1972 Mercedes-Benz SL-Class #- Idle issues

Engine idles fast, then idles slow.
Also, has high oil pressure. (May be unrelated). Not driven much in past year.

I doubt they are related.
On a 72, carbureted, correct? Is it gummed up? Does it have original throttle return spring? The spring might be getting weak.
Oil pressure: You might not be changing the oil frequently enough.
Your oil pressure gauge might be faulty. Unusual to get high oil pressure on an aging engine.

Nope, they had Bosch fuel injection. Time to find a old MB mechanic wizard.

I would check around for vacuum leaks.


The dreaded CIS f.i. system?



Thank you both. Will search for vacuum leak, then for grey-haired MB mechanic wiz.


The engine doesn’t have an oil pressure relief valve, so that means the oil filter does.

Try replacing the oil filter.


Consider a vacuum leak as mentioned and I believe these things also used an idle control module. Maybe the latter has decided to go belly up.

I’m not a Benz expert but I think the fuel injection is similar to the early VW systems. This means there are trigger points for the fuel injectors and those are located in the ignition distributor underneath the ignition points breaker plate. Those trigger points are one of those out of sight/out of mind items.

Maybe the trigger points have given up the ghost by years of arcing, fiber blocks being worn down, or springs broken/weak. Here’s a pic of the trigger plug assembly.


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Excellent suggestion. Thank you for the detail and the photo.


This car predates that by several years

100% dinosaur

It might be Bosch D-Jetronic . . . in any case, EXTREMELY EARLY efi

Dinosaur it may be, but still far better than CIS…

CIS came along about 1977 or so and was used by most European car makers until way up into the 80s.

One of the biggest curses of CIS is fuel injector poor spray patterns and suffering from the human trait of incontinence.

1972-1975 used Bosch D Jetronic in North America

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The Porsche 914 also used the D-Jetronic starting in’69.

cis and the later cis-e were great WHEN they were working properly

As long as there were no vacuum leaks

As long as somebody hadn’t buggered up the adjustment(s)

As long as the idle air control valve was working properly

As long as cold start was working properly

As long as you didn’t have to retrieve codes

And so forth


Do they make Depends to fit CIS?


Nope; no Depends but it could have used them. Cleaning and testing fuel injectors along with replacing them if necessary was a good paying gravy job. Two hours on a 4 cylinder VW.
There is a special cleaning and testing tool used for that purpose but as with many of them the injectors needed to be replaced as even after pressure cleaning the spray pattern was all over the place and they would dribble fuel out a drop at a time.

db4690 mentions vacuum leaks and that is correct.
The way the system works is that it relies on incoming air to raise an air sensor plate which moves a plunger in the fuel distributor.
Think of a soda straw with a tiny crack in it and you’re trying to suck soda out of a cup. Can’t do it. Same principle with CIS; any air leak can cause the sensor plate to become balky at the least or refuse to move at the worst as the air plate is working against fuel pressure and engine suction can’t overcome it.
Some cars would idle a bit rough because the owner did not fully seat the oil dipstick after checking the oil level.

As mentioned, great when it’s working. Most of the time somewhat problematic.