Need a little help with an 88 240 that will run, but is almost undriveable. The car will start relatively well, however the idle is real rough. When power is applied the engine shakes even worse, almost like it is running on less than 4 cylinders. If the RPM can be brought up to around 2500 or 3000 the car will run pretty well, well enough to be driven on the highway. I'm not real good with a voltmeter so I have been following the replace anything you suspect approach. The AMM went out about 2 years ago and the symptoms were similar, but replacing it again did nothing. I also threw in a new coolant temp. sensor since I had one around. I cleaned the throttle body and the IAC as well. I pulled the plugs and aside from a little carbon build up they do not seem too bad, quite a bit of white fluffy powder between the wire connector and the plug itself so I cleaned that up and applied a little connector protecting grease. I drove it around the block today and while driving the door ajar buzzer kept coming on, maybe I have an electical problem? I did check and retighten all the electrical grounds that I could find.

I'm not real sure what to look at next, I am unsure about the condition of the o2 sensor. The car has about 160k and I doubt that it has been replaced in the last 60 thousand miles, could it cause these types of symptoms?

Any suggestions and straight forward diagnostic techniques would be appreciated (don't worry about explaining too much, like I said diagnostic testing is not my best subject.) Thanks for your help in advance.

Greg Normington

A rough idle that smooths out at higher speeds is usually a vacuum leak.

Visually inspect all your vacuum hoses.

If you find no problems visually, use something flamable like engine starting ether or an unlit propane torch. (Be careful!) The engine will smooth out if it sucks something flamable in through the leak. Check for leaks around the intake manifold and at the fuel injectors. Is the brake assist working properly? You can get a vacuum leak if that fails.

The oxygen sensor can cause a poor idle, but generally not so bad that the engine shakes. Usually the only symptom of a failed oxygen sensor is loss of gas mileage.

1st off unhook the battery cable, pull all fuses out of
the fuse holders and scrub the prongs with a wire brush.
Then replace with german glass fuses, Check your wire harness
for any cripy or breaking wires. My 87 did odd things and idled bad
till I changed the harness and converted to glass fuses.