What brand of tune up parts are you using? They can make quite a difference. You want a Bosch cap and rotor, Bosch or Bougicord wires, and I've found NGK plugs to give the best results. Platinum plugs are a waste of money, and can cause poor running, Bosch Copper plugs are OK but avoid the made-in-India ones as the quality doesn't seem to be as good. I have no personal experience with it, as I've never had to change one, but I suspect Bosch O2 sensors are better than generic brands.

I would go with used AMMs, as I have a couple of good ones from cars I've parted out. If I had to buy one, used (original, not rebuilt) would be my first choice, then a Bosch remanufactured if I couldn't find one in a junkyard.

Sorry but please clarify...what you are experiencing?...what is the problem with the car?

You start out saying: "In my quest to address a slightly rough idle"

BUT you are asking in your Headline: should I replace the AMM with a used/rebuilt/new.

you are getting answers to that question (New/Used/rebuilt AMM)....but if your problem is a "slightly rough idle" ???

I wouldn't go replacing an already replaced AMM.

What do you mean by a "slightly rough idle"
There are many other things that would cause a "slightly rough Idle" and a bad AMM isn't necessarily the next item on the list.

Make sure you take a good look at the plug to the sensor.

Sometimes one of the pins slip back and make a bad connection.

If a pin has pushed back in the plug, gently pull it out and bend the locking tang out a bit so that it seats properly.

On out '88 745, this happened twice, and the car would barely accelerate going forward, but go like a raped ape in reverse! The weight of the cable was disconnecting it.

I eventually removed the boot from the plug and made a line across all the wires so that I could see if the pin was backing out. I would look at the plug each week when I checked fluids, etc.
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '95 855, '95 854, the first three are mine, heh, heh, 415,000 miles put on 9 bricks

I've had good luck with AMM's rebuilt by Python and Fuel Injection Corp and bad luck with ones from Cardone. jp

When I bought my 1988 240 it ran like crap (I still bought it cause I figured it was nothing major). It turned out to be the AMM. A good test is to disconnect the amm. If it runs just about the same then that is likely the problem. I replaced it with a rebuilt unit from autozone and the car has been fine since. It cost about $150 and that is well over 2 years ago. Good luck.

I don't know if the AMM can cause a rough idle. Apparently others have had this happen. I have had 2 AMMs go bad. In each case, the engine would barely run. If I disconnected the AMM, the engine would start and idle but had very little power.

In both cases, I replaced the AMM with a rebuilt ordered from one of the parts houses on the net. Each came in a Bosch box with a Bosch rebuilt label. These cost about $180. They ran fine.

I think the swapping of the AMM with a known good one is the best way to check your AMM.

Before doing anything else, check for air and vacuum leaks. These will cause a rough idle and cost little to fix. Spray carb cleaner around the injector seals and see if the idle changes, if so, replace the seals. Do the same around the intake manifold gasket, the throttle body gasket. Inspect the 'accordian' air hose for cracks and chafed holes. (Take it off and wriggle it to see into all the folds.)

Clean the idle air control motor and check the hoses connected to it for splits.
Check the hoses on the flame trap for splits.
Check the vacuum hose to the fuel pressure regulator and all other hoses connected to the intake manifold.

Clean the connector to the AMM with a spray cleaner such as Oxygard, the grease with a silicone grease (Sylglide) and reconnect.

As someone else said, clean all grounds and the fuse by the battery if your model has this.

You have done lots of great work, just keep going. Don't spen the big bucks on a new AMM until you've done all this work firdt. Good luck.

What year is the car?

Is your idle better when the car is fully warmed up?

Is the crankshaft sensor original?


Here are a few random thoughts (mostly assuming a 1989-on 240):

After replacing lots of sensors/what-not, cutting power to the ECU/ICU (via battery/fuse) will reset its adaptive memory and allow it to "properly" adapt to the new sensors.

Something I did with my '90 240 that I'm convinced made a very significant improvement is to clean all the grounds in the engine bay and lube them up with Oxgard. Specifically, battery posts, main chassis ground, FI/ignition grounds on each side of the fuel rail, and both ends of the engine-firewall ground strap. I also replaced the infamous 25 amp fuse and holder. Everything looked pretty nice to begin with... but things were noticeably better when starting after cleaning the grounds/contacts.

As for air mass meters, the day may come when we've gobbled up all of the used ones, but until then you can still get about 10 used ones (some even warranteed) for the cost of a single new Bosch one. Rebuilt and aftermarket ones are rumored to have some funkiness, which I've experienced myself... namely a "dead" spot on hard acceleration when the engine is cold. That one is now the "spare in the trunk". I'm currently using a junkyard AMM of unknown provenance, and the car idles pleasantly and still got 34mpg (it's a 5-spd) through the mountains last summer.

Good luck and Happy New Year!
-Ryan
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Athens, Ohio
1990 245 DL 130k M47, E-codes
1991 745 GL 280k (Girlfriend-mobile)
Buckeye Volvo Club

Buy a known good used one and disconnect the exhaust heated aluminium tube between the airbax and the exhasut manifold. If that is still attached and the airbox thermostat is no good it will fry AMM.

Before you do that clean the idle air control valve and check the is does work. Dan

Um... you can certainly see if you can borrow one form a firened that is known to be good. I have had a slightly 'off' AMM that was a problem from almost when the car was new. It also never seemed a problem after the first 2 inutes of operation and the car got GREAT fuel mileage....just ran the car a hair too lean. So I kept it for summer, and got a used one for winter.

But....in the same car...I had a bit rough running that was actually something else. Adjusting the timing. You didn't say what year the car was, but...check your timing. Since then I have seen a few cases where people end in setting the car to TDC after changing something significant, or maybe after the timing belt, and not then resetting the timing. It actually amazed me how often I did see it.
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Legum servi sumus ut liberi esse possimus.

used.