Hard to start. Is this normal?

[OLDGOLD]

Ever since I've had my 1980 300SD. The engine always needs at least 10 seconds of glow before the car will start smoothly. Even when the car has been running for a prolonged period of time and I turn it off; if I wait 20 seconds, the engine will just crank and crank. It will if I restart immediately, but not much longer. I've been told that even in the summer months (Texas) if your compression is good, you should be able to start without GP's. I find this claim to be dubious at best, as my car is a b*!$@ to start with just one glow plug out. I have the manual GP button and nearly new plugs.

It's gotten down to the 20's F here in Dallas lately and I've tried plugging my block heater in one night, and not on another and it hardly seems to make any difference. Not sure what the deal is, but I think it should start easier. I had to glow for nearly a minute and make 5 cranking attempts with a little fuel added to get her started that morning,

I want to add that the engine sounds good and drives great. I feel like I'm getting plenty of power and the battery is nearly brand new with a 4/19 date on it. I daily drive her almost 50 miles and use Chevron or Shell conventional oil. Let me know if having to glow at least 15 seconds for smooth start up in anything above 50F is normal?

Last thing to add... I do have a small exhaust leak at the back of the exhaust manifold just in case that means anything.

[Alec300SD]

What you describe is not normal.
Someting is amiss with either air, fuel, or ignition(heat/compression).
Usual suspects for poor starting:

Air incursion into IP.
Clogged fuel filters.
Clogged fuel vent.
One or more glow plugs out/or  begining to fail.
Glow plug relay failing...shutting off too soon. (Not applicable with manual glow)
Poor spray pattern on one or more injectors.
Low compression from tight valve clearances, (or worn engine), when was the last valve adjustment?
Excessive cranckase pressure (blowby) activating the shutoff valve.
Slow cranking due to weak battery, poor electrical connections, and/or thick engine oil.
Wax crystals in fuel due to cold, water in fuel, or poor quality/contaminated  fuel.
Dirty air filter.

[OLDGOLD]

Air incursion into IP. / I've replaced all lines going to and from the IP within the last year.
Clogged fuel filters. / Replaced recently. Even purchased the Hebmuller with copper inserts.
Clogged fuel vent. / I haven't looked into this yet...
One or more glow plugs out/or  beginning to fail. / I hope not as I've replaced all with Bosch GP's within a year
Glow plug relay failing...shutting off too soon. (Not applicable with manual glow)
Poor spray pattern on one or more injectors. / overhauled injectors with Monark Injectors
Low compression from tight valve clearances, (or worn engine), last valve adjustment? / Done 15k miles ago, will do again soon
Excesive cranckase pressure (blowby) activating the shutoff valve./ There is quite a bit of blow-by
Slow cranking due to weak battery, poor electrical connections, and/or thick engine oil. / Battery is new
Wax crystals in fuel due to cold, water in fuel, or poor quality/contaminated  fuel. / Use only Shell V-Power and TK-7 fuell additive
Dirty air filter. / Just inspected and looked good. Although, I will be replacing with a K&N soon

[Alec300SD]

My best guess from the symptoms is...chronic air incursion.

Another possible cause which would explain your symptoms is a leaky primer pump coupled with a failing lift pump check valve.
Easy test, activate the primer pump and see if it leaks.

When the secondary fuel filter is changed,  it is best replace the o-rings (or o-ring and crush washer).
When the primary air filter is changed and the hoses are not refreshed, you can get hard starts after a hard run.
The stretched rubber hose ends can loose their ability prevent air incursion (especially at high engine rpm).
The short hose from the primary fuel filter to the lift pump is very prone to premature radial cracks at the cut ends, due to the relatively hotter environment.

I do a twist test on fresh hoses when installing them.
If you can rotate any of the fuel hoses by grabbing and twisting, you will have found a potential air leak.

You may also want to check that all of the banjo bolts are still tight.

[UTn_boy]

You mentioned one thing that is usually not a good sign......excessive blow by.  The reason for this is worn piston rings and/or cylinder bores.  Both of which will lower compression.  Sometimes excessively tight valve lash will cause too much blow by, and since your engine is due for a valve lash adjustment I'd definitely do that first.
Afterward, you'll need to perform a compression test to see where you are.  The OM617 turbo compression norms are 24-30 bar (350-435 psi), with minimums of 220 psi as well. Both have a maximum of 3 bars acceptable difference between cylinders.  It's very common for these engines to become hard to start when they're excessively worn.  One day it just won't start at all.  On the other hand, you could have an injection pump that isn't timed correctly to the engine, or as Alec mentioned, the pump itself can be suspect.  It could be the lift pump, a leaking primer pump, check valves in the lift pump that are weak or stuck open from debris, worn plungers and barrels in the pump, or all of the above.  Again, do valve lash adjustment and compression tests before anything else.