Diesel hot start problem

[RunningTooHot]

This is more of an annoyance than a problem, however I would like to solve it. I cannot figure out what would cause a diesel to be more difficult to start when it is hot than when it is cold.

My 84 300D starts fine when it is dead cold (a little bit of stumbling & misfiring for just a few seconds, then it smoothes out perfectly). Once it is fully warmed up, if it is left sitting in a parking lot for more than a few minutes, it takes quite a bit of cranking to get started again. If I tip into the throttle to assist in starting, there will be quite a voluminous cloud of smoke from unburned fuel burning off. This really flies in the face of conventional wisdom regarding diesels in general -- it should be easier to start once it is hot. I have searched through the archives and cannot find a similar problem under the same circumstances.

The injectors are new, valve lash is set properly, compression is good, glow plugs are new & working properly,etc. etc. The only thing that I can think of is possibly that the fuel pressure in the injector lines is bleeding back down. I have seen references in a couple of posts to some type of check valve seals in the injection pump, yet I am not sure if this type of problem would be caused by that. If that IS the cause, could someone please outline the procedure for replacement of the check valves? (I cannot find the procedure on the service CD) Any feedback would be greatly appreciated!

Thanks again


RTH

[JimSmith]

This does sound like a conundrum, and I have had the same symptoms before with my 1982 240D. I believe either air is leaking into the injection system when it is hot and depressurized, so that to get it running you have to purge the air, or the starter is getting ready to give out and at elevated temperatures the insulation resistance is such that you do not get the necessary rotating speed to fire the engine. There may also be some other modes of failure for the starter when it gets hot.

The first idea, air leaking into the fuel system when it depressurizes, does not explain why the air is not there when the car is cold. It is possible that it shrinks in volume and is small enough when cold to be flushed through the system without getting it airbound. Sounds like a reach to me.

The weak starter theory sounds better, and on my 240D I changed the starter and the engine mounts when I noticed the driver's side mount was broken in half. The starter was lifting the engine and kind of tossing it around the engine compartment, which was also making the actual torque available to turn the engine less.

I bought the "heavy duty" starter from FastLane, which is listed for the 300D Turbo, after confirming it would fit, and installed it with some new engine mounts. No more problems. Starts immediately hot or cold. Jim

[mreid]

When you say a few minutes, how few? I have a similar problem with my VW diesel. The problem is that after the engine cools the glow plug circuit will not activate because it is to warm but the pre-combustion chamber has cooled to the point where it needs a little extra heat. It is a narrow window where this occurrs so I live with it.

[RunningTooHot]

Thank you for your replies! Sorry that it took so long for me to respond -- I have been up to my eyeballs lately

JimSmith, good hypothesis about the air leakage & reduction of volume of the air bubble when cold. The suction lines are in good shape and not leaking as far as I can tell, however I think it would be prudent for me to replace them anyways considering the age of the car. The next time the engine is very hot, I will go under the hood and use the manual pump to see if any air bubbles present themselves in the return line.

This being my first diesel, I'm not sure exactly how fast the starter should spin the engine over. I will try to take notice of whether it seems slower when the engine & starter windings are hot.

Mreid, I believe that the glow plugs are functioning correctly, however the light on the dashboard does not directly correlate with when the glow plugs are actually receiving voltage. I will hook up a test light directly to one of the glow plugs to check their functioning when the engine is hot.

I hope to be able to poke around at this later this week -- Thanks again guys for the assistance!

RTH