Engine won't start unless Oil cap is removed: Theory

[Classy6]

This was a problem a friend was telling me about yesterday, neither of us had encountered the issue before and I've tried to do a little research this morning but can't find anything overly conclusive. Thought I would share in the hope of someone a bit more knowledgeable on the subject chiming in with some theories.

Car in question was a ford focus 1.8 turbo diesel (not common rail) on a 53? plate I believe.

Wouldn't start was the initial fault but the owner had recently changed fuel filter so thought initial issue was fuel bleeding. Spent 15 minutes bleeding fuel, wouldn't run off easy start but was very close. Lots of fumes coming back out the inlet pipe when attempting to start so..

At random he took the oil cap off and the engine fired into life but with huge amounts of blowby from the cap. Put the cap back on, cuts out again. Visually checked over the PCV hoses for kinks, took a few off to check blockages etc, all looked OK. Removed the pipe off the rocker cover, where it comes off the PCV and when the engine's running again massive amounts of blowby and pressure, similar to that of exhaust pressure.

By chance, he jumped in the car, brake pedals absolutely solid. Friend concluded a vacuum pump or worn engine. Car is no longer available for any testing, so this will just be hypothetical conclusions based on the above information.

Anyone able to link explanation to theory? Or have any other thoughts?

[Classy6]

Thanks for replies so far.

Things to bare in mind was that the engine ran perfectly at idle and rev'ed up fine whilst the cap was off - I'd imagine that would throw out the camshaft broken.

Exhaust blocked is plausible but doesn't explain the lack of vacuum knocking the servo out.

[Classy6]

stevieturbo said:

Presumably same car that was mentioned over on PF ?

So has a compression test been done ?

Has the intake path been checked for blockages ?

PF? Not sure what that is sorry.

No compression test done, was a roadside check over. Though as above, ran perfect when cap was off, rev'd freely & quickly.

Intake pipe to turbo was off the airbox, ruling out air filter. If there was a blockage prior, would expect it not to rev as freely/quickly as it did?