bbearden

So my wife is still having gas mileage issues with her 01 ML 320. We had the kickdown switch on the transmission replaced and the car has 60K miles, so it is doubtful that the plugs are the cause. It has a new air filter and the MAF is operating properly. Additionally, I checked the tire pressure and it is around 35 psi all around. We are still averaging between 10-13 mpg which is a far cry from what other users on this forum are getting. Anyone know of an area that should be checked out? Its at the dealer now and their diagnostics revealed no problems last time, aside from the kickdown switch. Thanks in advance.

MJ50

do u drive mostly freeway or city?
if city, that's about right...

bbearden

Thats about an 80/20 highway/city split so it sounds a bit low to me. I'm hearing others getting 18-20 so I can help but think WTF!

sosh

Do not believe that anyone gets 18-20 with 80% city. The last 6cyl ML I had was a 98, all others were ML500's. However I believe with city driving as I recall I got about 15 and about 21 on the road. With my W164 ML500 I get 13-14 around town and 20 on the road. That is also very similar to the 02 ML500 I had also.

bbearden

Given that she drives 80% highway and 20% city, something is off. There is no way I can even get 15 with 100% highway driving.

sosh

Sorry about that I transposed the hwy/city. If you are getting your mileage figures from the trip computer that could be a problem with something wrong with it. Could be her driving habits also??? There are lots of other sensors that have a bearing on mileage as well and they all should be checked out. Another possibility is the CATs if clogged they could be the problem. Are you using 91 octane?? Lower grade fuels reduce mileage. Possibility of a dragging brake or emer brake??

bbearden

You bring up some good possibilities that could cause this issue. We have been manually calculating the mileage and I told the service tech to check out the cats since that can affect the MAF readings I'm told. We always use 91 octane fuel and to my knowledge the rear brakes are not dragging. I should probably re-attack that last point to be sure. Will let you know what I find.

mihir_d

My mileage had dropped to 12 mpg city/16 mpg freeway....
My mileage now is 17 mpg city/22 mpg freeway....

Between 66K miles and now (101K miles)...these all 3 events have improved mileage...

1. Changed the CATs under emissions warranty at 72K miles...
2. Changed MAF soon after that...
3. Changed spark plugs at 94K miles....

I think, this was the real cause of poor mileage on my ML...

1. MAF went bad around 66K miles....makes the ECU believe, oxygen is leaner than what it really is...ECU floods engine with fuel....result, premature build up on spark plugs...

mileage drops....no other signs like CEL etc...the drop in mileage is puzzling...but I just ignore it in the short term...

2. Coincidently, CATs start going bad around the same mileage...leaking CAT means, ECU is made to believe, the oxygen is richer than what the MAF is reading....that cause ECU to cut back on fuel..result, mileage improves inspite of bad MAF....

So around 66K miles, mileage drops, but just in a month, the mileage is back up for another 3-4 months, until the truck's exhaust starts sounding funny...took it to the dealer...diagnosis...cracked CATs....

3. Once CATs are fixed, mileage drops again....took it back to dealer, dealer now checks MAF and informs, MAF is bad...replaced MAF, mileage improves marginally..likely cause, premature build up on spark plugs has done the damage....

4. Finally at 94K miles, I replace the plugs....mileage jumps leaps and bounds...

In short, CATs, MAF and plugs or a combination of any of them will cause mileage to drop....

bbearden

I do believe that I have a combination of a few of the problems that you had. Thanks for sharing these issues. I specified for the techs to check out the cats tomorrow and I was considering changing the spark plugs myself. As for the MAF, it might not be a bad idea to swap it out but is it possible for a MAF to go bad and Mercedes STAR diagnostics not to pick it up?

mihir_d

They typically do a voltage test on the MAF...the voltage has to be within spec. to call the MAF good. In my case the voltage was just a tad out of spec.

But a voltage on the MAF that is on the higher side within the spec. will contribute to poor mileage..

Instead of swapping the MAF, I suggest, you spray clean it...if you dig this site, I have posted instructions with pictures on how to spray clean the MAF with a special electrical cleaner...

Another thing, my experience with the trip computer has been that it is very accurate in computing mileage...I would rely on it 100%

bbearden

Yes, I have cleaned the MAF. I try and clean them on my Benz's every Qtr with CRC MAF cleaner. Very easy preventative maintenance. Will definitely check what the MAF reading comes to be.

mihir_d

Again, if you dig this site, you will find I have posted how to DIY plugs with pictures..take one advice from me...you will be saving tones of money on labor by DIYing this...don't compromise on the plugs...the OEM plugs at the dealer are about $16/pop = $200 for a V6..suck it up and get the OEM plugs...nothing else...you can get Bosch platinums for $5/pop elsewhere, but you could potentially introduce other mysterious issues that might be difficult to solve later...

Plugs have a life of 100K miles on the ML...so although $200 for plugs hurts (and may even sound outrageous), it only hurts once in 5-6 years...

bbearden

So the dealer calls me up this afternoon and tells me that there are three problems:

1) The cats are clogged up and that they are going to have to replace them

2) The MAF sensor has been determined to be faulty, and

3) As a result of the items above, the spark plugs are are most likely fouled out.

Starmark covers the first two items and I am going to handle the plugs myself. Thanks for all the help in diagnosis. I basically told the service advisor the problem and he he reaffirmed that it was in fact the issue when he called this afternoon.

sosh

From what I understand but not having done it plug replacements are difficult from the standpoint of access and removal of the plug boots and wires. Why not let the dealer do this?? Also, with your mileage the plugs should not have to be replaced yet so why not have all else done and see what happens before changing the plugs?? Remember there are 12 plugs to change and use the oem plugs.

mihir_d

If you read my story above, my MAF went bad first, then the CATs...even after both were fixed, the improvement in mileage was marginal....I am convinced that either or both of them can cause the ECU to flood the engine with excessive fuel (that causes the drop in mileage symptom)...when that happens, you are going to cause premature build up on spark plugs....

There was no one to advice me at that time on what was causing the mileage not to come back to normal after the CATs and MAF were replaced......I thus, just lived with it...finally the real improvement in mileage came when I replaced the spark plugs some 25K miles later....had I known this before, I would have changed the plugs right then....now, looking back...I am able to understand what exactly happened between the 66K - 94K miles that caused my mileage drop...

DIYing the plugs is a bit tricky and labor intensive, but is worth the effort....you are easily saving at least $350 in labor....

BTW. I am surprised to hear Startmark will cover MAF. the MAF is a consumable and usually not even covered under the factory warranty....

sosh

I certainly did read what you posted. However, this guy has only 60K on the vehicle quite a bit less than what you had when you replaced the plugs and his are not due to be replaced untill about 100K. So perhaps it would be best to see if he gets the improvement he is seeking without the plug change. As for his doing it himself we do not know if he is experienced and its certainly safer to have the dealer do it. The downside of an inexperienced person doing this himself is the possibility of cross threading the plugs and damaging a head and/or getting the wires off with the boots without damaging the harness after the plugs and such have been installed for 6 years and 60K. That could be the best $350 he ever spent if he screws up. Oh yes, the MAF is covered by at least the original warranty. I had 2 changed under warranty on the 02 ML500 I had and the problem was discovered by the dealer during a routine service A and service B.

bbearden

I am experienced with spark plugs and this car is a hell of alot easier in terms of space than previous cars I have changed plugs on. I am going to wait and see if the mileage goes up and if not, I will buy the plugs from the dealer. He wants $600 to do the job...ridiculous! I know the procedure for removing the plug boots and can do the job w/o paying the undereducated suckers at the dealer $400 in labor.

As for the MAF, I am surprised that you said it wasn't covered by factory Starmark warranty. I've never heard anyone consider that a consumable or wearable item.

sosh

If you are addressing my post I did not say that the MAF was not covered by warranty, in fact I said the opposite...that it is covered. MAF's are consumables and eventually must be replaced. As for the plugs if you are familiar and have done this you should be fine but keep in mind that you most likely do not have a lift to get the underside plugs and the more difficult ones I am told are the bottom rear ones. Some guys have had to take the inner fenders out to get to them. Good luck, let us know how it works out.

mihir_d

sosh, I replaced my plugs at 94K miles with with 100K mile life in mind, however looking back, I am convinced my plugs went bad at 66K miles..the most likely reason, premature build up on them due to bad CATs and MAF.

No harm in trying out without replacing them first, but it helps to know...if the mileage doesn't come back up (as it didn't in my case), the plugs need to be changed. I wish, there was somebody to advice me at 66K miles about this...I would have changed the plugs right then, instead of waiting till 94K miles..

The MAF is a consumable and I have heard conflicting stories about whether warranty would cover it or not...my local dealer at least refused to cover it...

bbearden

Changed my plugs with NGK for a cost of $2.99/plug. Runs so smooth now and I think my mileage is back up to par. Thanks for all your help. Once you know how to diagnose MAF failures, brake switch failures, and bad CATs, it is pretty easy to determine problems in these cars.

MJ50

did u DIY or go to a shop?
maybe i need to change plugs for better gas mileage?

bbearden

I did the work myself in the driveway and it was very simple, took about 90 minutes to complete. Plenty of working space and is easy for a novice mechanic.

bbearden

After 1/4 tank of city driving there appears to be no marked change in gas mileage, stil around 10-12 MPG! After a new CAT, MAF, and spark plugs it still gets zero improvement in fuel economy. Any body have any ideas? O2 sensors should have popped up on the diagnostic if they were going south, right? Likewise, I think they would have been able to find any bad ignition coils. Could the injectors be dirty or partially obstructed? Doubtful with only 60K miles. Is there a chance that it might take some time for the system to adapt to all the recent changes? Should I reset the ECU? Running out of options here...

mihir_d

You can now keep theorizing what could be wrong....but putting after market plugs certainly has introduced a permanent mystery into the equation...

Don't reset the ECU. The truck has to be put on a dyno to make it re-adapt fully...

sosh

At this point the only questionable things are the NGK plugs. I would put the OEM plugs in it so you can remove this from the puzzle. Do not know why you went this route anyway?? You also said you replaced one CAT, how about the others?? In any case a 1/4 tank around town is really not a true test. How about some highway mileage??