bbearden

True, the plugs were chosen as this is what Mercedes recommends in some of its newer models and I have had nothing but good experiences with them in other vehicles in the past. Bosch is such a source of problems with Mercedes that I did not want to buy their plugs. After a half tank of mostly highway driving, I averaged nearly 150 miles so 150/11 gallons = 13.6 which is better but still not quite what I had expected to get. You are right about the other CAT, maybe I need to ensure it is not clogged or leaking.

sosh

Do not know who suggested NGK plugs but I have never heard of them being recommended. Not they they are lousy plugs but OEM is and was the plug of choice. I have never had an issue with them in any of the 6 MB's I have owned. In addition I have never had an issue with any Bosch product in any of my vehicles. Let us know what you discover with this contiuing saga.

bbearden

I was reading in the forums and it was mentioned that Mercedes now recommended their use in the newer GL class SUVs. I did not confirm this but I do know that the OEM Bosch plugs I pulled out were running fine with a nice light tan coloring, granted some of their electrode had worn down. There was no indication that they were not firing properly so my intuition is telling to rule out plugs from the equation. I can't imagine O2 sensors not tripping the CEL or throwing a code at the dealer.

In terms of Bosch part, a number of parts are made by Bosch in our cars. The MAF, knock sensors, Crank/Cam Position Sensor, brake stop lamp, O2..... A number of the sensors that plague our cars with trouble are made by Bosch. I'm not saying their plugs are necessarily bad, but I don't think they make quality products given the failure rates for the other components in our cars.

Until I isolate the problem, I guess I will enjoy 13.6 MPG (as compared to 18 MPG for the SLR!). Gotta love it when the dealer cannot be trusted to find the problem. Happy driving!

sosh

I disagree with you about Bosch products but here are two other thoughts. Is the heat range of the plugs to OEM specs. I remember years ago with motorcycles that the NGK heat ranges do not match those of the Euro made plugs even though they claim compatibility. In addition, does your fuel where you are contain the 10% ethanol like ours does down here? That will have a detrimental effect on mileage.

bbearden

Thats a good point. At the parts store they were listed as compatible but I cannot full guarantee the heat range is equal to the OEM plugs. 10% ethanol is used in the fuel here as well and you're right as that would affect mileage to a degree. Thanks for the insight. Its always welcome to get an accurate diagnosis.

Radomir

It's just an engine and just plugs we're talking about... you're not troubleshooting a nuclear reactor by putting wrong plutonium rods into it

Time to step back.. if those plugs didn't help.. then it wasn't plugs to begin with.. and spending $200 on "OEM plugs" (you can get 'em elsewhere for less) isn't gonna help...

If you're getting 13mpg on highway in a V6 320, something's obviously wrong and since you replaced MAF, CAT etc.. it's probably elsewhere (ECU??)... has anyone hooked it up to a diagnostics computer and read ECU parameters? You may be better served by posting on the mb shop forum (not here)... that's where some mechs hang out

just to add -- hot/cold plugs won't bring 20mpg down to 13.. so don't worry about that just yet.. just make sure they're properly gapped.. if they are and it didn't help then it wasn't plugs..

good luck.

bbearden

Agreed! I really have narrowed it down to only a few possible alternatives at this point: O2 sensors, the other CAT, or the ECU. Those are the only things to my knowledge that could possibly cause such a major loss in economy. I checked the plugs and they are burning nice and tan with the factory 0.039 gap that I checked myself.

Pottie

The Cat, New Plugs, New Air Filter it does change significant on Gas Mileage
I have 00 ML430 and now is running about 16mpg in the city...and from road trip in freeway I m getting easy 18mpg...
This about 240miles for 15GL and is about 270miles for 15GL..off course normal driving not sprint too many times...cz V8 engine not easy to stop....for V8 with that gas mileage I wont complaint too much..

bbearden

Nice, yeah my fuel economy is up to. My fuel gauge was faulty as well and giving me incorrect readings which certainly didn't help the situation. Fixed for now and enjoying around 18-19 MPG on the hwy.

dfallen2015

So I do not know if anyone will read this, but here is something very, very, very interesting.....I bought an 01 ML320....Lots of problems. I have been fixing things on this slowly....However, about 4 months ago I finally got this baby ready...besides the whole suspension thing with the torsion bar this baby purrs and runs like new, actually better...let me explain....Changed the MAF. Replaced the spark plugs with OEM Bosch. Fixed a small leak, flushed and cleaned the transmission out. I change my oil every 3 thousand miles myself. I replaced a throttle positioning sensor. I am not a fast driver, but I do about 75 80 on the highway. I do a lot of highway driving....I record my millage very carefully, then always fill the tank and do the math to get my MPG. My average in the city is about 17.4 to 17.8. On the highway, I average 24, and like today after 179 miles I pulled off 26.8. And this is consistent. I drove to San Fransisco and averaged about 345 miles and decided I better fill it, it took just under 14 gallons, so I was just at about 25MPG. I don't get it I also own and drive a 99 Lexus rx300 and if I get 17 to 18 on the highway I am lucky. I had it smogged a few months ago and the place said I was far below the threshold. This baby has 219,000 miles and seems to want to keep going. I had the suspension, replaced all four with KYB's rear struts are finally loosening up after like 20 thousand miles. but the front, so hard to get that torsion bar just right.....and the Four-wheel drive works so well, I dragged a 1953 ranchero out of 7 inches of mud like nothing....so much power... just thought I would share my positive experience....I mean there are problems, but nothing that affects me driving and enjoying the first vehicle that only requires a few tools to work on.......I know a lot of people hate these cars, I wish they would make them like this still....so solid...so easy to work on....

Mt. T

I do 24MPG on highway, at less than 100KM/h.

ML320 2001, new plugs, MAF and TBD cleaned. Stock tire size, look at the recommended pressure in your driver door.

Mt. T

These V6s are incredible, considering they came out in 1998. Let's not forget they are 5000lbs trucks too.

Holy5417

It has the pressure regulator on it. It's VERY over looked since it's kinda hidden.