I replaced the fuel pressure regulator on my 1991 Volvo 240 because the engine was idling poorly and there was fuel present in the fuel regulator vacuum line. I also disconnected the MAF (before I discovered the fuel in the vacuum line), but found that this made no difference in the idle, so assumed the MAF was not at fault. With the new fuel regulator in place the car will not idle at all, and runs poorly, except at high rpms. However, if I now disconnect the MAF it runs a lot better, still not perfect, but the fuel economy is miserable. I assume the MAF is faulty as well, but why would it make no difference to disconnect it with the old, faulty fuel pressure regulator in place?. Is it possible that I damaged the MAF by disconnecting it without turning off the engine? If I replace the MAF do I have do disconnect the battery in order to clear the control unit? Thanks in advance for any advice.

 

a 91 is LH2.4 and has the ODB module under the hood, doesn't it? read your fault codes. (note, this is *NOT* OBD-II like 96+ cars, you don't need a code reader).

see the articles linked here... Engine and OBD Diagnostic Codes

they are talking about 700/900 but a 91 240 has pretty much the same engine as a 91 non-turbo 740/940.

 

Technically, it is not a good thing to disconnect or connect the AMM while running. In practice, at the shop, we will disconnect it while running but not reconnect it. Any time you replace sensors, it is always good form to isolate the battery so the ecu can re-learn from scratch. Please note that it may take up to a week for the ecu on LH 2.4 to completely relearn, depending on how much and how you drive. By all means check your codes; it's not a panacea but an excellent place to start.

 

It is MANDATORY that neg terminal is disconnected for at least 15 min when playing with/ replacing MAF.

 

Well, there you have it...the edict has been issued. Of course, deliberately vague...but nonetheless, an edict.
I presume you are referring to AFTER the service; allowing the ecu to reset? Kind of why I said "good form". Not re-setting the ecu does not mean it will not re-learn...it just means it will take longer to re-learn...sometimes a week or more, particularly turbo cars. Re-setting the ecu just allows it to happen at a faster rate. While the ecu is re-learning, the performance of the car will be substandard.