I've been having problems with my truck for about a month now, I've got a 1993 W250, mostly stock except for a few gauges, headlights on relays, auxillary radiator, banks exhaust, and a Viper alarm system, all installed by previous owner(s).

It's got the A518 auto trans, which was inspected shortly after my troubles began and found to be in perfect condition.

The problem I'm having is that on the freeway, once the engine is hot, it will intermittently lose power, like it's shifting out of Overdrive. My boost drops to zero, my speed rapidly declines, and the tach (ISSPro) needle bounces erratically, until I let off of the accerlator pedal completely for several seconds, then something clicks and it behaves normally again. Once the engine's hot the problem is pretty much constant, never fails to show up. Also, my acceleration seems pretty sluggish all the time, regardless of engine temp.

In trying to remedy this, I've replaced the fuel pump with a Cummins OEM model (diaphram type), fuel filter, cleaned or replaced everything from the lift pump back to and including the tank, and replaced the TPS (which I determined to be faulty) with a potentiometer. DOing all this, I've managed to rid myself of the bouncing tach needle, but all the rest of the symptoms are still there.

The crank position sensor is at the correct gap, and I intend to test that, and the PCM tomorrow, but I'm at my wit's end here. Desperate to get my truck back to it's old power and reliability, I'm open to any ideas and suggestions.

 

Oh, also worth noting, my truck used to smoke a lot under hard acceleration, nice dark grey/black smoke, especially when cold. It no longer does this, can't get anything out of it. I think the whole problem started when I ran low on fuel.

 

You mention a few gauges...does it have a pyrometer? If so, what does the pyro do when this starts to happen?

When you say something clicks, do you mean you literally hear a "click" noise and the truck regains normal operation?

 

It does have a pyro, but it's unreliable, have to tap it a few times before the needle moves, I can't say I ever noticed it behaving oddly during my power losses though. The other gauges, which do work, are a boost meter and an oil temp gauge.

Not so much an actual click, it more feels like the transmission shifts or something, and then the whining sound as the turbo begins to spool back up again.

I'm grasping at straws here, but what kind of symptoms would be caused by clogged injectors? I'm wondering if maybe something may have made it past my filter and stuck in the nozzle of one of the injectors, as the truck does seem to exhibit much more vibration than I remember it having, especially during acceleration, and I wonder if that might not be the result of one cylinder not getting enough fuel?

 

Is the turbo a mechanical wastegated model? I'm not overly familiar with the 12-valve engines, but it sounds almost as if the wastegate is opening on it's own...possibly a weak spring?
If it is the turbo, a drop in boost should also result in a spike in EGT. If it's not the wastegate, it could be a bad connection between the turbo and the intake, and it only shows itself after it has warmed up and flexed the boots a little bit.

 

In order to actually clog an injector you'd have to pass something a lot bigger than your filter should EVER let past. Most of the worries with bad filtration are the result of debris causing an injector to not close completely, hang open and dump fuel into the engine, or wear the nozzle tip prematurely...this usually wasn't a concern for the 1st-gen engines, and only became a problem with the 2nd-gen and up.

 

It is mechanically wastegated, but if that were the problem, shouldn;t I see a cloud of black smoke, as the fuel/air mixture goes to far in the fuel direction? Or am I completely off my mark on that?

Also, just noticed that the underside of the turbo, around what I assume to be the oil outlet/drain, is wet, like it's been leaking.

 

You're right in that no air should cause a lot of smoke, but if you're at high enough RPM it might not be as noticeable. You'd definitely see the EGT shoot up though.

I'm not familiar with the first-gen turbos, so maybe someone else will chime in about the leak. Generally speaking though, there's only three things that can leak from a turbo: Oil (internal turbo failure or maybe bad valve guides), fuel (overfueling from the cylinder getting pushed out the exhaust) and coolant (bad head gasket or cracked block draining into the exhaust). None of them are good, but an internal turbo failure is the most likely and also the easiest to fix.

You don't have an exhaust brake do you?

 

No, no exhaust brake. My pyro sensor is tapped in before the turbo, would i still see a surge in egts if my boost zeroed? The spring on the wastegate actuator still seems very strong, and the actuator responds well to an iincreased manifold pressure.

Oh, and merry Christmas.

 

electrical? Like a bad fuel solenoid?
Or maybe pulling air somehow?

 

And a merry Christmas to you too.

Yes, you'd definitely see a big EGT jump if you suddenly lost boost and had the same amount of fueling.

If it's not the turbo, it might be one of the silicone/rubber boots in the airway to the intake manifold. A leak isn't always obvious and may only show when it hits a certain PSI. This causes all the pressure to vent out, which lets the boot seals again, and the turbo begins to rebuild pressure. If you're actually hearing a noise or feeling a mechanical movement, it most likely is NOT a leaky boot though...

Any kinks or weak spots in the fuel lines between the filter and the fuel tank?

 

Well, After carefully examining the rest of my fuel system, I think I may have found the problem...



i guess my question now is, how far am I going to need to disassemble my injection pump to clean this gunk out of there? And will the pump have to be removed from the engine?

 

Wow that is nasty, and yes it probably came from the bottom of the tank. You may be able to pick up a few spare fuel filters and a tank of B10 or B20 that would get it cleaned out nicely if you really don't want to open everything up. I am interested to know the thoughts of more experienced members prior to seeing you try that idea though.