To start with the car is a 1990 740 GL non turbo. I bought it 2 days ago and i have been in the garage messing with it since i brought it home. So the car drives fine when the engine is cold, but when it gets warmed up it chokes and loses power above 2000 rpm. The rpm gauge goes erratic and the whole car surges. It is very difficult to decipher if it is air, spark, or fuel. I absolutely cannot figure it out. I ran the codes and it appears its popping for faulty signal to/from air mass meter, and RPM sensor signal absent intermittently. Do the symptoms i described sound like it could be this? i replaced the plugs and wires but i haven't touched the ignition coil, distributor or rotor yet. Thanks in advance for the help.

 

crank position sensors have been known to get erratic when hot, but I'd reset the codes and see if they come back, those could be old codes. I've never heard of a MAF thats thermally sensitive, anyways, they are in the path of fresh air, so they don't tend to get hot.

the ECU (fuel injection control unit) relies on a 'coolant temp sensor' aka CTS to tell it how warm the engine is so it can adjust the mixture accordingly. See here for how to diagnose it.
http://www.swedishbricks.net/700900F...eSensors1.html

 

So the crank position sensor is the same as the rpm sensor?

 

Ive been reading through the forums all day looking for people with similar problems, i don't want you to have to repeat things you probably have a hundred times lol. I am going to try to wiggle the cable around and see if i can tighten it up or anything. Is it tricky to get out and can it be done without removing anything? Have you ever heard of air mass meter, wiring, problems ever effect the way a car drives like that though? I would very much not like to have to spend the money on i new one if i don't have to.

 

Or maybe that could be one in the same correct? doesnt the crank position sensor connect into the air mass meter? or am i way off

 

the crank position sensor is on the back of the engine, near the transmission bell housing, and has nothing to do with the MAF/AMM other than that they are both connected to the ECU (actually, the CPS is connected to the ICU, which passes the signal onto the ECU).

the CPS is an inexpensive part, and not /too/ hard to replace, although its kind of awkward reaching it.

 

I'm going to give replacing this a try. It can't hurt to put a new one in there either way. What do you think about the air mass meter code? It wouldn't let me erase them so i do not think that they are old codes.

 

press the buttton for 2-3 seconds to get the code, then after the code is displayed, hold the button down for at least 10 full seconds to reset that code. repeat for each displayed code. then switch the ignition off, and on again, to verify you just get code 1-1-1.

 

i will do that. I just went out in the garage and now the thing won't start up at all. Only thing i did was feel around behind the engine block for the crank position sensor

 

the codes also will not reset. i did what you said, but after i hold it down for 10 seconds, i let go and the light goes off then comes back on its own for about 5 seconds. and the codes are still there.

 

when it goes out then on again, try pressing it for ANOTHER solid 10+ seconds. I usually stretch that to 12-13 seconds to be safe.

 

Disconnect the battery for 20+ sec, the light will reset. The other way many times doesn't work.

 

Replaced the sensor and she started right up! Thanks for the help guys i really appreciate it. I still have to take her for a test drive and see if the the engine stops surging. Ill post back with the results of the test drive.