My 150,000 mile 2000 Discovery II started leaking antifreeze @ the head gasket, passenger side nearest the firewall. I have owned this since new and have done the routine MX on it.

I made plans to have the gasket replaced but in the meantime, while a family member was driving it, they allowed it to run low on coolant, but shut it down when the red light on temp gauge started flashing. After allowing it to cool, and replenishing coolant, it ran normally.

Assuming the worst that a head was cracked, I purchased replacement head sets from Atlantic British, plus considering the mileage it did not seem unreasonable some valve work may be coming.
At the same time, I had timing chain & gears, water pump, & lifter rods replaced.

A visual inspection did not reveal any cracks on the heads after they were removed.
Also, my mechanic did say the engine interior had a sludge build-up when he opened it, though what I saw on the heads themselves when I returned them was not what I would say is a heavy accretion, particularly for an engine with this mileage. For the record, oil changes were typically done in 5000-6000 mile range with 10W40 oil; Since I live in a rural setting & it is 20-25 minutes into the city, the engine has always been operated at normal operating temps and not just for a few minutes and then shut down.

The vehicle runs well now, except it now has a low oil pressure issue.

At idle, the low oil pressure light comes on (turns off @ 1000-1100 RPM) and the lifters apparently start collapsing. Once the RPM increases, valve train noise goes away and all seems audibly OK.
For a brief period, the engine made no valve train noise, though the low oil pressure light would come on at idle, particularly after the oil was warm.
I drove it @ 50 miles and changed the oil/filter from 5W30 back to 10W40. This since the engine had been open and there is always the chance of debris finding its way in, possible defective oil filter and changing oil weights. No perceptible change.

I took LR back to my mechanic and, as I understand it, he placed an apparatus on the oil filter housing that monitors oil pressure. He states the pressure comes down to @ 5 PSI at idle and is 45-50 PSI @ 2000-2500 RPM. Also, the oil pressure fluctuates @ 15 PSI given a constant engine speed.
These numbers are after engine is warm.

I am seeking help diagnosing the new low oil pressure issue, which never existed before the engine was opened. While I have tried to answer potential questions, no doubt I have overlooked some, so please ask. Your help would be greatly appreciated!!

 

did you replace the head bolts as well as the head gaskets?

 

Yes, head bolts were replaced.

 

Your oil pump is shot and should have been replaced when you replaced the chain.
I wouldn't drive it till this issue is resolved.

 

Mike-thanks!

Which is best; replace just the pump (@$100) or the entire front cover (@$500) which I believe includes the pump? Seems labor difference would not be that great, but I am not sure hence my asking.

Also, I am perplexed as to why this manifested itself after the new heads; any ideas on that?
This seemingly appeared out of no where coinciding with the head replacement so I would like to try and address all at this time.

Thanks for your time.