MG MGB and MGB GT Tech Talk
I am new to the MG world and looking at buying a 1976 MGB. Car appears solid, no rust (under coated), shell and finish look very good and the top is in good shape. 118k miles. Owner wants $2800. Engine sounds smooth and no smoke. Only thing is that the oil presure reads about 55 PSI at idle and changes little when the rpms are brought up. Is this something I need to worry about? Could it be the gage? Is there something I can check?
Sounds ok to me.
If memory serves me correct , most older engines were sitting around 20 psi at idle and 50-60 at full rpms.
I just finished a rebuild with new oil pump and seeing 55-60 at idle. 70 psi at full rmp.
Maybe someone will quote the manual for you.I'm in work and dont have it available.
Steve
according to BENTLY book.. ( 1975-80 GUIDE)
sytem pressure: running.... tween 50-80 lbs /sq.in
idling........tween 10-25 lbs/sq.in
At idle was the RPM around 750, where it should be? The seller may have made sure the engine wouldn't die in front of you by tuning it way up, which would explain the oil pressure reading.
Every engine I have built has around 50 lbs at idel and 70-75 at speed. If the oil is not up to temp(which takes about 30 miles) my engines read 75 lbs. Asked the guy if the engine has be worked and if so how many miles ago. If it hasn't the rod bearings are worn. The factory manual calls for rod bearing to be changed every 50K.
If the gage goes to zero when the engine is off, it's probably working, since it's actually an oil gage, rather than a meter off of a pressure sending unit. One thing that may be happening if your seller isn't on the up-and-up is that he put in 50W oil to quiet any valve noise or bearing knock. It will make the engine run quieter, and will also cause higer idle oil pressure. My 79 has 25-30 at idle and 60-70 at road rpm. It has 69K miles on it and has never had any engine work done. If the pressure isn't changing with rpm, I'd be curious why.
It kinda appears like every is hinting that something is wrong . Why? Rick mentions 55 psi at idle! This pressure is a great idle pressure.
Why would the use of 50w oil be a problem ? The manual calls for 20w50 , unless you were referring to a 50w** oil.
My last dumb question is :
As Tom has mentioned , as I have experienced , and as a lot of people have mentioned ; when new bearings and new oil pump have been installed , the idle pressure is around 50 psi !! Why does the bently manual state idle pressure around 15-20 ???
My last thought : isn't it possible that the oil pressure relief valve is releasing too soon causing the pressure not to raise over 55 psi !Is that even possible? Is it bad for an engine to have a pressure not go above 55?
Steve
I was referring to straight 50W oil. It was an old trick that let the engine run quieter before it was warmed up completely. So some poor yokel (like me) started the engine, said "wow, that sounds pretty good" and shut it back off. If a test dirve happened, it was usually too short to warm the engine all the way. 20-50 oil, on the other hand, has the viscosity of 20w when cold and 50w when warm, so it isn't nearly as thick when cold. I was just kind of wondering why idle and running rpm pressures were so similar. If the oil pump is doing its thing, shouldn't it increase pressure with rpm? The relief spring could be relieving at 50 psi, but since that seems to be the normal pressure for a broken in engine at road rpm, it's surprising that it doesn't drop at idle. I guess I'm just a suspicious sort of guy. After all, I figure if the car being sold is "just a perfect little car that never ever gave me any trouble", why is it for sale. Guess when I was young and foolish, I got burned too many times by "cream puff autos. My motto is usually "Plan for the worst and hope for the best". Hope all is well and the car is as it appears, 'cause they sure are a blast to drive when they're working right.
I don.t have a clue where bently got its figures on oil pressure. I have never had an MG with a good engine go below 50 lb's at idel when warm. I would be concerned if one of mine droped to 20 lbs.
Mine drops to 20-25 at idle when the rpm is around 700. If it's cold idle, around 1100rpm, the pressure is around 50 or so and out on the road, it is more like 70. It used to be around 50 at idle (warmed or cold) until a Brit car mechanic worked it over, just a couple months after I got it. He since died and his shop closed by his family.
I was reading your posts about oil pressure, so I decided to check mine. I have a 78 with 71000 miles and 10/40 oil. In cold weather (30 deg) the idle pressure is 75 at 900 rpm. When warm, idle pressure is 60 at 750 rpm and 80 at 2500 rpm. No engine work has ever been done (I'm the original owner.)
I'm sure you have some bearing wear but it sounds like your OK. I bet you keep her up real good. Regular maintinance is all it takes to give them a long and happy life.
I am amazed by the great response to my novice information needs. Thank you very much for all the info. I feel pretty good about the oil pressure now.
That does it. Replacing the rod bearings after 50k is the last straw. If ever I wasn't sure about the MG being un-worthy of keeping, I am now. So, in the spring I will get it running one more time and goodbye. Sell it for sure
and say a prayer for the poor soul that buys it. Thank you guys. I have read in interest over the past month or so sence I found this site. But now I'am cured. I am not as capable a mechanic a most of you. So I don't have the "right stuff" for working on these cars. I got to have at least a little faith that the manufacturers of
a car had some idea of what they were doing. Before I bust my butt on it. The late model Bs especially were nothing more than a very badly designed, obsolete and inferior product produced by a desperate company about to go under as a last ditch effort to pay their bills. The earlier MGs still had some fun in them but the
late models should never have been made. But you guys know this already. Sorry I didn't mean to rub it in.
Best of luck to all.Merry Xmas.
