Diesel Newbie: P0251 Code, runs rough above 1,800 RPM

[DeepSouthRick]

I'm a diesel newbie who's trying to help his dad diagnose some problems with his 98 GMC Sierra 2500 with the 6.5 Turbo Diesel.

My experience is with gasoline engines, so I'm reading everything I can about the 6.5 and common troubles.

He bought the truck used, and the former owner said the Injector Pump had been replaced. I would presume a new FSD came along with that. The former owner didn't know.

The Problem:

The truck idles perfectly and starts flawlessly. However, at about 1,800 RPM, the engine begins to sputter and miss. If this were a gasoline engine, I would suspect fuel starvation.

Heat doesn't seem to be a factor, as this problem occurs at startup.

I put my OBDII computer on it, and found one code: P0251. Dirty optical sensor, maybe? I cleared the code, and it hasn't returned. The sputtering occurred unchanged as the engine temp rose and settled in at 178. The sputtering occurs whether sitting in park, mild driving, or under hard pulls.

I drove the truck for about five hours one day during this Georgia heatwave, and the sputtering SEEMED to calm after about 30 minutes on the interstate, and get somewhat worse after about 45 minutes of Atlanta city driving (high that day was 104, I think). But it could've been my impatience with Atlanta traffic and the broiling heat. For the most part, the symptoms changed very little througout a wide range of driving conditions.

From what I've learned the past few days, relocating the FSD or adding a heat sink is probably a good idea. But I'm not convinced just yet that the FSD is the problem, because I would've expected to see a more dramatic change througout the day that I drove the truck. I hate to tell my dad, who's retired, to drop $500 on an FSD relocation kit and "hope" that fixes it.

I printed the Diesel Page's "what to do before swapping the FSD" page and am working through that list. But so far, no obvious culprit has emerged.


Rick, west of Atlanta

[HammerWerf]

Rick,

First order of business, Welcome to The Diesel Page. As you have discovered, lots of good reading, good folks, and good humor.

The code P0251 is important. The search function is helpful in cases like this. My 99 Suburban went into (as my Wife described it) running very bad, would not shift, idle was terrible, and I just bearly made it back home...FIX IT.

The terrible running is the ECU recoginized something wrong with the injector pump, set the P0251 code, and did everything it could to convince you not to drive very far. This is Limp Mode. Restart tends to reset that part of the computer.

Here are two threads to read that will give you a flavor of the error. Use the search function, search by description " P0251 " and you should get a bunch of articles to read. Go to the bottom of the 6.5l page and expand the time frame you can search through.

http://www.thedieselpageforums.com/tdpforum/showthread.php?t=25923

http://www.thedieselpageforums.com/tdpforum/showthread.php?t=24732

Check you mileage, could be a warranty replacement.


HammerWerf

[DeepSouthRick]

Thanks for the tips!

Even with my lack of diesel experience, my gut (and experience with OBDII systems) tells me this is an electrical problem, and not a problem with the injectors or the pump.

I never just throw parts at a problem. I like to diagnose it, fix it, and be done with it.

I feel a little responsible for the upkeep of this truck, since I went with my dad when he bought it. I checked it out, even drove it around myself, and everything was fine at the time so I gave him a thumbs up on the purchase.

I'll follow up with the other threads and let you know what I find. It'll probably be a few days before I have a chance to work on his truck again.

For what it's worth, from the descriptions I read, I don't think it's going into "limp" mode. I would expect consistent symptoms with any sort of PCM-induced mode. Switching the truck on and off doesn't change the problem.


Rick, west of Atlanta


Rick,

First order of business, Welcome to The Diesel Page. As you have discovered, lots of good reading, good folks, and good humor.

The code P0251 is important. The search function is helpful in cases like this. My 99 Suburban went into (as my Wife described it) running very bad, would not shift, idle was terrible, and I just bearly made it back home...FIX IT.

The terrible running is the ECU recoginized something wrong with the injector pump, set the P0251 code, and did everything it could to convince you not to drive very far. This is Limp Mode. Restart tends to reset that part of the computer.

Here are two threads to read that will give you a flavor of the error. Use the search function, search by description " P0251 " and you should get a bunch of articles to read. Go to the bottom of the 6.5l page and expand the time frame you can search through.

http://www.thedieselpageforums.com/tdpforum/showthread.php?t=25923

http://www.thedieselpageforums.com/tdpforum/showthread.php?t=24732

Check you mileage, could be a warranty replacement.


HammerWerf

[noelb]

This maybe a red herring but... I remember someone posting with similar sort of problem. What reading do you have for TDCO? Should be between negative 0.5 and say negative 1.9.

I may be right off track but...

[daustin]

Hey there Rick, another fellow GA resident!!! I'm a little north of Atlanta, above Marietta. Your problem sounds like it might be a fuel delivery problem - maybe lift pump. The lift pump is on the frame rail, and the OPS (Oil Pressure Switch) is wired into the circuit. The IP itself may pull enough fuel to run up to a certain point, but can't pull enough once the RPM's get up to feed the beast with enough. I've heard of this several times, might be worth a check. When you turn the key on, the lift pump should run for a little while. If you don't hear it when you turn the key on, have someone turn it on and get under the truck and feel it, it should vibrate a little. If it's not running then, try to jumper it - see if it's the pump or what's controlling it. The OPS keeps it running once the motor fires and there's oil pressure in the motor. Hope that helps ya.
Don

[moondoggie]

Good Day!

If it runs like a gasser with an ignition miss (at high rpm and / or high load), the 1st place to look is the Lift Pump System (NOT the lift pump, the whole system).

On newer trucks like yours, I think the lift pump is powered by a PCM-controlled relay (experts?). That doesn't exclude the OPS (oil pressure switch, which is actually a switch & oil pressure gage sender in one unit) from having the same problems it had on older trucks like my 95s.

You can test the LP system in your driveway; it might be good to have a helper, even a kid, as long as the kid can turn off the engine if / when you start screaming.

Fill a small container partway with diesel fuel. Find the fuel drain valve. (I don't know where it's mounted on yours; on my 95s, it's bolted to a bracket on the [single] thermostat housing, I don't know where they moved it for your dual 'stat engine.) One rubber tube goes under the IM, the other points downward & will (if the valve is opened) squirt fuel on your driveway. Drop the rubber tube that points downward into this container. With the engine idling, open the brass fuel drain valve. If the container starts filling, your LP system is OK; if the fuel stays about the same level or starts going down, close the drain valve RIGHT NOW - your LP system isn't working, at least not during "run" mode. (There's different circuitry that powers the LP during cranking.)

If your LP system isn't OK, you can then test further to see if the LP isn't receiving power (fix) or if the LP itself has failed (replace).

Let us know...

Blessings!

[chambie]

I may be off on this also.......... but my 97 1 ton would run and idle good until it got to 1800 rpm's and then it would miss and load up(it blew a ton of white smoke) acting alot like a fuel filter on a gasser. I took it to the shop and they replaced the PCM and reset the injector timing.
Not sure if this helps, was just a thought.
Lindsay

[DeepSouthRick]

As promised, here's my follow-up:

Like I said, this is with my dad's truck, so it wasn't something I could just run home and do in a day.

Short version: Problem Solved!

I read, read, and read some more, considered all the suggestions on this thread, and also considered my own experience with OBDII systems (my diesel experience hardly counts, since the last diesel systems I worked on were tractors, forklifts, loaders, etc., and that was 25 years ago).

Anyway, after checking everything three times, I knew the problem had to be the PMD. Of course, I hated telling my dad he had to spend that kind of money, but I had ruled out everything else. I suggested he order the SS Diesel Heat-Sync kit, to avoid merely cooking another PMD. However, I could NOT get enough slack on the PMD wiring harness -- not even close -- so he had to order their 8" extention.

The new PMD was the solution, and the heat sink adds a nice trick touch. He said he feels like he has a "hot rod" now. Runs smooth as silk, with all the power he needs.

I can't believe the GM engineers ever thought that burying an electronic module deep in the engine valley was a good idea. Still, it's not as bad as some of the kludge engineering I've seen on S10 pickups.


Of course, the hot rodder in my is always ready to kick in, so now I'm thinking... what else can we do to improve performance? (grin)

Thanks again to everyone who helped!


Rick, west of Atlanta