What you all think the cause may be?
 

My '87 300E has been running very rough lately. It starts right up, every time. However, it acts like it is not running in all cylinders. I notice the power is substantially weaker, and it sucks down gas like there is no tomorrow. It takes a quarter tank to drive 25 miles!! The gas econometer, when idle, stays in the middle. Acts as if I am flooring the gas pedal. The only time the meter goes down is when I am coasting or when sitting at a light, throw the car in neutral.

Most time the car acts like it is going to die when sitting at a light or stop, but it never does. It just coughs and sputters.

I suspect some kind of vacuum leak, or a badly needed tune up. I don't want to throw money in it unless I know at least it may help the problem. The car has a little over 300K on the clock.

If it is vacuum related, how do I know? Same question on the tune up. Is there a way to check if I need a tune up?

Anyone has any idea, or it ready for the heap yard?

Since the economy gauge is simply a mechanical manifold pressure gauge, I would suspect a vacuum leak. A vacuum leak would cause a lean running condition.

I had similar symptoms. I was getting about 125 miles per tank with powerloss. Also, the economy gauge sat at almost the middle (slightly to the left). I poked around a little and then tested my O2 sensor - it was bad. I changed it and now get a little over 400 miles to the tank. Still working on the economy gauge problem as I occationaly still get a hiccup when accelerating.

tune up
 

1. change your fuel filter
2. check you distributor cap and rotor
3. with the car running in the dark open your hood and see if anything is badly arching, if it is you need to replace that spark plug wire and or spark plugs.
4. check your air filter
5. check all vacuum lines carefully or just listen for a vacuum leak.
6. replace your spark plugs they are relatively cheap, cheaper then a tank of gas.....