I recently bought a 1985 240 DL for my daughter. The car has 139 k on it. It idles rough and appears ot be running rich( if you look close you can see black smoke coming out of the exhaust. I found old paperwork in the car from a preivous mechanic that said it needed a new mass air sensor to fix the problem then. I don't know if this is the problem or not. Any help would be greatly appreciated.

You'll want to buy a Bentley manual.

If you unplug the air mass meter and it improves the situation, that's a good quick diagnosis that points towards getting a new one. There is a diagnostic for the AMM that you can do with a voltmeter but I don't know it off the top of my head. It's in the Bentley.

Another common rough idle culprit is a dirty throttle body. If you're not sure that it's the AMM and want to try other, cheaper things first, clean the throttle body. All you'll need for that is a new throttle body gasket, some carb cleaner, a clean rag, and maybe some pipe cleaners.

If it ends up being a bad air mass meter shortly after having had it replaced, you'll want to check into the air intake flapper valve that lives in your air filter housing. Unfortunately the failure mode has that valve all the way on hot, which will fry a new air mass meter pretty quickly.

A bad oxygen sensor can also make you run rough and rich. Try to find the wire coming up from the exhaust to the firewall in front of the passenger. Unplug it. If the car runs better with the 02 sensor unplugged, I would suspect that. A generic one (which needs to have the wires spliced onto your existing connector) is $40 or $50. One that plugs right in is closer to $100.
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Andy in St. Paul, '91 745 Regina 198K mi, '89 244 142K, '87 245 RIP

Thread reading. How do you actually perform the oxygen sensor. DO you unplug it while the car is running? Or do you unplug it then start the car? Also in this post I read about testing the air meter. Is this done while the car is running?

Having rough idle on a 90 245 230F auto.
Also could having a little to much oil (reading just over max) cause rough idle?

Scott SC
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Scott sc

It sounds like a faulty Fuel Pressure Regulator. Pull off the vacuum hose to it and smell for gas the diaphram may be broken. Only about $40 and very easy to replace so you might just start out there. If this is it change the oil as it may be contaninated with gas.
If you haven't done it yet clean the throttle body, IAC valve and flame trap. Spark plugs may need replacing.
Dan

Butting in with a rough idle question. I have a 90' 245. 230F auto with about 170K. I have thus far cleaned the throttle body, change the plugs, wires, cap, rotor, flame trap and hoses. Figured it must be a vac leak some where. I have looked in the Bently for vac hoses but did not find a section on this.

The lower flame trap hose was split at the top and I also found that there was some build up of debris in the tube where the lower hose fits to the engine. This debris must have been bulding up because of that split hose. I did not clean this out did not want the debris (dust and oil)to fall back into the oil pan, could this cause a rough idle problem and would it be a problem if this trash made it to the oil pan?

Scott SC
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Scott sc

It sounds just like a bad fuel pressure regulator. I went thru the AMM and wasted time and money on that. It was $25 FPR and now it runs like a champ. The black smoke was really bad, it always ran and started.