[robfrey1]

I drive a 1999 525tds which is somewhat rare in the US. When the engine is cold and has been sitting for say overnight, the engine fires right up without issue. But if it's a hot engine and it's been off for more then a minute or so, it takes about 10-15 seconds of cranking to start. There is no diminished power when driving. Fuel filter was recently changed but no improvement. Also, when engine running, the see through fuel line running does not show any air bubbles.

Any thoughts?

 

[Hoooper]

How many miles?

 

[robfrey1]

it has 106,000 miles. I would say the hard start has been occurring for the last 10,000 miles though.

 

[diesel fan123]

I would have your mechanic check the fuel rail (or wherever the fuel supply goes to the injectors) to find out the fuel pressure when hot.
If that is good, next would be a compression check/leakdown to see if there is adequate pressure and look for leaks.
If the engine has manually adjustable valve clearances, the clearance has tightened up.
(I have two cars that do this if i fail to adjust the valves every 15k miles)

 

[robfrey1]

definitely not leaking fuel under the car, but the valve adjustment I have not considered.

 

[jims2321]

possibly of a vapor lock in the fuel line?

 

[Hoooper]

Possibly a faulty drain back valve, not sure what the low pressure pump demand is but its possible the pump only runs to prime when the engine is cold. I would suggest putting a fuel pressure gauge on it or reading the rail pressure sensor (assuming there is one) when cold, hot, running, etc. to make sure your pressure is correct for all those conditions.