12 mpg on interstate at 70 300TE
 

I've been convinced that the axle ratio in my 300TE is the reason for the rpm's I have observed.

Perhaps I have a different problem here.

Let's assume for a minute given the axle ratio that I could expect 20 mpg in this car cruising at 70 on the interstate. Given a 19 gallon tank less a 2.4 gallon reserve, I'm looking at 16.6 gallons indicated on the fuel guage. At .5 tank I should expect to burn 8.3 gallons before the reserve light indicator lights up.

105 miles/8.3 gallons = 12.6 mi/gal. (observed)

Something is wrong here.

I should reasonably expect 20 mi/gal x 8.3 gallons = 166 miles before triggering reserve.

What other factors might cause this reduction in economy.

Please see my prior post -- "No top gear 300TE" from early this morning

A slipping odometer, a very common problem, will reduce the "appearant" milage considerably! Check the odo against some interstate milage markers before panic!

As to the gears, the TE starts in 2nd unless you floor it and hold the kickdown switch closed, or start off in S or L rather than Drive. Give that a try before thinking you aren't in top gear.

You will be pulling 3300 to 3500 rpm at 70 mph, I think, or something at least higher than 3000. Pretty low gearing.

Actual highway milage should be in the low 20s, mine is. 19 in town :(.

I would also check the mixture, or at least teh tailpipe. If you have black soot around the end of the pipe, you are running rich and need to get that fixed.

Peter

Do I understand you're using the fuel gauge indicator to calculate gas mileage? Can't do it... it's too inaccurate. You have to measure from fillup to fillup, topping off the fuel tank the same way each time. Mileage is best calculated over an average of sequential fillups. The more the better.

I do appreciate your input.

The numbers I listed in the posting were an estimate, either way the estimate using the fuel guage over the past few days shouldn't be off by 40%.

I do track mileage on my 300TD over an extended period tank by tank.

The TE holds 19 gallons of fuel including the reserve. On Thursday evening I filled it up with fuel, it has 210 miles on the trip with the reserve light on. All interstate miles at 70-75 mph

210/(19.0-2.4) = 12.6 mi/gal.

It's been like this for a few weeks now.

I drove the car to Cincinnati and back about a month ago (approx 300 miles on a tank of fuel driving at higher speeds (80-85). I've lost approx 90 miles to the tank even driving at a slower speed (70 mph).

No offense, but I don't need to track it for several tankfuls to see that something is up with the fuel consumption of the car. I am going to start now, tracking the fuel consumption, but have seen no reason to keep the record until now.

You guys have a lot more experience with this model than I do, but it seems obvious that something has changed. It's a symptom I'm sure. One individual suggested a bad O2 sensor or rich mixture.

Does anyone have any other ideas? Would the O2 sensor failure result in an immediate change or would it be noticed over time?
How would the fuel/air mixture get changed without someone changing it?

Just ideas, I am open to all suggestions.

I will begin tracking the fuel consumption on this car more closely now as it is an issue.

Is it unreasonable to expect 20+ MPG from a TE on the interstate?

It was doing better before.

Fuel distributor failure can result in suddenly increase fuel consumption. I've seen this several times, it cannot be fixed by adjustment. Appears to be a result of corrosion of the spring diaphram in the fuel distributor, shows up as incorrect differential pressure.

Another possibility is sticking or stuck piston in the mass air flow sensor, so that the fuel pressure regulation system is "stuck" at high flow (the amount of fuel is determined by the position of the "flap" in the air flow meter, transmitted to the fuel distributor via a plunger that operates a "controlled leak" betweek the upper and lower FD chambers). If the plunger is sticking, it will run unneccessary fuel down the intake on cruise, same as if you had the throttle open further. Can be fixed by replacing the o-rings and cleaning the plunger, or replacing it if scored.

The only other posibility is a failed O2 sensor or computer, allowing very rich running, but this is unlikely due to the nature of the feedback system -- no current flow to the electrohydraulid actuator EHA results in basic mechanical mixture setting without electronic feedback control.

You need to get the car to a shop that can perform the correct diagnostics, anything else is just guessing.

You should get 22mpg at least on the highway.

Peter

Hello,
I have logged a W124 260E(USA) fuel consumption for almost 75,000 miles. On any interstate trip at 70-85mph (when I drive) it does at least 28-30mp/usg with a/c in operation and two person+luggage. In urban usage, 22-24mpg in summer(a/c in use) and 20 mpg in winter. Other drivers in the household always get at least 3 mpg less. IIRC, it has a 3.27 final drive, so it is about 21mph/1000rpm in top gear with stock 195/65x15 tyres.
Just my 2 cents.

Hey Nassar, whats up! :D



Most 103's tend to get between 20-27 on the road depending on ambient temps, loads, grades and speed. 12 is a bit low. Whats the base CO setting like?





Joe

Thought I would find a possible answer to my recent gas milage decrease. I got 295 miles on my last tank.... and had 410 miles on my tank in Fall of '02. Thats a big decrease... but what could be a possible reason? I have a small hole in my rear exhaust... right after where the center and rear mufflers unite. I'm going to replace both lower ball joints soon and get an alignment, but I haven't had the time. (School... and goiung on an over seas break). Oh.. I just thought about the air filter. Mine will need to be changed soon, but how big of an affect does this play on milage? So to sum up my post.. what are possible reasons to my poor milage? Where should I begin to look? Car currently has 189,005 miles.

would affect emissions readings more than economy but it could affect economy somewhat. when I had a hole in my 300SE, the economy improvements were only slight even after dealing with the problem. hopefully it will fix yours

I'm not sure if your 300SEL has the same type of engine & engine controls as my 300TE, but I'll tell you what I've done to my car to restore the fuel economy. I found a couple of vacuum leaks at both ends of the hose leading from the idle-air control valve, replaced that hose and solved low end driveability and idle flakiness. Replaced the OVP relay (not cheap, but very effective). Fuel economy improved significantly upon replacement. Replaced the spark plugs, plug wires, fuel filter. Oxygen sensor is next (this week hopefully).

I hope some of this info helps you. The OVP relay replacement was an expensive, yet instantly effective solution. My TE has 202,000 miles and runs perfect now.

The 300sel has a 3.0 m103 motor

Very likely to be a bad O2 sensor at this age. Won't always set the check engine light, either.

Check the tailpipe -- if it's black and sooty, you are running rich. Check out the O2 sensor and fuel system.

Sadly, a bad fuel distributor can also do this.

Peter