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Oil Pressure Gauge not returning to zero

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Mark_Barton

Mark_Barton

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When I picked up my TR4 it had a Smiths oil gauge in it that I wanted to replace with a Jaeger one to match the other gauges. It also never returned to zero, even with the engine off - it always stopped at about 25psi.

I found a Jaeger gauge on e-bay and replaced the Smiths one (which still reads 25psi even disconnected from the car). The Jaeger replacement read zero when I received it, but after a short time in the car, has started doing the same thing.

Does anyone have any experience of this, or any explanation as to why? The Jaeger gauge I got had the flat glass so I've got a third (!) to match my other curved glass gauges. I guess it's not the end of the world - it behaves normally in all other ways, but I'd rather it went back to where it should. Is there anything I can do before I fit the next one to clear this up?
 
Mark-

When you start, does your gauge ever go off scale on the high side? That's about the only thing I can think of that could cause the gauge to go bad. It is very strange the 2nd one is doing the same thing now - I doubt you have residual pressure but you could always check by disconnecting the line. If you are high siding the gauge, though, that may be due to a faulty pressure relief setting I think - you'll need to get that sorted out otherwise you'll probably keep repeating the problem.

Randy
 
First of all, make sure that the needle isn't hitting dial face. I've also had an OP gauge not returning to zero even though the needle wasn't hitting the dial face. I took it apart, and it turned out that somehow the needle where it attaches to the pivot inside had become dislodged. The gauge still worked, but didn't return to zero. Once I got it all back in place, it worked fine.
 
Thanks. I think it's probably the pressure relief valve - the pressure does go off the scale when cold.

Haynes tells me the valve's built into the head of the external oil filter. Presumably they mean the housing where the pump is attached to the engine, rather than the (now spin on/off) canister? Is this something I can check during an oil change?
 
Yes, the relief valve is built into the body that your filter screws into, its part numbers 90-97 here in the Moss catalog

I think when you change oil thats a pretty good time to take it apart if needed - but I'd get other opinions first before going that route as I've never messed with the relief valve personally. It could be that the valve is stuck shut and just needs a good cleaning. To calibrate, the TR4 factory manual makes it sound like a pretty simple operation. Back off the locknut and adjust until you get about 70-75 psi pressure at 2000 rpm. It says to rotate the screw counter clockwise to reduce the pressure appropriately. This may not be possible with a warm engine, as you could be running at a lower pressure anyway and the valve isn't doing anything. When its cold, the pressure will be at its highest and apparently in your case you are getting high enough pressures to peg the gauge and damage the bourdon tube enough to miscalibrate it.
 
I'm thinking mine did this once and it was a bit of something in the oil line to the gauge. Undoing the gauge and letting some oil (engine running) pump onto a rag cleared it.
 
Mark_Barton said:
Thanks. I think it's probably the pressure relief valve - the pressure does go off the scale when cold.
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You might want to check the accuracy of the gauge then. I'd believe it was wrong before I'd believe the pressure relief valve stuck closed; it's just a ball & spring.

On my TR3A, I could make the oil gauge read at least 20 psi too high, just by overtightening the fitting.
 
OH yeah, another question is what grade of oil you are using. Back when I ran straight 40 weight year-round, it would peg in the winter time. Just couldn't flow through the relief valve fast enough.
 
Had the dash apart today to replace a bulb, so I've bled a bit of oil out of the line, and re-tightened it to a firm finger-tight - I'd used a wrench last time. Will see if the pressure reading changes, otherwise I'll have a look at the relief valve next time I change the oil.

I'm using 20W50 oil, and once warmed up the pressure doesn't exceed 70 on the gauge. Could that be too viscous for NY winters?
 
Mark_Barton said:
I'm using 20W50 oil, and once warmed up the pressure doesn't exceed 70 on the gauge. Could that be too viscous for NY winters?
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I wouldn't <span style="font-weight: bold">think </span>so. 20W is recommended down to 10F, had it been colder than that when you saw the gauge peg?
 
Since this has happened to two gauges, I'd lean towards an overpressure situation rather than the gauge itself being the initial problem. Still weird, though. Is this a new engine rebuild? Sounds like you have pretty good pressure regardless there.
 
The gauge has always pegged on a cold start, even back in October when I bought it. Granted that was on the Smiths gauge which was already reading 25psi with the engine off, but it happened with the replacement too before the weather got properly cold. I think I tend to agree with Randy that it's probably overpressure.

It's not a new engine rebuild, I don't think. There was no history with the car, and no evidence of a recent rebuild either.

I'll have a look at the relief valve next time I change the oil, and I won't fit the new gauge until I have - in the meantime, I guess too much pressure is better than too little!
 
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