Hello,
I recently purchased a 91 240 Sedan (B230F, 2.4 Jettronic, AW70 automatic trans). The car was starting and idling very rough when cold. The guy I bought it from said it need a new airbox thermostat. I researched this a bit and decided to do some routine stuff first: Tune up (new plugs, wires, cap, and rotor), oil change, and radiator flush. I put some engine cleaner before the oil change. After this stuff the car started and ran a little better (drove it around a few miles to get some parts).
I came home and changed out the air filter. After that, the car would stall after starting and running for a moment. I unplugged the MAF sensor and the car would run better indicating that the MAF sensor was fried. I took everything apart from the airbox to the throttle body, cleaned the MAF, throttle body, and replaced the air hose that connects to the throttle body because it had a hole it it. Put it all back, no dice. The MAF must be toast I am thinking. What should I do?

 

So the current issue is the engine dies after running for a few seconds after start? If you give it throttle, does it keep running? Did you adjust the throttle position sensor so it clicks when the throttle closes? Also, did you verify that the idle air controller is working properly.

Really, the MAF shouldn't be in the picture for idling as long as your throttle position sensor is properly adjusted. The idle air controller adjusts the amount of air necessary to keep the engine running at around 700 RPM.

 

the flapper in the air box should be wired closed IMHO.

btw, M46 is a 4-speed manual with an electric overdrive. a 1991 240 is probably an automatic AW70.

 

That's not a reliable test. Plus MAF sensors are not cheap. I would purchase one based on that.

You have a bit of testing to do.

 

further, there's a lot of crappy quality aftermarket rebuilt MAFs out there. in general, the replacement parts market is full of junk, especially if you buy by price.

 

+1.

I'm an advocate of pulling plugs to see what they say. When it is staling and dying, pull a sparkplug. Is it wet? Is it normal (that is to say coffee colored)? It comes back to spark, fuel and air. And you may get nothing from pulling your plugs, but it is a relatively quick way to rule out some obvious things.

 

note I was reading in some Mercedes factory service documentation, you should never pull a spark plug out of an alloy head when its hot. always wait for it to fully cool down or risk ripping out threads. makes sense to me.

 

Yes, with MAF plugged in, the car stalls after starting and running for a moment. If I unplugg the MAF, it will run. If I push the throttle when MAF is plugged in, it will rev up but after a moment of hesitation. If I let go of the throttle, it dies. I tested the resistance between terminal 2 and 3 as indicated by the Haynes manual and it test within normal range. There is another test in the haynes manual (burnoff test) but Im not sure how to do it under current conditions (car wont run with it plugged in)
I will test throttle switch this evening to see if it clicks. Would this affect how the car runs with MAF plugged or unplugged?
I do not know much about the idle air control or where it is located. I will do some research and see if I can get this tested as well. I have a day off tomorrow (Sunday and will be working on my car all day.
Thank you all for your responses and help..

 

thats a pretty good sign you got a bad MAF.

sadly, most replacement ones, especially the cheaper ones are a crapshoot, and very unreliable. Even the "Bosch" rebuilt ones are a crapshoot, the most reliable seem to be the very expensive Volvo branded ones, whcih are also rebuilt. AFAIK, new MAF's for these cars are no longer available.

your best test is to find another car with the same MAF, and swap them. ALL LH2.4 240/740/940 use the -016 MAF (LH2.2 used -007, and the somewhat rare LH3.1 uses -001). if the problems 'follow' your MAF and the other one works great in your car, swap back and go looking for used ones at junkyards, most any non-regina and non-LH3.1 240/740/940 from 1989+ (turbos from 1990+) have the -016. original ones are usually better quality than most rebuilds.

 

if your not using a scope or a known good maf tests are not valid.

 

If pierce recommends against it, I'd go with what he says. The thought (and the problem described) never had been a problem for me... but I will have to consider this a bit...

As to MAFs not being available... I know you can get them from FCPeuro, and their return policy is pretty good.
Volvo 240 Air Mass Sensor Parts | FCP Euro
I've ordered quite a bit from them... and I've contacted them about problem parts before, and they've been pretty good about getting back to me. IF you can get one from a junkyard, that is the way to go, but FCP might be acceptable. Fortunately, I've not had that problem on my car yet... Just be sure you get one for LH 2.4. LH 2.2 is easy to tell apart. Up where the connector hooks up to the MAF, there is an adjustment screw. This is not there on the LH 2.4. If in doubt, you can google both and see pretty well the difference.

 

the connectors are different too. but mostly its the -016 PN for the LH2.4 and -007 PN for the LH 2.2. a lot of rebuilt ones look radically different than the original as they put a sort of heatsink over them where 'bosch' nameplate/cover was on the original.

 

I pulled a plug, it was not wet but covered in black soot. Probable from the rich mixture of running while MAF was unplugged I am thinking.

 

indeed, with the MAF disconnected, the ECU reverts to 'limp home mode', and runs very rich to be safe (too lean can burn holes in pistons and fry valves). its amazing it can run at all, since the computer has no idea how much air or what the throttle setting is, all it knows is RPM and 'throttle is at idle'.

running it like that for very long will likely foul your catalytic converter as well as the plugs (nothing a good italian tuneup shouldn't fix, 30 minutes or an hour on the freeway at high RPMs in 3rd [OD off] once you get it running right).

 

Today, I cleaned the flame trap (ordered a new one but put that one back for now a little bit cleaner) and cleared one of the hoses connected to the housing. I reconnected the MAF and started the car. It wanted to stall so I gave it some gas, then it idled erratically for about 10 minutes. Dropping to a very low idle to where it would almost stall, then surge and do it again. As the car warmed up, the idle evened out to fairly normal. I drove the car around for a couple of miles and it drove pretty good. It felt a little weak though.
I will see how it does this eve when it gets cold again. I am now thinking it is not the MAF. Based on some research, I think I may need to clean Idle air valve. Any thoughts?