Stumble and Blue Exhaust
 

My 1993 W250 CTD is letting me down after a few years of flawless service…

The only recent changes of note were these:

• I parked on a steep angle with low fuel and ‘ran out’ even with a quarter tank. [tapdshut] I cranked it enough to ventilate the injector lines before I really thought it out, but shut ‘er down.
• I put in five gallons of diesel, and loosened alternate injector lines until I got the prime back

After getting a gulp of new diesel the truck ran happily for a few days, then started to stutter/stagger/stumble near WFO at highway speed. I backed off the throttle through the mountains, and had no further issues, since the roads didn’t require anything approaching half-throttle. The problem returned on the way down the mountains, but just near WFO. With a short trip home and a downhill direction, I motored on home.

Over the next couple of days, the problem came and went, so I went to the parts store and purchased a new fuel filter / water separator. I couldn’t really tell if the contents of the fuel filter were watered down, but it was simple enough to replace the filter and lever-pump the fuel pump to restore prime. It lit right off when I cranked the CTD, and idled rough a bit before settling down. I felt pretty good about it until I revved the motor. [yuk]

I haven’t test-driven it yet, but parked, with no load (hence no boost) to speak of, there’s good, steady idle and good mid-range performance, but at or near WFO, I get stuttering and with the stumbles, puffs of blue smoke out the tailpipe. It would seem the fuel filter was not the problem… any suggestions?

Randii

I searched a bit...

This thread seems to be the closest link:
https://www.dieseltruckresource.com/...d.php?t=113572
...except I'm running the stock governor spring.

If I have a weak lift pump, have I hurt the truck by not shutting it down immediately and driving it a bit starved on fuel and short on timing up top?

Randii

I think the fuel system is sucking air. Pressurize the tank a little to see if any of the connections outside the tank are leaking, it's not uncommon to have leaks where the line connects to the lift pump and the tank due to tired o-rings.

The other thing that happens sometimes is the pick-up tube gets a hole worn in it by a screw in the assembly. If you don't find a leak outside the tank, drop it (or pull the bed) and lift out the pick-up assembly for a thorough examination. What makes me think this is the problem with your truck is the fact that you lost prime with a good amount of fuel still in the tank.

Hey Dave do you think it might be the fuel heater O-ring? I had similar issues with mine and after throwing a new lift pump and fuel filter at it and after I cleaned the entire area with brake fluid I notice that fuel was leaking from above the filter whenever I started it. I replaced the O-ring on the heater and it's been fine since. Randii an easy check is to have someone watch the area above the fuel filter while you start the engine.

Sure, that's another good place to check and easier than dropping the tank. :) It's the losing prime with low-ish fuel level that makes me suspect the tank pickup.

I will check the lines and the fuel heater... but don't make too much of losing the prime, since I'm pretty sure that was just courtesy of low-ish fuel and a significant fore-aft tank tilt.

BTW, no problems with start-up after -- it holds prime nicely since that one off-angle parking job.

Randii

Hi Randii, and welcome to THE website for all things smokey!:cool:

Finally got a chance to look at this... there's no visible leaks at the tank or at the fuel filter...and now cold idle is crummy. It'll stay alive with my foot in it at startup and smooths out a bit once warm.

What are the signs of a lift pump going down? Ive read that the VE pump will pull through a dead lift pump, what're the symptoms of this? Idle is slightly lower, but if I shift lower in the rev range, the truck runs happily down the road (slightly down on power), until I rev it up into boost, and then it starts staggering.

I guess I need to plumb in a fuel pressure gauge -- what's the best way to do this on a first-gen truck? Can I just tee in a gauge or do I need to get the drilled banjo bolt referred to at the bottom of this thread:
https://www.dieseltruckresource.com/...d.php?t=217701

For 50 bucks, I'm thinking of replacing the pump. Nearing 200K, it can't hurt and might help.

Randii

Drilled banjo bolt, there's really no good place to tee.

Throwing a lift pump at it sure won't hurt. :)

I have a new fuel filter, new fuel pump, and new fuel pressure gauge which suggests that there's nothing wrong with the fuel syste, on the supply side of the VE pump. Pressure gets as high as 8 psi when coasting and revving and as low as 6 psi when giving it throttle. Cold idle is just occasionally weak, and California-cold is hardly enough to fire the intake heaters.

If I shift lower in the rev range, the truck runs happily (possibly slightly down on power), until I rev it up near WFO, and then it starts staggering.

I'm open to other suggestions, but I'm getting pretty close to taking it down to Dean at http://www.orangevalediesel.com/ for diagnosis. This is frustrating, since this Cummins is pretty darn simple -- I have a hard time believing it isn't fuel delivery somehow... [dummy]

Randii

The fuel supply is OK according to your gauge readings. It sounds like case pressure in the pump is low. Could be trouble with the vane pump at the front or it could be a stuck pressure control valve. I would pull the pressure control valve on the VE and see if things are gummed up and sticking. It's under the funny looking two sided nut all the way at the front. Jim Lane did a nice writeup on resealing it for the sticky, check there for info.

I found the article...
https://www.dieseltruckresource.com/...15&postcount=9

How does the pressure control valve work, when it works? Is it a sliding interface or a spring/ball valve?

Randii

It's a spring loaded spool valve.

I had to do a little research... my hydraulic theory is a couple decades old!

Cool dialog and pics on spool valves:
http://www.babcox.com/editorial/bf/bf030542.htm
http://www.hydraulicspneumatics.com/...ydraulicValves

Armed with this and Jim's pictures, I guess I'm game to pull the spool valve and eyeball the O-rings. This whole adventure started after running the pump dry of fuel... O-ring damage on a sliding spool ain't exactly a big leap.

Before I do, though, I will be jumper-ing the KSB to see if boosting case pressure helps on the highway... seems like it couldn't hurt for a short trouble-shooting burst.

It runs so **** well in the mid-range that I can't believe timing or injection is hugely flawed, but I wonder about the regulation and/or ultimate compression from the vane pump. Failure in the former is IMHO likelier than failure in the latter.

BTW, a good link on VE pump theory: http://mebonty.monobasin.net/vepump.html

Randii

If the spool valve is stuck, jumpering the KSB may have no effect.