"Economy" panel meter

On a 1989 300E, with 103 engine.... I get good position on the "Economy" gauge/meter on the dash.
But occasionally, the meter goes almost to full
RED position (for a mile or two) then jumps back
into the white zone.

What functions/parameters from the engine feed the
computation to operate this "Economy" meter/gauge?

Thanks!

[J.HIDALGO]

Everytime you accelerate the economy needle should go to the red. Basically, as you "exercise" the engine, it produces more vacuum therefore, needle goes to the right/red...
If this happens during normal cruising speed (no sudden acceleration) then, that is another story.

It must be 'another story'. It happens after
a hard deceleration, then the needle 'jumps'
back to left/white and there is a noticible increase in power at same time. Yes, it happens
with no varying the accelerator, level ground.

BnOBOB

[sixto]

Actually, the left/white zone is more vacuum and the right/red zone is less vacuum. The higher the vacuum while cruising, the higher the mpg.

I believe the economy gauge reads manifold vacuum directly, as in there's a vacuum hose that tees off the vacuum hose going to the transmission modulator.

A really uneducated guess would be that a solenoid or switching valve stays open too long then closes.

The manual talks about checking the vacuum hose for oil (that shouldn't be there). Could be that you have a leaking transmission modulator or something like that causing the faulty economy gauge readings.

Sixto
91 300SE

[J.HIDALGO]

Ooops! I had it backwards. The principle is the same, though. You probably have a vacuum leak somewhere...
Thanks Sixto!