what exactly is a frozen piston?

One that won't move.

The thermostat (as are all thermostats) is a device that moves with temperature. In this case it moves the additional air slide piston first and secondly the piston transmits the movement to the internal linkage of the mixture governor.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Just a reminder - the 'thermostat' steve is referring to is the one on the fuel injection unit, not the cooling system thermostat you asked about earlier, which is in the housing at the rear end of the upper radiator hose.

Happy Motoring,
Mark

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DrDKW

So I have 2 thermostats.. 1 in the cooling system, and the other in my fuel injector pump.

Can both of these be causing me to run rich, or just the pump's injector. I can understand how the injector thermo could me causing me problems, but not how the coolant thermo can affect the fuel mixture.

What could cause a piston to freeze?

TIA

The thermo-sensor/thermostat on the fuel injection unit senses engine coolant temperatue to adjust the fuel mixture from rich(engine cold) to normal(engine warm). If the cooling system thermostat is stuck open and the engine always runs cold then, naturally, the thermo-sensor will tend to maintain a too rich condition.
Of course, the thermo-sensor on the injection itself or the 'piston' that it operates could be defective/stuck. There are other sensors, mechanisims and valves on these mechanical injections that can fail. Before you attempt repairs, find a detailed shop manual on this injection system to see what you might be getting into. Experimenting with gas and fuel systems can be hazardous. You might want to check out that Houston MB shop that Steve suggested.

Happy Motoring,
Mark

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DrDKW

Yeh,man,if that shop Steve suggests has someone experienced in old Bosch mechanical injections,that's the way to go.There are adjustments on the pump,but you better know what you are doing.If,for example,you try to adjust the idle mixture with the engine running you'll lacerate your fingers and destroy the pump.There is a screw to adjust the main rack,but again if you aren't experienced,you can soon find yourself in quicksand.Also there are 2 white screws to adjust upper partial load and 2 black screws to adjust lower partial load,but you don't even want to go there.Find someone who understands these adjustments and he or she should be able to get the bugs out of your SL.Then you can put it on the road with the top down,listen to that beautiful exhaust note,and thank a professional mechanic for a job well done.
Good luck.
Peter

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Auto Zentral Ltd.

I seriously doubt my wallet will be thanking him!

But anyway, I'll give it one last shot this weekend. (figers crossed)

Every once and a while I feel the need to remind people that I make my living fixing cars.

I try really hard not to be offended by the public's horror at the cost of auto repair. I usually do not bother to point out that for a small diagnostic charge (usually under $200) the correct answer can be had without the real expensive process of parts substitution.

Other than the youngsters on this site I predict that my personal income is probably in the lower 25% of all that venture to this site. I can say that I love the challenge of fixing cars and I have been blessed to be able to say that I have spent my whole life doing exactly what I wanted to do.

I spend as much time talking with technicians as I do on this site and know them from all over the country and locally. I personally believe that as a group they are honest, hard working, free-spirited, individuals. If there is a weakness to the industry it is probably do to the level of training given to some pretty low paid technicians.

I can tell you that in my shop if we have failed to successfully solve a problem it probably has cost me more than the customer. While we all make our money hanging waterpumps and brake pads, diagnostics is both where the fun is and where the money isn't.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Steve - I know what you are saying - perhaps people get disgusted with ALL mechanics - because they have been taken to the cleaners or not been happy with the work done by untrained or unscrupulous mechanics. The good get tarred with the bad. I myself became a shadetree mechanic because where I live good mechanics are hard to find. My first bad scenario happened in 1990 when I took my Aerostar to a Ford dealer for a tuneup and got hit with a $200.00 labor bill to install a cap and rotor, air filter and 6 spark plugs. They said they had to jack the motor to do it. OK - I understand. The part I didn't like was after I drove it for a week, I popped the hood to check oil and found 3 bolts about 2 1/2" long sitting on the rad by the hood latch. When I asked them about it - they said "if it is running right -don't worry". I worried until I got rid of the van. Stupid thing that a pro should not have done. When I got my Mercedes I thought - the techs at the Dealership MUST be good. - I took my car there for a miss at idle - $400.00 later and one new spark plug wire - it still missed. The answer " it can't be- the computer says it is ok" When I showed them it wasn't - they said they had no idea. I was ticked off but decided to work on the car with my friend who does mercedes repairs as a hobby. When we started checking under the hood - the computer box was loose - they took out the nuts that held it in place and forgot to replace them. I now maintain 4 VW's, my Mercedes and an Olds. My daughter just took her VW with battery problems back to the repair center that sold the battery to me to check the battery - $20 later the guy said there was nothing wrong with it. I learned about checking batteries on the net and found out one cell was shorted. When we took back the battery with this info they replaced it under warranty but would not refund her $20.00(oh yes it was the problem). I know all this sounds like puny nit picking -BUT - when I am expected to pay premium dollar for professional diagnosis and repair and I get sloppy work and poor attitude in exchange - I decided to do my own repairs as my standards seem higher than theirs. As many threads show on this site others have had bad mechanic experiences. It is truly unfortunate that quality shops and techs like yourself suffer because of some shoddy shops out there. I guess it is just human nature to classify all the industry the same. You ever notice on this site how people always ask for a quality shop in their area??? When the choice is aggrevation or do it yourself, I choose do it yourself. Yes I have spent money needlessly for parts - but I wrote it off to education.Too bad cars were not small enough to send in the mail - I'd rather have an honest pro like you do the work than me. If your shop does quality work and your mechanics are pros - you will always be busy - a running car is not the goal - Customer Satisfation is. It is also the best advertising. Sure you will get the deadbeats that will try to get things for nothing from you - but I bet they are in the minority. Most people (especially those who own Mercedes) don't mind paying if they get what they paid for.

I fully second the comments in the previous post. Steve, going thru this thread makes me pretty confident that you are one of the best out there, technically as well as professionally. You are providing your expertise free of charge on this forum, and that is something that we all truly appreciate. And as has been said earlier, man if I was within a few hundred miles of your shop, I would never go to any other mechanic.

But the unfortunate reality is that in my 11 years of experience in maintaining a mercedes, unfortunately, 75% of the shops/mechanics I have come across have been either incompetent, or unscrupulous, or both. And this is when 50% of them have been at authorized mercedes dealers, in 3 different cities. I can give you tons of examples of authorized dealer technicians giving WRONG advice, just to make money. To top it off, more often they have not had a clue as to what the problem was. This problem is exacerbated by the individual called "service advisor" at these dealers. He is probably a used car salesman, who behaves like a salesman, not like a tech, had zero knowledge, and will say any crap just to make money. I guess that's his job. And he is the one you have to deal with, or through. You won't believe the kind of quotes I have obtained, and the unnecessary repairs that have been suggested, or the "diagnostic" charges that have been levied without any result. I had a rough idle at cold, and 1500 dollars later, it was solved when the oxygen sensor was found totally dead. Shouldn't the tech have noticed it the first time he tried to adjust the fuel mixture - the output was constant. and I can go on and on. Believe me, since you are a tech, you probably haven't experienced the frustration of a person who doesn't know too much about cars, being taken for a ride again and again and again, by people who you would think one could trust - the authorized dealers and independent MB specialist. There is only one, I repeat, one one guy I found to be both honest and competent (he is in Queens, NY). Anyone else I went to, first asked me to shell out at least a grand.

So that's the reality out there. Again, Steve, its all the more commendable, and a relief, to see people like you around. Maybe you should branch out, set up shops all over the country, and get these bad guys out of business. I will personally make sure everyone I know comes to you for repairs.

Rad-man

Listen to Steve-he has been helping me with my rich mixture-see the related posts on this Vintage section of the board.

To re-emphasize, the thermostat is the FI thermostat which is the vertical housing on the back of the pump with two hose connections. I ordered a new thermostat (PN 001 203 95 75), figuring that after 30 years, I would give the old thermo a rest.

Also see Steve's response re: what to check first regarding rich mixture before adjusting the pump.

regards,

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Regards,

Tom S.
1971 280SL Tunis Beige Metallic
1971 280SL Tobacco Brown (13K miles)
1970 280SL Deep Red
1994 E320 Cabriolet
1999 E320 Wagon 4Matic
2002 LX 470
1992 Land Cruiser

thanks for the post. It's a bit of a relief when people have some of the same problems as I do. (in a good way).

I put my finger over the hole and the engine died!! (I'm happy because I'm hoping I finally found the culprit to my problems).

I took off the filter and started the engine up. I immediately heard air being sucked into the engine. I left the car on for a good 20 minutes and it would stall if I put my finger over the hole.

Is there a difference b/c the Bosch or the Berh? Hopefully this 40$ part will fix all my probs. thanks a lot, guys!

Just so I understand, the thermo should open when the engine is cold, and then close when the temp reaches operating temp?
Why exactly does it die with my finger over the hole? Shouldn't this simulate the thermostat closing? So why would the engine die with the thermo closed?

Because your idle speed has been adjusted to compensate for the fact that you are always running with the same amount of additional air. After you replace the t-stat you will have to adjust the idle air screw as it won't run when the car warms up as you have surmised. Remember engine speed is determined by air not fuel! To raise engine speed air must be added.

BTW the t-stat does not open or close it just gets longer as it warms and shorter when it cools. This action moves the piston below the t-stat which is the air slide. REMEMBER it may not be the t-stats fault. If the air slide piston is frozen open a new one will not fix it. I would verify the movement before replacing. I would replace though as I think the t-stat will not be very good after trying to push a frozen piston for a while. It is very likely that it will be frozen as no movement usually leads to such. (or is such).

The piston is in the housing below the t-stat cylinder. They are easily cleaned/freed up. Drain your coolant and pull the t-stat housing (cylinder) - two screws. With the t-stat removed push on the piston where the t-stat plunger sat. It should be spring loaded up. It should easily push down and spring back. If not remove the other two screws and take the housing to the bench to free up. NOTE: there are shims beneath the t-stat and beneath the housing. Besure to replace them as they were.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

so pretty much this is where the cause of my rich mixture lies?

After 30 years, the thermo and piston have stopped working properly, and to fix these I need to clean the piston and replace the thermo?

I guess this was my cars way of teling me it wanted a new distributor, wireset, and plugs. oh well