What does it measure? Where does the needle have to be while in park, while accelerating, and while cruising?

thanks joel

The economy gauge is for those who have no feeling in their right foot. It tells you when you are stepping on the gas. Obviously if you don't step on the gas you will use less fuel. It does help to be moving down the road when using less fuel in order to get good mileage. In other words the reading at idle will be maximum but since you aren't moving you can't get good mileage.

It is a vacuum gauge on most MBs. Later models have calculated mpg figures which are calculated using load values from the engine management system and road speed. On BMWs since the mid 80's their indicator which looked like a vacuum reading was actually similar to the new calculated readings. In other words it was smart enough to know that at high manifold vac readings such as idle/no speed there was no movement so Zero mpg. Its gauge while very high on deceleration (foot off the gas) would go to zero once stopped.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

thanks steve. the reason i posed this question is because there is sprited discussion on the subject in other forums.

the consensus is if needle not pegged minimum at park, it signifies vacuum leak.

my car readings as follows:
Park/Nuetral - 1/8 off minimum
In gear - 1/4 off
In gear,a/c on - 1/2 off

are these acceptable readings? do i have to check for vacuum leaks? where do i start?
thanks, joel

To tell you the truth I have never paid much attention to actual readings. I only pay attention to relative readings.

I am sure that idle in park should give the highest reading. Full throttle would give minimum readings. Any condition that reduces engine speed at constant throttle will reduce the reading. Thus at idle as you add load (trans or A/C) the gauge reduces.

I have a fair idea what the actual vacuum readings would look like at various load conditions but the economy gauges aren't numbered gauges and probably vary from car to car a fair amount.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

joel,

You never stated the car model. If it is an early model with a simple vacuam guage, it should peg in the black zone at idle. If it doesn't, AND you have a rough idle, I would suspect a vacuam leak somewhere.

Good luck,

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Larry Bible
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

larry,

The car is an 1989 260E. Vacuum leak? where do I begin to search? thanks

joel,

I've never had to track down a vacuam leak on my 124 car. I would start looking at all the rubber fittings. These seem to harden and crack with age.

Also, do some searching on earlier posts. There was one a few months ago with some suggestions by Steve Brotherton. If I remember correctly, he talked about a rubber connector that was either under the dash or passing through the firewall.

I would exhaust underhood possibilities before going under the dash.

Good luck,

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Larry Bible
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles