Engine Runs rough when water temp gets to 175 deg. ???

[ESO]

Dear Guys,

Merry X'mas....

I really do need help and advise, I am such an unhappy person as my W116 (280S) keep having problem with idling. I picked up the car from my friend 2 months ago who had it since new in 1977. I have had the suspension kind of rebuilt, installed a rev counter replacing the clock only unit. Major oil changed, etc.

I have had the carburetor cleaned and adjusted but still, every time when the temp gauge gets to approx. 175 deg, the car will start running rough. Specially when I have my foot on the brake. From time to time it even cut off. However, when the car is running, it runs pretty good and smooth. I have read that some people replace their Solex Carburetor to Weber 38 which cost approx. USD650 for the whole kit which sound fair.

Now, please advise based on your experience, do you think I should buy a Solex rebuild kit (USD85) and have the original carburetor rebuild OR I should buy a set of Weber kit which cost a bit more (USD650).

Weber Kit : http://69.0.202.131/html/kits/k248.htm

Solex Kit : http://rep.racepages.com/parts/racepages/quote.jsp?year=1976&product=S1011-11283&application=000268102&part=Carburetor%20Repair%20Kit&category=S&dp=false&showdc=false

[koan]

Hello ESO and Merry Christmas to you,

If the rough idle and stalling happens only when you have your foot on the brake you may have some sort of leakage in the brake booster.

It's normal for the idle speed to drop when the brake pedal is pumped as this feeds air into the engine and leans it out but a single application and hold of the brake pedal should not affect the idle.

To verify if this is the problem you could remove the vacuum hose to brake booster  at the booster and block off the hose end with a rubber stopper. Then try the brake pedal when the engine warms up.

DO NOT drive the car like this as without brake boost it WILL NOT STOP very well.

Make sure what you use to block the vacuum hose is a good tight fit, there is a risk something too small is liable to be sucked in.

k.

[ESO]

Hi Koan,

Merry X'mas and thanks for your reply. I will surely try and test this tomorrow as my mechanic friend's workshop will be opened. Anyway, it is quite weir as the car only runs rough at traffic jam when it is in gear and with my foot on the brake. Other than this, the car do runs well.

Besides, I was wondering if it might caused by having the rev counter installed....Do you think so? My car original came with a clock only unit but I have installed a rev counter onto the car, since then it started acting unusual. All I did was to get a wire from the coil connecting to the rev counter.

Will test and then keep you updated.

Thank you

ESOO

[koan]

The rev counter wouldn't be the cause of the problem, it operates by "sniffing off" an insignificant fraction of the current available to the ignition coil.

I suppose there is the  remote possibility that the wire to tach you installed as resulted in the brake light switch on the pedal grounding or applying voltage to the coil that upsets the ignition, but that is a really very remote possibility.

If the wire you installed is the only charge made, disconnecting the wire at the coil would eliminate any  interaction.

What happens when idling in gear with the handbrake on but no foot brake (car not moving) ? Just wondering if the engine idle RPM is set too low.  If you push the brake pedal down under these conditions what happens?

Do the "disconnect the brake booster" test and if the engine still stalls it's not the brake booster.

k.

[alabbasi]

Could it be your auxiliary fan kicking in?

[ESO]

UPDATES.....


Have done a few test this morning as follow. Checked the brake vacuum hoses, they seems to be okay.

Started the engine, wait until it warms up. Put the car in drive gear, handbrake on, then started pumping the foot brake and guess what? After pumping for a while, the idle speed started getting rough. Will my IDLE problem caused by a leaking brake drum?

Thx

ESO

[koan]

Started the engine, wait until it warms up. Put the car in drive gear, handbrake on, then started pumping the foot brake and guess what? After pumping for a while, the idle speed started getting rough.

Every vehicle with a brake booster that derives vacuum from the inlet manifold will do exactly what you describe. You mentioned the car had on occasion stalled when you had your foot on the brake, not when you pumped the brake.

The engine probably has a number of vacuum lines running off to various things, most MB engines have thermal vacuum "switch" some where on the engine that opens when it warms up, if the plumbing to this is disconnected or broken it would cause the poor running.

Rough running when warm could be an indication that a good tune up is required - by someone experienced with your engine. They should be able to find any vacuum problems.

Hopefully this would solve your warm idle problem, then if you still have the stalling with your foot on the brake, then worry about the booster.

k

[ESO]

Koan,

Thx for your advise, this issue really taking a lot of time just for investigating where exactly has problem. My friend actually help and made some adjustment to the engine by advancing the timing as well as adjusting the idle speed. Now, the engine seems to be okay but again I have to really spend time testing it.

Do you have any advise on how to proceed a very detail vacuum leak check for this car? I do not have much experience on this car.

Thank you

ESO

[koan]

There should be a section in the manual on the engine vacuum system and a diagram of your specific setup, do a search of the w116.org library.

The diagram maybe found in the engine compartment on the radiator support.

Read the manual and have ago, if you have a problem ask questions.

A useful tool is a hand vacuum pump.

I still think a tuneup by someone familiar with your engine will be quickest and easiest solution.

k

[ESO]

Hi Koan,

Thx again for your reply. Now that the engine has been tuned again and appears to be okay. However, according to some research on carburetor system, I have also noticed that the Webber 38 system seems to be good as a replacement. The price USD650 seems reasonable too as it can improve the fuel economy and reliability of the engine. Based on your knowledge, do you think it is worth doing this conversion?

Besides, I have been advised to check for Vacuum leak which was a good hint as the vacuum leak do lead to this rough idle.

Happy New Year.

Regards
ERIC