Fuel Economy Guage goes into red when engine off
 

I took the car in for service and they replaced the distributor and spark plugs. Now the fuel economy gauge goes to the extreme right (red zone) when I switch off the engine. Prior to the service, it used to go to the extreme left (i.e. no fuel being delivered), which I beleive is normal. The economy guage works as per normal when I am driving.

Is this a problem and what would cause this to happen?

VJ

The fuel economy guage on my 190E measures vacuum in the intake manifold and when the car shuts off the vacuum goes away, so the guage needle goes into the red. For my car I believe that is how it is supposed to work. Jim

The fuel economy guage on my 190E measures vacuum in the intake manifold (more vacuum means less air flow and less fuel usage while less vacuum means the throttle is open more, with more air flow and more fuel usage) and when the car shuts off the vacuum goes away, so the guage needle goes into the red. For my car I believe that is how it is supposed to work. Jim

Thanks Jim for the great explanation. I am ok with your explanation.

Not that it matters now, but I wonder why before it went to the left?

VJ

the needle is supposed to rest on the left side.
take it back.

Joel,

What "puts" it back on the left when the engine shuts off?

Jim

I believe the left is the resting position.

I just went out and checked my car. The needle rests in the red. When you start it, the needle goes too the left at idle and when you give it throttle it goes back towards the red (right). I am going to the local dealer to pick up a temperature sensor for it and will examine any 190's he has on his lot and get back. If I can get any attention from someone in the shop I will ask them. I understand this thing to be a crude (no calibrated numbers) vacuum gage and nothing more. In that case, when there is no vacuum the needle should indicate the throttle is wide open, which is in the red zone. What would push it back to the left? Jim

economy needle
 

I was out snooping around car lots the other day looking at mercedes. Some of them had the needle on the left and some had them on the right. If it works while the car is on, I would not worry about it.

Just got back from the local MB dealer, and saw 5 cars with economy guages (4 were 190's and 1 was a 124 body) and all of them were in the red. I asked the service manager what the deal was and he looked like I asked him to derive Schroedinger's equation. So, by observation they are all in the red at rest with the engine off. If there is a reason to make the guage read differently at rest, please enlighten me. Jim

Make that one 126 body. Jim

So do you think Schroedinger could have predicted the probability of needles waving from the left or right side?

Schroedinger might have thought this was more an exercise for Heisenberg, what with his uncertainty principle and all.

I was perplexed by the vacuum gauge. While I had my instrument cluster out for odometer repair, I did notice that the Eco gauge was peged out in the red. I sucked on the nipple on the back of the gauge and the needle went to the middle region. The gauge measures the amount of vacuum available. A gauge pegged out in the red means you are using almost all available vacuum either due to heavy acceleration or due to leaky vacuum hoses somewhere in your system. Mine hangs out around 3/4 most of the time.

"I sucked on the nipple"

Careful there big boy! Don't want to wake the moderator.;)