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?

[GreenV8S]

Massive blow-by might be caused by piston failure or head gasket failure?

[Mignon]

Sounds like the camshaft might have broken due to valve to piston contact leading to the vacuum pump not being driven and blowby from the inlet due to bent valves and from the breathers due to broken piston.

[buggalugs]

Blocked exhaust?

[Disco_Biscuit]

I had a similar problem on a petrol Astra, had so much blow by that wouldn't run with the rocker cover breather pipe connected. Disconnected it ran fine.

Can only assume not enough oxygen entering the inlet.

[Elliot2000]

Agree about the blocked exhaust, had same thing on a couple of bmws with blocked cats or dpfs. With the exhaust fully blocked- it was the only way for the gas to escape

[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.

[stevieturbo]

Presumably same car that was mentioned over on PF ?

So has a compression test been done ?

Has the intake path been checked for blockages ?

[james7]

stevieturbo said:

Has the intake path been checked for blockages ?

This would be first on my list to check.
I have seen it with other makes of engine where the egr valve is slightly open and causes carbon build up to the point of blocking everything up.

[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?

[Petrolhead_Rich]

Classy6 said:

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.

Possible that it was pressurising the vacuum pump through the drive from the camshaft to the pump?

+1 for blocked exhaust, so engine is choking on it's own exhaust without the oil cap removed, but as the car is gone, we'll probably never know?!

At 2003 will it be young enough to have a DPF? if not then perhaps the CAT is blocked? either way, a quick unbolt of the exhaust would have confirmed it either way!

[SoCalDave]

Looks like there's pressure due to the blocked exhaust too.

[tiandebeer]

Hi i know this is already an old thread but i seem to be having a similar problem with my 2010 vw polo vivo 1.6 . Most of the time it runs perfectly and starts perfectly but now more often than not it doesn't want to start unless i open the oil filler cap . I've asked many mechanics but nobody seems to know what the problem is if anybody knows how i can correct this please let me know ?

[DVandrews]

If the induction is blocked, no air can be drawn into the engine. When you open the filler cap air can enter the engine void and from there through to the inlet manifold via the PCV connection.

Dave

[andybracing]

DVandrews said:

If the induction is blocked, no air can be drawn into the engine. When you open the filler cap air can enter the engine void and from there through to the inlet manifold via the PCV connection.

Dave

this is very possable, the vaccume is irrelivant as its created by a pump not the engine, either a inlet problem, or an exhaust problem, easy to find eitherway