M1123 Low Power

[Hummer Guy]

I'm having this strange issue where I'm barely having any power, especially at 45-55Mph. This happened a month ago and I thought it was my bad brake pads in front that was dragging, I changed them but Im still having the same issue. The strange thing is that everything was working properly until about an hour later, it started to be really low on power, the engine isn't giving any signs of bogging out or running weak, it just feels like I'm pulling a 20k Ibs trailer in the back. I drove the truck again this morning for almost 2 hours and the same thing happened, I started off strong and was able to get to 60mph with no issues whatsoever, about an hour later in driving, it started acting like a boat anchor again. I also notice its throwing out a little black smoke while trying hard to accelerate at 45-55. I pull the fuel gauge sender off and the tank looks like it just have some specs of dirt sitting at the bottom of the tank, I didn't see any gelling in the tank or anything crazy.

 

[Mogman]

Get a laser thermometer and see if the brakes are getting hot, also have you rehabbed the fuel system? clean tank? change the filter? replace the lift pump?
Does fuel run out of the fuel filter water drain when it is running?
Also when you lose power try loosening the fuel cap and see if that makes a difference.

 

[Hummer Guy]

Get a laser thermometer and see if the brakes are getting hot, also have you rehabbed the fuel system? clean tank? change the filter? replace the lift pump?
Does fuel run out of the fuel filter water drain when it is running?
Also when you lose power try loosening the fuel cap and see if that makes a difference.

Yea, I changed the filter and lift pump 5 months ago. I haven't tried draining the valve when its running, but with the truck off, I had diesel come out two different time. I haven't clean the tank because it looked pretty clean inside from the time I changed my filter and lift pump

 

[Hummer Guy]

I put some of the power service additive in it last night, it ran decent this morning when I took it out for a drive, it manage to do 55Mph pretty easy but about 15 minutes later it started to get weak again and only was able to do 443-45. I pulled over and remove the fuel cap and it still felt weak but I was able to hit 47mph this time. My thermometer broke so I wasn't able to check the brake temp but I put the truck in neutral and I was able to push it easily without feeling the brakes holding it. I think this some fuel problem but I could be wrong, it didn't start doing this until about 10 miles after I filled up at walmart

 

[Mogman]

Check to see if fuel runs out of the water drain when running and cold, then try it again when it is running "weak" if there is no flow close the valve immediately as it is likely you will be pulling in air.
It does sound like a fuel restriction issue.

 

[frauhansen]

I had the same thise days when I filled up with dirty diesel. It ran well for a while and then started to die slowly. There was nothing coming out of the water drain either.
Replaced the diesel filter, which was completely clogged, and the problem was solved.

 

[Coug]

Runs good at start, then loses power...

Could be heat related, and the injection pump needing replaced. I'm not as familiar with with this possible issue, but something to keep in mind if everything else checks out good?


As was mentioned before, you can open the fuel filter drain to see if anything comes out. I would make sure the truck is parked somewhere it can sit until you get it fixed when you do this, as a failed lift pump or other fuel restriction can make the system suck in air instead of pushing out fuel, and it's a pain to get restarted if this happens. (mine had a failed lift pump and once the valve was opened it it couldn't be restarted until the lift pump was replaced)

Also, don't assume that a part is fine just because it was replaced a few months ago.

 

[Hummer Guy]

I pulled the fuel filter and it does have a little brown gunk at the bottom of the filter and housing, I was expecting a bit more buildup in it, but we'll see what happens when I get the new filter. The water separator only had very small black debris on it

 

[Mogman]

Checking the water drain as suggested was a way to test if you had lift pump pressure, did you test that???

 

[Hummer Guy]

Checking the water drain as suggested was a way to test if you had lift pump pressure, did you test that???

No, I didn't test that yet, if I still have no power with the new filter then I will give it a try

 

[86humv]

A2 and 11 series trucks have a CAT in exhaust area before short muffler......if all fails, drop the cat out and see if its clogged.

 

[Hummer Guy]

I'm a little impatient, does anyone knows if there's a napa number for the filter or the gasket?

 

[Hummer Guy]

Check to see if fuel runs out of the water drain when running and cold, then try it again when it is running "weak" if there is no flow close the valve immediately as it is likely you will be pulling in air.
It does sound like a fuel restriction issue.

Ok so I cleaned the filter and housing and then injected some of the injector cleaner in the housing to prime it easier. For the first 10-15 minutes, the truck was running better than it ever did, I got up to 65Mph easily and it still had juice to go faster, it ended up getting weak about 10-15 minutes later and return back to barely doing 45-50Mph again.

When I got back home, I opened up the drain valve and fuel was flowing out fast, I'm going to let the truck cool off and see what the filter looks like, seems like I'll have to drain and clean the fuel tank most likely, I was really hoping to avoid the dropping the tank route

 

[frauhansen]

I had the same here... dirt in the tank from am very old source.

That was it looks like out of the diesel filter.

So i drained the tank by using the drain plug at the bottom of the tank. Not removing the tank, just drain.
After that, put 5gal of fresh diesel in it and drained it directly. Put 5gal more in it an drove some miles. Drained it agoin. 5gal more and some miles more, i see clear diesel when drained. So all fo the dirt was drained an in the filter in the front. Cleaned this Filter (see pic above) and problem solved!

 

[TNDRIVER]

Ok so I cleaned the filter and housing and then injected some of the injector cleaner in the housing to prime it easier. For the first 10-15 minutes, the truck was running better than it ever did, I got up to 65Mph easily and it still had juice to go faster, it ended up getting weak about 10-15 minutes later and return back to barely doing 45-50Mph again.

When I got back home, I opened up the drain valve and fuel was flowing out fast, I'm going to let the truck cool off and see what the filter looks like, seems like I'll have to drain and clean the fuel tank most likely, I was really hoping to avoid the dropping the tank route

Pull the fuel level sender out of the tank then insert eyeball. Way easier than pulling the tank. Having said that, the tank will probably have to come out. If the tank is clean replace "all" the rubber fuel lines, remove the one way valves and clean or discard them, "remove" the fuel filter housing from the truck and thoroughly clean it. Anytime you drain the fuel filter drain it into a clear jar so you can see what you have. Eyeballing the stuff squirting onto the ground is difficult for an experienced mech. to discern, useless for an amateur. If you are not acquainted with diesels there is anti-gel and biocide to think about next. The "paper" filter is a consumable, the green water separator, if handled gently can be rinsed and reused, absolutely no holes . These trucks are NOT a buy and drive vehicle. You own tools, right? If not buy tools, lots of tools. My two cents.

 

[Hummer Guy]

I drained the tank and then drained it again with 5 gallons of diesel, once my new filters come in, we'll see what happen, unfortunately, its still horribly weak, but I think the paper inside of the filter is probably clogged up so bad that I can't possibly get it clean. My biggest concern is the injector pump dying, but we'll see

 

[Hummer Guy]

Pull the fuel level sender out of the tank then insert eyeball. Way easier than pulling the tank. Having said that, the tank will probably have to come out. If the tank is clean replace "all" the rubber fuel lines, remove the one way valves and clean or discard them, "remove" the fuel filter housing from the truck and thoroughly clean it. Anytime you drain the fuel filter drain it into a clear jar so you can see what you have. Eyeballing the stuff squirting onto the ground is difficult for an experienced mech. to discern, useless for an amateur. If you are not acquainted with diesels there is anti-gel and biocide to think about next. The "paper" filter is a consumable, the green water separator, if handled gently can be rinsed and reused, absolutely no holes . These trucks are NOT a buy and drive vehicle. You own tools, right? If not buy tools, lots of tools. My two cents.

Yep, did that, the tank doesn't look bad at all but there is a little dirt sitting at the bottom, I manage to get almost all of it out. I also use the power service clear diesel cleaner in the fuel that I drained out, I had the truck as reliable as a buy and drive vehicle, but now it has return back to its govplanet state, one thing breaks and then the next thing breaks

 

[Mogman]

I plum forgot about the check valves as mentioned above, have they been removed?
Most IP issues are it just will not start and it will not start when hot due to a worn parts in the IP, a loss of power is not USUALLY a sign of IP issues.
There is a filter on the input of the IP that can get clogged if the fuel filter is not maintained correctly, it is not meant to be serviced installed on the engine but I would think it would cause issues all the time, it is possible you have a return issue to the tank, the check valve or another problem, if the return is restricted it will cause a loss of power up to and including stalling of the engine.
The fact you had good pressure at the drain valve says you have a good lift pump, the fuel tank pickup is not clogged or that the tank vent has an issue.

 

[Hummer Guy]

I plum forgot about the check valves as mentioned above, have they been removed?
Most IP issues are it just will not start and it will not start when hot due to a worn parts in the IP, a loss of power is not USUALLY a sign of IP issues.
There is a filter on the input of the IP that can get clogged if the fuel filter is not maintained correctly, it is not meant to be serviced installed on the engine but I would think it would cause issues all the time, it is possible you have a return issue to the tank, the check valve or another problem, if the return is restricted it will cause a loss of power up to and including stalling of the engine.
The fact you had good pressure at the drain valve says you have a good lift pump, the fuel tank pickup is not clogged or that the tank vent has an issue.

I didn't remove the check valve, where would that be located? The only reason I didn't consider that is because the truck always start instantly, even after sitting for over a month, the fuel didn't drain back in the tank from the check valve

 

[Mogman]

I didn't remove the check valve, where would that be located? The only reason I didn't consider that is because the truck always start instantly, even after sitting for over a month, the fuel didn't drain back in the tank from the check valve

There are two of them one suction and one return, they are directly in front of the fuel tank in the fuel lines with short pieces of rubber fuel line on each end, you will need some good quality rubber fuel line in 5/16" and 3/8" about a foot or so each.