Car wouldn't start
 

I was going to drive the SD to work this morning, but it wouldn't start (drove fine yesterday). It was around 20F outside. The battery is strong as the headlights were coming on full power and the car was cranking, but it wasn't getting anywhere. Just "crank crank crank crank". I was not getting the sound that you get in the cold weather. The glow plugs were definitely coming on and the oil pressure gauge was all the way at the top.

The 300D started right up.

I didn't have time to investigate, but I was thinking that possibly the fuel is gelled. I find this weird as I drove this car last week with it being down into the teens with the same tank of fuel (however, the tank was more full).

Any thoughts on where to start?

Try leaving the key in the glow plug position.... without moving it... for 45 seconds before engaging the starter....
this is from the FSM.....
that pretty much maximizes your glow plug heat.... there is a chart in the FSM giving the temperatures the GPs get to at different times....

It's warmed up to around 28F and I finally got the car to start. I'm wondering if one of the glow plugs is out as the car started no problem in 15 degree weather a few weeks ago. Same fuel as before.

Glow plugs have always been the problem when I've had cold start problems on a well running engine. You can put an ohm meter on them without removing them. Some may be definitely out of range. You'll have to pull them and hook to battery cables to check the actual glow especially if the ohm meter doesn't find any suspects.

Either make or buy a reamer if you end up pulling them. Options are on this site. Buy Bosch only.

A few things to keep in mind:

- the dashboard GP light operation does not equate to a guarantee the car will start (the light going off doesn't necessarily mean the plugs are hot enough for the car to start).

- the full preglow relay cycle is about 30 secs. You should be able to hear the relay click on when you turn the key to the preglow position, and click off after the 30 secs. If you are in a noisy environment and cannot hear it, another trick is to leave the overhead light on inside the car... it should flicker and brighten a little when the relay clicks off.

- if you have any doubts as to the health of the glowplugs, leave the relay on for at least one full cycle.

- as Junkman said, Bosch are the preferred brand but only purchase from a reputable supplier (no eBay or Amazon stuff) and make sure the Bosch boxes have a holgraphic seal with a security code on it. Believe it or not, there are chinese knockoffs running around that burn out in a few months. Don't ask me how I know that (thank you, Amazon vendor).

Pull the larger of the two connectors at your glow plug relay. Check each pin to ground with an ohm meter. You should read a very low resistance. Zero means its shorted and needs to be replaced. A high reading, +10 ohms, also indicates a bad glow plug.

I just bought a set of wrenches just to do the glow plugs ,one side has the open end while the other is closed with a ratcheting feature plus a swivel end ,made the job soooo much easier.Harbor freight for $20. and was well worth the cost.Its a 10mm for the glow plug while the small screw on the end is an 8mm which the wrench set did not have ,was able to find an 8mm wrench instead which worked great .Ive noticed while pulling mine that youll need to keep a long screw driver against the wire to release the peskey small nut of the end ,it wants to spin the wire with the bolt.A very important part about these small screws when tightening them back on is to NOT OVER TIGHTEN THE SCREW ,you will end up ruining the glow plug .It might be a good idea to just pull the whole injector lines while doing the job ,I get away with bending mine out of the way once the holding clamps are off but wouldnt recommend doing it this way.Im still having some cold startup issues ,mine I believe is due to a long needed valve adjustment which will be completed this week ,Ill chim in here to give a report on the starting behavior of the car after its performed ,could be linked to alot of mb diesel issues on the no start gremlin ,take lash readings on just how out the valves were before adjustment.

Small correction - I'm assuming you have an OM61X engine, chasinthesun, as you're talking about valve adjustments. On the later OM61X engines with the pencil style glow plugs, it actually takes a 12mm wrench to remove/install the plug. You're correct that the wire nuts are 8mm. Just wanted to avoid confusion.