My '89 240 GL is blowing high emissions, and the mechanic determined the MAF sensor was not working and the fuel mixture was very rich. After replacement, the mix is still full rich -- the mechanic says it's all over the place, starting fine but almost immediately switching to full rich. He's replacing the air temp sensor today and will replace the PCM next. He's clearly puzzled by it all, and that's worrying both me and my bank account.

Is he missing something or is this the correct path to be following?

BTW, the car has run great through all of this -- even the repairs! You'd never know there was a problem until they sniffed the emissions.

I'd like to see your test results - usually full rich helps to LOWER NOX (but raise CO).

In my opinion replacing a fuel ECU is a waste of money. Air temp sensor? You mean coolant temp sensor?

More likely its the fuel pressure regulator (or the vacuum line between the FPR and the intake), o2 sensor, or the CAT.

I left the test results with the mechanic, but it was over the limit in hydrocarbons and carbon dioxide (I think those were the two). Here's what I know they've checked and found no problem with: all the vacuumn lines, the fuel pressure regulator, the catylitic converter, and as I mentioned, they replaced the mass air flow sensor.

Is the next step logically to try a new computer module?

Measure the the voltage from the O2 sensor on the fat green wire, while connected, during idle. It should swing very quickly between .1 and .9 VDC.

At what point does the temp guage normally read?

I'm going to get the car back for the weekend since it's running fine, so I'll check the voltage this weekend before taking it back to the shop on Monday morning.

The temperature gauge historically runs right about the mid-point, and that hasn't changed. It typically warms up to that point in during the first 10 minutes of driving and stays there, fluctuating very little in winter or summer.

i agree, check o2 sensor. this is a little more far-fetched, but could have leaky fuel injectors. also, just because hc and co high doesn't always mean rich running, could mean tune-up needed -plugs, rotor, cap, wires, clean pcv trap, air filter, timing
good luck

The tech guy continued to check all the systems with no positive results, so today he replaced the computer. It fixed the problem immediately, and the car passed the emissions test. It blew zeroes on the two emissions that had been high. It was certainly an expensive fix, but seems ready to go on to the goal of 400k miles (it's currently at 268k and runs great).

Thanks for each of your suggestions as I walked through this with the shop.

Just to finish this thread in case it might help someone else with a similar issue, the final fix was to replace the computer, which instantly solved the fuel mix issue. The emissions tested perfectly (blowing zeroes for the previously offending elements) and it has been running great.

One other unexpected result is that that my gas mileage improved from 15 MPG to just over 23 MPG!

Had the same problem with my 1987 240 DL.

The master Volvo mechanic in my area replaced the catalytic converter and problem solved for $500 or so bucks.

Hope that helps.