Economy gauge quit

[Jack M]

HI, Restarted my 1986 560 and the economy gauge did not function. Rough idle also. Worked fine and smooth before dinner.

I am assuming a vacuum problem. Where does the vacuum line for the economy gauge come through the firewall and connect to the engine? Or is it under the dash somewhere?

What color or id marking is on it?
Thanks, Jack

[rocky raccoon]

The fault is probably not at the economy gauge or it's vac hose. You do however have a massive vac leak. Based on experience, I would look at the main vac source hose.

Between the intake manifold and the firewall at the approximate center of the engine you will find a small pipe with a right angle rubber fitting attaching to the vac hose. That hose leads along the top of the firewall to the U.S. driver side where it terminates in a three-way connector. One of the hoses in that connector goes to your vac gauge.

The right angle fitting on the intake manifold is in an area of high heat and oil vapor and is usually the first to deteriorate. It is hard to see and hard to reach unless you have tiny hands. The rubber connectors are available from the dealer at low cost or an assortment can be found at most big box auto part stores. Inspect all your connectors under the hood.

[Jack M]

Thank you Rocky. The end of the hose that you described had come off the metal pipe coming out of the main vacuum on the engine. I slipped it back on and everything is 100% I will get a new piece of hose for a tighter fit.

It is always a pleasure to receive such accurate information from all who responded to my questions over the years.

Thanks again, Jack

[WINTS]

My gauge had never worked, and I thought a vac leak was the cause. At this oil change, I asked my mechanic to look over the system, as I couldn't see anything really wrong. Within a minute he quickly popped the dash, and sure enough, the hose had come unattached at the gauge...