fuel enters the pump. I am checking all these fuel pressures at the valve at the water neck. This tells me that the restriction (if there is one) is between the filter bowl and the pump. The hose from the filter to the valve is new.


Is the return line off the pump the one that sits right at the front at a 45 degree angle? If so, this fitting is a little valve and also holds pressure. I put it in my mouth to see if I could release the pressure and was able to (don't try this at home!).


SnowDrift