Please be patient with this newbie. I tried the search function and though I've learned some, I'm not sure as to the source of my problem. My 1983 380SL (167,000K) had the frequency valve unplugged when I bought it. It runs o.k., hard starting when warm, but recently has taken to stalling on quick stops. Plugging the frequency valve back in causes a rough idle and it won't idle at all until it warms up. It also has a bad hesitation off idle with the valve plugged in. The valve does "buzz" when plugged in, by the way. From what I've read here, I suspect the oxygen sensor or the frequency valve itself? The car blows black (carbon?) at start up, actually marking the floor. Can the O2 sensor be checked at the connector? Thanks in advance for any help.
Paul

ok, i'll bite. the fuel pressure on the car needs to be checked first. if the line pressure, rest pressure or the control pressure is wrong, hard start when warm and/or poor idle may result. also, need to look for vacuum leaks.

the freq. valve is only a fine tuning mixture adjustment after the o2 is translated by the injection ecu. have never seen a freq valve cause a running problem. when the system is running and adjusted right, the freq valve will go-buzz-off-buzz-off in even pulses. so, coolant temp sensor, o2 sensor ok, and no other obvious running problems-leaking injectors, vacuum leaks, weak plugs, stretched timing chain, i could go on. lotsa variables, unless you have the correct gauge to check pressure and access to an ignition scope, farm it out and let someone who's been around the block deal with it. otherwise, we could throw parts at it and see what fixes it, maybe 200-300 if you're lucky, higher if you're no so lucky. good luck, chuck.

I agree that you should check out fuel pressure and fuel flow, and find any vacuum disconnects or leaks first.

If you want to do a quick check of the O2 sensor, though. I have found the easiest way to do it is to unhook it at the connector, which is usually in a little slot in the undercarriage. You can also use Pin 2 on the computer's big connector in the passenger footwell. Use a multimeter between the O2 sensor and ground. With the engine warm and running, you should see something in the .5 volt range.

If the O2 sensor is dead, the computer will think the mix is lean and try to enrich it, which is not going to help the running.

A bad idle control valve can cause stalling as well.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Thanks for the quick replies, guys; this forum seems very nice. I will check fuel pressure, hadn't considered that. The idle changes substantially when I plug/unplug the frequency valve, so something is happening there. When I first unplug it, it surges and then settles into a faster idle than before. Do these frequncy valves ever get dirty, causing a fuel flow problem? It will idle fairly well cold with the valve unplugged but will not idle at all cold with it plugged in; the idle decreases gradually until it dies in about 15 seconds. I will check the voltage at the connector under the dash for the O2 as well. Thanks again and I'll do some checking.
Paul

Hmmm ...

There is an oil temp switch on the front passenger side of the engine near the air pump. It opens at 61/18 degrees F/C and that's when the lambda contol unit goes into closed loop and starts trying to control the mixture. I don't think it's supposed to do anything to the mixture until that switch opens.

You might test that switch for ground with a cold engine and open with it warmed up. It is Pin 3 on the control unit.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Thanks, Chuck. I'm impressed with the interest and willingness to help you show. I'll check that switch; that would explain the difference in hot/cold characteristcs. The very noticeable hesitation with the frequency valve plugged in has me puzzled. It always had a slight hesitation with the valve unplugged that I didn't address, perhaps a fuel pressure symptom? Well, I have plenty to check tonight. I have a service manual on CD to give me fuel pressure specs. Thanks again; this board is great.
Paul

You have an early KE-Jetronic system, where the basic mixture is controlled by the air flow flap on the fuel distributor and adjusted to keep the O2 llevel in the exhaust pretty much constant with the frequency valve and O2 sensor.

Here is what I would do:

Verify that the ignition system is up to snuff - a missfire will cause the O2 level to go too high in the exhaust, and the computer will keep adding fuel.

Verify that you don't have vacuum leaks. These can be anywhere, but the most likely are rubber connectors on the vac controls, door locks, manifold half seals, idle control valve hoses, or injector seals, roughly in that order. Check the brake booster and transmission modulator, too. Fix any and see what happens.

Make sure the cold start valve isn't dripping, and that the warm-up regulator is working, if you have one (KE might not, mixture will be controlled by the frequency valve).

Verify that the temp sensors have the correct readings and are connected, and that the wireing is OK.

Next, check the O2 sensor as described above. You can check if first, but if there is something wrong with ignition or vac, you won't necessarily learn anything.

An exhaust gas analyzer will make this so much easier.....

If you have access to one, set the base mixture with the allen screw buried between the airflow flap and the fuel distributor to get the proper CO level with the frequency valve disconnected, the check is with the valve connected.

By this time you should have a decent running car. If not, either the computer or the fuel distributor is bad.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Thank you Peter. I will check for vacuum leaks this evening along with the other checks mentioned. The ignition system was my initial project and I changed plugs, wires cap & rotor and helped things some, but I still couldn't plug in the frequency valve without trouble. I know I keep saying this but I am very impressed with this board. I belong to another board, motorcycle related, and as good as that board is, I must say this one is even better .

Look for a big vac leak -- I believe that the engine runs rich with the frequency valve disconnected or bad (way rich, that is, with clouds of black smoke). If it runs OK with the valve unplugged, chances are the mixture is actually very lean -- fix that (vac leak) and it may be OK.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Thanks again, Peter. That would explain why it won't run cold with the valve plugged in. I'll search for vaccum leaks tonight.
Paul

I believe that power to the valve opens it, returning fuel to the tank, and therefore leaning the mix. If it is off it is closed and the mix is richened.

So my guess would be that the warm-up compensator has gone south and the engine is relying on a closed (unplugged) frequency valve to get enough fuel to run when cold Plug it in, the valve starts to open, the mixture is leaned and the engine dies.

You can test the WU compensator by measuring its resistance - it's on the CD.

Your hesitation when warm could be caused by any number of things, including a bad mixture or vacuum leaks.

When you get it warmed up, connect the valve and measure the dwell or duty cycle or even the voltage at Pin 3 in the diagostic socket and let us know what it is. Ideally it will be wavering around 22 degrees dwell, 50% duty cycle, or 50% of you battery voltage.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

The frequency valve IS supposed to be operating when the engine is cold - I just read this off the CD. Until the oil temp hits 60 degrees, the duty cycle should be 60%, which would equate to a dwell reading of 27 degrees. Meaning that the valve is open 60% of the time and diverting fuel to the tank.

This would definitely cause warm-up problems if the compensator is not working correctly, and uplugging the valve would help.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

Man, you guys are great. Chuck, THANKS again...Now I show my rookieness in things Mercedes related...the CD has pretty good illustrations as to which valve is which, so I hope I can find the WU compensator. The stalling after warm up could be fuel pressure or vacuum leak related so I have my work cut out this evening. I can't say enough good things about you guys. I don't know if I can repay you but I guess that's not the point. The point is these German cars....a real learning experience for a life-time GM guy. No comparison between GM dullness and this SL, in case anybody was wondering . I will post my findings, hate to be redundant but THANKS AGAIN.
Paul

Chuck, I'm sorry to see you're having problems with your '81, of course at this point I can only help you by hoping you find the problem. On my '83, I measured 11.68 volts or approx 84% of battery voltage at terminal 3 of the diagnostic connector, engine warm and frequency valve plugged in. I couldn't find any vacuum leaks, and resistance across the warm up compensator terminals is .017 K ohms (I couldn't find the spec on the CD). There is only .58 volts on the harness side of the connector (for the WU comp), with the engine cold...normal? I just realized I forgot to measure O2 output, will check that tomorrow.
Paul

there is 12v on 1 side and ground on the other side of the connector. the 12 comes on when the key is on. if you have a cold start problem and are checking pressures, always leave the key off because the internal heater will make the control pressure rise will checking. once you have established base cold readings, then plug it in and start the car. this way the ambient temp and the heater will raise the pressure together. if the voltage is off and the control pressure regulator is otherwise working properly, the control pressure will rise slower. good luck, chuck.