GMC Duramax Ambulance

I usually post on the 6.5 boards and dont know that much about the duramax other than the normal diesel stuff. A friend of mine has an ambulance company and one of the ambulances is a gmc duramax. Im not sure of the year so Ill throw out the problems its having: Sometimes when running it loses power and will only run about 30 mph. It only happens when the fuel is under half a tank. They can stop and sit for a few minutes and it will run fine then repeat some miles down the road. The shop has swapped out the turbo (no idea why imo),cleaned the fuel tank,replaced the fuel pumps, and maybe thats it. To me it sounds like a fuel delivery problem but they have changed the filters and pumps. Any ideas?

Would be helpful to know what codes are being set. The symptoms you're reporting could be due to a fueling issue or even a transmission issue.

When was the fuel filter last replaced?

Jim

30 mph could be a bad thing for an ambulance!!! What year is the cab/chassis this is built on? What is the 8th digit of the VIN?


The Duramax ECM has a 'limp' function that is usually associated with codes, setting the SES light.

Does the 30mph limit back to normal function come and go like a light switch? If so, its probably a limp code, and they have cycled the key a few times, resetting it.

I'd have them dig deeper for codes, and ask if anyone has installed an aftermarket programmer or power tune. If so, have it removed - with the weight of that work body, and a heavy foot, they are probably limping it with transmission slippage protection.

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Originally Posted by BamaChief600

I usually post on the 6.5 boards and dont know that much about the duramax other than the normal diesel stuff. A friend of mine has an ambulance company and one of the ambulances is a gmc duramax. Im not sure of the year so Ill throw out the problems its having: Sometimes when running it loses power and will only run about 30 mph. It only happens when the fuel is under half a tank. They can stop and sit for a few minutes and it will run fine then repeat some miles down the road. The shop has swapped out the turbo (no idea why imo),cleaned the fuel tank,replaced the fuel pumps, and maybe thats it. To me it sounds like a fuel delivery problem but they have changed the filters and pumps. Any ideas?

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I have been in contact with an Ambulance Company with same complaint, and I also think from GA. Last I heard they were installing my twin lift pumps which should correct the problem. If this is not the same Company then we'll have to look to getting them set up provided the fix works.

One of the NEW fuel pumps was bad. All is well....thanks folks.

What fuel pump? The HP fuel pump, in the engine valley? Aftermarket lift pump(s)? What year? Is this a chassis/cab, or a van (van has an electric primer pump)? Inquiring minds want to know.

2004 and I think it was aftermarket. They do there own work. I believe it was just the lift pump not the IP.

Chassis cabs and some special applications use two fuel tanks. There is a factory ECM-controlled electric fuel transfer pump between the two that works to maintain fuel levels. There have been a few problems reported by owners in the past, but not recently.

Jim

I made a follow up call to the shop that did the work on the Ambulance that I worked with. My twin lift pump setup fixed the issue on both units.

Cool - what was the problem?

My '04 and '06 both have dual tanks, the smaller rear tank added by the upfitter when the beds were added. No issues - the factory gauges are even suprisingly accurate as well with normal, smooth decline/readings from 'F' to 'E', even though the smaller rear add-on tank drains completely, first.

Sometimes after the trucks are shut down, I can hear what sounds like an electric pump working for a few seconds, but have never investigated. Maybe the upfitter added a lift pump - I'll have to look someday when they are up on the rack.

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Originally Posted by Mark Rinker

Cool - what was the problem?

My '04 and '06 both have dual tanks, the smaller rear tank added by the upfitter when the beds were added. No issues - the factory gauges are even suprisingly accurate as well with normal, smooth decline/readings from 'F' to 'E', even though the smaller rear add-on tank drains completely, first.

Sometimes after the trucks are shut down, I can hear what sounds like an electric pump working for a few seconds, but have never investigated. Maybe the upfitter added a lift pump - I'll have to look someday when they are up on the rack.

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That's the transfer pump. Your engine is supplied fuel only from the main tank. The reason your rear tank empties first, is the fuel management pumps from that tank to the main, as fuel is burned, until it's empty. The pump you hear is a transfer cycle in progress. Still, no fuel lift pump.

...got it, thanks!

How does the gauge stay so accurate throughout the range of 'F' to 'E'? With your description, it would seem that the needle would sit on 'F', or at least at some point in the range, until the rear tank was completely empty, then return to normal function?

(Also...does the refridgerator light REALLY go off when you close the door? :D )

I'm not sure about either of those questions. If I were to venture a guess, because the PCM is in the loop, it averages the 2 sender inputs. Lucky you....with an accurate gage, and all. GM never got it right with one tank, and one sender, let alone 2. Perhaps the two cancel out the inaccuracy.

The refrigerator is an age-old mystery. I remember seeing a 1950's "modern household technology" cartoon (spoof), and one was a refrigerator with a little door in the big door, just so you can watch a little guy run out and flip off the light as soon as you close the door. It's amazing what sticks in our heads after all these years. Can't remember what I had for breakfast, but stuff like this is permanently embedded.

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Originally Posted by DmaxMaverick

The refrigerator is an age-old mystery.

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My cat knows, but he isn't talking!

And no, I did not put him in there... Little rascal crawled in there of his own accord when my back was turned to put stuff on the counter. Spun back around and shut the door... Fortunately, I was back in there in a few minutes and a somewhat unsettled grey streak headed for the dining room... :p