Revs dropping

[karmannghia60]

Hello
When the engine has been running for 1/2hr or more and at operating temprature, the revs drop and the car often stall (auto transmission). What could be the issue?
Thanks
Raf
Melbourne Australia
RHD 1969 280SL

[hands_aus]

Hey Raf,
anything else you could add?
eg., does it re-start easily straight away or does the problem persist for 20 minutes, 2 hours or more?
Your statement says that it 'often stalls'. Are the circumstances always the same eg., at traffic lights, a certain level of fuel in the tank.
Have you changed fuel brands or servos?
How long since the fuel filter was replaced?
Can you hear the fuel pump spinning?
As you can see I am thinking a fuel problem.

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL #114, auto, ps , 717,717
BEST OF THE BEST!

[merrill]

Raf,
could be a couple of things, however After you answer Bob's questions
we may be able to narrow your search.

Try this, when it gets warm and tries to stall, give just a little gas.
does the car continue to run?

Matt
Austin Tx
66 230 sl - "white"
78 300 D - Blue
98 C230

[karmannghia60]

Here are the answers
does it re-start easily straight away or does the problem persist for 20 minutes, 2 hours or more?
Yes, it starts easily but the revs will be low so as soon as I put it in drive or reverse, it could stall again

Your statement says that it 'often stalls'. Are the circumstances always the same eg., at traffic lights, a certain level of fuel in the tank.
Yes at traffic lights. Nothing to do with fuel level

Have you changed fuel brands or servos?
No, I use premium (98 octane) plus Valve Master

How long since the fuel filter was replaced?
A while, 2 or 3 years but very low usage

Can you hear the fuel pump spinning?
Yes. It is NOT a fuel delivery issue. It is simply the revs drop to about 500 or 600 when the engine warms up. Obviously such low idle revs is not enough to keep the engine running once gears are engaged

[hands_aus]

Raf,
Thanks for the reply.
I see your car is an auto.
On the top of the inlet manifold is the Constant Speed Solenoid (CSS) which is used to increase the engine idle speed when in gear.
You can watch it working with the bonnet open. Put the transmission in gear (forward or reverse) the plunger should move the throttle linkage enough to increase the idle speed.
Is it possible that the CSS is not adjusted correctly?

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL #114, auto, ps , 717,717
BEST OF THE BEST!

[J. Huber]

Sounds a lot like a linkage problem to me (of which the CSS Bob mentions is part).

James
63 230SL

[karmannghia60]

Thanks for the tip, I will check it out on the weekend
Raf

[karmannghia60]

Solid as a rock, this thing does not move a mm. Is the plunger supposed to move in/out if I try to pull/push it?
Could be ceased I guess. The car sat in the garage for the last two years because I was overseas, only getting used for couple of weeks every 6 months or so.
Is the CSS repairable so should I get a new one? Bob, where do I get it from in Aus?
Thanks again

[hands_aus]

quote:Originally posted by karmannghia60

Solid as a rock, this thing does not move a mm. Is the plunger supposed to move in/out if I try to pull/push it?
Could be ceased I guess. The car sat in the garage for the last two years because I was overseas, only getting used for couple of weeks every 6 months or so.
Is the CSS repairable so should I get a new one? Bob, where do I get it from in Aus?
Thanks again

Raf,
The plunger should move by hand. If not try to free it.
Remove the wires from the solenoid, apply 12 volts and see if it works without the engine working.
Test to see if 12 volts is being applied to the solenoid with the car running at idle, in gear.

Don't rush out, a new one costs an arm and a leg.
If absolutely necessary to replace it, you might be able to buy a working one from John Green in Canberra http://mbspares.com.au/
or Chris Stewart in Melbourne. < Chris Stewart bikesncars@optusnet.com.au >

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL #114, auto, ps , 717,717
BEST OF THE BEST!

[karmannghia60]

Good news & bad. I freed it and tested it, so the CSS is now working. However when I come to a stop at the lights, the revs will still be low till I just give the accelerator a slight nudge then the revs will pick up and stay up

[hands_aus]

quote:Originally posted by karmannghia60

Good news & bad. I freed it and tested it, so the CSS is now working. However when I come to a stop at the lights, the revs will still be low till I just give the accelerator a slight nudge then the revs will pick up and stay up

Raf,
At least the engine keeps going.
There could be some play in the lower linkage rod that connects the pedal to the pivot point on the lower RHS engine block.

Bob Smith (Brisbane,Australia)
RHD,1967 early 250 SL #114, auto, ps , 717,717
BEST OF THE BEST!

[rwmastel]

quote:Originally posted by karmannghia60

It is NOT a fuel delivery issue.

quote:Originally posted by karmannghia60

The car sat in the garage for the last two years because I was overseas, only getting used for couple of weeks every 6 months or so.

I would just check the fuel system because of non-use.
- Clamp the flexible fuel return line at the fuel tank.
- Disconnect the flexible fuel return line at the fuel tank, drop the end into a large (2 liter) glass jar, and remove the clamp.
- Turn the ignition key to position 1 so the electric fuel pump runs but the car does not start.
- After 15 seconds, turn the key off.
- Measure the fuel, should be about 1 liter.

Fuel delivery problems in these cars are VERY common.

Rodd
Pagoda Technical Manual
please contibute: http://www.sl113.org/wiki/pmwiki.php
1966 230SL
2006 C230 Sport Sedan