Yup the afm is a larger mechanical unit with a piece of plastic scewed into it (the potentiometer) this is the thing that breaks. MB only sells the whole unit unfortunately, but you can by the potentiometer seperately now.
m

Just drove the car. It is worse than before (now it has poor idle and acceleration), but they said the poor idle may be due to putting on a bad set of new wires. They will put on another new set tomorrow. They said that the acceleration is probably due to a faulty air flow meter.

They told me that you have to buy the whole air flow meter from Mercedes, not just the potentiometer. Are you sure you can get an official Mercedes potentiometer separately?

So tomorrow, a 3rd set of wires and then discussion of the Air Flow Meter subject.

Can the potentiometer be bought from a Mercedes dealer separate? If not, where to get it separately?

Is this the potentiometer from one of pesuazo's previous posts? I found one for $41 on a eurodirect.com, but will that fit on my 89 420 sel? Are they all the same?
Can I get the potentiometer without the air flow meter from the dealer?


Is the potentiometer the part pictured at this link?

http://www.peachparts.com/shopforum/tech-help/82080-sudden-surge-when-decelerating.html?highlight=potentiometer

Is this the same one for my '89?

After one week in the 2nd shop, the car runs worse. It now idles really bad at idle, stalls when you give it gas more than gently, and won't start unless one pumps the pedal. Essentially all they did is put on new fuel pumps and a coil, another set of wires, and then replaced them with the other ones since it did not make a difference. The put on a new fuel filter, which they said did not look good, but said the car ran even worse after that. Will keep you updated.

Matt, the potentiometer can be bought seperately. Prices from $40-80. the picture does show the correct part. The part number is in my thread you found. I really do not think it is the pot. However, I am still think you have an ignition issue. However, it would be nice if we could see the eha and dutycycle numbers too. This may help confirm the problem is fuel or ignition.
m

mattsuzie:
Yes, that is the pot.
To make sure, you can read the part number right off the pot itself.
No, the dealer does not carry the pot by itself.
The old version ended in 015, the redesigned ends in 035.
You are correct, Eurodirect has it really cheap (42 including shipping).

I thought joe p was going to work on your car?

Thanks Gentlemen. I will let you know what the culprit is.

So far I talked to 2 Techs and the dealer. No one will just put in the potentiometer, have to put in the full air flow meter if to repair unless I do it myself.


I will let you know what the culprit is.

That does not surprise me, luckily it is a very easy job.
m

I would take the risk of installing the pot myself ($42), rather than pay $1K for the whole unit "to see if it fixes the problem".

By your description the techs are just throwing parts at the problem, with no success.
New wires (3 times), new fuel pumps, new coil, new fuel filter, now new AFM?
Proffesional mechanics are supposed to diagnose and repair, not throw parts at it at your expense.

You and I can do stuff like that.

If you do not feel comfortable changing the pot, perhaps we can meet a weekend and work on our cars together (may be 87-300E_in_NC will join us, his mechanical skills are excellent). He has a spare pot, which we can test. Or may be swap pots from our cars.

Just let me know.

Thank you for your help! Will let you know what trail I take.

Every mechanic I talk to, despite the feedback that I convey from what I learned on this forum all say that, no way, the potentiometer is hard to put in by itself and they will not do it. They also have said that anyone on this forum is just simply very talented and ingenious.

I hear you and I understand that you have done it successfully.

Yes, all new parts have been thrown at it except the fuel distributor and air flow meter. They also said that I have good compression and was happy about that for it has a ton of miles. I wonder if the putting in the two new fuel pumps temporarily somehow made something worse? Can increased fuel pressure further damage a failing fuel distributor?

All 3 techs have said that there is only so much one can test the air flow meter and the fuel distribtor and the only real way to know what to replace is to go by a hunch through experience.

What is frustrating is that 1 tech has one opinion (AFM), another tech says fuel distributor, and another tech say could be either.

Hey Matt, Check something for me. I just got done undoing this one. The 560 shop car needed a chain soooooo, I rolled a new chain and guides in it. In the process of 458 distractions I missed the l/h cam by one tooth. Car idled a lot like yours once warm and ran a lot like yours hot. Once I took and popped the valve covers off and looked I just dropped the tensioner and moved it a tooth. Runs like a top now. (hey we all have our moments)

It would be worth a look anyway. Just pop the valve covers, put it on TDC and see where the cams are in relation to the tdc marks on the bearing towers. Might be the culprit. I still dont think the car needs a fuel head.


Joe

One of the MAESTROS in the forum have spoken


:D

The latest is that the original tech put in a used fuel distributor and another set of wires. The tech says that the stumbe at idle is gone, but there is a "miss" on cylinder #5 and that it there is a 1/2 sec stumble during acceleration. He is still working on it.

The last tech said that my compression was around 150 psig on all cylinders, but the original tech that the compression is much lower than that due to mileage.

look here for the potentiometer:

http://www.peachparts.com/shopforum/tech-help/82825-source-airflow-sensor-potentiometer.html

Started it last night and ran it for a few minutes. It is a lot better. Not sure though if it is at 100%. I found out also that the fuel distributor was replaced with a used one (it was not rebuilt). He said that is is due to the two cyliders having lower compression - will see.

In any event , I think the car is as a good as it is going to get at this point. Perhaps that slight hesitation could not be felt before because the fuel distributor was so bad.

It definately does not shake as it used to.

Good news Matt. So I take it that it was the fuel dist and not the wires that where the problem.
m

For the most part, yes Martin, you are correct, it was the fuel distributor that was the bulk of the problem, not the air flow meter/potentiometer.

However, the original tech did admit that he put on the wrong wires when he put on the first new set. The correct set is on now. It is hard to say how much the wrong set of wires contributed to the problem as a result of the Fuel Dist. being so bad.

When I did a quick and short test drive, it ran pretty good while cold. The big test will be driving around when the engine is warm and when there is hard acceleration.

Thanks for your help. It was starting, idling, and running so bad after I let the 2nd tech look at it, I was really worried.

I drove it today for several miles and the first good news :D is that the incredible shaking at idle is totally gone. There is a quiver at acceleration, but I think that may have been there before and may be due to the high mileage as the tech mentioned.

My 300 has the same mileage and it is as smooth as ice. The 420 is still pleasurable to drive, it is just not as smooth as the 300. I guess I have to finally come to terms that one day I would have to deal with the engine's high mileage. It turned 236k, but still runs very strong for a 15 year old machine.

Overall after a 1 month fiasco and being in the shop for 2 weeks, I finally got the car back!!

Damn. maybe I could visit my sister in Charlotte and we could ALL get together. My dad's 300E has that problem. It was partially solved when the mechanic here put a shim in cylinder #4. That fixed part of the idle problem. I think the rest is the pot, but my dad won't buy the $42 part just to SEE if it works. Then again, the same mechanic bought a $92 OVP relay to see if it fixed the ABS problem. It didn't. I told my dad about the sensor, but he was determined to have am mechanic tell him, not me. Sorry to ramble. Let me know when you guys are doing that and I could maybe talk my mom into visiting my sister while I meet up w/ you guys for awhile.
Thanks
David

Could it be injectors that he needs? There is a guy here who said that fixed the problem on his 300E. I know, different cars. BUT, maybe thew injectors are getting dirty?
David