Rough idle/bogging on accel.

I've been working on my recently purchased 79 280SE and have some troubles. I replaced the plugs, wires, distributor cap and rotor. It is still idling rough, although not as bad as it was. When I give it gas from a stop it bogs down, almost dies, and sounds like it is pinging a bit. After a few seconds with the throttle down it picks up and accelerates (not very fast but that is another issue). When it does build up some revs it seems to run very smoothly.

Any ideas what the trouble could be? Vacuum leak perhaps?

Vacuum, mixture, and timing are the big ones. The one issue with the m110 engine is it is quite finicky and my car will barely run if something is incorrect in the tune.

Check the vacuum lines first though. Look for bad rubber fittings and cracked lines. There are ways to test for leaks. You might also have to find someone to adjust the CO with a smog machine.

Another issue with the car is CIS fuel injection and fuel contamination, this system is very dependent on fuel pressure to work correctly and clogged filter/strainer will lower fuel pressure. Without proper pressure the mechanical injectors won't open. You can actually take a long screw driver to the injectors and listen to them opening and closing. The injectors will make a distinctive buzzing sound as they open and close with fuel pressure. You might want to put some fuel injection cleaner and fresh gas in the car if it sat, but be careful not to use too much cleaner as you can dislodge crud and clog the fuel distributor.

If the car give you real trouble you might need the Probst CIS book and a fuel pressure tester. About a 70-80$ investment, equivalent to 1 hours labor at an independent MB garage.

Here is a link to CIS troubleshooting guide from the benzworld w116 forum.

http://www.benzworld.org/forums/w116-s-se-sel-sd-class/1223523-bosch-d-jetronic-k-jetronic-troubleshooting.html

Good luck!

What he said^^^. Bad vacuum would stop my car from running. While timing set for high rpm in the race car leads to some stumbling and pretty much on/off throttle at idle. If it is a street car, I would think timing would be the issue, unless there is a leak/crack in your vacuum hose.

Steven

Those old Benzes will leak vacuum EVERYWHERE. CIS does NOT like vacuum leaks. As mentioned earlier it's also important to verify you have the correct fuel pressure at the correct time or you are shooting in the dark.

When checking vacuum, be sure to check the A/C and door vacuum systems. In fact, it's probably a good idea to disconnect those and cap the hoses, see if that helps. I've seen more than one car where there was a vacuum leak in the A/C controls and someone has fiddled with the mixture trying to overcome that. That means when you get the vacuum system fixed the car runs like complete crap. So if you disconnect the A/C and door vacuum systems, tune the car and it runs great then runs like crap when the A/C and door systems are reconnected, you have some more fixin' to do. Good luck with replacing the defroster flap vacuum motor, it's a bitch.

By the way, this kind of thing is why Mercedes went with an electric vacuum pump on later cars. That way if a door vacuum motor barfed the car would still run. But of course it winds up making the pump run all the time, causing it to burn up. Can't win for losing.

I also saw a vacuum booster which leaked between the M/C and the booster. Didn't hear a thing, but spraying carb cleaner around the MC/booster junction would change the idle.

Having a car that was tuned around a problem is an issue also as Jensenman said. I had a vacuum line to the transmission become disconnected and the car went erratic. I had my mechanic retune mixture so the car would run correctly, when I finally found that the vacuum line was disconnected to the transmission I plugged it in and the car stalled out rich. I had to have the mixture retuned once the vacuum line was hooked up to the transmission.

When you get it running correctly it will be worth the trouble as mercedes in proper tune are pretty reliable.

How about that? A Mercedes with a CANOE stuck in its CIS.

Jensenman wrote: Those old Benzes will leak vacuum EVERYWHERE. CIS does NOT like vacuum leaks. As mentioned earlier it's also important to verify you have the correct fuel pressure at the correct time or you are shooting in the dark. When checking vacuum, be sure to check the A/C and door vacuum systems.

Hmmm...the vacuum door locks don't work, I'll check that out tomorrow if I get a chance.

Welcome to CIS land.

I'll trade you some pressurized smoke testing time for a bottle of Chimay Blue.

I may have to take you up on that. How about a bottle of Stone Double Bastard instead, I already have one in my fridge .

I'm gonna go home on my lunch break and bring the Merc back to work to change the fuel filter and hunt for vacuum leaks since it is dead here and I am working till 8pm.

There used to be a cheap CIS fuel pressure tester but it went the way of the dodo. The cheapest one I found so far is like 100$ so the 50$ tester went away.

CIS is really a simple fuel injection system, the problem is getting all of the parts to play nice together. Not to mention almost every part in the system is over 100$. I don't understand why people think it is rocket science as there are only like 10 parts to it. The only form of fuel metering happens in the air throttle body(sensor plate) which basically controls the amount of fuel flow from the fuel distributor to the injectors.

If you are going to do the fuel filter you might want to change the tank strainer at the same time, I hear it is a bitch to do and you should plan on changing the fuel hose from the tank also. If you see any disintegrating fuel lines it might be a good idea to change them as the working pressure for the CIS system is only around 80psi.

Benzbaron: Do you have a picture of the throttle linkage in your car? The linkage coming from the firewall that has two "fingers" has a ton of slop in it. I imagine there should be a bushing there but I don't know what it is supposed to look like.

Full throttle in the car is about 1/4 at the throttle body.

I'll see if I can get you one later, but my throttle has been fooled with so it isn't the best example. I'll see if I can hotlink a photo from the EPC for you.

If the throttle is bonked the first thing to check is that it is mounted correctly to the firewall if it is like the 107. On my car the throttle mounts to the firewall using a round plastic ball which is compression fitted into the firewall. What happens is the ball pops out and slips down which doesn't transfer the motion from the linkage to the throttle. I think there are like 14 pivot point in the throttle linkage, wonder why people are going to drive by wire.

I found the pic you need on the EPC but can't figure out how to copy it. The russian parts site is down also. I'd sign up for EPC(it's free!) and go under w116.024(280se). The pic you need is under "control."

I think this site will work but you better brush up on your russian

http://www.detali.ru/webCat2.aspx?MID=QZX&RID=TOY

Maybe it won't work!

EvanB wrote: I may have to take you up on that. How about a bottle of Stone Double Bastard instead, I already have one in my fridge .

The stone will work, just so long as it's a 750ml.

Get the obvious leaks dealt with first, the smoke is a great way to locate obscure cracks and gasket issues.

Well I didn't have time to replace the fuel filter but I did take a look at it and it looks fairly new. The bogging lessened after driving it for awhile, I can now put the throttle down from a stop and get only slight hesitation. It is still running extremely rich though, as in fuel out the tailpipe/gas fumes burning your eyes.

I shimmed the throttle linkage with some folded paper temporarily. I can now get a little more than 1/4 throttle and it did help greatly. I can only imagine what it will be like when I can go full throttle.

On this picture I am missing at least #17, maybe more. I'll have to take a closer look tomorrow.