My 450 SL with 180000 miles on her has a strange idle problem.
When cold started, the idle goes to 1500 RPM and takes 4-5 minutes of running to come down to around 800 RPM. If you pull the two wire connector from the small vacuum relay by the coolant tank, the timing will drop to 0 BTC from 10 degrees BTC, and the vacuum will then be allowed to go through the relay. The vacuum relay connector has 12 volts on it all the time. This has the effect of blocking the vac. from getting to the Dist. all the time, hot or cold.

So far I have confirmed, Valve settings, Injectors cleaned, MAP sensor checks OK, Trigger Points cleaned, Dist. points and rotor clean, plug wires ok, coolant temp sensor replaced, air temp sensor ohmed ok and timing set to 10 BTC with no vacuum.

Anyone have any other ideas??

Thanks to all,
Mark

Mark:

Check the 212F switch for a ground, and look for a shorted wire on the relay circuit. It has voltage any time the ignition is on. It is supposed to be switched on (to vent vac) when the temp is above 212F, the AC is on, or the car is in third above 1500 rpm, I think. May also be oil temp related, don't know.

You could also have a stuck relay, stays on when not energized.

Idle should be 1200 plus when cold, should drop to 800 at 150 F indicated on the temp guage. If you have a tired thermostat, you may be taking too long to get to operating temp. Vac should be applied on cold start, retarding the ignition timing.

If the vac retard isn't the problem, check the operation of the temp switch, the thermo time warm-up regulator, and the idle air warm-up valve. They can get tired and not shut off the extra air soon enough. And you will need to check both temp sensors for correct operation -- the one in the intake manifold and the one in the air intake horn.

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!

Mark,

If all the other suggestions don't pan out, locate the idle air valve and give it a good smack on top with the flat side of a good sized ball peen hammer. I used to love doing that to clients cars in the old days. I never did charge for the acquired knowledge of knowing where to hit the engine. Seriously the air valve is made of two pieces of aluminum that walk apart from heating and cooling so many times. If it has separated some, a good hammer smack will move it back and bring the idle right back down.

Peter

__________________
Auto Zentral Ltd.

Thanks Guys,

Peter,
I think this may have something to do with the Air Cond. system. Ever since I have had the car, the clutch on the compressor has always been engaged. It does not matter where the switch or lever settings are on the console. Someone in the past may have altered the wiring.

Your indication that the RPM should drop to 800 when the temp reaches 150 degrees is about what it is doing. It takes about 4 minutes to get there with a air temp. of around 80 degrees (car sitting). Does this sound too long??

I have already checked the cold start system and it seems ok. The two temp sensors should be ok, I relplaced the coolant temp sensor and the resistance readings of the air horn sensor are correct and it slightly changes the rpm when the wires are removed. The vac. relay does work, if you pull the plug while cold, the rpm will drop and you hear the relay click. Put the plug back in and the car races back to higher rpm as it blocks the vacuum.
I thought my next step would be to take the Aux. Air valve off and clean it.

What would be the best way to defeat the Air Cond. engagement system to see if that is the culprit?

Thanks for the help, again

Mark

Easiest way to is to pull the AC fuse so that no juice goes to the solenoid. I don't know where it is, though.....

I have the Chilton's manual, very small diagram, very hard to read. I'll see what I can find there.

Do you have the automatic climate control? If so, it may be 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!

Thanks Peter,

I will look for the AC fuse. I do not have a Automatic system. The car has a 3 position switch for fan control and dual levers to adjust defrost and temp settings.

I have removed the air bypass valve and cleaned up the brass thermo. bulb and coolant areas (kind of nasty). The RPM appears to drop quicker now. It takes around 3 minutes instead of 4.

The other problem is fuel mileage. Around town it plain stinks!
8.4 MPG in the last tank. BUT, on the highway the previous tank got 17.8 mpg at a constant 70-75 miles per hour. What gives??

The car is going to a Upholstery shop today for new leather seats and a new Convert Top.

Thanks,
Mark

Two things on the milage:

1 -- get a new, OEM thermostat. The engine stays in startup mode (rich) until the temp gets to 150F, and you will never get there in cool weather and a bad thermostat. I had the same problem, along with overheating. Seems the thermostat won't either close tight OR open all the way when shot.

2 -- find someone with a good CO analyser and some time -- look up Steve Brotherton's post on setting the CO and timing properly. If you don't have the mixtuer (CO) set right, these things can be real gas hogs.

The fact that you get good milage (and 17 mpg is good!) on the highway makes me think you need a new thermostat. It will reduce the high idle time, too -- mine heats up in a couple of minutes now.

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 Peter,

I have a new thermostat and will be putting it in when the car comes back with new seats and top. I am currently trying to buy a exhaust gas analyzer on E-Bay. I will let you know the outcome in a week or two.


Thanks Again,
Mark