High Oil Pressure - 1971 MG Midget

After putting the first 200 miles on a rebuilt 1275 engine in my 1971 Midget I’m trying to find the cause of high oil pressure.
The car also smokes until the pressure drops down, once it’s warmed up.
This is part one of hopefully only two videos on this subject. It’s too cold for working in an unheated garage, so part two will be a few months from now.

[@jimkirby3659]

Here are my thoughts. You state the engine was recently rebuilt and now has about 200 miles on it. If the engine rebuild included new piston rings along with cylinder boring/honing, it’s likely the blue smoke you are seeing during the first few minutes of driving until the engine fully warms up is due to the new rings not yet being fully seated against the cylinder walls. A little oil gets by the rings and stays on the cylinder walls on the down stroke only to get scraped up into the combustion chamber on the up stroke where it’s burned and comes out the tailpipe as blue smoke. Once the engine is fully warmed up, the pistons and rings expand tightly against the cylinder walls and do a much better job of sealing out the invading oil. If my suspicions are correct, the production of smoke should decline as more miles of driving are accumulated and the rings continue to seat. Hopefully the smoke will be completely gone around the 500 mile mark. As I’m sure you know, to assist in the seating of the rings, don’t install synthetic oil in the engine during this break-in period. As for the smoke at idle, it appears to me in the video to be somewhat darker in color. My guess is that darker smoke is more likely from the choke being closed while the engine is idling causing the engine to run rich. I’ve always been told that black smoke comes from gas when the mixture is too rich, blue smoke comes from oil when the rings, valve seals, or PCV valve leaks, and white/gray smoke comes from water/coolant when there is a head gasket leak. Regarding you engine’s oil pressure, I think it’s fine. For a newly rebuilt Midget engine, one would expect readings that tend toward the upper limit. It’s my understanding that the pressure relief valve is set to 80psi. That means it cracks open at 80 and won’t let the pressure exceed 80 - in that way, it only acts as an upper limit to the pressure, it doesn’t set the engine’s working pressure. Your engine’s oil passages determine the engine’s working pressure. Once your engine warmed up, the pressure dropped down to around 75psi, so that’s what it took to push the oil you’re using through your engine idling at 2000 rpm. I would expect the pressure relief valve was closed at that time as it should be. if you were to lower the pressure relief valve's spring compression to say 60psi, it would open at 60 and let the oil flow bypass the engine which requires 75psi. That would be a mistake. If you prefer a lower working pressure, you might consider using a thinner oil. Also, as your newly rebuilt engine wears in, you should see a slight drop in working pressure. For comparison, my well broken-in little Midget runs around 60psi. On the off chance you aren’t familiar with The MG Experience website, mgexp.com; you might want to check it out. It’s a great forum to find out just about anything there is to know about all things MG. I’m enjoying following along with you on your journey to restore your outstanding Midget. Keep up the good work.

[@midgetmakeover]

Hi Jim, Thanks for the excellent reply! I suppose my concern was that I didn't remember seeing the oil pressure sitting that high for the first 100 or so miles, but it could have been. With the car finally being on the road after so long I have just been very glad to drive it around. I have been running standard oil from the start with ZDDP added, and I've tried to follow some advice found in several places about how to drive during the break-in period. The engine was machined, slightly bored, and new pistons, rings, bearings etc. throughout. Hopefully it wears in over the coming warm months. I have spent many hours looking through mgexp.com, which has been very helpful. Thanks again.

[@charlesv20]

@@midgetmakeover , have you check your richness? Does the smoke is improve when the choke is on? As you, my 1275, in a frogeye, with cold weather, run hight pressure oil, but that’s dissapear when getting hotter.

[@dimadee]

Not sure if you got this sorted. 80psi on a cold engine with choke is not unusual for these engines. High oil pressure will not make it blow smoke. The oil pressure is measured with a hot engine. Many of us run 15w40 or 10w40 as the 20w50 is like honey when it is cold; this means high oil pressure and low flow.

[@awalk5177]

Looks good to me and just needs running in. On these A series engines they should have small O rings on the valves to reduce oil passing down the valve stems. I would be interested to see after running in what the smoking was like when the rings have bedded fully with the bores.

[@waynedimmery4553]

Don’t worry about oil pressure fix carburetor

[@midgetmakeover]

It definitely needs some adjustments.

[@stevehill5637]

Nice car!

[@midgetmakeover]

Thank you.

[@michaelhart895]

Oil pressure relief valve

[@mgman71]

I don't think 80psi cold is bad at all. First thing i'd check is compression on all cylinders, 10% allowance between cylinders. Really only a few places for oil to enter the cylinders. Rings, PCV system, rarely a head gasket, valve stems (typically intake)

[@midgetmakeover]

Thanks mgman71. When I last checked, compression was very close across all cylinders. It’s been suggested that more break in time may still need to pass before the engine will stop smoking before it warms up. Looking forward to playing with it again in warm weather soon.

[@michaeledwards6509]

Smoking is from rings or coolant..head gasket

[@leerobertson6940]

My 1972 midget is exactly the same and it’s an original 22k mile engine. No smoke on mine tho

[@brettgofgosky1911]

I have a 1970 mgb that is literally brand new showroom condition. Same yellow color too

[@mikes6961]

Rings didnt seat properly? My cars pressure is high too, just assumed it was the gauge. Positive pressure to me is a good thing.

[@JerryMotorsport]

Hmmm an unconventional problem there; usually not enough pressure as things like the oil pump wears (got me wondering what our car runs at - had a quick look at one of my videos and it was just over 70psi on a semi-warm engine at 140ºF; will check next time I cold start). Could your pressure be high due to new components and running in? Don't want to state the bleedin' obvious, but presume you are using the recommended oil grade. Also is it particularly cold where you are at the moment? Was thinking oil shouldn't be forced into the combustion chamber unless there is a head gasket breech. Will be interesting to see if there is a clear cause if if you've just got health pressure and a slightly smokey A-series engine (modern engines are so clean and tight by comparison). Good luck.

[@midgetmakeover]

Thanks Jerry, My concern with high oil pressure was forcing oil out of the engine or past seals etc. I'm running standard/recommended oil for the car. The weather was certainly getting cooler when I started to notice the issue, maybe 15C. I'll have to wait for warm weather to take a closer look. Hopefully not another blown head gasket.

[@michaelfraser5723]

bit like blood pressure

[@davidburman451]

The oil is quite dark perhaps dirty already? The rings arent seated yet or perhaps install with the gaps aligned causing blow by into oil....also depends on the state of the bore...was it bored ? Was it honed and pistons re- installed in original bores.... I dont recommend leaving full choke on so long...It does not have to wait until the temp guage finally shows warm before taking it off. Just enough choke to start it then push it in to fast idle position only... Any thing else gas washes the cylinders and rings over time causing wear. Hope it clears up for you

[@midgetmakeover]

The cylinders were bored. Hopefully I installed the rings with the gaps positioned correctly. It was several years ago. Thank you for the choke advice. I’ll reduce the time I keep it out. Could be the rings just haven’t seated yet. It’s been 200 miles since the rebuild, and some have suggested it can take longer than that.

[@davidburman451]

Definitely cold now...I just noticed the Ontario plate. Keep us posted ! I have a Spitfire thats in pieces Im restoring and a sweet 74 Midget that needs some magic in the spring....Im in Ontario as well. Just a thought a bad PCV diaphram or rubber valve guide seals will also pull oil into the cylinders. I always loved the rims on the 71's...one year only.

[@waynedimmery4553]

Thicker oil cause’s higher oil pressure when cold

[@michaelfraser5723]

the spring is too strong

[@kurtk409]

Did you ever solve this? My 73 midget runs at 70psi hot, 50psi at idle hot

[@midgetmakeover]

I’m just taking the advice of others that it’s not too high when cold for a rebuilt engine. It drops to about 40 when hot at idle.

[@raedshomegarage1064]

hi so kindly let us know did that solve the problem?

[@midgetmakeover]

Unfortunately it did not. I’ll dig deeper when the weather warms up.

[@roselh1]

I would compare that spring to another before you do much more

[@midgetmakeover]

That's a good suggestion. The new spring, and plunger/valve are both cheap and worth trying. Thanks.

[@waynedimmery4553]

Idle it down to around 6 or 7 hundred rod. Your idling 2000 way to high and your tail pipes saying flooding carburator

[@waynedimmery4553]

Idle is way to high and your smoke is raw gas it’s flooding try a 5 / 30 none synthetic oil

[@glpilpi6209]

These engines because of their tolerances should only be used with a 20/50 multigrade.

[@brettgofgosky1911]

Bad head gasket