M103 Excessive fuel consumption - PeachParts Mercedes-Benz Forum

alienman · #1 08-09-2009, 06:27 AM

Hi guys,

My indie mechanic recently replaced the water pump and thermostat in my '90 300se, and I've conincidentally started to notice a huge drop in fuel economy. Whereas I used to get around 20MPG, I'm now getting around 12MPG! I've noticed that the engine runs a little cooler than before (80C), and I'm not sure which thermostat he used.

The first thing I thought to check was the duty cycle. I found that the car was running a little too rich, so I adjusted it down to around 50%, fluctuating. I'm going to check the coolant temp sensor and air intake sensor later today, but was wondering if it's possible to get valid duty cycle readings if the coolant temp sensor is faulty? Also, what else can cause such a huge drop in fuel economy?

Thanks for any input you might have!

lee polowczuk · #2 08-09-2009, 12:46 PM

Possible o2 sensor failure... although that's a huge drop ...even for a bad o2

tinypanzer · #3 08-10-2009, 10:55 PM

Check your temperature sensor (not the sender). The sensor is the 4 prong unit located back by cylinder #6.

Use an ohmmeter to check both elements, both should read identically. I have posted the resistance/temp charts here previously as well as a how-to.

If your temp sensor goes, your fuel economy drops radically. Ask me how I know.....

pawoSD · #4 08-11-2009, 12:11 AM

My 300E was doing the same thing until I replaced all the temperature sensors.......now it gets about 20/21 city and 24/26 highway.

wbain5280 · #5 08-11-2009, 01:52 AM

Did it occur to you that the wrong thermostat was installed?

pawoSD · #6 08-11-2009, 02:11 AM

Quote:

Originally Posted by wbain5280

Did it occur to you that the wrong thermostat was installed?

? 80C is about normal....the 12mpg issue does not have anything to do with it running at 80C....it is a sensor issue. The computer is running it too rich.

alienman · #7 08-11-2009, 03:59 AM

Hi guys,

Sorry for the late reply, I've been preparing for a hurricane coming our way (Hawaii) which it appears to be scaled down to a tropical storm now. I didn't get a chance to check the 4-pin coolant temp sensor but will do so tomorrow.

I'd like to check out all the temp sensors and order all the faulty units at the same time to save on shipping but am not sure how many there are. Besides the 4-pin, I know there's an air intake temp sensor. Are there any others, and are their resistance/temp values the same?

Regarding the thermostat, I'm not able to confirm which one was used just yet, as the mechanic is on vacation this week.

Thanks again!

Cal Learner · #8 08-11-2009, 07:01 AM

Quote:

Originally Posted by alienman

...Besides the 4-pin, I know there's an air intake temp sensor. Are there any others, and are their resistance/temp values the same?

Two others:

1) one-pin sensor located at about cyl 5 for temp gauge in instrument cluster P/N 005-542-26-17
2) two-pin sensor located at about cyl 4 for high speed auxiliary fan P/N 008-542-45-17.

AFAIK, only the 4-pin coolant sensor and the intake air sensor (007-542-21-17) use the resistance/temp scale shown in the FSM.

alienman · #9 08-12-2009, 03:20 AM

Hi guys,

I finally had a chance to check out the air intake and coolant temp sensors this evening. The readings were in line with the resistance values posted elsewhere in the forums.

I also re-checked the duty cycle and it was the same as I had set it before (~50%, oscillating) both at idle and at 2500rpm. Vacuum connections and lines checked out ok with the mity-vac.

I swapped out the O2 sensor about a year ago and it appears to still be working according to the multimeter readings. The IACV is functioning and I resoldered the connections in the OVP a year or so ago.

What else could cause such a huge drop in economy? I'd appreciate any ideas on what else I could look into. I'm thinking clogged fuel filter, leaking EHA, injectors, or cold start valve? I'm going to pull the spark plugs tomorrow to see if they have anything to say.

Thanks for any input you might have!

alienman · #10 08-12-2009, 03:35 AM

p.s The car doesn't have cold or hot start issues, though it burps once in a while when idle. Also, no black smoke. The car seems to run fine save for the economy issue. Thanks!

Ara T. · #11 08-12-2009, 04:47 AM

Are you sure the odometer is working 100%?

alienman · #12 08-12-2009, 05:26 AM

Hi Ara,

My odometer stopped working about 6 months ago, and I haven't had a chance to swap the gears out yet. I only use the car to travel to/from work along the same route everyday (about 15 miles round trip). The car used to average 25 trips (18-19 mpg) before the reserve light would glow, but since the water pump was replaced, it's only been able to make about 12 trips (12 mpg) before it would light up. I'd like to try and figure out the problem and fix it myself to save some money, but in the worst case scenario, I'll take the car back to the mechanic to have them (hopefully) figure it out!

pawoSD · #13 08-12-2009, 09:02 AM

Fixing the odometer on these cars is so simple, why does no one just FIX it right when it breaks??

alienman · #14 08-12-2009, 07:48 PM

Hi pawoSD,

While I haven't done it before, I agree that it looks like a pretty easy job to those swap gears. Unfortunately, my situation is that I don't have a garage to work in and have to make do with street parking. I already have the replacement gears and am looking forward to the opportunity to swap them and to also paint the needles while I'm in there.

alienman · #15 08-14-2009, 05:46 AM

Hi guys,

I pulled the plugs today and found that my mechanic replaced them with NGK TR5's on the last tune-up. Other than being the "wrong" type of plugs for the engine, they looked okay to me. I'm going to pick up a set of H9DC0's from the dealership tomorrow and swap them this weekend, though I don't think that's the cause of my loss of fuel economy.