ML320 loses and gains power - PeachParts Mercedes-Benz Forum

abuff · #1 07-02-2003, 06:48 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?

tabo · #2 07-03-2003, 10:45 AM

Are you bringing it into the dealer? It could be anything! insufficient fuel pressure, air mass sensor (I sell these for 1/2 price off dealer), O2 sensor!.. Thats horrible right after warranty..

coachgeo · #3 07-03-2003, 10:54 AM

I would bet its your Calalytic Converter. Timing is about right for this to possibly happen also. You may find that it is still under warentee cause its possible they warentee that longer than the other parts This happend on my rig. A piece has broken loose in your Cat. it finds its way block off the exaust creating an exahust brake you could say. (back pressure)

if not that there is something in your fuel tank blocking the fuel pick up. It gets sucked up against the pick up at times.

I vote for the Cat problem

abuff · #4 07-03-2003, 11:17 AM

Wouldn't something in the converter alert one of the sensors on those pieces? My computer shows no faults.

On the blockage, your idea and mine were the same. We put a flow meter on it and get full flow throughout the system. On the other hand, we didn't have the meter on it when it failed.

Thanks very much for your help.

Alan

coachgeo · #5 07-03-2003, 11:23 AM

I dont know about sensors picking up on it. Sorry. With just a minimal understanding of the sensors out there I would think no. Sensors are all before the Cat. aren't they.

This did on happen to me on an MB. It was on a 97 Jeep TJ.

abuff · #6 07-03-2003, 11:44 AM

Thanks. I have them checking the converter as we speak. The dealer says that there are multiple sensors in the catalytic converter and that flow should be uniformly restricted if the problem you describe occured. I asked them to check it anyway.

Alan

stevenstevensteven · #7 07-06-2003, 06:59 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.

#8 07-07-2003, 11:05 AM

Had a similar problem on a 1998 ML320 and the culprit turned out to be the catalytic converter. Although the vehicle was out of warranty (63k miles), the converter was replaced free of charge under the federally-mandated 70k mile/7-year emmissions warranty.

In my case, there were several stored codes which indicated ignition failures in individual cylinders. The diagnosis was that the blocked cat was causing back pressure problems in the engine. The engine management system was then interpreting this as ignition failure and consequently shutting down individual cylinders to prevent raw fuel entering the cat. This explained the drop in power.

For information, the ML has four O2 sensors; one on each exhaust manifold before the cat and one after the cat. From my limited knowledge, the first two are used by the engine management system to control the fuel/air mixture. The two after the cat are tell-tales for emmissions monitoring purposes to verify that the cat is performing properly.

Hope this helps.

abuff · #9 07-14-2003, 12:27 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

coachgeo · #10 07-14-2003, 01:10 PM

Glad to hear you got it resolved.

Gilly · #11 07-14-2003, 04:59 PM

Did they do the fuel pump update on it too?

Gilly

need2speed · #12 07-14-2003, 07:15 PM

Quote:

Originally posted by abuff
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.

WOW! The same thing happen to my wife's ML two weeks ago. Uphill in the mountains and the car just bogs down...no power and no revs. Happened on-and-off for a couple of hours.

Took it in to the dealer but there were no fault codes. Just this weekend the "check engine" light went on, even though the hesitation problem hasn't returned.

The car is going back in this week. I'll suggest they examine the fuel pump.

abuff · #13 07-16-2003, 09:55 AM

Yes, the replacement fuel pump was an updated version that supposedly doesn't have the problem.

ApeGeneral · #14 07-24-2003, 04:05 AM

I understand that I need to do the fuel pump mod too sooner or later because I didnt do it during my last service... does the warranty cover it? I have the extended warranty.

abuff · #15 07-24-2003, 08:25 AM

My warranty (extended) covered it, but not until it failed. You'll have to check with the dealer to find out whether preventative measures are covered.