One of my 240Ds starts great in the cold, the other starts terribly - PeachParts Mercedes-Benz Forum

mabbonizio · #1 12-26-2018, 09:11 AM

I have two 240Ds. One is an 82 123 240D with 170K miles, the other is a 72 115 240D (motor swap) with unknown miles.

The 82 starts right up in just about any temp and idles smooth.

The 72 will start in colder temps, but really chugs away for about two or three minutes. Literally the entire car shakes side to side until it smooths out.

Both cars have all their services up to date so neither are neglected.

I would think this could be indicative of low compression however here's the kicker.

82 - burns about 1 qt of oil per 1k of high way driven miles.

72 - burns zero oil. Literally the oil level never changes between oil changes, this makes me think the motor has decent compression. Thoughts?

Could the 72 start so poorly in the cold due to the loop style glow plugs?
I do have what looks like a tiny fuel leak at injector 3. I'm wondering if the pre-chamber loosened up when i changed the injectors and it has a minor compression leak?

Maximan1 · #2 12-26-2018, 10:29 AM

How are the glow plugs wired on the '72? My '79 had loop style glow plugs, but they were wired incorrectly so that only the first and last plugs were heating. Until I fixed it, the car would be very difficult to start in the cold and shook a lot until the middle two cylinders started firing. I suppose that an engine with better compression would have a more pronounced shake.

Just my .02¢

t walgamuth · #3 12-26-2018, 10:31 AM

Yes, I agree with the suspicion of glow plug function. If wired correctly the loop style will be in op if one glow plug is bad.

barry12345 · #4 12-26-2018, 11:37 AM

You may have too much fuel drain back. Creating issues with start up. A check for air flowing out the relief line from the injection pump with the fuel when first starting up is something I would also check. There should be none. Otherwise the engine is not getting as much fuel to start on as it should.

The valves in the lift pump are supposed to act as anti drainback as well for example. I have wondered for a long time just how many times this has been missed with posters having cold start difficulties.

The loop plugs are not as efficient overall as the special pencil type including the size adaptors that can be retrofitted to your engine. If the car is going to be used in really cold conditions they should be considered. Since the one car I have has them. Plus is not used in colder weather. I have no issue with the loop plugs remaining.

So a gradual deterioration can exist. Yet fortunately it involves very little money when checking things out yourself and rectifying anything that is found to be substandard. At thirty to forty years of age the possibility that something will be found is pretty fair. If not found you know that everything is good.

I also wanted to mention the availability of the pencil plugs with the larger bases. May soon be harder to even find.

Cranking to basically reprime the fuel system is hard on the battery and starter. Then the engine running rough until the air is eliminated. Low existing fuel pressure in the base of the injection pump is not going to help load the elements well either. It is unlikely but you can have an engine that burns little base oil. Yet the valve seats can be a mess. So the compression is not great.

These engines are pretty straightforward still simple things that should be checked from time to time do not get done. That they are very old means the possibility also exists they never have been.

Stretch · #5 12-26-2018, 11:38 AM

Once you've stopped fiddling about with glow plugs and you know they are working it is time to consider servicing checks such as valve clearances, compression check (might be worthwhile), but most importantly it is time to check the timing chain elongation in accordance with the FSM - this is the so called 2mm lift test.

It is always best to check the camshaft position / chain elongation before even considering the fuel delivery side - the timing chain "health" has a direct effect on the injector pump position - by correcting the chain by either replacement or correction via the offset keys (see FSM) you may not even have to mess about with drip tests. (Which for many a novice is an opportunity to end up with something functioning worse than what you've got now!)

Diesel911 · #6 12-26-2018, 12:30 PM

People have said that updating to the pencil type adapter glow plugs had a significant difference on starting. But, those plugs are also getting harder to find.

As Stretch indicates there is a bunch of things that can effect the ability to start.

On the engine that burns Oil people have said that if the little O-ring is bad in the fuel supply lift pump it sucks oil out of the Fuel Injection Pump and mixes it with the Fuel and you burn it.
Another oil loss can be leaks.

You might try switching to synthetic Oil for one period and see if it is possible that you are having some sticking piston rings. People say the synthetic oil frees them up in about 200-300 miles of driving. I don't know if the synthetic oil is more resistant to burning or not.

Concerning what barry was speaking of on the fuel leaking back to the tank from the fuel supply/lift pump. Before you start do a lot of pumping on the Hand Primer to be sure the fuel is up from the tank to the lift pump and see if it starts and runs faster. If it does then there is a good chance the fuel is leaking back to the tank. The lift pump repair kits don't cost a lot.

Diesel911 · #7 12-26-2018, 12:33 PM

I did not read all of the posts in detail. The fuel pressure relief/overflow valve controls the fuel supply pressure inside of the fuel injection pump housing and that is what feeds the fuel into the elements. If that is not holding the correct pressure you can also have a bad idle.

On the ones you can take apart you can stretch the spring till it has a free length of 27mm (at leas on the 300Ds and it usually restores the pressure holding ability. Don't loose the ball bearing that is used as a valve.

t walgamuth · #8 12-26-2018, 01:37 PM

I'd check glow plug function first, its the most likely and the easiest to check.