MGB An oil pressure odysseus

[PAUL161]

That is 65lbs idling when hot, say 63 to 65! At speed it'll run up to 75lbs, around 73lbs is normal when hot after running a while. A new spring was installed when engine was rebuilt and if I remember correctly, it was a tad longer than the old one, but don't quote me on that, remembering anything beyond yesterday is a gamble. PJ

 

[max71]

After putting in a new spring/packing/valve I seem to get 60 at speed when warm but can drop anyway from 50-25 at hot idle. Any theory why idle would drop so low when speed is within reason (albeit not as good as yours but with 10 pounds)?

 

[PAUL161]

I would say the pump isn't putting out to full capacity due to wear. It's only a guess, but I don't know where else to go. A new pump, not cheap, might solve your problem. Then again, cam bearings, main bearings anyplace pressurized with oil and not up to specs could be the culprit. PJ

 

[max71]

Yoda - you should read the very overly long initial post. I did put a new oil pump in just in case. The only thing wrong with the uprated one in there was that weird scoring on the end that rotates within the pump (other end of the vanes). Otherwise it looked perfect. Checked everything - looked new, clearances perfect.

Even though I put in all new bearings (except for the 2 main ends which can't be done in the car easily). Weird.

 

[DrEntropy]

WRT that relief valve, I have seen the mating face of more than one of 'em with uneven wear. That allows by-pass even at the low RPM's.

It's too late now but when we rebuild MG engines, it is a habit that we use a bit of valve grinding paste and a wooden dowel in the ID as a tool to "seat" them. Then the block gets the usual "boiling" and cleaning.

 

[max71]

I saw some wear on the old valve body when I put the new assembly in. I'll take a photo later

 

[Brinkerhoff]

By itself the "low" oil pressure you have shouldn't cause any problem . I took a 70,000 mi. TR6 engine apart with only 5 lb. oil pressure at an idle and the rod and main shells looked like new and it ran just fine. The problem you MAY have is if you've run grit through it and still are.

 

[max71]

I've wrestled with the concept that the dingle left over from the hone was slowly melting and adding grit. Oil/filter are changed at 3000 miles. Shouldn't this have gotten most of it? I just changed the filter again at a couple hundred. Oil doesn't have any grittiness to it.

 

[max71]

By itself the "low" oil pressure you have shouldn't cause any problem . I took a 70,000 mi. TR6 engine apart with only 5 lb. oil pressure at an idle and the rod and main shells looked like new and it ran just fine. The problem you MAY have is if you've run grit through it and still are.

What was the ultimate cause of the low oil pressure if the bearings were ok?

 

[max71]

I know its two choices and for now the only choice is driving it and watching the OP. Being a daily driver I do worry because its used daily and previously I was supposed to use it for some trips which didn't bother me at all. True, about the electric gauge, I don't know if it's been off since the rebuild or not. I took apart the sender and all that was wrong as some oil on the fork that mates to the tab for the electrical connection. Once wiped off it read right. But I did convert to mechanical for better accuracy.

Having the car since 1973 and after 6 rebuilds and over 500K where the OP was always checked (did get a different gauge and sender at one point) My OP was always much higher. 65LBS at speed now is ok for me with the packing in a new spring/valve. Its the low end OP which is something I've never seen from any of the engine rebuilds and odd not a pound difference after I renewed the bearings in the car. Makes me wonder if pulling it and just redoing the crank/rod bearing would even have an effect or not.

The pressure not to have it down for long means I'll have to have a short block built properly when budget allows and just do the swap. At this point its just a big mystery that's all. Car runs great, no knock. Plenty of power. Shop I'll eventually use for the short block and switch is mystified why it has the low OP at the low end. Makes no sense until they tear the short block apart and see what ultimately is causing it.

So I'm resolved in the fix. Just curious what it could be causing this.

 

[max71]

Here's the promised pic of the pressure relief valve.

 

[Rut]

That pressure relief valve doesn't look like it would hold pressure very well, can you tell what the mating surface looks like? I know there's some leakage past the valve, but that one looks too rough.
Rut

 

[max71]

Since I took it somewhere to do it, I didn't have them use a bore scope or anything to look inside the block. It took them an hour to get it in so I didn't want to rock the boat.

Again, this is the old one. I had a complete new assembly with packing that replaced it. It looks like the oil pump end (pic in post #5), but not as severe.

 

[max71]

That's the plan. I'll keep you guys updated.

The only caveat is that the pressure suddenly dropped 25lbs one afternoon in traffic. If it slowly dropped over time I would consider it normal. So I'll watch to see if it starts dropping lower and lower.