MK IV core plug popped out when starting the engine

I have a problem with my Mark IV 3,5L DHC (Chassis 617003) with which I hope anyone here can help me:
When I tried to start the engine a few days ago (the starter turned), there was a really big bang and I found a core plug under the car. In the MK IV service manual plate 7 (see attached) it is indicated number 48. Then I tried to fix the core plug again which was really difficult because there is no space to place a tool/lever. After I had it fixed provisionally I started the engine, it ran but after a few seconds another big bang and the core plug fell out again. Without the core plug the engine could not be started. Had anyone ever heard of this phenomenon?What can be the cause and how the problem can be fixed?
I would be great to get an answer from here.
Uwe (from Munich


Hi Uwe, and welcome to JL!

I hate to say it, but you might have a blown headgasket, or a crack in the head - what you describe can happen when combustion pressures can reach the water jacket.

You can do a bubble test. Rig a fitting which allows you to feed compressed air into a sparkplug hole. Turn the engine to TDC for a Cylinder, leave the transmission in gear and chock the wheels, fill the coolant up, then apply 5-10 bar to the cylinder. If the headgasket leaks, or there is a crack, you will see bubbles in the coolant.

Just be careful - the compressed air will turn the engine over, so you need to make sure that the crank cannot turn.

Off the top of my head, there are coolant and fuel mixture paths in the head. If a plug on the fuel mixture channels is open then fuel mixture will be too lean to burn due to air sucked in through the open hole rather than just through the carbs. Too lean a mixture and the engine will not run. A backfire through the intake path could blow out a plug. If the plug blowout is on the coolant side, then gasket or cracks are among possible concerns.

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Thank you for your answer Andrew. , I will see what I can do (with assistance of a friendly professional) and will report the results

Thanx Andrew, I will see what I can do (with assistance of a friendly professional) I’ll keep you informed.


Core plug No. 48 is part of the intake air passage. What we usually think of as an intake manifold going to the six intake valves is actually within the head, not outside. There is normally no coolant involved on this core plug. It is on the left in this picture. On the right is the coolant passage.
Undoubtedly a backfire caused it to blow out.

If it was on the front you could take the radiator out and have working room. For the plug at the back you will have to take the head off and hammer that plug back in. It is best to use a round bar about the same size as the plug with a flat end as a tool to install it. The goal is to have it flat or nearly flat. If you can’t find a big round bar you could use a big socket turned backwards on an extension and hammer on that. Some people put a dimple in the center, but that is not going to be as tight and often will not hold.

BTW the coolant system is not pressurized, it is open to the atmosphere.

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If the core plug is at the back you could drill a hole in the bulkhead to let a rod through for hitting it.


BUt to get to the root of the problem, It probably wasn’t installed correctly. As Rob says , if oyu look at factory installed plugs , they are almost flat. And because there is no water behind them they are usually original and a good indication of what they should look like.
But Looking at pictures of engines core plugs are rarely showing signs of being installed correctly . In fact , the reluctance to install correctly is so prevalent that some entrpeneur has devis a replacement that does not require the operator to read instructions and either use some witha screw operated flattening arrangement.
Or instead of installing correctly , one can drill holes in the engine and screw straps over the plug. Unbelieeeeevable.
Here is a pic of the same core plugs in an SS100 engine. When I posted this on the XK list regarding core plugs, An irate lister complained that it could be right as he;s never seen a Jaguar engine without over head cams.
It would be worth check all your core plugs { which aren’t “Freeze plugs”] and make sure they are all installed correctly as per the hubbard Spring website. Easierto do at home than by the side of the road.

Another observation to guide towards the intake channel rather than coolant is to note what happened when plug popped out. If the coolant level was full in the system before the plug blows out then coolant would flow out the hole until emptied below blown plug hole level.

The cause might be lean fuel mixture on cold start.

…Jesus wept.