oil pressure falling after rebuild ?
Discussion
I rebuilt an austin healey engine recently , new main and big shells , thrusts , pistons , followers , rocker arm bushes , pressure relief valve and spring ,
rebuilt oil pump , cam , cam bearings , new ali cylinder head , basically everything that needed doing after a cam follower and cam lobe failed .
when cold on start up I have 80 psi which drops to 60 + when warm . After 2 hours driving with 50 + lbs on the gauge at 2000 + rpm
it drops to 10/ 15 lbs at idle !
This has me a bit worried as prior to rebuilding it always had 25 psi at idle , which is what the manual states as normal .
the engine has just reached 500 miles so drained and good classic car spec oil was put in plus a tin of STP .
What could be the cause of this falling pressure , ?
the crank was checked and polished to within minimum specs and ovality .
oil / water gauge has been rebuilt with new capillary tube,
all suggestions welcomed , thank you .
rebuilt oil pump , cam , cam bearings , new ali cylinder head , basically everything that needed doing after a cam follower and cam lobe failed .
when cold on start up I have 80 psi which drops to 60 + when warm . After 2 hours driving with 50 + lbs on the gauge at 2000 + rpm
it drops to 10/ 15 lbs at idle !
This has me a bit worried as prior to rebuilding it always had 25 psi at idle , which is what the manual states as normal .
the engine has just reached 500 miles so drained and good classic car spec oil was put in plus a tin of STP .
What could be the cause of this falling pressure , ?
the crank was checked and polished to within minimum specs and ovality .
oil / water gauge has been rebuilt with new capillary tube,
all suggestions welcomed , thank you .
Gramos
It sounds similar to mine (1965 3000) on all pressures save that on cold mine is closer to 70psi and does not fall quite as low as 15 on a hot idle; probably 20-25. My engine was rebuilt to a fast road spec by Denis Welch Motors in 2013 and has gone on to do about 5000 miles since then with no issues. Also the car only uses an egg cup of oil a year compared to the 1litre every 250 miles pre rebuild!
It sounds similar to mine (1965 3000) on all pressures save that on cold mine is closer to 70psi and does not fall quite as low as 15 on a hot idle; probably 20-25. My engine was rebuilt to a fast road spec by Denis Welch Motors in 2013 and has gone on to do about 5000 miles since then with no issues. Also the car only uses an egg cup of oil a year compared to the 1litre every 250 miles pre rebuild!
It sounds to me like the oil is working too hard and viscosity is failing. Presuming you don't have an oil temperature gauge fitted; a temporary fitment would give you more information from which to diagnose the issue. Has anything changed with regards the cooling system? When that is underperforming a symptom is failing oil pressure as the oil system attempts to take up the slack.
Regards Andy
Regards Andy
Oil is classic car high zddp 20/50 w plus a can of STP oil treatment.cooling system perfect condition plus electric fan , Ali rad , now runs cooler than before after extensive block cleaning of water galleries ,160/170 f on a hot day . I imagine that the oil is equally as cool .the car is not driven hard ,I will try to find another gauge ,as the current one was changed/ rebuilt whilst the engine was out , could it be a faulty gauge ?
It could be that on initial start up and 80psi press, that your oil pump pressure relief valve is lifting but not resetting correctly once the pressure drops back, i had this once on an Audi quattro after an engine rebuild. A small piece of dirt had found its way in and was hampering the relief valve to re-seat correctly.
Did you fit a new relief spring, If not did you check the length of the spring? The spring can shorten (compress) over time with constant opening and closing, after starting a cold engine.
Did you fit a new relief spring, If not did you check the length of the spring? The spring can shorten (compress) over time with constant opening and closing, after starting a cold engine.
This isn't related to the problem, but is 80psi a bit high? My Healey-not-Austin engine was at 80 after rebuild and the engine builder said wind the relief spring setting out a bit so it wasn't so high.
I don't know if it's adjustable on an AH, mine has locknut, Sprites have shims, I guess 100/4, /6, 3000 are adjustable?
I don't know if it's adjustable on an AH, mine has locknut, Sprites have shims, I guess 100/4, /6, 3000 are adjustable?
phumy said:
It could be that on initial start up and 80psi press, that your oil pump pressure relief valve is lifting but not resetting correctly once the pressure drops back, .
Certainly seems as if the pressure relief valve isn't doing what it's supposed to. a) not opening to relieve the high pressure when cold
b) not closing to maximise pressure when hot.
These two statements are rather hard to reconcile.
If one has eliminated the probable, then the possible . . .
a gear-type pump suffers internal leakage when worn and the effect would vary with temperature, particularly if the PRV were stuck shut.
Is the oil pressure gauge an electrical one? They are not always reliable. If it is electric and you replaced the sender in the course of the re-build, it's very likely that the changes that you've observed are due to a sender with a different sensitivity range.
Many of the replacement senders seem to give a wider pressure range than the older ones. I've seen this on my car (totally without any other change). Higher pressures read higher and lower read lower. The consequence is that with a cold engine the pressure at 2 or 3,000 rpm may be off the scale, while at hot idle the gauge reads zero.
It's worth checking with a mechanical gauge or swapping the sender.
Many of the replacement senders seem to give a wider pressure range than the older ones. I've seen this on my car (totally without any other change). Higher pressures read higher and lower read lower. The consequence is that with a cold engine the pressure at 2 or 3,000 rpm may be off the scale, while at hot idle the gauge reads zero.
It's worth checking with a mechanical gauge or swapping the sender.
As above really. I've had more oil pressure senders than you can count on both hands (one shorted out and smoke was coming out of the dashboard until I opened the bonnet and ripped the wire off) and at the moment the oil pressure relief valve is playing up - again. So test the oil pressure with a wet gauge to see if it is the sender, and if AOK there then clean the release valve. Oh, and I use Halford's 20/50 for the E-type. Works fine and not expensive.
The classic tale told by my old mate .
Installed newly built engine in his Mallock race car . No oil pressure !
Remove engine and replace with a mates spare engine , no oil pressure!
Change gauge and bingo !
Go racing and his mate blows his engine up and needs his loan engine back .
They did 4 engine swaps in 60 hrs !
So check the gauge and senders .
I only ever had one failure of pressure on a rebuild. That was on an AUDI coupe and was cured by back filling the oil pump that had air locked itself. Since then I prime oil pumps and filters during assembly .
Installed newly built engine in his Mallock race car . No oil pressure !
Remove engine and replace with a mates spare engine , no oil pressure!
Change gauge and bingo !
Go racing and his mate blows his engine up and needs his loan engine back .
They did 4 engine swaps in 60 hrs !
So check the gauge and senders .
I only ever had one failure of pressure on a rebuild. That was on an AUDI coupe and was cured by back filling the oil pump that had air locked itself. Since then I prime oil pumps and filters during assembly .
lowdrag said:
Just as well you don't have an F-type then. Cranks and journals are no longer available.
Seriously? I had heard there was a supply issue with some underbody cross-members that are prone to rusting, but you'd imagine engine parts would be easier to find, for a while yet at least.Gassing Station | Classic Cars and Yesterday's Heroes | Top of Page | What's New | My Stuff