I think I accidently stumbled across something today.
I changed Oil and filter in my 2006 XJ8 VP today. After I added the first 5 quarts of Oil I started the engine to fill the filter before adding the last two quarts, Put car in reverse backed off the ramps and noticed the engine was running rough and had a bit of shacking going on put car in park and goosed the engine to see if it would smooth out the RPM went to 2500 and would not come down. turned off Engine and restarted and was same ( rough idle and when throttle was pressed it went back to 2500 RPM) turned off engine. Finished filling with Oil and replaced filler tube cap. Startedengine and all was just fine. So can it be said over time valve cover gaskets PCV valve seal Oil filler cap O-ring and any thing else associated with engine being tightly sealed may cause rough idle and RPM variations.
Owning an older Jaguar is like trying to rise a spoiled child you never know what might P-them off.

 

I have to say that is a rather "unusual" way of adding oil after a a change.

Why not add the recommended oil quantity then start the car and after a while check the level and add any amount required?

That way you would not be revving up an engine that is actually quite low on oil.

 

So what system is responsible for the rough idle and jump in rpm?
Would it be the Evap system ?

 

Running without the oil cap is probably the reason but not 100% sure. Rough idle for sure but the jump in rpm is odd.

 

You don't want to run an engine two quarts low on oil. For the filter, what you do is pour as much fresh oil into it so it won't spill out when screwing it into the engine block.

 

The engine is sucking unmetered air from the oil cap port which is why it's running hesitantly. There are tubes from the valve covers (pvc) that go into the intake to remove moisture from the crankcase.

 

+1 it was an air leak

I half-expected a horror story about how much oil sprayed all over the engine bay and has been burning off ever since...

 

I certainly would have added at least 6 qts. of motor oil before I started the engine, and then checked the oil level.

 

I think BlackKat is right - any unmetered air will create problems (see threads on lean engine codes that many of us have had to run down). The oil fill opening would obviously let in unlimited air - much more than a cracked vent hose or intake manifold gasket. I suspect the ECM was running it at the high RPMs just to try to keep it running. Even the O ring on the oil dip stick could create a problem....

 

Hi Bob,

Thank you for reporting your discovery - it's an important issue. The reason for your rpm jump and rough idle is that by leaving the oil cap off you created the equivalent of a major leak in the crankcase breathing system. This system feeds vapors from the crankcase back into the engine air intake to be burned in combustion. The volume of vapors is carefully metered to maintain a relatively constant slight vacuum in the crankcase and is controlled by the Engine Control Module so it can account for the vapors when determining how much fuel to inject to maintain the target Air-Fuel Ratio.

When a high volume of unmetered air is inhaled by the engine, the oxygen sensors detect the additional air and in response the ECM adds fuel in an attempt to keep the AFR within target. But because the ECM can't measure how much extra air is being inhaled (because the air is bypassing the Mass Air Flow Sensor), it can only guess at how much fuel to add, which results in a constant over- and under-correction.

Since overfilling the engine oil can also be harmful, I usually add the specified oil capacity minus 1 liter, then run the engine at idle for a few minutes to fill the filter and oil passages, then shut off the engine, allow it to sit 5 minutes, then top up the oil a little at a time until the level is exactly at the maximum mark.

Pre-filling the oil filter as AJ16er mentioned is a great idea unless the oil filter on your N/A AJV8 is mounted horizontally, in which case it's not really practical.

EDIT: Sorry for my late reply. For some reason when I opened your thread I didn't see most of the later replies. Apologies to all.

Cheers,

Don