Economy guage stuck 1985 500sl

[qsprn]

I have a 1985 500SL with under 80000 miles but not perfect. It always starts fine when cold. Recently when starting warm it acts like it is flooded
and I have to hold the accelerator to the floor. It seemed to be fine after that. Last week however, it started a little harder and the Economy guage on the dash went all the way to the right and stuck there. When idling, it seems to be running a little rough but appears to
run fine when driving. I also noticed a mild tapping when I lift the hood with the engine running. I tried tapping the dash to see if the Economy guauge needle would let loose but no such luck. In the past the needle would always move right when pushing down on the accelerator and return to normal as the rpms leveled off. All the other guages seem to work fine. I assume I have some fuel issue, but am limited mechanically.
Before I take off the air cleaner and do some damage, I would appreciate any help you can give. I may need new spark plugs as well.
Thanks
Bill

[Jack M]

Hi This happened to me a short time ago.

You probably (90%) have a vacuum leak. Meaning that the hose that goes to the vacuum gauge from the engine has come off its connector or some other hose has.
Remove the air cleaner and get out your CSI Flashlight and examine every one of those 25 year old hoses and connectors. their are lots of them.

Age and vibration cause them to come loose. You will find it. Start at the firewall drivers side and take your time.

Good Luck, J

[ejboyd5]

Originally Posted by Jack M

You probably (90%) have a vacuum leak.

I'll move that up to 98%

[SunGuy]

Originally Posted by ejboyd5

I'll move that up to 98%

I agree. Look at the main vacuum line coming off the right/passenger side of the manifold. Use your flashlight, this is hard to see. You should see a vacuum hose, plastic step down tip, and the nipple on the manifold; most likely trouble spot. If connection is solid and no cracks trace the vacuum line accross the firewall, you'll find the leak.

Bob

[Werner W]

This happened to my 85 500SL some time ago. On the driver side under the hood at the corner where the hood meets the fire wall by the brake booster, there is a four or five way rubber vacuum line "T" connection. One of the vacuum tubes / lines came out of the rubber "T" connection. When I plugged the vacuum line back in the economy gauge worked again.

[jplinville]

Originally Posted by SunGuy

I agree. Look at the main vacuum line coming off the right/passenger side of the manifold. Use your flashlight, this is hard to see. You should see a vacuum hose, plastic step down tip, and the nipple on the manifold; most likely trouble spot. If connection is solid and no cracks trace the vacuum line accross the firewall, you'll find the leak.

Bob

My money's on the line at the back of the manifold. I'm willing to bet that the rubber elbow came off. It's a simple job, if you have small hands, and the part will cost around $3.00. My 560 did the same thing when I bought it, and that is what the culprit was.

[qsprn]

Thanks to Jack M, ejboyd5, SunGuy, and jplinville.
You were right on. In fact I had two bad vacuum hoses. One between the
engine and fire wall and one in the front of the engine.
Economy guage works as before. I did get frustrated and enlisted some
help, but the problem is solved thanks to your input.
bill