ML320 Stalling on Long Grades (long)

[markjenn]

I've got a weird issue with my 2000 ML320 (70K miles) and I'd appreciate any suggestions or advice. I like the truck, but if I can't solve this problem, I may have to sell it.

The problem is that the engine will abruptly and completely lose power when climbing long mountain grades. It is exactly like it is running out of gas - it will be humming along at 75 mph one second and totally and completely lose power the next.

It never completely dies; I always am able to limp along at reduced speed with the engine cutting in and out until reaching the top of the grade, where it will regain power and be fine. The problem is fairly intermittent - I've had long trips over mountain passes where the problem didn't occur, but other trips where it has occurred several times. It never happens driving around town or on level terrain. The amount of fuel in the tank doesn't seem to matter. What brand or grade of fuel I use doesn't either. I get no CEL, temps are fine, the engine runs fine otherwise.

The MB dealer has taken a run at the problem, but since it can't be duplicated in the shop, they're just guessing. I have tried to convince them its the fuel pump since this is problematic area on the truck, but they say it checks out fine. The first two times I had the problem, it was very hot, so I thought high ambient temps were a factor. The dealer thought they could intermittently replicate the problem with high underhood temps as a crank position sensor which they replaced, but the problem has since occurred in winter conditions so this was a red herring.

On my most recent trip (in winter temps), the problem was particularly bad in one stretch. A friend with me suggested that we loosen the fuel cap in case it was a tank venting problem. This solved the problem for the remainder of the 1K-mile trip over numerous mountain passes. I've had the problem go into remission several times so I don't consider this definitive. But a tank venting issue would be consistent with the fact that the problem only occurs after a long climb at high power which would pull a lot of fuel over a long period of time - if the tank wasn't venting well, this could start to pull a vacuum in the tank that would prevent the fuel pump from working properly. Having said this, I've never noticed a "whoosh" when opening the gas cap.

Another interesting item is that with the fuel tank cap loose over thousands of miles, I don't get any kind of CEL. Shouldn't I get a CEL with the tank cap loose? This would suggest some kind of problem in the tank venting system.

Final piece of info consistent with a tank venting problem. With the fuel tank gauge bottomed, the truck used to take about 18.5 gallons to fill when new. Recently, I ran the truck to the absolute bottom of the fuel gauge (I was worried I would run out), but I could only get about 16 gallons into it. Either the gauge calibration has changed dramatically or something has happened to drastically reduce the fuel capacity. Could this be the tank collapsing due to lack of venting?

This is a real puzzler.

- Mark