Code 251 1997 Suburban

[dsnyder]

So my new to me 1997 Suburban started missing on my way to work today and then it stalled and would not start for a while. Definitely was acting like the PMD was at fault. However, there are some other factors.

1) I have only had this vehicle for about 250 miles and during that time it has run flawlessly.

2) The fuel gauge was reading about 1/4 tank when things started acting up.

3) The previous owner told me he had recently replaced the PMD and relocated it.

I went to check the PMD and the previous owner had just plugged it into an extension harness and left it loose in the side of the engine compartment bolted to a heat sink that was also loose. No ground. It was very hot.

I unplugged it and got it to cool down a bit and plugged it back in and the vehicle started.

I was about a mile from a gas station and I thought maybe the gauge was reading wrong and I was out of fuel. I put in 10 gallons. It drove the rest of the way to work and then later in the day home with no issues.

When I got home I put the code reader on and got the code 251. I took the good PMD set up out of my now defunct 1994 and installed that correctly with a ground wire in the front bumper.

So now to my questions... I read somewhere that when 6.5 act up at around 1/4 tank it can be fuel line issues... basically fuel starvation because of a clogged return line or something. Would that trigger a code 251? Or, could the ungrounded, hot PMD trigger the code 251?

Finally, I read the 251 can be a bad injection pump. If that is the case, will the pump out of my 1994 work in my 1997?

Thanks in advance for any thoughts. This group is the best!

Dave

[john8662]

251 - Injection Pump Cam System.

Check that your lift pump is working and supplying fuel pressure to the engine when it's running.

Otherwise, the injection pump is pulling fuel from the tank by itself, and any small leaks in the system will suck air worse and cause fuel aeration. Mainly the lines after the fuel pump and the fuel filter housing itself.

Think you're getting air in the fuel.

John

[Kennedy]

The 251 is related to the optical encoder sensor. It sounds like it is time for a new pump. Check out the basics like clean undyed fuel etc, but it is highly likely that a pump is in your future. We've been doing almost exclusively brand new pumps these days. I do trust my rebuilder, but I don't trust most any others out there.

[dsnyder]

Since it crapped out on me Monday, I have had not more issues.

The two things I did were fill the tank and replace the PMD.

When it gets down to 1/4 tank again if it repeats the problem then I think I'll know that air is getting into the system.

Could air in the fuel cause a code 251?

If is does end up being the injector pump, will the one from my 1994 work in the 1997? Both are electronic but is there a difference in the years?

The lift pump comes on with the glow plug light. I hear it come on but when the glow plug light goes off, the sound stops. Once the engine is running I can't tell by sound of course but I assume it runs again when the engine starts. Is that the proper way? Does it turn off until there is oil pressure from the running engine?

Thanks for the answers... hoping it is not the pump.



Dave

[DmaxMaverick]

First, your 94 pump will work on your 97, but not the other way around.

Excessive air in the system can cause a P0251. As John said, it's an optical sensor issue. Air can cause this.

Don't assume the pump is running. Confirm it. If it isn't running while the engine is running, but does during pre-run, suspect the OPS.

[dsnyder]

OPS = Oil pressure sender or Optical Pump Sensor? I'm guessing Oil Pressure Sender.

That's good news on the 1994 pump working on my 1997!!!

Is there some sort of in fuel tank issue where the return line can suck air if the intake is somehow clogged? I vaguely remember reading a post about something like this where when the tank fuel level gets down to 1/4 the return is exposed to in-tank air and because of the clogged intake is sucks fuel from the return.... when 1/4 fuel, it then sucks air?
If the lift pump was not working, and the intake was clogged, the injection pump could suck through the return line. Any logic in what I'm thinking?

Thanks again for helping me!

Dave

[DmaxMaverick]

OPS = Oil Pressure Switch.

The (otherwise working) IP will not pull air through the return line(s), under any circumstance. The return outlet is checked. A plugged fuel inlet line can exploit any slight leak and cause air intrusion by sucking it in, or out-gassing. It IP will ultimately just cavitate and the engine won't run. Blocking the return line at any point will cause failure of the soft lines, and make a mess.

If the engine starts after allowing the PMD (and IP) too cool after a stall, may not indicate the PMD. If you have a spare, known working PMD, switch it immediately. If it starts, it may be the PMD. If it doesn't, the problem is more likely a tired IP. Adding fresh fuel to a low tank will not only cool the fuel, it also increases the fuel viscosity. Either of these can (temporarily) revive a stalled IP that's worn out.

The lone ground wire in the PMD harness, grounds the PMD. It MUST be grounded at the IP. Anywhere else will, sooner or later, cause issues that may indicate a wrong problem.

If the lift pump isn't working, fix it. If you have a fuel supply line leak, fix it. If the PMD is failing, fix it. Fix what you know is not working, then troubleshoot remaining symptoms as necessary.

[Kennedy]

Could air in the fuel cause a code 251?

If is does end up being the injector pump, will the one from my 1994 work in the 1997? Both are electronic but is there a difference in the years?

The lift pump comes on with the glow plug light. I hear it come on but when the glow plug light goes off, the sound stops. Once the engine is running I can't tell by sound of course but I assume it runs again when the engine starts. Is that the proper way? Does it turn off until there is oil pressure from the running engine?



The pumps may swap depending which model pump is on the 94. The only pump that will not interchange with the rest is the #5068.

Check the lift pump operation by opening the water drain valve with the engine running. If fuel comes out it is running. If the engine stalls it is not.

Look up "optic filter harness" These seldom if ever really fail, but I usually jut remove and plug in direct. This is also a good time to check out the electrical connector to the encoder. Typically the lock tab will get brittle and break off. A zip tie will hold the connector seated.

[dsnyder]

Thought I'd give a bit of an update-

I verified that the lift pump is working and I changed the PMD. Since the one stalling occurrence (that set the code) where the gauge showed 1/4 tank of fuel and I added 10 gallons after basically coasting into the gas station, the engine has run flawlessly.

Today, with 1/2 a tank of fuel showing on the gauge I filled the tank.

Here is what is interesting... with 1/2 a tank showing on the gauge, the tank took 36 gallons of diesel.

I'm thinking at 1/4 tank the tank must have simply been empty. The pump sucked in air and the set the code when the vehicle was starving for fuel.

Make sense?