ML320 loses and gains power - PeachParts Mercedes-Benz Forum

abuff · #1 07-02-2003, 12:01 PM

My 1999 ML320 has developed a peculiar problem at 50,000 miles. It started going up a steep grade at high altitude. All of a sudden, the car slowed, then sped back up, then slowed, etc. I took it to the side of the road, turned off for a few minutes, started back up easily, and began to drive again. Same symptoms.

I had the car towed. The dealer had no ideas as to the problem. There were no indications from the computer and no lights on the dash.

It took a couple of weeks for this to happen again, then a few more weeks before the next occurance. During this time, we drove a lot, including some very steep grades, including the one that started the problem.

Finally, two days ago, it happened again. Desparing of making my destination, I turned around and the problem happened on the down slope! After several stops, I got it to a mechanic who made it fail, but has been entirely unable to diagnose the problem.

Again, it slows down, then speeds back up. Shifting down makes the problem worse. Sometimes it slows down to the point of taking the car to a stop. It never stalled until the mechanic had it, but many times it took a minute at idle to get the rpm's to go back up.

Help! Any ideas?

#2 07-02-2003, 02:44 PM

This purely a guess, but your symptoms sound like fuel vapor lock. Does it run fine when it's cold and only does this after the engine is hot? If not vapor lock then some kind of fuel restriction (partially clogged fuel filter) or the MAF sensor would be suspect.

abuff · #3 07-02-2003, 02:59 PM

Yes, it certainly does sound like a fuel problem. Yes, it only happens when it's hot. Unfortunately, it doesn't happen a lot of times when it's hot, too, so I've (possibly incorrectly) ruled out vapor lock.

On the fuel restriction, we've had it on a fuel meter and get plenty of flow, even under heavy load (accelerating up a steep slope). By the way, it didn't fail then, either.

Thanks very much for your help. Any other ideas?

#4 07-02-2003, 04:57 PM

you might want to move this question over to the ML forum, maybe those guys have experienced this before and can help you. I still think it's fuel related though, vapor lock is a weird thing and is not always a sure thing when the engine gets hot. Weather conditions can play a role with vapor lock as well. Sometimes it will happen and sometimes it won't......

Good Luck

abuff · #5 07-02-2003, 07:54 PM

Thanks. I'll try the ML postings. How would one cure a vapor lock problem?

Alan

#6 07-02-2003, 08:19 PM

I'm not sure that's what it is, and if it is I have no idea what the fix would be. Most vapor lock issues were solved in the early 90's with high pressure fuel systems and fuel injection so on todays vehicles I'm not sure. And you said that your fuel pressure was good so I'm out of ideas. I do hope the ML forum can help you though. I noticed your new to this forum, thanks for joining in on the finest Mercedes Forum on the internet. There are some very knowledgeable people here including several Mercedes Tech's, so enjoy your ML and the Mercedes Shop Forum.......

ejsharp · #7 07-02-2003, 08:45 PM

It sound very much like a problem with your fuel relay. If your car is similar to my 300E regarding the fuel relay... then there is the strong possibility that the circuit board of the relay has hairline cracks where the componets of the board are soldered to it. Te components that normally cause a crack are the heavier miniature electromagnet and contact point assemblies. There are two or three on the board. Look on the back of the board where the pins from these littlle guys poke through the board and soldered. You may see circular cracks around the pin. If so buy new relay or resolder the spot (s).

Because of the weight of the electromagnet, a significant change in the angle of gravity can cause the heavier parts with cracks to shift on the board and by so doing lose contact with the circuit of the board... thus causing your symptoms... intermittant or no fuel pressure.

joe p · #8 07-03-2003, 07:31 AM

Sounds to me like either the intake boot is split or the a/f meter is going south. There also were a few 112 engines who had EGR problems.

Joe

stevenstevensteven · #9 07-06-2003, 07:08 PM

abuff,

Well, I just got home from my July 4th weekend in the mountains and I had exactly the sample problem! As we were rising in altitude, up to 3,000 ft, my car began to loose pwr. We lost pwr intermittantly, on uphill grades, downhill grades and flats. It began shortly after a gasoline refill stop, so I wondered whether it could be bad gas. I added treatments, but the problem continued intermittantly during the 200+ mile drive home. My fuel pumps don't seem to be sucking any air, as I think that I would hear the loud squeal that they make when they grab air. I had'nt thought about the cat converter, my car just passed 152K, and as the 2nd owner I've never replaced it.

Abuff, I'll be watching this thread to see what you find. I'll also post info as soon as I have some. Perhaps we can help each other solve our problems.

Benzmac · #10 07-06-2003, 07:42 PM

The ML had a MAJOR problem with fuel pumps. We have replaced several.

Most of the time they will just quit, but, if you are having an issue where fuel delivery is the most likely candidate, I would have the fuel pump checked promptly.

ejsharp · #11 07-06-2003, 08:33 PM

Have you checked the fuel relay yet? Its a little black box about the size of a pack of cigarettes, with plug-in pins comming out the bottom. Look for circular cracks around the soldered pins ON THE CIRCUIT BOARD located inside the box... particularly where the heavier parts of the board are soldered to it. The bottom will pull out of the box to expose the circuit board. Mine is located behind the battery and behind a removable plastic shield and against the firewall.

stevenstevensteven · #12 07-06-2003, 10:02 PM

I don't have any experience with the Cat Converter. Is there a simple way to check to see if this is the cause of the problem? Anyone want to guess on how much I should expect to pay to replace one of these for my '89 260e.

As far as my fuel relay, I replaced mine about 6 months ago, but I didn't check the board that I plugged it in to. I'll have to take a look.

Thanks.

abuff · #13 07-14-2003, 12:28 PM

I want to thank all of you for your help. I don't think I'd ever have gotten this fixed (unless it died completely) without your help.

After going through all of your suggestions, it turned out to be the fuel pump. Apparently ML320's have a problem with fuel pumps that vapor lock. In my case it was terribly hard to isolate because it only happened after long periods under load.

I've pushed it to the max with the new pump and it hasn't failed yet.

Alan