series 1.5..small amount of coolant is coming out of the radiator cap after 10 minutes of driving. I tested the cap using a pump style device and it doesn't even register any pressure. When I look at the cap, its just a simple rubber diaphragm inside a metal cap. When I look up a replacement cap on xks.com or terrys jag parts it shows an exact same cap?.. and terrys calls it a "blanking no pressure" cap..how can a radiator cap be no pressure? I thought that is one of the things that increases the boiling point in cooling systems? The design of the cap looks like it cant withstand any pressure. To be clear, this is the radiator cap. The overflow reservoir has a cap on it that has a spring on it.. I would think both caps should be a spring style? Any help is appreciated.

 

I'll disclose right away that I'm no authority on E-types

There are/have been various models of Jaguar over the years that have used the blanking cap and pressure cap combination so the idea itself isn't all that strange.

Whether or not it's correct for an E-type, I can't say.

What you're calling an 'overflow' reservoir is probably an header tank, which IS designed to be pressurized and have a pressure cap. Whether or not you also have an overflow tank as well, I can't say. If you do, it would not be pressurized.

Cheers
DD

 

The system is similar to my 1977 MG Midget 1500 which came out in 1974, and also like an XJ Series 3 I once had. The highest point of the system has a blanking cap that has no blow-off mechanism, it is just a seal. The system is filled from here with coolant, and should be almost full, or just below the level of the blanking cap. There is also a separate expansion tank, normally mounted lower down in the engine bay somewhere which takes the pressure cap. The SNG Barratt web pages for a Series 1.5 shows this should have a pressure of 7 lbs. Normally the expansion tank is filled initially about 1/3rd full, when filling the system, with the rest of the space available for coolant expansion. It sounds like you need a new blanking plate, or the housing for the blanking plate as there should be no leakage here at all.. There must be a good seal here, and you need to tighten the cap until it comes up against its stop.

 

Ditto to what Fraser said. Do not ignore the rubber hose and connections between the 2 tanks. Might want to repeat your pressure test with the hose removed and the connection blocked. That will isolate the problem area.

 

Concur with comments... the “pressure cap” sits on the expansion tank, the one on the radiator is a blank. You should see a hose between the two parts plus a small diameter hose from expansion tank to evacuate over pressure, this hose just drains under the car