83 300D vacuum issues / rough idle
 

I posted this in tech support with no responses for a few days.

83 300D 230,000miles. I get the failure to turn off. I replaced a piece that fits the vacuum hoses on top of the engine and all was well. Then I had my mechanic replace all the engine mounts and dampers in an effort to fix a rough idle and now it's taking 10 seconds to turn off again.

All the hoses under the hood are making well seated connections. I did notice one line going into the firewall that you could pull in and out with ease. I didn't try to pull it out all of the way....

How do I track down the leak? Is this something for the MB Dealer? I'm starting to lose faith in my indy mechanic as he is honest but horrible at diagnosis apparently. (my rough idle has been going on for years. since beginning of year I've replaced injectors, had valves adjusted, fuel filters replaced, dampening screw replaced and adjusted, and the engine STILL rocks badly while in Drive with the AC on)

Thanks in advance.
Matt

There is one black vent line that goes through the firewall and vents under the dash. Is your "loose" line black?

You clearly have a vacuum leak again. The only way I know to track it down is plug one branch connection at a time untill you locate the one that's causing the problem. Pay attention to the soft tees and connectors, they are more likely to leak than the hard tubing.

Good luck

First step is to check the vacuum pump. Attach a vac. gauge to one of the line running off the main vacuum source line that goes from the pump to the brake booster. Plug the other ones. Start the engine and take a reading. It should be in the green and steady. If it is low or jumpy, rebuild or replace your pump after testing your brake booster.

If your vacuum source is good, then it has to be one of your components leaking off the vacuum.

Connect a mityvac type vacuum hand pump to the fuel shutoff valve on the back of your IP (a round metal bodied valve with one brown vacuum line entering it at a rubber connection). With the engine running, pump the hand pump a couple times. If the engine quits immediately and holds the vacuum, then you know that at least your fuel shutoff valve is good. If it does not shut off after a few pumps or if the vacuum leaks out, then you need to replace the valve.

If your valve is good, try plugging off each of the lines running through the firewall (the end towards the vacuum pump) in turn and seeing if your vacuum shutoff problem stops. That way you can isolate it to one system (AC, door locks, etc). Door locks is a common culprit.

Good luck

Thank you guys.

I'm going from memory here but I could have sworn the vac line I saw passing into the firewall was a multi color (brown dark red ?).

I am not a mechanic, but this sounds like something I could accomplish; I appreciate the guidance.
Matt

There is supposed to be one unplugged line...for whatever reason the AC system needs an atmospheric pressure line or something along those lines (no pun intended :D ).

It's usually black, maybe with a white stripe, but you would know if one of your major component lines was unplugged.

This vacuum diagram appears to agree with my 82 300D, I assume the 83 is similar:

http://www.ultrahot.com/vacuum/egr_82_300d.jpg

This shows the vent line as black, which agrees with mine. This may help you the other color lines. Also, my car has a vacuum diagram (with color codes) on a sticker under the hood. The black vent line isn't labled as "vent" on the sticker, it just ends.

Craig

have
 

you had a compression test? a low cylinder will cause rough idle. of course the 5 cylinders hop around quite a lot even when running correctly. did you do the engine shocks? you need a baseline for comparison, so you need to find a grizzlled veteran who has a known sound 5 cylinder and have him/her listen and you listen to theirs and yours and see if yours is hopping excessively. of course the shutoff prob is vacuum, the above advice is good there.

good luck

tom w

Thanks Tom. I regret working two problems into one post.

I am afraid the rough idle may be a bad cylinder as I DID replace the engine shocks, and everything else mentioned. The only thing I've read on these forums that hasn't been done is timing chain check and a compression test. Starting it right up, everything is fine, even with AC on. But run it for 10 minutes in this Florida heat so the engine warms up and I have to drop to N every stoplight lest I'm doing the Panzer Boogie. Also I drove my mechanic's station wagon for a few days while he replaced the AC last year and it had twice the mileage and idled like a champ.

The weird thing is I got 23.7 MPG before I replaced the injectors, driving in the city (no AC, optimal driving conditions to-from work at 45mph) a few months ago. I cannot imagine a cylinder is bad if I"m getting sticker mileage. :confused:

that
 

mileage might be attainable with a low cylinder. 45 mph is a pretty good speed for mileage if not too many stops.

tom w