Lowish oil pressure and highish consumption
Discussion
Just trying to decide what other work to do when I do the diff, and this might be the tipping point.
I've always had quite a low hot idle, about 25-30 psi. On really hot days in standing traffic it can get down to ~20 psi. I use 5W-50 Millers oil and I've changed the oil pressure relief spring. This is coupled with quite high oil consumption. In the past month I've gone through about 2-3 litres, in less than a thousand miles (with each journey usually being 20 odd miles of mixed B road, motorway and dual carriageway).
Could this suggest the oil is overheating slightly when at a standstill? Worn oil pump? Maybe worn valve oil seals?
Other oil pressures are spot on. Above 2k rpm the oil pressure is 40+. Cold oil pressures are about 70-80.
I've always had quite a low hot idle, about 25-30 psi. On really hot days in standing traffic it can get down to ~20 psi. I use 5W-50 Millers oil and I've changed the oil pressure relief spring. This is coupled with quite high oil consumption. In the past month I've gone through about 2-3 litres, in less than a thousand miles (with each journey usually being 20 odd miles of mixed B road, motorway and dual carriageway).
Could this suggest the oil is overheating slightly when at a standstill? Worn oil pump? Maybe worn valve oil seals?
Other oil pressures are spot on. Above 2k rpm the oil pressure is 40+. Cold oil pressures are about 70-80.
I've been through this so perhaps I can help.
Firstly, if the 20 psi you are seeing is on the electric gauge I wouldn't take much notice of it until you have put a proper wet gauge on which will be far more accurate. Having done that, if you do have 20 psi then be reassured that it is sufficient oil pressure to prevent catastrophic failure. A good rule of thumb is that you need 7 psi per 1000 rpm to float the bearings properly. A Rover V8 would really struggle to get anywhere near 20 psi at hot idle, as would a Jag straight 6 as used in the E type. Indeed, most people would agree that volume of oil delivery is at least as important, if not more so that oil pressure, in fact the 40+ psi that soem people report at a hot idle will be causing unnecessary drag as the engine creates oil pressure that it doesn't need.
Having said that, 20 psi is considered low for an AJP, though there does appear to be quite some variation in what people see at the pressure gauge. This could be down to several reasons.
Low pressure is down to one of two things; either there isn't enough pressure being generated (ie the pump is kn@ckered) or the pressure is being lost somewhere - most likely where there is excessive clearance between the crank and the main bearings. Normally this would clearly indicate worn main bearings, however, having rebuilt my AJP I feel fairly well qualified to say that the original build tolerances are poor. In my case I found there to be two problems. Firstly the crank had been machined 2 thou undersize - this can only have happened from new as there was no ovality, and secondly the size of the main bearing journals increased when comparing the front journals to the rear ones - the result of a blunt tool having overheated as it was used to line bore the block (or a numpty operating it). In the end I chose to grind the crank to 10 thou undersize, fit corresponding bearings and live with the slight difference in main bearing journal sizes. They are all within tolerance, however, the front is at the tighter end and the rear at the looser end of the acceptable clearances. I also had the oil pump rebuilt, however, I am satisfied that in my case the problem was oil pressure being lost rather than not it being generated. To give you an idea, pre-rebuild I had about 17 psi at a very hot idle, I now have 28-30.
So... If you have low oil pressure, I wouldn't necessarily be that worried about it if it has always been the case. It may simply be that that the tolerances in your engine are larger than on some others. However, if it has gradually got worse then it is probably a sign of worn mains.
Regarding the oil use. 2-3 litres / 1000 miles is high. So long as you are sure that it's not leaking out then it is likely to be being burnt. This would either be due to poor oil control because the oil rings or bores are worn or because it'd leaking past the valve stem seals and being burnt. The latter are fairly easy to change with the heads in place whereas the former require a proper engine rebuild - and everything that goes with it...
Firstly, if the 20 psi you are seeing is on the electric gauge I wouldn't take much notice of it until you have put a proper wet gauge on which will be far more accurate. Having done that, if you do have 20 psi then be reassured that it is sufficient oil pressure to prevent catastrophic failure. A good rule of thumb is that you need 7 psi per 1000 rpm to float the bearings properly. A Rover V8 would really struggle to get anywhere near 20 psi at hot idle, as would a Jag straight 6 as used in the E type. Indeed, most people would agree that volume of oil delivery is at least as important, if not more so that oil pressure, in fact the 40+ psi that soem people report at a hot idle will be causing unnecessary drag as the engine creates oil pressure that it doesn't need.
Having said that, 20 psi is considered low for an AJP, though there does appear to be quite some variation in what people see at the pressure gauge. This could be down to several reasons.
Low pressure is down to one of two things; either there isn't enough pressure being generated (ie the pump is kn@ckered) or the pressure is being lost somewhere - most likely where there is excessive clearance between the crank and the main bearings. Normally this would clearly indicate worn main bearings, however, having rebuilt my AJP I feel fairly well qualified to say that the original build tolerances are poor. In my case I found there to be two problems. Firstly the crank had been machined 2 thou undersize - this can only have happened from new as there was no ovality, and secondly the size of the main bearing journals increased when comparing the front journals to the rear ones - the result of a blunt tool having overheated as it was used to line bore the block (or a numpty operating it). In the end I chose to grind the crank to 10 thou undersize, fit corresponding bearings and live with the slight difference in main bearing journal sizes. They are all within tolerance, however, the front is at the tighter end and the rear at the looser end of the acceptable clearances. I also had the oil pump rebuilt, however, I am satisfied that in my case the problem was oil pressure being lost rather than not it being generated. To give you an idea, pre-rebuild I had about 17 psi at a very hot idle, I now have 28-30.
So... If you have low oil pressure, I wouldn't necessarily be that worried about it if it has always been the case. It may simply be that that the tolerances in your engine are larger than on some others. However, if it has gradually got worse then it is probably a sign of worn mains.
Regarding the oil use. 2-3 litres / 1000 miles is high. So long as you are sure that it's not leaking out then it is likely to be being burnt. This would either be due to poor oil control because the oil rings or bores are worn or because it'd leaking past the valve stem seals and being burnt. The latter are fairly easy to change with the heads in place whereas the former require a proper engine rebuild - and everything that goes with it...
Thanks for that James. Most of that confirms what I already suspected, and from reading previous topics about low idle pressure a number of people had said theirs was that low but had never really worried about it.
Mine has been that low ever since I got it (2.5 years), but for nearly two years of that it's been off the road! Think I'll get the diff out and then decide what to do with the engine.
Mine has been that low ever since I got it (2.5 years), but for nearly two years of that it's been off the road! Think I'll get the diff out and then decide what to do with the engine.
Hmmm, good point about the oil weight. Can't remember off the top of my head why I went with 5W-50, I'm sure I saw it recommended somewhere. Think I will change back to 10W-40 on my next oil change.
I know the timing cover is leaking slightly (still), as is one of the cam covers, but I think this only makes up a small part of the consumption.
I know the timing cover is leaking slightly (still), as is one of the cam covers, but I think this only makes up a small part of the consumption.
itiejim said:
5-50 is also quite thin when hot - I'd run it with 10-40 as TVR recommends. Some even use 15-50 (the mobile 1 race stuff). This will increase oil pressure and may help the consumption slightly - though your can't really get away from the facts is you have significant wear.
Er... I think you may have this the wrong way round. The higher the number, the 'thicker' the oil at a given temperature. The first number is at low temperatures, the second at high. So in other words 5w50 compared to 10w40 is thinner when cold, but thicker when hot no? Toadusmodus said:
Er... I think you may have this the wrong way round. The higher the number, the 'thicker' the oil at a given temperature. The first number is at low temperatures, the second at high. So in other words 5w50 compared to 10w40 is thinner when cold, but thicker when hot no?
No - the oil is thinner when warm.itiejim said:
Toadusmodus said:
Er... I think you may have this the wrong way round. The higher the number, the 'thicker' the oil at a given temperature. The first number is at low temperatures, the second at high. So in other words 5w50 compared to 10w40 is thinner when cold, but thicker when hot no?
No - the oil is thinner when warm.For multigrade oils you will see two numbers (for monograde oils only one). The first is followed by a “w” and is commonly 0, 5, 10, 15 or 20. The second number is always higher than the first and is commonly 20, 30, 40, 50 or 60.
The first and second numbers are not related.
The “w” number (0, 5, 10, 15 or 20)
When multigrade oils first appeared, a low temperature test called “w” (meaning “winter” not weight) was introduced. Using a “Cold Crank Simulator", the test measures the oils ability to flow at low temperatures.
ALL oils are THICKER at low temperatures than at high temperatures but the lower the “w” number, the quicker the oil will flow at low temperatures.
The second number (20, 30, 40, 50 or 60) This number is known as the SAE (Society of Automotive Engineers) number and is measured in “Centistokes” (cst) at 100degC.
Centistokes (cst) is the measure of a fluid's resistance to flow (viscosity). It is calculated in terms of the time required for a standard quantity of fluid at a certain temperature to flow through a standard orifice. The higher the value, the thicker the oil.
One thing is for sure, that engine has a serious oil habit. I used to consider 700/800ml every 1k miles as quite high, but my Cerb had done nearly 60k miles and was tracked quite a lot.
Edited by Spoof on Tuesday 22 February 13:32
Blimey, I'm certainly confused now! Just had to re-read this all about 3 times to work out where I've gone with this one.
You're both quite right that I was mistaken in relation to the latter part of the viscosity description, however, for some reason I'd also misread Toadmundus' post which I thought the poster was suggesting that oil got thicker as it warmed up! Apologies for all confusion caused (mostly to me by the sounds of it)!
I'm now off for a stiff drink and a long lie down...
You're both quite right that I was mistaken in relation to the latter part of the viscosity description, however, for some reason I'd also misread Toadmundus' post which I thought the poster was suggesting that oil got thicker as it warmed up! Apologies for all confusion caused (mostly to me by the sounds of it)!
I'm now off for a stiff drink and a long lie down...
Edited by itiejim on Tuesday 22 February 23:33
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