Hello all, I’m new here and new to the Jaguar scene. I’m not completely new to classic cars having fully restored two vw beetles some years ago.
I’ve just bought a fully restored rhd xk120 ots and I’m working my way through it trying to get everything right so I’m happy with it (bit of a perfectionist)!
I’ve not done any miles on it yet as I’m waiting for uk registration.
I have several little things to solve but one thing is really worrying me - the oil pressure or lack of it. Now I have a fully photographic record of the rebuild and at least from this it looks like it has been done properly. New main and big end bearings, new -8 pistons as it was a -7 car etc.
The big but is that the oil pressure seems very low, at cold start 45psi at idle and then when warm dropping to 5psi at idle and at 2000rpm it is about 35psi when warm. It has 20W50 mineral oil in it.
I have spoken to the vendor (bit of a language barrier but he seems honest enough) and he says that when he purchased the oil gauge he say that it was reading low and purchased a second from the same supplier which again produced the same results. This was confirmed on another xk120 he had at the time. He also said that he tested the oil pressure with a workshop pressure gauge and it was good.
I have had a quick look at the pressure release valve on top of the filter and the pressure adjustment screw was about 1/4 of a turn off fully in. This hasn’t made a difference. I haven’t yet checked the seating of the valve (not the easiest thing to get to).
If the oil filter part is good is there any reason when the gauge my not be measuring the correct pressure or do I start to think the worst?!
Welcome to the best source of knowledge on the Internet for these beasts!
The very first thing you need to do is get a new known, mechanical gauge, and T into the system to determine what the pressures are. Jaguar generally reports that 10 psi/1000 RPM is good, but if you have five psi at idle and 35 to 40 going down the road, that’s entirely adequate.
It is also possible that the pressure relief valve is either worn, or has a chunk of dirt in it.
I have a friend coming with another gauge tomorrow so that will be the first thing checked - will report back. I guess that just leaves the pressure relief valve after that, I will remove it to take a look at the seat…
Be aware that I had an issue like that on my E type: at idle I barely had any pressure at all. I clamped off the return hose and immediately ended up at 80 psi, so I knew it wasn’t the oil pump. I took the oil pressure relief valve out– I do not recall if your year 120 has the internal relief passage or the external – and the valve itself looked perfectly fine. I put in a new one, and end of problem!
Thanks again. Unfortunately my friend is ill so no spare gauge as yet - not good for my stress levels this isn’t On the subject can someone tell me what size the thread is on the union on the top of the oil filter that feeds the pressure gauge please - the large end that screws into the top? I’ll get myself a test gauge, they’re not expensive, but I need to make sure I have a compatible union!
Brilliant thanks, so if I understand it correctly the top of the filter requires a 1/4" BSPT thread?
I’ll be keeping an original Xk120 dual oil pressure/water gauge but I just want to check the health of the engine first, or at least know where the problem lies. Engine pulls well, sounds well etc, there are no bad symptoms but I’ll feel a whole deal happier when if I know the pressure is good.
In my youth I had a MGA with exactly your low oil pressure issue. Replacing the oil pump with a rebuilt unit brought the oil pressure back up to normal readings. This should also have been a given when your engine was rebuilt. What I didn’t know then though was that an adjustment of the oil pressure relief valve might have been enough to effect a fix. So like the other posts, perhaps focus your efforts there and on gauge accuracy first before going deeper.
That might be true, but I wouldn’t guarantee it. The thread that was referenced was for an E-Type which uses an electrical oil pressure sender, unlike your XK120 which has a fitting for an oil pipe which goes to the gauge.
Edit: Actually the E-TYPE oil filter housing doesn’t use a 1/4" BSPT either. It has a parallel BSPP thread which requires a copper washer to seal it. If you check out the photo, you will see that the moved sender has a sealing washer to the T fitting.
Thanks Chris and David. Very useful re the union thread,next job is to take the union out, I have had a helpful offer from Context Pneumatic here in the UK to ID the thread for me, I’ll let you know when I know for sure as it may just help someone else!
Still hopeful it might be the guage or seating of relief valve or something similar!
It has been my experience that some suppliers of BSPP and BSPT hardware don’t know the difference and may supply one while claiming it is the other. And sometimes the thread listing merely says BSP. And sometimes previous mechanics have mixed the threads, so you will do well to identify both the male and female threads in a given joint and ponder what should be there instead of just copying what is there.
I don’t think the values you mention are alarmingly low. A few psi at idle and 40 at 3000rev/min is normal for a warm engine. The causes of a low reading go, in order of likelihood, sender, gauge, relief valve, pump and last of all bearings. As already suggested, the first step is to check with a mechanical gauge.
Hi
I have just been hear with a XJ series 2 4.2
my guess is the thread forms are the same
the oil light switch is BSPT if you look you can see its tapered
I dont know were you are but i am in England
so you can use this gauge it comes with a copper pipe
to get on to the engine use this it comes tapered that goes in to the block
the fibre washer goes on the pareal end and the pipe attaches to it
do not over tighten the tapered end or you might crack the block
seen it done on BMC A series engines
Super thanks all. Still not had time to sort out a pressure gauge and connection parts - I realise this is the first thing I need to do really. Whilst I’ve been waiting I thought I’d have a look at the oil pressure relase valve. I couldn’t shift the housing/valve seat with with filter in the car (need to be like Mr Tickle) so I removed the oil filter. Anyway I’ve got the oil pressure valve off…
So that’s a bit of a bodge for a start. I’ve use some fine grinding paste and lapped the valve into the seat just to be sure and I will renew the spring just in case.
Speaking of bodges, while the filter was off I thought I’d clean the inside. Below the bottom plate was 1/2" of thick sludge, another concern for sure. Clearly when the engine was rebuilt he didn’t venture into the lower reaches of the oil filter. I would have thought this would be obvious which leaves me wondering if there have been any other corners cut!
If you remove the oil filler cap on the cam cover and there’s oil reaching the camshaft, then there’s nothing urgent to worry about. Be aware that some messy oil mist may come out.
If I were to of done that, I would’ve at least used two nuts of the same color…
In further addition to some other posters assertions about oil flow, I just happened to notice that on my school bus which is a great, big, 10 L six-cylinder, with pistons and connecting rods much more massive than a Jag, runs 40 psi smokin’ hot and about 12 psi at idle. Pah-lenty!!
On 2 rebuilt xk120 motors I see 60psi at cold start ( relief valve set point), 50-60 hot while driving (3000rpm), and 20psi at hot idle. One motor has gone around 5000 miles.
As others have already implied, I’m sure these pressures are well above the minimum needed to keep the motor safely lubricated.
The numbers you stated (if accurate), seem in line with a motor that is perhaps a bit worn down.
Hence the bodge with the nuts shown previously. The good news is that the new spring also seems quite a bit stronger so I am now hopeful that when I get chance to test, I might well see an increase in oil pressure!
On the subject can someone tell me what size the thread is on the union on the top of the oil filter that feeds the pressure gauge please - the large end that screws into the top? I’ll get myself a test gauge, they’re not expensive, but I need to make sure I have a compatible union!
