1967 W108 250SE irregular idle - PeachParts Mercedes-Benz Forum

Tassie Tiger · #1 06-03-2006, 06:01 AM

Hi all,
During cold starts the idling is rough and low RPM's. I must rev the motor a few times for it to settle to an even idle. Occasionally when in drive, the motor hesitates and I must give it a hard rev which seems to bring the motor back to good power output. The cycle continues until the next cold start.
I have replaced all filters, points and plugs have been replaced, timing and dwell has been set to specification. Oil in the MFI pump is to the correct level. What could be the problem? I would appreciate your comments.
Also today I noticed that my oil pressure gauge is always at maximum pressure and the needle right against the stop whether at idle or high speed. When motor is turned off, it drops off to zero. What is causing this and how can it be rectified?
Regards,
Tassie Tiger

Arthur Dalton · #2 06-03-2006, 09:58 AM

Pegged OIL Pressure gauge is normal on that 129 engine. Be happy..

On the idle, there is a small hex plug on the CSV, It is a fuel test port . Take it off and turn the key to Run position and see if any fuel comes out the port. If it does , you have a leaking Cold Start Valve..
There is also a Warm Up device on the top of the pump that has an air filter . Those get plugged and will also cuase your cold idle condition. Try the CSV first and post back findings

Tassie Tiger · #3 06-04-2006, 04:52 AM

Hi Arthur,
I removed the small hex nut as suggested and turned the key to the run position. No fuel leaked out.
With regards to the small air filter, I can hear suction during the cold start. The suction ceases after the motor has warmed up.
Regards,
Tassie Tiger

Arthur Dalton · #4 06-04-2006, 05:01 AM

OK

A quick check of fuel flow would be my recommendation
Take the fuel line going into the filter at the the injection pump off.. [ the main filter]
Take some fuel line and slip it over the line bubble flare fitting and put the other end into a container.. turn the key to run position and fuel will flow to the container.. you are looking for 1 liter of fuel in 15 seconds time.
...Submerge the line in the container as it fills and watch for air bubbles.. there should be none..

See what you have.

ctaylor738 · #5 06-04-2006, 05:26 AM

I have learned that it it not unusual for the enrichment plumbing to stick. Here is a post from the 113 board that has information on how to take it apart and clean it, with some excellent pictures.

This is a really good site for the mechanically injected engines.

http://www.sl113.org/forums/topic.asp?TOPIC_ID=5151

Arthur Dalton · #6 06-04-2006, 05:43 AM

The warm up device has already been confirmed as working.

After the fuel flow test has been confirmed , my suspect will be the cone check valves on the pump lines leaking by.
There was a modification on the 250se injection pumps.
If you have the R18Y pump, you have the cone valves. If it is the R18z pump, you have the modified Ball valves .
The earlier cone valves would allow the pressurized fuel in the injector lines to leak back into the pump and drain the injection lines over-night.. so , at first cold start, the lines have to bleed/re-pressurize themselves....
If you have the earlier pump [ pre 6/'67 - just check the last pump R #] , you can replace the cones valves with the ball type . I have those part #s , if needed .. or you can go to the same 113 Forum C.Taylor mentions and look them up ..
http://www.sl113.org/forums/topic.asp?TOPIC_ID=3557&SearchTerms=ball,valves

All the subsequent pumps had the ball valves and that eliminated that problem [ they also found that they could eliminate the rack solinoid at the same time, as the ball checks solved both problems]
Another trick MFI guys have learned to use is to turn the key to run position for 15 secs before start to allow the electric pump to pre-pressure/clear all the lines from the tank
http://www.sl113.org/forums/topic.asp?TOPIC_ID=5196
.. this does help starts, but does not remedy the cone valve problem.

Tassie Tiger · #7 06-04-2006, 07:17 AM

Thanks for your responses.
I will check the fuel flow during next weekend and advise. My MFI pump is R18 Z so I assume it has the modifed ball valves. Is it possible that some of them are leaking and hence the uneven idle at start up from cold? I will also check the internet link for information on enrichment rods.
Regards,
Tassie Tiger

Arthur Dalton · #8 06-04-2006, 11:18 AM

If the ball valves leak back, they can be cleaned with a carb cleaner.
They are just a check valve and dirt is usually the cause for them to mal-function.
The cone valves were just a poor design type to use as they are prone to wear spots at the same sealing surface, whereas the ball don't.
The much later pumps [R22 >] increased the balls to 4mm, but that change was not due to leakage.
I see you have done much since starting to work on your system and you have a good knowledge of the parts and their functions.
I have a PES pump Crono/Tech article that is well written and covers these third generation pumps. You may want to have it for your reading.
If you have any interested in such, PM me and I can give you that info.

Tassie Tiger · #9 06-06-2006, 05:39 AM

Thanks for your advice Arthur.
I will try some cleaner additive over the weekend. I used to use the car as a daily driver, however, now only used during weekends so it may take some time for the cleaner to do its magic.
I would be interested in learning more about the injection pump and would appreciate receiving any further information.
Regards,
Tassie Tiger