1986 Jaguar - hot engine


#1

We have a 1986 Jag sedan, with only 23,000 miles, in mint condition. We are the only owner. The car has had regular service and about 4 years ago a complete checkup of “everything” .

Twice in the past 3 weeks, my husband drove the Jag a distance of about 10 miles and then put it into the garage. when I came home about an hour later. the garage interior felt as though it were on fire. The hood of the Jag could not be touched. I threw the garage doors open and opened the hood of the Jag; the engine was also too hot to touch. Admittedly the day was hot (98 degrees) but, my car, which I had just driven 25 miles was not that hot. It took 4 hours for the engine to cool.

It happened again this past week, in 90 weather. My husband said that the temp gauge on the dash was normal when he drove it this week.

I am concerned about a fire hazard. My husband thinks it is just the weather.

Any thoughts???


#2

I doubt if anything is wrong. It’s July!


#3

Take it to a shop, have them check/fix the temp indication (an infrared thermometer works great for this) then address the cause of the overheating if it exists. Overheating kills engines.


#4

Yes, it’s July, but you know better than us if this hot hood, etc., is unusual. If it’s a new behavior (you’ve had it 20 summers, right), take it to a trusted Jaguar mechanic, quick. You also might want to find a Jaguar-specific discussion, where you’ll have a lot of Jag owners to ask.


#5

I’m assuming this is an XJ6 series III 4.2 Liter straight six engine and not the 5.3 V12.

The 4.2 engine does not have any notable overheating problems, though it does use a thin tin head gasket that can sometimes weep coolant though this would be visible by drip marks on the engine block sides. The temp gauge should run on the low side of the normal lettering on the gauge usually around the ‘r’ of Normal though in the high 90’s it will creep to the midway position.

There are a few things that can cause a hotter than normal engine:

  • Make sure you have coolant inhibitor in that engine, it won’t lower the temperature but if you need work on that engine it will make it easier and prevent snapped head studs, a very common problem during cylinder head removal.

  • The series III 4.2 injection engine suffers from badly coked inlet valves if persistently used for short trips, the coke build up will cause the head to run hotter than normal (since the coke glows red hot) and though the alloy head will do a good job of dissipating the extra heat it will get very hot to the touch. The only fix for this is either a head job or remove the inlet manifold and blast the valves with walnut shells through the inlet ports.

  • The 4.2 is also timing sensitive, even a slightly retarded engine will run a hot head though it can generally handle the heat.

  • The 4.2 doesn’t use electric fans it has a single clutch mounted fan, a dragging fan will cause a slightly higher temperature but this would normally show on your temperature gauge.

  • The 4.2 also uses a very large (for the engine) cast exhaust manifold, these do get hot and take a couple of hours to cool down, it does get hot in that engine bay when parked, also check that you have the hood liner in place, it doesn’t make the engine any cooler but it will absorb some of the heat from the cylinder head directly below the hood.

  • And finally, the 4.2 is fairly robust and can run unpressurized without overheating while moving due to excellent through the grill air cooling around the engine. However an unpressurized 4.2 will get significantly hotter when switched off since the unpressurized coolant can reach higher temperatures than is normal. 2 favourite culprits are the engine coolant filler at the front of the head, make sure the brass neck isn’t distorted and that the flat cap doesn’t have a split rubber seal - very common. The second candidate is the heater control valve on the firewall directly at the rear of the engine, once the diaphragms start to rot (very likely on an 86 car) you will lose coolant pressurization and will never see the leak since it will be small and drips onto the rear of the engine block and boils away from engine heat. These problems can be easily identified : run the engine till hot, switch off then just turn the ignition on without starting up and watch the temperature gauge, if it climbs rapidly to 3/4 or above you know where to look.

If I were to guess with the cars present mileage and your mention of short trips I’d go for the inlet valves. An infrared check should pick this up straight away.

If this is a V12 engine let us know, that’s a totally different animal ~ it has dual thermostats for a start.


#6

Thank you so much!!! This gives us a great idea where to start to look for the rpoblem/solution. And yes, this car has been used for short trips almost exclusively.