M119 rough idle
 

Hey all,

We're in need of some assistance on a car we've got in at the shop. Here's the scenario:

1992 500SL; Initial complaint was a rough idle, but the car was also due for its 30K (major) service. Completed the service then moved onto the idle problem. A lot of things pointed to evidence that this car hadn't been serviced in a LONG time which we noticed while doing the 30K. Anyhow, we found a number of things which needed attention, as follows:

-numerous vacuum leaks (which was clearly obvious by the high idle and roughness)

-dead injector on cylinder #7 (we replaced all injectors, reset lambda)

-distributor caps and rotors in poor shape (replaced caps and rotors - plugs were replaced as part of the 30K)

-all ignition leads checked out fine

-overload relay blown (replaced it)

-a couple of the air injection elbow connectors were cracked (replaced those)

-replaced any cracked vacuum lines (down under intake - main issue was no vacuum present at the EZL, ran a new line, good vac there now). Reconnected a line under the intake that was off.

-motor mounts checked out fine

Soooo......The miss is gone but we still have a slight rough idle, but it's rather sporadic. We cracked each line at the fuel distributor, had some rusty fuel come out, but it didn't really change anything overall once we "purged" them all. I don't think we have a fuel issue by the way the car is running. We sprayed some brake clean around the nozzle holders and the air tubes (which are still supple), and the idle will clear up. I scrutinized the air tubes carefully and I don't think we've got an issue there either so I'm leaning towards the injector holders. While we replaced the nozzles and seals, we didn't do the holders. Problem is that it's hard to isolate whether it's the holders or the air tubes without throwing parts at it, which we don't want to do. Everything else we did on the car was needed just to establish a "baseline" diagnosis since this car has been rather neglected. Additionally, the original miss was cured (idle cleared when we tapped on #7 injector with engine idling), but this "roughness" is still present....and driving us crazy! Reminds me again why I named my shop MB CLASSIC, not MB MODERN, hehe :D

Any of you out there with more experience on the M119s please fell free to chime in. Fortunately I've got a W108, 109 and a 113 in there now to keep me sane. :wacko:

This is a WAG so take it with a grain of salt.

30k on a 92 so this thing has probably sat a long time? Probably the fuel ain't so great.

How full is the tank?

Maybe topping off the tank with some 93+. Then run the living tar out of it. Italian tune up, thrash hard for 50-100 miles.

Maybe it just needs to run?

Aaron..if you have an infrared heat gun find the exhaust manifold runner that is the coolest..that would be your miss..Also if you have a SCOPE then use the parade & see if the firing voltage is equal.

MBDOC, how would a crack in the exhaust manifold cause an occassional miss? I'd have thought that the intake manifold was more relevant...

Not questioning the diagnosis, just curious about the cause & effect issue.

Thanks...

Infra-red heat gun measures the surface temperature of the item that it is pointed to. If you check THE EXHAUST RUNNERS ONE AT A TIME & COMPARE THOSE TEMPS the coldest runner suggests that that cylinder is misfiring..

Has nothing to do with cracks in the exhaust..

Thanks for the info. It makes sense that the coolest runner would be the one attached to the misfiring cylinder.

I misread your first post, there were no references to cracks in the manifold.

Did you do a compression test?

Just curious, I had one like that, turned out to be a cracked block.

any final results to solve the rough idle?

The fuel distributors on those are very problematic and frequently cause idling issues. Have you tried switching the fuel lines around to see if the problem stays on the same cyl or stays with the same port of the fuel dist?
Usually the M119 can tolerate a considerable amount of vaccum leak from the intake without a well definded miss.

If after everything has been checked and re-checked and still without resolution, it may need a valve job. As a Damn good M/B tech I can tell you that the Very Single BEST tech I've ever worked with reminisced about M119 needing valves and seats ground to cure rough idle problems when they were still under warranty. The guy was a huge pain in the butt,l but he was never wrong...ever, literally one of the best in the nation.

The reason I say that is coming from the marine side where its not uncommen to have to deal with engines that sit a lot. I have seen more then one that just runs like crap until a few hours are put on it under load.

Engines like to run.