Why does my diesel run fine but start so poorly?
December 8, 2009 2:02 AM   Subscribe

My diesel-powered car doesn't want to start but once it does, it runs fine. What's up with that?

I've got a 1984 Mercedes Benz 300D that for the last two days doesn't want to start but, after a few touchy seconds, runs fine. Right after starting, it seems like it isn't getting fuel or air or something. It just runs very wispy, in fact "gasping" is the word I want to use. I can have the pedal to the floor and it barely stays running.

Then, all of the sudden, it starts revving and running better.

Possibly useful info: we are in a sudden coldish snap (dipping below 30 F), the fuel is a few months old (more than 3 but less than 6), and I just had an oil change a week ago, before this started, and nothing was noted.
posted by esereth to Travel & Transportation (13 answers total)
Could be the fuel pump, I saw the same on a Toyota diesel when the bearings in the fuel pump electric motor were going... After the pump "warmed up" a bit it would work.

Are you letting it sit with the key turned three-quarters of the way, for at least 30 seconds before cranking to warm up the glow plugs?
posted by thewalrus at 2:23 AM on December 8, 2009 [1 favorite]

replace glow plugs? these things wear.
posted by gijsvs at 2:53 AM on December 8, 2009

Best answer: This suggests that summer-grade diesel fuel can begin to cloud with wax crystals at temperatures as high as 40F.

This says that difficulty starting is a possible consequence of these crystals, which can restrict fuel lines and filters.
posted by jon1270 at 4:02 AM on December 8, 2009 [1 favorite]

I think I remember my 84 300SD owner's manual suggested blending a little regular unleaded gas with the diesel fuel to prevent waxing or gelling. Diesel fuel coagulates in the cold and it doesn't flow nicely to the engine. Thats the source of lots of cold weather hard starting when it comes to diesel engines.
I'm going to vote for glow plugs, too. Since diesel combustion relies on heat and pressure, instead of spark as in a gas engine, the glow plugs heat the combustion chamber before the car starts and for a little while after in order to optimize the heat in the combustion chamber. If those glow plugs aren't working, it'd be hard to start the engine.
I'd change the glow plugs and the glow plug relay. If you're handy, you can do it yourself in a few minutes.
posted by Jon-o at 4:15 AM on December 8, 2009

Best answer: Glow plugs could be it. Also, I find it helps when it's really cold to cycle the glow plugs two or even three times before trying to start it.

I've never driven an MB diesel, but don't they have cold-start knobs? If so, are you using it?
posted by bricoleur at 4:25 AM on December 8, 2009

And have you drained the water from your fuel filter lately?
posted by bricoleur at 4:26 AM on December 8, 2009

nthing replacing the glow plugs. This has cured cold weather start problems on two diesels I've owned in the past. I'd also try mixing some new diesel in with the old, or replacing it altogether if possible.
posted by ruperto at 5:29 AM on December 8, 2009

Best answer: Glow plugs, totally.
posted by Pollomacho at 6:48 AM on December 8, 2009

I would pick up the most recent copy of Popular Mechanics as this month's edition has an extensive section on diesel motors and the maintenance needed. They suggest checking the fuel filter and air filter as these two components can greatly affect the performance of the vehicle. Or, as others have said, it might just be the cylinder not getting warm enough.
posted by kasperj74 at 8:28 AM on December 8, 2009

In addition to the plugs, consider changing the air filter.
posted by Thorzdad at 9:08 AM on December 8, 2009

Best answer: I have a 1983 300D with over 360,000 miles. The colder it is, the longer I let the glowplugs heat up before starting it.

When you turn the key to the glowplug position, you know how the glowplug light illuminates, then goes out? Well, the glowplugs are still heating up even after the light goes out. If you listen carefully, some seconds later, you'll hear a click as the glowplug relay finally stops the current. When you hear that, turn they key off, then back to glowplug position again. Let the relay click off a second time, and then start the car. I bet it's better.

If that doesn't solve the problem, it's also possible you have one or more nonfunctioning glowplugs. It's pretty straightforward to troubleshoot and repair, for the gory details and photos look here:
or you can visit a dealer.
posted by 2xplor at 10:58 AM on December 8, 2009

Best answer: Also, if your fuel filters were bad, you would notice problems at all times when driving the car, not just upon initial startup.
posted by 2xplor at 12:01 PM on December 8, 2009

Response by poster: Okay...

The summer fuel thing makes a lot of sense and fortunately I was pretty low. So I'll filled 'er up. I think letting the glow plugs go extra long is really helping, too.

The glow plugs I will try and tackle this weekend, just to be on the safe side. The air filter is new so I won't touch that. I hadn't even heard of draining water off so I imagine that needs doing, badly. I'll try and figure it out.

Thanks for all answers!
posted by esereth at 7:32 PM on December 8, 2009

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