300D intermittent rough start when warm - PeachParts Mercedes-Benz Forum

bplust · #1 07-25-2015, 08:05 PM

Hi all,

My 1984 300D (214,000 miles) recently began starting rough, when warm, occasionally. There doesn't seem to be any noticeable pattern to it.

It starts cold very well after a recent valve adjustment and plug change. It idles nicely, has plenty of power for the hills of Vermont, shifts as it should, and is very nice to drive.

However, if I'm running errands around town (I live 8 miles from town, so my car is up to temperature by the time I get there), sometimes it will crank but not start, or it will crank for a second, and start, but run rough with no power. A quick shutting down of the car and starting again makes the car run as it should. When this car starts as it should, warm or cold, it's a quick flick of the key to get it going. It starts up so quickly (when it starts properly) that it really is just a flick of the wrist. I feel the need to emphasize this because I have met many 123 diesel owners who feel that cranking for 5-10 seconds is "just how it is".

Any ideas as to what could be the cause of these intermittent starting issues would be greatly appreciated.

Thanks in advance for any replies,

Bryan

bplust · #2 07-25-2015, 08:12 PM

Here are some pics of the car from one fine day, when I used it to pick up a 70 year old two man chainsaw.

Zacharias · #3 07-26-2015, 03:08 AM

First question: when you try to start it hot, are you pressing down on the throttle pedal at all? My first diesel, a w116 300sd, behaved that way for years (particularly on hot weather) and I found I had to press the pedal down about 1/3 to overcome it. I am just throwing that out as my experience.

What intrigues me about your issue is the fact that turning the key back to KPO then attempting to start again apparently results in a quick restart -- if I am understanding you correctly.

I wonder if you have an issue with the vacuum module on your ignition lock and for some reason it isn't releasing the vacuum on the shutdown diaphragm on the injection pump until you turn the key a second time.

That is all that pops into my head, anyway.

vstech · #4 07-26-2015, 06:28 AM

My first thought is of alternate fuels. If it has been run on WVO, the ip could be worn.

Check linkage, if it is resting in the partially shut down position, that could do it...

Diesel911 · #5 07-26-2015, 10:51 AM

Idle speed?

I did not have the starting issue but had the hot rough running issue of which part of was My Fuel Inlet Hoses were hard and when the Engine was hot apparently the Plastic Filter warped under the hard Hoses and allowed air into they system. Replacing the Hose with 5/16" Fuel Hose from the local autoparts store fixed that part.

Back off your Rack Damper/Governor Idle Pin and adjust your idle to spec and then adjust the Rack Damper/Governor Idle Pin.

Look up the long thread on the Fuel Pressure Relief/Overflow Valve. The Spring inside gets compressed and if you have a valve that can be taken apart you can stretch the spring back to a free length of 27mm.

I am not sure if this is the beginning of a Fuel Supply/Lift Pump needeing to be rebuilt. Generic kits for that had were around $17 with the Bosch kits being more.
Mercedes sells a valve kit but there is no little O-ring in the Kit that is needed to do a rebuild.

Maxbumpo · #6 07-27-2015, 10:53 AM

I like the idea of an air leak into the fuel system, which causes the injection pump or the lift pump to loose prime. Does your car still have the original priming pump on the injection pump? They are white with a knurled handle on top, unscrew to free the plunger and then pump-pump-pump, then screw it down when done. The original primer pumps are notorious for leaks. The replacement is a smaller black cylinder, just push it down to operate.

Another possible leak source are the flexible fuel lines daisy-chained between the injectors and back to the fuel filter assembly. If any of those are wet with fuel, they need to be replaced.

bplust · #7 07-27-2015, 12:28 PM

Hi all,

Wow, thanks for all the replies! I will respond in order of received:

Zacharias:

1. No, I am not depressing the pedal at all. I will try this.

2. Yes, you are understanding me correctly. A quick flip back to "off" and then back to start will reliably start the car.

3. Possibly. Although sometimes it starts right up, sometimes it takes two times.

vstech:

I became the second owner of this car two years ago. I have detailed service records from the original owner, and I am confident that he never ran it on alt-fuel.

The linkage is good, identical to my other 300's, which do not experience the starting issues that this car does.

Diesel911:

Idle speed is consistent with my other cars.

I actually removed my plastic inline filter and haven't put it back on when I last did my filters. I had the cartridge, and thought I had an extra plastic one on hand, but was wrong, so I do not have one on at the moment.

I will look into your other suggestions. Thank you for all of the info.

Maxbumpo:

I assume you meant the original primer on the LIFT pump. If so, not, I replaced it with the black Bosch primer, as the original white one started leaking 8,000 miles ago.

Regarding fuel leaks, when I acquired the car, I replaced all of the rubber lines. About 140 +/- miles ago, my #1 (frontmost) injector started leaking very slightly when the two halves of the injector screw together. Odd... never seen this before. I tried snugging up the upper half with the injector installed in the engine, but this only slightly slowed the leak (I think). I'm probably going to replace my nozzles this week, as they are 214,000 miles old, and with the injectors out of the car, will be able to tighten them up properly in a vice.

This leak at my injector is the only fuel leak on the car.

Thanks again for all the replies, I really appreciate it!

Bryan

dkr · #8 07-28-2015, 02:23 PM

I ran into this same issue with my 84 300D last year. The three ordinary possibilities are the safety neutral switch, starter motor, and wiring / voltage drop. I am assuming the car has not been abused with WVO and maintenance has been done -- i.e. the car hasn't been sitting for a decade and recently resurrected.

You can generally test the safety neutral switch by putting the transmission in neutral and then trying to start it. If this is the problem, the engine should not be cranking in park. You would just hear a tiny click sound as though your batteries were totally dead.

The starter motor will usually start to fail at higher temperatures. If the engine starts to crank a little bit but is unable to start, it points to a starter motor failure. I put a rebuilt in mine and it failed on a road trip. Ironically, I think the original starter motor would have lasted longer, but unfortunately rebuilt Bosch and new Chinese are about the only options.

It is also possible you are not getting enough voltage to the starter motor. You could try putting your voltmeter on the batteries and tracing it down through the connections to the starter motor. You need about 12.5-12.6V to crank the engine.

Have you tried jumping the connections with a paper clip when you can't get the engine on?

Dkr.

bplust · #9 07-28-2015, 02:30 PM

Thank you for the reply, dkr.

The car, while it has low mileage for being 31 years old (214,000 miles), has never sat for more than 5 months. It was driven year round before I acquired it, and now that it's in my possession, it does not see salt. I live 1/2 hour from Canada in Northeast Vermont, and they salt the roads like crazy up here, and winter is long. So from mid/late November to late April, the car is not driven, I just start it up every couple weeks and drive in back and forth on my driveway to keep everything freed up.

I just ran outside and put the car in neutral, and it fired right up.

I do not think the starter is on the out, nor is it not getting ample voltage, as it cranks very fast, and even when I'm experiencing my starting issues, there is no difference in cranking speed; it just won't start or starts and runs rough with little power.

Bryan

dkr · #10 07-28-2015, 02:47 PM

Well aside from those issues, there's compression and glow plugs.

A W123 with good compression should start when the engine is at temperature without waiting for the glow cycle. Later versions of diesels time the glow plugs for this.

You can test it by getting the car up to operating temperature and then turning it on like a regular gas car, bypassing the glow plugs. If it cranks but does not start, it points to a compression issue.

If it does not start immediately without glow plugs, you could do another test by running 2-3 glow cycles before cranking the engine. If it starts up immediately without running as rough, it would point to a compression-related problem and that would be a good thing to check next.

The glow plugs come into play here slightly, but it would be more of an issue for cold starts. How old are the glow plugs?

Dkr.

bplust · #11 07-28-2015, 02:53 PM

I'll comp test it when I replace my nozzles (hopefully I'll get to it sometime this week).

It just seems very odd that it either starts RIGHT up, or it just cranks and either won't start or starts and runs rough. Shut it off, start it again, runs like it should. Strange. Hopefully I'll have some time this weekend to play with it.

dkr · #12 07-28-2015, 03:24 PM

Well, aside from that any issues with fuel leaks could cause the same problem. Once, I changed the pre-fuel filter and just had a real pain starting the engine again. I spent several hours without being able to fix it. It turned out there was a pinhole leak in the clear filter that you could never see.

Dkr.

bplust · #13 07-28-2015, 03:25 PM

I changed the main filter under 10,000 miles ago. For what it costs, I might just replace it again. I do not have the prefilter on my car at this time, so a leak in that wouldn't be an issue.

Maxbumpo · #14 07-29-2015, 11:58 AM

No pre-filter is not a good long-term condition, that filter protects your lift pump and also makes your main filter last longer (big stuff gets filtered out before it gets to the main filter). However, clogged filter is probably not the issue.

I like the idea of stepping on the accelerator pedal once before attempting to start, to make sure the fuel rack in the injection pump is not sticking at idle or stop position.

You could also try temporarily adding a clear vinyl hose section into the fuel return line, and watch for bubbles in that while the engine is running, and that will help determine if you have air leaking into the system.

I'm really leaning toward a problem in the injection pump itself at this point, which is probably not cost effective to replace just to solve an occasional no-start condition for which you already have found a work-around.