Missing, Stumbling, Help.

[black_noise]

Been a while since I dropped in on the site, havnt had any problems.:D

I got a 95 6.5TD 4x4 truck, It had the lift pump and oil pressure sending unit replace last NOV. running good ever since...

untill a week ago. :mad:

Drove it for 40 miles, parked for a few hours, then it would idle and "miss" and then miss while driving and chug and sputter when trying to accelerate.

Replace fuel filter- parked it overnight, started it, let it idle for over and hour, and then test drove it for a mile, no problems at all, no missing. Then I shut it off for a half hour, then started it and it would miss every 10 seconds or so, not die, but then I drove it back home the 40 miles, and it would miss real bad trying to accelerate softly or normal (didnt try to get on it) untill you get up to about 55 MPH, then it wouldnt be AS bad, but it would still miss some.


I work at a GM dealer, and the truck was sold new to this guy, I guess it has been only serviced by us, but it was had like 4-5 lift pumps, 1-2 Oil Pres units, 1-2 Injection pumps, and it has had the tank dropped and flushed out once before. Is all this common on a 95 with about 130K?


Any help is greatly appreciated, I havnt had any experince with diesels, only worked on the Gas motors.


Im gonna test lift pump pressure monday night, and whatever else you guys can steer me into testing.



EDIT: As I said, im still new to diesels, so Im still looking around the site, Im also gonna check into the PMD, sounds to be right up my problem road.

[a5150nut]

:rolleyes: I'm gonna crawl out on a limb here and. . . .

Try a piece of clear tubing in your fuel RETURN line and see if your getting air bubbles in the fuel. From what you have said about all the work done on this truck I would guess there is an air leak in the suction line.

Try a search for "fuel problems" or "air in fuel" and you will find lots of information here in previous discussions.

I am sure you will hear more from the rest of the gang soon. :D

[1629dill]

I came to the site today to check on almost the same thing. I have a 1999 Suburban that has started to miss and studier when you first start to drive it. Once the temp is up it runs fine. It may threw a miss in now or then. I have replaced the fuel filter and it was great for a day. The next day it was up to its old tricks. As ideal it runs just fine.

If I start looking for air will it show up at idle and where do I out the hose?

[joed]

1629dill:

Was your injection pump replaced under warranty by chance? After the IP was replaced in my 98, it would do the same thing with the on-set of colder weather - stumble until it reached operating temp, then it would run fine. I attributed it to a sticking Fuel Solenoid - my scanner showed the fuel solenoid closure times to be long (over 2ms). I tried a different FSD - still the same. Others have noticed similar issues with the newer built IPs - they seem to be set up this way, right or wrong.

It never seemed to get worse, and I was out of warranty, so I just lived with it.

Joe.

[1629dill]

I added a new FSD cooling system from Kennedy Diesel last spring. It came with a #5 calibration resistor. Could that be part of the problem? The injector pump was replaced about 4 years ago.

[Mark Rinker]

Sounds oddly familiar to the running characteristics we had with a '94 K3500 that developed pinhole leaks in the fuel supply line from rust. We eventually replaced the entire line, after cutting/splicing two winters in a row...

[black_noise]

well the other night I check LP pressure, it would spike to 4 when firing up, then after about 1 second of running it would drop to 3 then over the next 5 seconds it would wither down to almost 0, then it would die, I was also getting bubble to the inlet of the IP, Im not sure if thats cause of a bad connection on the gauge or not.... I dont this it was, but if thats the cause then Im suspecting the fuel filter not being installed right, I will check that and the LP tomorrow, testing for power and flow right at the pump.

[black_noise]

So I hooked up a headlight bulb to the lift pump wires to see if I was getting juice.... according to the manual, it should get power to prime the line, then after running it gets power if the oil pressure sending unit is working and the motor has oil pressure.

I dont get power either way. So the 10 amp fuse underhood also has no power going to it, so that means that the relay is bad and not letting power through? Right?

[DmaxMaverick]

You have a 1995 model. They are unique, in one respect: the fuel pump prime stage. The fuel pump will (or is supposed to) have power to it any time the key is in the "crank" position. You can test this. With the gear range selector in any position other than P or N (set the E-Brake, engine off), turn the key to start. The engine won't crank, but the pump should run. This bypasses the OPS, and allows for a fuel system prime during cranking. The pump should run while the engine is running, once the oil pressure comes up.

Fuel pressure on these is rather subjective. Anything more than 0 PSI is sufficient. A new, optimal pump will produce 4-7 PSI at idle, but should never drop to 0 at any time. Your truck has 3 fuel pumps: Lift pump, fuel transfer pump (IP internal, low pressure, between the inlet and IP rotor), and high pressure (IP internal, high pressure, rotor to the injectors). The IP only needs a constant positive flow fuel supply to operate correctly. The key is positive flow, pressure is irrelevant.

[WelderSki]

Did you check the oil pressure switch? It supplies the fuel lift pump with power.

On my '94 Chevy Sub, I was having stalling problems, I got under to take a look at the frame mounted fuel pump, (OEM is 3-5 psi) in the fuel line only. I think adding a prepump 150 micron filter/water seperator back a foot or so from the pump,(it could also have a fuel heater in its outlet) and a tee fitting after it for a vacuum 0-30 hg gauge, I'm going to change the lift pump to a 7-10 psi one, supplying fuel to the rear of engine mounted, 30 micron filter, and add a fuel pressure gauge to the outlet side of it, or in the fuel line, before the IP. Would give me more info, while in the cab, driving etc. I'm also thinking of raising the engine mounted fuel filter, with a new bracket 2 or 3" higher for ease of changing and giving better access to the oil pressure switch. (testing and replacement)

It's in the shop now having the PMD, replaced and relocated to the intake manifold air box, on an aluminum finned heat sink. I don't think that is the best solution. While we know that engine temps, will shorten the lifespan of the PMD as mounted to the IP, in it's location in the engine valley, it's engineered by Stanadyne, as mounted to the IP, to provide heat to the IP, which likes warm fuel. And provides it to the injectors, now by relocating the PMD to make it function longer, the heat it gives off, is being drained off with the heatsink.

I've given this some thought, about moving the PMD away from high engine tempertures, while still providing heat to the pre-injector pump fuel, by mounting it on a bracket closer to the IP, but off the engine into the air stream, with a different type of heatsink. And may also include a third 10 micron fuel filter/seperator before the IP. Right now it's on the drawing board, (computer screen) and when I've finished it, I'll post the drawings for review and comments. I'm still searching for online wiring diagrams of the IP, oil pressure switch, and aftercooler fan circuits. Anyone know where I can find them, let me know. Thanks !

I hope that helped !

Semper Fi !
Ski

[black_noise]

PROBLEM SOVLED -

After double checking all the wiring to the lift pump, and manually jumping off the battery to see if it works, I replaced the fuel relay, and tought I had it solved, but I think that all the jumping the lift pump manually just primed the line real good. I ran it down the street and it started doing it again.


SO..... More wire checking and found out that I was getting the signal to the cluster for OIL PRESSURE but not to the lift pump.


I know, I know, the OPSU (oil pressure sending unit) is like a killswitch for the LP if the engine it running and doesnt have oil pressure, but I figured that since it and the LP were replace last December, that it was still good. HA, I guess not.


So this time instead of an ACdelco, I went with a lifetime Borg-Warner OPSU, hopefully last longer than a year.



Anyway, thanks guys for any and all imput, I know that all you guys hear is new people coming around with the same problems over and over, but thanks.

[JohnC]

Just for everyone's information, the oil pressure gauge and the lift pump circuit are completely independent of each other, just in the same housing at the oil pressure switch. Also, the purpose of the oil pressure switch is to shut the pump off if the engine stalls, as in a wreck, when the fuel line is severed, not to protect the engine from a loss of oil pressure.

[black_noise]

True... thats why I didnt look at it untill I had looked almost everywhere else first, I always had good reading on the gauge inside, but like you said, 2 seperate circuts in the same sendor.

[1629dill]

Still working on my problem! I have check the fuel pressure at the filter an I have 6 to 6.5 psi. Is that good?