I have an 87 740 GLE that I bought in May. I've been doing some minor work on it (I'm a novice mechanic and learned most of what I know from this forum) to keep it running through the summer. It's been doing will and getting about 25 - 28 mpg, but over the last two weeks it's dropped drastically to somewhere well below 20. I have to commute every day for work so this is becoming an expensive problem. I could use some suggestions on areas to check for problems or ways to improve the efficiency so I don't have to fill my tank each week. Thanks!

 

CTS is the first thing that comes to mind. thats the coolant temperature sensor used by the ECU (fuel injection) and ICU (ignition) computers to tell how warm the engine is... a cold engine needs to run richer. this is NOT the same as the temp sensor hooked up to the dashboard.

Another thing is air leaks in the intake path between the MAF/AMM (mass air flow/ air mass meter, 2 names for same thing) and the intake manifold.

Another thing is a failed fuel pressure regulator. disconnect the vacuum hose off the back of it, and check for any signs of gas there. if the FPR's diaphragm blows, it will squirt gas into the manifold.

 

27-28 is really good! both my 240 and 940 average about 22 when not towing..

like peirce said check real good for vacuum leaks, he little rubber hose ends on the plastic lines are always springing leaks where you cant see em and make sure the MAF is working properly

 

our 87 240 sedan has always gotten about 25mpg around town/county, and 28-30 on long sustained highway runs if you're not leadfooting it.

my 740 turbo is much worse 17-18 if I'm lucky zipping around the county.

 

My 240 gets right around 26mpg with the 5-speed m46. Haven't done a lot of long sustained drives since I put that in it, but I'll know that in the next couple of months driving 150 miles on way to and from school to get home on weekends when I can.

Another thing to check... and this may sound silly, but once upon a time my family had a vehicle that suddenly started getting quite bad mileage. After some investigation, it was discovered that there was a hole in the fuel line. If you car doesn't seem to be suffering otherwise, you might investigate that a bit.

 

yeah, to get the 28-30 number, I've only seen that on multi-hundred mile days where I've gone through full gas tanks entirely on the freeway at the speed limit, like driving from LA to the SFBA.

 

A DRASTIC drop in mileage must be caused by something!

Could be an intake leak making it run too rich; or an ignition miss, or even a brake drag, or even an underinflated tire.

 

also... a 1987 is in the biodegradable eco-junk wiring years (81 or 82 to 87). harness deterioration could cause all sorts of random problems.