High Idle Hell

Hello,I have a 450SL 1979--THE IDLE IS VERY HIGH UNTIL IT REACHES THE RIGHT TEMP, THEN IT'S OK-- ANYONE HAVE AN IDEA WHAT IS WRONG ?? THANKS DENNIS

Originally Posted by DENNIS

Hello,I have a 450SL 1979--THE IDLE IS VERY HIGH UNTIL IT REACHES THE RIGHT TEMP, THEN IT'S OK-- ANYONE HAVE AN IDEA WHAT IS WRONG ?? THANKS DENNIS

When you find out, let me know!

My 80 450SL does the same, and I've looked for obvious such as vac leaks, but I'm still looking. I'm still waiting for a CD to arrive, until then it's only poking in the dark! BB

I HAVE BEEN LOOKING FOR THIS ALSO,IF I FIND OUT I WILL REPORT IN.THANKS----DENNIS

[dangill]

While I may not have as many years as some tech's here
I have spent a LOT of time with the 1980 vintage engine
and its arcane control systems.

The high idle speed is the norm until the engine is HOT. Both my
1980 SL and SEL will reach 1800 rmp before beginning a gradual
slow down to 650 rpm.

I spent weeks trying to understand how the major and minor devices interrelate because I was convinced that no-one would design what is essentially a "Choke System" and would make it so complicated and do it deliberately.

From my memory, here is an overview:
There are at least three major units involved.
Warm up compensator, idle speed air controller, and a distributor
with a vacumn controlled retard. When the engine is
cold the warm up compensator changes the differential pressure in
the fuel distributor using a heated bi metal element acting on the
bellows within the unit. As the operating temp is reached the strip
normalizes the external pressure on the bellows which then normalizes
the fuel distributor pressure and leans out the mixture.
The idle speed air controller also works on a temp controlled air
flow allowing more air to flow if the engine is cold.
The vacumn controlled distributor will retard the spark until about
50-60 degrees c is reached so about the time you would expect the engine to slow down the spark is "advanced" (removed from the retard position)
and the engine speed leaps forward again.

I am writting this from work and possibly could add some details
when I get home and get my paper manuals and notes if anyone
is interested.

I've been up front of the intake and ya, there are several items with vac hoses running all over the place!

I'm pretty sure now, after a drive Sunday, that my thermostat must be wide open as the engine never quite makes it to 80c and it takes an hour to get rvrn close. After running 15 or 20 minutes, and all of a sudden, my idle jumps up and smooths out. It doesn't have any power. Compression is fairly consistant between cyls 170 lbs.

How common are injection issues? The odometer stopped at 178K and nobody knows when? My guess is well over 200K and had been totally neglected!

I might be thinking more about selling it now and may place an ad soon! BB

[ctaylor738]

I think it's a wear issue.

My 13K 280SL blips up to about 1200 at start and quickly comes down to 800 or so and then gradually drops to 650. My 182K 280SL goes to 1500 at start and stays there until it hits 80 degrees, then drops very quickly down to about 800.

There are at least three devices that add extra air/fuel during warm-up: auxiliary air device, the warm-up compensator, and the cold start valve.

The manual doesn't really explain how they work together to regulate warm-up speed - it has separate sections on each device.

Hello guys, when my car reaches operating temp. the car runs great, then when it is cold again, it all starts over again. If that is normal then so be it---but, what a engineering screw-up. THANKS AND HAPPY HOLLIDAYS--DENNIS

[230/8]

Sounds like your auxiliary air valve is working properly. This is fairly rare, be glad. It is thermostatically controlled, much like the radiator thermostat. In fact, it uses a thermostatic pellet just like the coolant thermostat. When the engine is cold it allows more air in to raise the idle speed, like a choke, and as the coolant temperature rises it closes off and reduces the air and the idle speed. It is the large aluminum fitting at the top front of the engine with two very large diameter air hoses running to and from it to the manifold. Your cold start valve also is plumbed in near there to enrich the fuel mixture when cold starting.

Check your in-gear hot idle speed against the data tag on the radiator support. Should be around 700-800 rpm. Set it with a good tach, not the dash tach, and see if your cold idle speed is more reasonable. The large brass screw adjacent to the aux air valve is for setting the idle speed. Don't mess with the screw under the air filter at the throttle body.

230/8

Hi Fellas. This morning before light I delivered my sick BENZ to a repair shop along with 3 major parts that the BENZ dealer/repair shop diagnosed my car as needing. So I searched the Ebay site and bought some used componants for a few dollars (not the $2400 that the dealer quoted). When the repair shop installed them the car wouldn't start. So he replaced all of the parts he took out and the damn thing won't start with the original parts???? Bummers...>I drove it there sick and now it is sicker!! Anyone out there with some experience and a few suggestions??

[jr811]

My 79 SL acts just like Dennis's - High Idle until it warms up then runs OK
I have replaced the auxiliary air value - this DID NOT help.

[230/8]

To busybenz and Dennis and jr811: Just a note to ensure understanding of a basic principle...the aux air valve is supposed to raise the idle speed when the engine is cold and as the engine warms, it gradually closes off the aux air passage and lowers the idle speed down to the normal setting. There is a piston inside the housing that slides up and down to open and close an air passage. It is fitted with a thermostatic pellet that is immersed in the coolant. It operates just like the other thermostat in the engine. In fact, one repair that is sometimes made is to replace the pellet with a fresh one removed from a standard thermostat.

If your cold idle is way high, above 1500 rpm, then someone may have tinkered with the idle air screw, the large adjusting screw nearby the aux air valve. A mechanic may have tried to make an adjustment when the aux valve was sticking, a common event. Usually the screw gets turned down to drop the warm idle speed because the aux air valve has stuck part open and won't close the air passage. If you are getting a higher cold idle and a low, or normal warm idle, then it sounds like your aux air valve is fine and does not need attention.

Again, check the warm idle speed, it should be 700-800RPM. If it is too high, adjust the idle air screw at the front of the engine to get the right speed. Then when the engine is cold, see what the tachometer registers, it should be around 1300-1500, more or less.

Bear in mind that these older models usually will not operate as well as we have grown accustomed to with our modern EFI equiped cars. A lot has changed in 30 years and new cars today are really good on driveability whether hot or cold.

As to the way it was designed, it seems they were trying to mimic the operation of a carburetted engine. They have high idle speed when on the choke and as it opens up the idle speed drops. But our old FI engines do not have a choke. The electric cold start valve briefly dumps extra fuel to start the engine, then the higher speeds provided by the aux air valve help to prevent stalling while the engine is warming up to normal temperature. This is why cold operation driveability complaints were fairly common, or so I am told by others who know a whole lot about these things...

Just some additional food for thought, for what it's worth.

230/8

I am amazed at the amount of good information on these threads. I am equally amazed at the complexity of my 380SL. It sure isn't the same as the old tractors or motorcycles I repaired as a teenager. This one has me beat! I'm storing it for the winter and will have it hauled 150km to a BENZ dealer next summer. In the meantime I will log onto this thread and watch the rest of you solve some problems and contribute what I can (what not to do). Seasons greeting.

[ctaylor738]

Jack -

Since we have some time, why don't you tell us more about the problem and the parts that were replaced? The 380's are not all that complicated, at least to get them running.

Thanks for your response Chuck and to the rest of you on this thread. I logged onto this thread because I have a 380SL and because I recognized so many similar problems that others had (Dennis and bad idle problems). My Beast did all of the idle things that most of you mentioned except that when it reached temp. and the idle reduced it stalled, except if I feathered the gas peddle and kept it running at +1400 RPM, and then it heated. The BENZ diagnosis said I needed 1) idle speed control 2) idle control valve 3) warm up regulator 4) oxygen sensor 5) Buzz valve 6) coolant temp sensor. When I obtained these items from a used 380SEL (same year) I took them down to a local garage to have them installed (I am not as dexterous as I once was, clumsy!). The garage replaced the parts and then the darn thing wouldn't start, let alone run. So they put the original parts back in and still no start, no spark! Now the mechanic is using a volt meter to trace the "no spark" problem and comes up with a small (1 inch square, 5 prongs, $178 from Toronto) relay from behind the glove box, a fuel relay that is putting out 7 volts and it shouldn't (he says holding the electronic diagram he downloaded off of the NET). I'm thinking that if the darn thing worked when I drove it down to the shop then why isn't it working now? I concluded that this mechanic was apprenticing at my expense. I also resolved to bite the bullet and have knowledgeable technicians look at it next summer when I get home again. One thing that haunts me is that it just may be that thingamajig fuel relay because of some of the comments on this thread. I have a few days left in CR before I have to return to -38C (-36F) country and I'd sure kick myself if it was that simple and that mechanic was right. How's that for self doubt? I have just enough time to order and install the part and spend the rest of the winter wondering or satisfied . . .hhhmmm??
I 'replaced' #1,2, & 3 above to finally answer your question Chuck.

[ctaylor738]

The fuel pump relay will stop a 380 dead in its tracks. It is fairly easy to test by putting your hand on the fuel pump while someone turns the ignition on and cranks the starter. You should feel it vibrate. If it does not, pull the relay out and look at it. Figure out where the pins marked 30 and 87 go in the socket. Make sure you have 12V at the 30 socket. Then jumper those sockets and see if the pump vibrates. If it does, then the problem is most likely the fuel pump relay. If it does not, then you may have a dead pump.

If the pump is alive, though, try to start the car. If it won't start, then there needs to be some serious troubleshooting.

One somewhat remote possibility is that someone has messed with the mixture and gotten it extremely lean, and that it simply is not getting enough fuel for a cold start.

If your problem is no spark, it is a completely different set of tests. Make sure of the obvious - the rotor is installed and all the wires hooked up. If that is all good, I would suspect the TDD box (silver, on the left fenderwell).

Good luck.