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1979 300SD Only Starts When I Use Starting Fluid

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Squiggle Dog's picture
Phoenix, AZ
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1980 300SD
1967 230

I received a free 1979 Mercedes-Benz 300SD the other day. Yay! It was on craigslist, and the owner said it could be driven home. Well, it had no alternator, belts, air cleaner, exhaust, fuse box, etc., so I decided I would tow it home instead.

The guy gave me an almost identical 450SE parts car with it (which was in great condition, and made it home and then wouldn't start, but that is another story) from which I borrowed some parts.

I put a battery up to the 300SD, and it cranked and cranked and cranked (very slowly, even though the battery was charged up). It just would not fire. But I shot a little starting fluid into the turbo, and it started right up. Pretty quiet for having no exhaust, too. I took it for a drive, and it drives well and has a LOT of power. The turbo is quiet and does its job.

If I shut off the vehicle and crank it over again right away, it will start up. If I wait a bit, it has to crank a lot before it starts. And if the vehicle sets for a while, it will only start if I use starting fluid.

If I give it a quick shot of starting fluid, it starts up and runs fine. But if I don't use it, it will not start. I'm not sure why.

The odometer shows about 175,000 miles. If that's true, then this engine should have almost no wear. The guy I got the car from said he replaced the glow plugs and relay. I noticed that what I think are the injectors are leaking at the base and where the braided hoses attach on each side (this is on the engine, they are in the same place a spark plug would be if it was gas).

When the engine is running, I can see bubbles coming out of the base of those injector things, and they are leaking a lot of fuel (that's why the engine is so dirty nasty). Could these parts be the cause of a no start?

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Squiggle Dog's picture
Phoenix, AZ
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1980 300SD
1967 230
Re: 1979 300SD Only Starts When I Use Starting Fluid

The glow plug light comes on, but I'll check out the glow plugs and see if they glow. I can probably buy new hose for the diesel injectors, and o-rings if they have them. Then at least they won't leak and get the engine all dirty.

Don't use starting fluid on the engine, eh? The guy I got it from said that's how he had to start it. Good thing I didn't decide to add a few cans of ether into the fuel to help it start better, huh? Haha.

I'll inspect and replace a few parts (including a larger battery) and stay away from the starting fluid. When I used it, I only did a very tiny puff, and then waited for most of it to evaporate first, and never tried it with the plugs warm (so that's probably why I didn't blow it up.

It might be a while until I do much work on the vehicle, as I am currently working on getting my 1967 230S fintail driveable.

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pch2021's picture
Rockledge, FL
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Re: 1979 300SD Only Starts When I Use Starting Fluid

jsut because the light comes on, it doesn't mean they are ok. Iwth this type of system, one good plug will intitiate the light.

I would start by changing all 5. they require a 12mm wrench and an 8mm wrench.

then changing the oil.

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Pierre Hedary
...happily servicing vintage Mercedes-Benz in sunny Florida...
1972 280SE 4.5 252K
1970 280se 3.5 coupe 108K
1971 300sel 4.5 121K
1991 420SEL TMU, but looks brand new!
Mercedes technical advice: 407 765 2867

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lkchris's picture
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Re: 1979 300SD Only Starts When I Use Starting Fluid

VLayton;121710 wrote:
Sounds like glow plugs.

Depends a bit on whether it will start when hot.

An engine that's run for a while and then shut off should restart in a minute or two with very little help from glow plugs.

Could be lack of compression, too.

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Dusty-NZ's picture
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Re: 1979 300SD Only Starts When I Use Starting Fluid

The fuel leaks are a big signal that air is also getting into the system. It will be hard to start because of the air.
You add a little starting fluid and the engine spins fast enough to prime itself again. Stop all the fuel leaks.
Do you have the correct battery , I have found this makes a difference to the speed that the engine spins over at , so that effects the starting ability also.

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Re: 1979 300SD Only Starts When I Use Starting Fluid

Your glow plugs are wired in series. When one goes out, the whole circuit is out. To check the plugs properly with an ohm meter, you must disconnect the wire to each one before you measure its resistance.

When you find the bad one, replace it.

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Virginia Beach VA

87 300SDL
95 E300D

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Squiggle Dog's picture
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1967 230
Re: 1979 300SD Only Starts When I Use Starting Fluid

Thank you for all the advice. I'm using a battery that is a lot smaller than what the vehicle needs--the engine cranks VERY slowly. I think that I remember it starting right up when I jumped it from another vehicle. I don't think I used ether that time. So yes, the battery may have a lot to do with it. I'll have to jump it again and see if it starts without the ether.

As for the fuel leaks, I wondered if maybe air is getting in there and not giving it enough fuel. Someone told me once that if a diesel runs out of fuel or gets a leak, that it has to have the lines purged for it to run again because of air bubbles.

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Re: 1979 300SD Only Starts When I Use Starting Fluid

If you have any thoughts of keeping the car beyond next week, you would do well to invest in the technical documentation for it.

Everything available is $19.99 a set at: http://www.classictechlit.mbusa.com/MB/

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Virginia Beach VA

87 300SDL
95 E300D

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Squiggle Dog's picture
Phoenix, AZ
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1980 300SD
1967 230
Re: 1979 300SD Only Starts When I Use Starting Fluid

Thank you for the link! Now I know where to get the service manual. It looks like they also have the manual for my 1967 230S.

I've learned shop manuals are a must-have. I planned on getting a manual for it anyway (especially since it is a Mercedes-Benz, a modern car by my definition, and is a diesel [which I am not familiar with working on, but I think I will grow to like]); I was just interested in getting the vehicle operational in a short time frame if it was a quick fix.

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1987 300TDT
Re: 1979 300SD Only Starts When I Use Starting Fluid

Never ever ever use ether to start an MB diesel, unless you are willing to take the risk of complete engine destruction with no warning.

I completely agree with the battery as the major factor in 'no start'.

The fuel leaks described thus far are likely failed fuel return lines (braided rubber lines that daisy chain from #5 injector back to main fuel filter) and NOT any problem with the injectors.

Also, valves may be out of adjustment/too tight, which reduces compression to the point that starting gets difficult.

Test glow plugs by measure resistance to ground, which should be around 0.7 ohms.

Diesels are very different, and MB diesels even more so. You will have much to learn if you want to properly DRIVE and maintain this car (yes, even driving a diesel must be done differently than a gasoline car!!).

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Max Dillon
Charleston SC
'87 300TD, 334k miles (need to pull the head again...)
'95 E300 Diesel, 335k miles (daily driving duty)
'73 Balboa 20

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