Took my 1989 240DL in because check engine light came on. Mechanic ran test on fault codes. He said that if all the codes were real than my car wouldn't be running at all. Car is running perfect. Just the light is on. He replaced the main relay. Two days later the check engine light came back on. Car is still running fine. Here are the codes it comes up with. Does anyone think this is an Air mass meter problem, even though the car runs so well? Also what is an Air heater hose. There is this accordion looking hose ( approx 3in dia )that the mechanic said doesn't need to be connected?

Fault codes in the EMC:
223 Idle valve signal missing
312 Knock enrichment from ignition module missing
131 RPM signal from the ignition module missing
121 Air mass meter signal missing
212 Oxygen sensor signal missing or erratic

Fault codes in the ignition module:
214 RPM signal missing

I see no reason to suspect the Mass Air flow meter over anything else indicated. If they were valid codes it would not run. Possible a momentary glitch in the electrical system such as bad power connection or ground somewhere caused the codes to set. A good place to start checking is the grounds at the base of the columns where the door hinges are. Remove the cover and they will be visible.

No need to run to the mechanic when light comes on. Learn how to read the codes and clear them so that you can monitor the situation. It's all in the FAQ section.
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David Hunter

You may have some bad ground connections so make sure all grounds on the engine are clean and tight.

If the 3" hose is the preheat hose from the exhaust manifold to the air box you don't really need it unless there is a visual emissions inspection in your state.

Dan

On the heater hose (the 3" diameter hose that runs from the exhaust to the intake. YES. Take it off then melt it down and make something useful out of it.

Andrew