The dreaded engine won’t shut off problem
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Joined: Dec 2025
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1985 300D Turbo 65000 original miles
The dreaded engine won’t shut off problem
My vacuum pump on a 300 D turbo is pulling 21 to 22HG but the nipples to the supply line are reading 6 to 10 HG. check the valve on the back of the injection pump holds vacuum and shuts off immediately with a hand pump. also blocked off the locks which worked perfectly and green line with golf tees still won’t shut off. Change the brake master cylinder round gasket on the brake booster still won’t shut off. my brakes feel normal, but maybe a little spongy possible the diaphragm in the master brake booster. I can get the car to shut off if I rev the engine to 2000 RPM and then turn the key off and shuts off. Took the check valve off the vacuum pump cleaned it real good still shows the star and the check valve is working normally. i’m thinking the supply line so I clean the nipples on the main brake booster supply line but still pulling not enough vacuum. when I disconnect the vacuum line off the back of the brake master cylinder and plug it and turn the key off. It shuts off immediately. if anybody’s got any ideas, let me know I’m open.
Junior Member
Joined: Jan 2004
Posts: 24
Likes: 3
From: California
1983 300TD, 1998 SL500 (sold), 2013 GL450
Have you checked the vacuum fuel shutoff? If this thing is leaking no amount of vacuum will cut off the fuel supply.
https://www.autohausaz.com/pn/142713...LglSDI1bZJocDj
https://www.autohausaz.com/pn/142713...LglSDI1bZJocDj
Junior Member
Joined: Jan 2004
Posts: 24
Likes: 3
From: California
1983 300TD, 1998 SL500 (sold), 2013 GL450
Maybe the vacuum pump? Increasing revs would
make the vacuum pump cycle faster, if you’ve already troubleshot the vacuum circuits and everything else has checked out good, maybe the pump is marginal.
make the vacuum pump cycle faster, if you’ve already troubleshot the vacuum circuits and everything else has checked out good, maybe the pump is marginal.
Thread Starter
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Joined: Dec 2025
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1985 300D Turbo 65000 original miles
Yes, I checked out the vacuum pump even the little valve down by the vacuum pump itself with the star and everything seems to be working good. Yeah the vacuum pump is pulling about 20 to 21 HG. it might be the key switch itself, but I already checked the solid brown line with the key on it Holds vacuum,when I turn the key off it drops. It just seems like it’s not getting enough vacuum off the little fittings on the main vacuum line it’s self. when I checked that line that goes to the three-way valve to the door locks the shut off and the climate control. It seems to be pulling about 11 HG. So yeah I probably will get another vacuum pump. That’s very difficult to find but Mercedes source out of Washington state seems to have some good products. I even cleaned those little nipples off the main vacuum line as well. But I know they shut off is good because it holds that vacuum and shuts off once I pump the hand vacuum pump. It’s always like that on these 617 engines but I sure like the car it’s fantastic and it’s the best car ever made. It’s just 40 years old but looks new only 65,000 Miles. and last summer I even replaced the shut off valve at work fantastic. I just think it’s a very very small leak. I don’t know if it’s from the EGR system or not I might block that whole system off.
Junior Member
Joined: Jan 2004
Posts: 24
Likes: 3
From: California
1983 300TD, 1998 SL500 (sold), 2013 GL450
When was the last time you replaced the brake booster line?
https://www.autohausaz.com/pn/1234307429
It sounds like you're getting adequate vacuum off the equivalent of the front of this line, but losing suction at one of the tees if I understand you correctly? This is certainly cheaper and easier than replacing a vacuum pump. You said that when you block off the end of this line to the master cylinder, everything works fine- do you think the brake booster itself leaking?
https://www.autohausaz.com/pn/1234307429
It sounds like you're getting adequate vacuum off the equivalent of the front of this line, but losing suction at one of the tees if I understand you correctly? This is certainly cheaper and easier than replacing a vacuum pump. You said that when you block off the end of this line to the master cylinder, everything works fine- do you think the brake booster itself leaking?
Thread Starter
Newbie
Joined: Dec 2025
Posts: 6
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1985 300D Turbo 65000 original miles
Last summer, I put a brand new brake master cylinder, plus the new O-ring that goes on the back of the master cylinder. Before I did that I took the master cylinder out of the car, cleaned it real well because there was some brake fluid leaked inside the brake booster. I checked the diaphragm and everything and it seems to be working on the brake booster plus I have great brakes now so that seems to be testing. OK I think you’re right. It may be the break vacuum booster line. I haven’t changed that and that’s an idea I was looking into. Thanks for the info on that, i’m here in Italy and it’s hard to find parts. I have to get them shipped from America if you could believe that crap! and there are some places in the UK that send stuff to me as well. I use Mercedes source out of Washington, but the shipping is astronomical, but he has incredible stuff for these cars. I put his electric diesel pump modification in and it sure makes a difference.
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Junior Member
Joined: Jan 2004
Posts: 24
Likes: 3
From: California
1983 300TD, 1998 SL500 (sold), 2013 GL450
Sounds like you were able to seal some small leaks. Congratulations, no easy feat. I’d call having to rev a couple hundred rpm to attain adequate vacuum a success. I remember vacuum pumps were getting hard to come by, I was able to score one a couple years ago shipped from Latvia via eBay, in case you ever need to go that route.
Thread Starter
Newbie
Joined: Dec 2025
Posts: 6
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1985 300D Turbo 65000 original miles
Yes sir you’re absolutely correct. It sure gets frustrating sometimes with these W123 turbo diesel but it’s a fantastic engine and the car of course being over 40 years old. I was lucky and found this car on an auction site in America with very low miles👍 I had the same car back in 1982 brand new and I thought one day I’d sure like to get that car again,It was so much fun to drive. I may look at a new aftermarket URO boost line like someone had suggested in the thread so I’ll be looking into that later but right now though it’s shutting off with just a couple hundred RPM, which is for me perfect. Also it’s getting cold here in Northern Italy and I think that had something to do with some of the vacuum lines shrinking a little bit but who knows. You don’t see these W123 in Europe much anymore and people always try to stop me and want to buy the car.🤣 if you own one of these diesels I would suggest putting in that Aux boost pump that’s for sale on the Mercedes Source website,sure makes a difference because the lift pumps are getting old on these mechanical injection pumps.I noticed a change in horsepower as well, especially after adjusting the valves and doing the regular maintenance.
