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99 6.5L Stalling out, tried "everything"(?!)

RDasher

RDasher

New Member
Messages
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Location
New York
Hi Folks!
I am a lifelong backyard mechanic without much diesel experience except for a backhoe and this newly acquired 1999 Chevy dump truck w/ 6.5L TD. The truck runs well when it runs, but suddenly starts to lose power, sputter and eventually stall as though it has been starved for fuel.

Indeed, when I attach my pressure gauge to the filter drain line, I observe sometimes as much as 5 psi steady, but then at some seemingly random moment it will drop and even go into negative pressure as it sputters and stalls.

I attached a volt meter back-probed into the center terminal of the OPS wire to Wire Harness plug and observed battery current maintained the entire time the engine is running, which I believe rules out the OPS.

I replaced the lift pump and verified the pressure (more than 10psi) and flow at the entrance to the fuel filter
-removed and cleaned the fuel filter canister.

I have replaced and relocated the PMD/FSD.

In August, I replaced the fuel tank and sending unit, so it would surprise me if there was any trouble there.
Yesterday, after a test drive and limping home of over 2 hours, I decided to test pressure and flow directly at the fuel feed where it enters the filter. I noticed that at some point that line was replaced and although it was in like new condition, it was bent severely as it was too long, so I thought maybe it was kinking. Figuring I had finally found the elusive trouble and a simple fix, I cut off a couple inches and reattached it without the bend.

Started the truck, pressure gauge on the drain line ... 5 psi steady, idled for 10 minutes, 5 psi steady. Drove it around backyard about 6 times performing beautifully then suddenly sputtering and sputtering and stalled. I am at my wits end.

Not sure how the system can go from running with 5 psi in the fuel canister with seemingly full flow from the LP, to a low pressure condition where I believe the injection pump is starving for fuel. I am going back out this morning and try to replicate last night's conditions and if possible watch real time readings of pressure from the filter canister and electric current from the OPS while I am driving.

Again, when it runs, it runs well, so I am hoping the IP is not at fault. Would a failing IP fail intermittently?

Please help if anyone has another idea or believes I missed something!
 
OK, a little update. When I started it today I had a voltmeter on the OPS (Oil Pressure Sender) wire going to the LP (Lift Pump). While running, it always had about 14 volts. I had my pressure gauge attached to the filter drain line and could watch it from the cab. As I increased rpm, the pressure dropped steadily from about 5.5 psi to less than 1 psi and if I held 2500 - 3000 rpm, the pressure would zero out. At first, slowing to idle would return the pressure to around 5, but as it warmed up the pressure returned slower and eventually didn't return, it just hung around 0 - 1 psi at idle. At that point any acceleration caused an immediate plunge into negative pressure. I didn't wait for it to stall out, but I know it will eventually and I don't want to strain the IP any more than it has been. What would change to starve the fuel flow when the engine warms up?
 
I just did a '95 K3500 dump for a customer. It had the same exact symptom where it'd idle all day long but would die as soon as it was up to temp and you tried to put any sort of load on it. It required a full injection pump replacement which we did with our ULSD upgraded pump, however, we didn't jump to that conclusion right away....

We first checked everything from grounds, fuel pressure and volume, PMD connections-wiring-resistor, setting timing on the old pump, and fuel quality....before condemning the pump.

The biggest culprit we run into that is hard to track down is PMD connections. To find them we either jiggle the connections around while cranking it over, or jiggle them around at an idle state to see if it stalls. Depends if you can get it to idle or not. Cheap extensions and worn out pins can wreak havoc. For this reason, every rebuilt pump now gets a new PMD harness and we also re-make the engine harness to swap with all pump jobs. Figure they are good for another 20 years after that.
 
I noticed you said you replaced the tank sending unit. Did you verify it came with the correct strainer sock during install? This is a big issue on this platform with the replacement parts because no matter how good your lift pump is, it can't work if there isn't enough fuel getting to it.
 
Thank you MrMarty51 and quadstar87,
The LP is new, and it was an AC Delco, but I just finished running through the trouble again with my pressure gauge connected to the top bleeder on the filter housing and I'm looking at the LP or supply end again. At idle pressure is strong and steady at about 5.5 or 6 psi, the pump produces over 10 psi on start up while awaiting the glow plugs. As the engine warms and I apply rpm or load the pressure drops to about 3, but soon can't even sustain that and drops negative .. at about -20psi the engine stumbles and stalls. I opened the filter housing and it was about 1/3 full without the filter in place! Air entrainment? Do I have a suction leak? Is something restricting in the new tank / sending setup ... which brings me to that question quadstar87. I didn't know there were different strainer socks. Are they different for gas vs. diesel? that would make sense. Damn, that could explain a few things. looks like alternative fuel source time .. I will test what I can and let you know. Thanks again to both of you
 
ak diesel driver said:
I would be suspecting the OPS.
Click to expand...
Thanks for your reply. The thing is there is 12 - 14 volt current coming off the OPS on the lift pump wire at all times even when the fuel system experiences serious pressure drops and the partial vacuum starts. The OPS never seems to change its operation. Thank you though, I bought a new one and am going to change it just cause .. cheap enough if it works!
 
snarl95 said:
Try your tests without the fuel cap on.
Click to expand...
Thank you for that suggestion. I did try it without the cap, but it has the same trouble. I notice a hiss when I open the cap and I think it is suction, but its hard to tell. Isn't the tank vented anyway?
 
Trying to find a way to connect to the female inverted flare fitting at the sending unit to try running from a 5 gallon can. The rubber o-ring set-up on an inverted flare seems to be asking for a leak and its not easy to find a way to adapt a fuel hose to it properly. Insight on those connections anyone? Thanks! Off to the hardware and parts store ..
 
You are on target now with what I was thinking, seperate fuel can.
Also, run a jumper wire to the liftpump straight from battery for now to eliminate questions from it having proper supply through the ops.

While at hardware store get 6” of 1/4” clear tubing and install as the ip return line that comes out the front of the ip. This is to watch for trail of bubbles as indicating air in line getting to the ip. Leave it there, you hae to replace it every couple years, but is always an instant diagnosis at a glance. I don’t think it is currently sucking air, because you would not see the gauge pull into a vacuum. And you say it is getting 20 inches of vacuum?!?:wideyed:

And you are currently seeing benifit of the fuel pressure gauge, imo the most important missing gauge from factory. Later, once problems are solved, run a tap on the line between ffm and ip, and feed a gauge in the cab- electric or manual doesn’t matter, but mount it permanent. Loss of LP function hurns up ip and noise from LP doesn’t mean it is working.

My money is on line restriction.
 
Leroy diesel has the fittings that screws on the sending unit and has a 3/8 barb fitting on the other end. That’ll be what you need. I could be wrong but I think the threads on the sending unit are metric threads just in case you go looking for something at the hardware store. If you can get it hooked up like you are talking about then that should tell you if it’s the tank sock or not.
 
Had to come back to this. The vacuum thing and warm engine is in my head. Stay on corse with current plan, just thinking future check before condemning ip.

There is a pump part of the IP that will suck fuel from the tank when the LP is dead. If that is working cold, but is loosing it’s pull when warmed up that could be the ip dying because of extended lack of supply from the LP (the whole why everyone should have a fuel pressure gauge on the dash thing) However... if you pour cold water over it while it’s warming up to the point that it starts to have the problem that should temporarily correct it to verify that is the problem.

However you should still have pressure in the line supplying the IP regardless of what that’s doing inside the pump, I THINK- GUESS- this is where we need a pump builder for the knowledge. Is it possible that part of the pump wear out and just cause massive bypassing from inlet to fuel return line? You could test that by measure volume of fuel coming out the return line at idle before the problem occurs, then measure it again while the problem is occurring. You would have to have a large increase in volume to show that is the problem. Cant imagine It increasing flow enough to draw that other fuel line down into a vacuum.

Seems to me there are two times when 6.5 owners decide to get rid of the truck. Unit needs a new IP, or when they blew the bottom of the engine out because of a bad harmonic balancer. If you need a new IP and plan on keeping the truck regardless, then even before a new IP would go on, I would suggest an LP upgrade first. The very best factory style LP is the AC Delco EP158. But IMO, it barely does the job for DB2, and when brand new barely hits the specs for the DS4 and Falls below that within a few months of operation. Get a pump on there that’s worth a damn before condemning the IP. I still think there’s a restriction somewhere- maybe even the new LP messing up?
 
Word from someone who has gone through what you are going through. COMPLETELY REPLACE THE HOSE THAT HAD THE KINK IN IT!!! I fought fuel starvation in my 1995 for years before locating a biodiesel softened hose that would kink completely off only when warmed up. You can't cut a kinked hose and expect the kink to unkink - the hose is damaged and restrictive.

You are following the correct troubleshooting and frankly with a vacuum showing up you are simply F%^&d. Something somewhere is completely clogging and the IP pump is pulling a complete vacuum that will boil diesel: aka diesel vapor lock. Fuel gelling is a possibility: loose the tank sock completely and use a heated filter before the lift pump. A heated water separator is the perfect tool for that job.

Continue inspecting the complete fuel system for any damage like a rock hitting a metal line, kinked or swollen hoses, bad heater in the fuel filter can...

What does your fuel look and smell like? Gasoline? Cloudy? Bugs in it? Water? Water can plug the tank sock.

http://www.thetruckstop.us/forum/th...vacuum-does-diesel-boil-at.35733/#post-410858

More info on bugs:
http://www.thetruckstop.us/forum/th...diesel-this-can-affect-you.35096/#post-470573
 
OK folks, Finally got it! Thanks to one and all for the suggestions. This truck has had a myriad of fuel issues since we bought it, so I've certainly gotten a crash course (ip return line leak, bad PMD, leaking fuel tank, faulty lift pump). So, when I changed the little piece of fuel line connecting the hard line from the LP to the filter the pressure would hold above 5 psi even when I accelerated hard at a stand still, so that solved one major trouble (Thanks WarWagon for pointing out it was still kinked and soft!). However, the fuel line pressure would still drop and pull vacuum after a half mile trip of bouncing and moving around! Interestingly, all the fuel I have taken is perfectly clean and free of water or anything (brand new tank). I found the right fittings and connected an alternate tank before the LP and sure enough it ran flawlessly with fuel line pressure never dropping below 5 psi. So I dropped the tank finally and pulled the sending unit and I do believe that sock is for gasoline (thank you quadstar87 and Jaryd). It sure as hell is not like OE at all. I still have the old tank so I pulled that sender just to see and that OE sock had a hole in the screen and had a fair bit of grit inside it. Spectra called that sending unit an exact fit for this truck, so I wrote them a note about the sock! I siphoned fuel from the bottom of the tank and used the tube like a vacuum to be sure I would get contaminants if they were there, but it was perfectly clean fuel, so I have temporarily just removed the sock, re-installed the tank and it runs perfectly now. So, I need a pre-LP filter/heater (WarWagon, thanks for that suggestion as well). Any one you particularly prefer? Anyway, I will keep an eye on new posts in case I can ever assist anyone here. Thank you all again!
 
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