Oil pressure on a 4.2 V8

[trooper1212]

I've just replaced the oil pressure spring which was a little bit of a bitch to do It'd be easy if the last person to do up the bolt wasn't a 300lb gorilla...
The old spring looked ok but it was about 2mm shorter than the new one, so I replaced it anyway.

I'm slightly confused about my oil pressure readings. I only have 10psi at hot idle, now I know that 10psi per 1000rpm is the rule of thumb but from postings on here I'd assume that most peoples Cerbs run greater than that.
I don't have any other oil pressure problems. I get 80psi at cold idle, 40 psi at hot 2000rpm, 60psi at hot 3000rpm. All well within the happy zones.

So how come I have comparatively low oil pressure at hot idle?

4.2 AJP V8 with 36,000 miles under its belt, clatter cams replaced with new whisper cams about 15,000 miles ago, valve stem seals replaced 6000 miles ago, just had a 12,000 mile service 500 miles ago. Running 15w/40w semi-synthetic oil. Using about 500ml of oil per 1000 miles, no suspicious clouds out of the exhaust, no unusual or excessive noise at either the top or bottom end, oil quick to circulate from cold.
Basically everything is exactly as it should be for a Cerb, apart from the lambda sensors playing up every now and then and the water temperature part of the gauge not working

So why do I have low oil pressure at hot idle?

Oil pump? Sender? Gauge? Piston rings? It couldn't be the lambda sensors causing overfueling and washing the bores could it?

[joospeed]

did you replace the spring because of the low pressure or has it occured since ?

[trooper1212]

Replaced the spring because of the low oil pressure tonight, its upped the high rpm pressure by about 5psi but made no difference otherwise. Couldn't see any bits of old spring in the hole and I bought 2 new springs from TVR power and they both were identical, so I don't think it's a dodgy spring.

[Julian64]

Hmmm, that doesn't sound right. Hot idle the lowest I've seen is about 40.

Of the 6-7 that have done their springs at my place, the result is usually very similar.

Cold 70-80 , Hot 50-60, hot idle 35-45.

Not that much variation considering the vageries of the Cerb. I wouldn't be happy with 10psi. A bird fart is higher pressure than that.

[carl_w]

FWIW I get 60psi when cold. 40psi at warm idle. 30psi (sometimes down to 25psi) at very hot idle.

[trooper1212]

Julian64 said:

Not that much variation considering the vageries of the Cerb. I wouldn't be happy with 10psi. A bird fart is higher pressure than that.

I'm not entirely happy with it, hence the post

On the other hand, I would be a lot more worried if I had low pressure across the whole range, or no variation. As it is all the other temperatures and speeds are fine apart from the hot idle. I'm thinking oil pump, but if anybody has other ideas?

[trooper1212]

Hmmm... what about oil temperature? Could the oil be overheating? Is the oil cooler working? How can I check?

The water temperature is on the money and I've not lost any coolant in 9000 miles of motoring, so the engine itself isn't overheating, but it could be that the oil itself is getting fried.

[trooper1212]

carl_w said:

FWIW I get 60psi when cold. 40psi at warm idle. 30psi (sometimes down to 25psi) at very hot idle.

Have you changed the spring lately? Figures like that point to a spring that isn't quite as spritely as it used to be, however if mine had settled on that after I replcaed the spring tonight, I wouldn't be concerned.

[andy4200]

Don't know if it helps but this is what mine runs at.

Cold - 60 psi
Hot - on the move - 50 psi - oil at 80 degC
Hot - idle - 30-35psi - oil at 80 degC
Hot - stuck in traffic - 22-25 psi - oil at 110 degC

I've never seen my gauge go below about 22 psi and I've not seen my oil hotter than 110 degC despite 2 hours in a jam at walking pace.

I know every car is different but just thought it might be a little info to help you figure out what's up...

Andy

[trooper1212]

andy4200 said:

Don't know if it helps but this is what mine runs at. Cold - 60 psi Hot - on the move - 50 psi - oil at 80 degC Hot - idle - 30-35psi - oil at 80 degC Hot - stuck in traffic - 22-25 psi - oil at 110 degC I've never seen my gauge go below about 22 psi and I've not seen my oil hotter than 110 degC despite 2 hours in a jam at walking pace. I know every car is different but just thought it might be a little info to help you figure out what's up... Andy

Thanks for that, I'm starting to think more down the oil overheating road now. How did you rig up the oil temerature gauge? The Cerb doesn't have a sender as standard does it?

[andy4200]

No, it doesn't have one unless it goes to the ECU.

I got Austec to fit the sender to the pipe that goes in near the oil filter while they were servicing the car. It's a couple of hours work as they need to take out the housing? for the oil filter and drill/thread the hole for the sender.

Joolz has done it for somebody but when I needed a service he was booked for the forseeable future .

After that I ran a wire through the back of the engine and in through the bulkhead where the throttle cable comes in. (Hindsight now tells me that the hole on the passenger side behind the water bottle may be an easier option). then down to the gauge fitted in the centre console part where my old speaker used to be. It's rigged up to the earth and lighting on the stereo head unit wiring.

Makes a difference as the oil definately takes a good while longer than the water to get up to working temps before I put the boot down. Usually about 12-15 miles on the roads round me.

Andy

[J_S_G]

Neil, how's it going mate?

Mine ran at 40 PSI solid on a run, 65-70ish when cold, and idled at more like 25, dropping to 20 when REALLY sitting in traffic. The latter seemed a bit low to me (and Joolz changed it to little effect at hot idle, although the cold pressure rose).

Didn't you mention a bit of smoke 'cos of the piston rings when you were still up this way? Can't imagine it's burning that much at 500ml per 1000 miles, though. (Hell, my Tuscan was at 1 litre per 300-400 miles before it finally went back to TVR for "upgrades").

10 PSI does seem really low... Wonder if your gauge is reading right? What RPM are you idling at - nothing silly-low, I take it? Is the 15w40 you're running it on decent Mobil 1, etc? I think Mobil do a 5w30 and 10w30 to change the upper viscosity as a last-resort if you can't figure it out.

[plug]

Can anyone tell me if 10psi on the speed 6 engine at hot idle and 30 to 40 psi when driving is to low? from what i have read on these threads it is. Does anyone know where the spring is located on the speed six?

[davidd]

Get your guage and sender checked. It might be that if the oil is hot the sender gets upset. I would have thought it is quite easy to shove a new one in.

D.

[carl_w]

trooper1212 said:

Have you changed the spring lately?

Nope.

[trooper1212]

J_S_G said:

Neil, how's it going mate? Mine ran at 40 PSI solid on a run, 65-70ish when cold, and idled at more like 25, dropping to 20 when REALLY sitting in traffic. The latter seemed a bit low to me (and Joolz changed it to little effect at hot idle, although the cold pressure rose). Didn't you mention a bit of smoke 'cos of the piston rings when you were still up this way? Can't imagine it's burning that much at 500ml per 1000 miles, though. (Hell, my Tuscan was at 1 litre per 300-400 miles before it finally went back to TVR for "upgrades"). 10 PSI does seem really low... Wonder if your gauge is reading right? What RPM are you idling at - nothing silly-low, I take it? Is the 15w40 you're running it on decent Mobil 1, etc? I think Mobil do a 5w30 and 10w30 to change the upper viscosity as a last-resort if you can't figure it out.

Hey James, things are good, I've read about your Tuscan troubles, but you knew what you were getting in to

What I thought was the piston rings turned out to be the valve stem seals, since having that done the oil consumption has decreased significantly and no oil is blowing out the exhaust. It still could be that but as the only symptom is low oil pressure at hot idle, I'm a little sceptical.

So, should I run a lower hot viscosity oil to increase the pressure? I've never really understood the weight numbers

Idle speed is 900rpm. I was playing a bit with the revs on the way to work. 10psi at 900rpm, 20psi at 1200rpm without load, 30 psi at 1200rpm tickover load. I could just up the idle speed I guess

I don't trust the gauge at all being as the temperature part isn't working, but as much as I'd like to replace it, there aren't any new ones at TVR. I could put a different guage in, but if I did that I'd like to change the rev counter and speedo to keep them all the same and that gets a bit expensive. If anybody knows where I can source a magnolia gauge I'd be grateful.

[andy4200]

plug said:

Can anyone tell me if 10psi on the speed 6 engine at hot idle and 30 to 40 psi when driving is to low? from what i have read on these threads it is. Does anyone know where the spring is located on the speed six?

It might be worth asking this on the Tuscan forum as they all drive S6 cars. I think the majority on here are V8 owners. It may take a bit longer for an answer here.

Andy

[trooper1212]

plug said:

Can anyone tell me if 10psi on the speed 6 engine at hot idle and 30 to 40 psi when driving is to low? from what i have read on these threads it is. Does anyone know where the spring is located on the speed six?

No idea about the speed six engine i'm afraid, but I have no idea about the V8 engine anymore either

[carl_w]

Well all the manual says on the subject is that the oil pressure should never drop below 10psi (except when you turn off the engine, obviously )

[trooper1212]

carl_w said:

Well all the manual says on the subject is that the oil pressure should never drop below 10psi (except when you turn off the engine, obviously )

Well, nothing to worry about then