RAVE also mentions high oil pressure could cause valves to float and misfire. Could cold thick oil cause high oil pressure? I'm using Rotella 5w-40.

 

Cold oil usually causes low pressure because it flows so slowly and is too thick for the pump to get it to the components quickly. That's why on diesels when you start one in cold weather, you monitor the oil pressure gauge. If it doesn't register oil pressure quickly, you shut the engine down to avoid damage to the engine. I can't remember, but there should be an oil pressure bypass that keeps the oil pressure from jumping too high in case of a blockage.

 

There are both. The filter has a spring loaded bypass that shunts the filter when oil is too cold to flow quickly. Normal oil PSI readings are high at both idle and revved up, when cold. As oil heats, filter bypass resets, oil thins, and at idle lower PSI than revved up. They also have a bypass valve and a pressure relief valve built into the front cover. They keep the oil pump from jacking up so high it splits the oil filter open, and set the max pressure for the pump before excess is returned to the sump. Rover shows exploded view of them in overhaul, tells you how to clean them, and there have been postings about wrong length spring inside. Now a D1 has a spec for 40 PSI, and a D2 50 PSI, and both share the same oil pump and same 4.0 liter block (some years). So I wonder if you could change the bypass spring in the front cover and have a little more PSI.....

 

I haven't tried that on a LR, but I have on other engines. Adding a barely stiffer spring can do wonders. But best to have a spring compression tester so you don't install a weaker or too strong of a spring.

 

im still convinced your issue is electrical in nature, and not related to any engine hardware... as you mentioned never being able to feel the misfires...

 

Yeah I'm just trying to exhaust every possibility within my abilities before giving it to a shop so they can scope and swap the ECU, among whatever other things they do.

 

What do the spark plugs look like when you remove them? are they uniform from cylinder to cylinder or do they vary. Throw in new plugs and see if it makess a difference. That is pretty cheap thing to try.

 

Is there a way to test the coil packs? Is there an external tester that could show a misfire.

 

I know the crowd is cheering you on in the electrical direction and that is certainly plausible, just don't discount the fuel option because it can also give you similar results. Another quick test of the pressure is to have the gauge fixed to the rail and with the engine off but the pump on pinch the RETURN line from the rail to the tank. Your pressure should shoot up around 70 lbs ish.

 

my primary reason for suspecting something electrical is that he says he doesnt actually feel anything when it sets the codes.

ive never had a car with an ignition or fuel issue not bump or sputter, even for one combustion event.