Cooling Problems





The Jag-lovers Guide to Overheating Problems

V1.0 (Jul 96)

Lawrence Buja

Jaguar XK engines are designed to run within a specified temperature range and overheating a Jaguar engine can be a very expensive mistake. With the thermal expansion of the aluminum head being two or three times that of the iron block, the XK heads are very susceptible to warpage when overheated. If the engine gets hot enough to seize the pistons, the damage can be catastrophic. The cost of repairing a heat damaged engine can range from a minimum of several hundred dollars if you do it all yourself to several thousand dollars if you have a specialist do it for you. Needless to say, it pays to watch your temperature gauge carefully and to take any signs of overheating seriously.

This guide is designed to help you diagnose overheating problems with your Jaguar. While principly aimed at Jaguar XK engines, it should apply just as well to other automobiles.

Table of contents:

A. Common causes of overheating:

01. Expired fan clutch.
02. Stuck thermostat.
03. Wrong/expired radiator cap.
04. Coolant leaks, external.
05. Radiator clogged externally.
06. Radiator clogged internally.
07. Engine core clogged internally.
08. Coolant leaks, internal.
09. Incorrect ignition timing.
10. Fuel mixture is too lean.

B. Other, less common, overheating causes.

11. Waterpump failures.
12. Bad auxiliary fan.
13 Exhaust restrictions.
14. Overfilled recovery tank.
15. Dragging brakes (Main or park).
16. High compression.
17. Low engine oil.

C. What to do now?

D. Coolant system pressure discussion.

E. Ignition timing discussion.

A. Common causes of overheating:

The basic theory behind water cooled engines is that a liquid coolant is circulated through the engine, where the liquid conducts heat away from the engine, to the radiator, where the coolant passes the heat to the outside air, and back to the engine. While this system sounds quite simple, there are a number of things which can disrupt it such as:

Inadequate air circulation past the radiator and engine.
Restricted coolant circulation through the engine, radiator or hoses.
Coolant leaks, which reduce the level or pressure of the system.
An improperly tuned, overloaded or, in the worst case, dying engine.

To get some clues as to why your Jaguar is overheating, there are some items which you should make note of:

When does is overheat? Always or only under certain conditions?
After the car is stopped, does the radiator feel warm?
Does the coolant level stay full? Are the obvious coolant leaks?
Is anything else wrong? Bad performance, white exhaust?

Given this information, you are well on the way to finding the source of the problem. Unfortunately, tracking down the cause of your overheating isn't always as straight forward as this list suggests and you may end up replacing several components before finally solving the problem.
But, don't give up, perseverance pays off and by carefully noting the symptoms and learning a little about how your cooling system works, most cooling problems can be solved by the home mechanic. Finally, given the potential costs of repairing a heat damaged engine, there is no shame in taking your Jaguar to a competent mechanic to fix an overheating problem if you don't have the time or inclination to repair it yourself.

Below is a list of potential sources of overheating with their symptoms, a short discussion of the problem and some suggested remedies.

01. Expired fan clutch (for cars with mechanical fans).

Engine overheats at idle/low speeds only.
Engine coolant level is normal
Radiator matrix is hot.
Mechanical fan spins easily from rest.

One of the most common causes of overheating is a bad fan clutch. The fan clutch is a fluid coupling which turns the fan a low engine speeds and slips at high engine speed. This provides large air movement through the radiator at low speeds, yet reduces fan noise at high speeds, when the fan isn't needed. With time, the fluid in the clutch degrades and slips at all speeds. Also, fan belts which drive the fan may slip or be missing entirely.
To check the fan clutch, with the engine off, give the fan a good push. If it turns more than three times, it needs to be replaced. Remember, this only applies to mechanical fans and not electrical fans. If you have an electric fan, it should be obvious whether it's working or not once the engine is warmed up.
Also check that any fan shrouding is intact and in place. This helps to funnel more air past the radiator.


$ Replace the viscous fan clutch.
- Tighten/replace the fan belt.

02. Stuck thermostat.

Engine overheats, both at standstill and 60mph.
Engine coolant level is normal
Radiator matrix is NOT hot.

Thermostats can stick partially open restricting the coolant flow from the engine to the radiator. If the large hoses to the radiator are not very warm to the touch after the engine has warmed up, then suspect that the thermostat is stuck closed. If the car never warms up all the way, suspect that it's missing or stuck open. Low temp in winter suggests thermostat stuck partially open, overheat in summer stuck mostly closed.


$ Replace the thermostat.

03. Wrong/expired radiator cap

Engine overheats, both at standstill and 60mph.
Engine coolant level is LOW.
Radiator matrix hot.

A hard or expired rubber seal on the radiator caps or an under-rated (3lb or 8lb) pressure cap on the overflow tank. For a SIII XJ6, replace the header tank cap with a flat non-venting cap and the overflow tank cap with a 15lb pressure cap (The header tank is located on the front of the coolant rail on the top, intake side of the engine, the overflow tank is on the fender wall). (See #22. System Pressure Discussion)

Solution: $ Replace radiator cap and pressure cap.

04. Coolant leaks, external (hoses, water pump, heater, etc)

Engine overheats, both at standstill and 60mph.
Engine coolant level is LOW.
Radiator matrix is hot.
Possible visible external leaks

In older Jags, coolant system leaks are very common. Leaks, even pinhole leaks, in any of the coolant/heater hoses can reduce the system pressure, causing boiling and overheating (See section D. System Pressure Discussion). Leaks past the water pump seals will do the same thing. Pull on the water-pump pulley, if it wobbles or feels loose, replace the water-pump. Other possible leakage points are the heater core, weeping freeze plugs and the transmission oil cooler.

- See Section C. "What to do now?" for DIY procedures.
$ Have your system pressure tested and replace the offending part.

05. Radiator clogged externally

Engine overheats, both at standstill and 60mph.
Engine coolant level is normal
Radiator matrix HOT.
Front of radiator matrix dirty/clogged.

A lack of air flow through the radiator. This is usually caused by debris (like leaves, bugs, dirt or trash) blocking the radiator or a/c condenser matrix.

Solution: - Remove and clean the radiator.

06. Radiator clogged internally

Engine overheats worst at high speeds.
Engine coolant level is normal
Radiator matrix NOT hot.

If plain tap water was used in your cooling system, your radiator may have developed deposits and is blocked inside. and needs to be back flushed or repaired.

Check to see if the radiator fins have started to debond from the core. On my E-type, I noticed this by checking the fins in the four corners of the radiator. Solder suffers from thermal fatigue and the solder that bonds the fins to the core crack. Result is less heat transfer. If you find the fins loose at the corners, it is time for a new core. And if you buy a new core, purchase a good one. The cheap cores have a higher lead content and a lower tin content with a resulting lower fatigue life.

- Clean radiator yourself: Remove radiator, fill with CLR (Calcium Lime Rust remover) to dissolve the CaCO3, let it sit for 24 hours, occasionally swishing the fluid around, flush and refit.

$ Or have the radiator rodded out at a radiator shop
$$ Or replace radiator

07. Engine core clogged internally

Engine overheats, both at standstill and 60mph.
Engine coolant level is normal
Radiator matrix NOT hot.

Internal rust or water deposits can clog up the internal engine water passages, reducing the amount of coolant flow.

$ Do a two-stage engine flush.

08. Coolant leaks, internal

Engine overheats, both at standstill and 60mph.
Engine coolant level is LOW.
Engine Oil is contaminated and muddy or coolant is contaminated or exhaust is
abnormally white and volumous or spark plug electrodes are wet when removed or
Coolant is present inside the cylinders.

Mixing of the oil and coolant inside the engine is a bad thing. It usually arises from either a bad head gasket or a warped head (due to the engine being overheated). A quick check is to look for the presence of coolant in the engine oil or cylinder bores. Another quick check is to run the car with the radiator cap off and see if a steady stream of bubbles appears in the coolant. The next easiest test is to do a compression test (this is very easy for a home mechanic). If the compression reading for one cylinder is abnormally high, this may indicate a bad head gasket leaking coolant into that cylinder. In the very worst case, there may be an internal crack in the engine block.

$$$ Pull head and get it straightened at a machine shop.
$$$$ Check for cracks in the block

09. Incorrect ignition timing

Engine overheats at 60mph.
Engine coolant level is normal
Radiator matrix HOT.
Performance is poor.

Don't overlook this one, many people have wasted lots of time and money replacing hoses, waterpumps and radiators when they should have simply set the timing correctly.
When the ignition timing is set too early, the fuel ignites too soon and the engine is working against itself. When the ignition timing is set late, it can also cause serious overheating. This can come from
1) A seized centrifugal advance.
2) Bad vacuum line to vacuum advance on the distributor.
3) Not disconnecting the vacuum line when setting the timing,
4) Faulty fuel injection sensors (temp, O2, throttle etc) causing the computer to set the wrong timing.

Remember, when checking or setting the timing, remove the vacuum line going to the vacuum advance on the distributor. Once the correct timing is set, rev the engine to see that the mechanical advance works, noting were the timing mark gets advanced to. Then reconnect the vacuum line and check that the vacuum advance works by making sure that the timing mark now gets advanced beyond the point reached by the mechanical advance only.

10. Fuel mixture is too lean

Engine overheats, both at standstill and 60mph.
Engine coolant level is normal
Radiator matrix HOT.
Engine "pings" under acceleration.
Altitude change from high to low elevation?

A lean mixture burns hotter and can cause overheating. In either fuel injected or carbureted cars, this can be due to a bad mixture adjustment or leaks in the vacuum lines (allowing unmetered air into the manifold). Other causes could be a weak fuel pump, clogged fuel filter or wrong float levels causing insufficient fuel delivery.

$$ Have the car tuned up.
$ Check the engine vacuum with a gauge (operate brakes)
Look for broken or disconnected vacuum lines.
Listen for air leaks.

B. Other, less common, overheating causes

11. Waterpump failures. Water pumps can cease working due to missing or slipping belts or worn internal parts. A symptom of this is that the radiator matrix will not get warm when the engine is hot.

12. Bad auxiliary fan. The electric auxiliary fan provides extra cooling when the coolant gets too warm or the air conditioning is engaged. If the auxiliary fan doesn't come on, you will experience weak air conditioning at idle with the engine overheating worst at low speeds. Tracing the aux fan circuits will require a wiring diagram and a multi-meter.

13. Exhaust restrictions. Plugged catalytic converters or bent/damaged damaged exhaust pipes can lead to overheating.

14. Overfilled recovery tank. If you are reading this because you've noticed water venting out of the overflow tank, but your engine temperature appears OK, then you simply may have overfilled your recovery tank. You don't need to do anything, the engine has already corrected the problem.

15. Dragging brakes (Main or park). If your brakes are sticking, this will cause your engine to do more work, generating more heat. The car will feel sluggish and the brakes will be weak after a high speed run. Check that the wheels turn freely, adjust brakes if necessary.

16. High compression. If the engine head has been shaved too many times or high compression pistons have been fitted, this will raise the engine temperature.

17. Low engine oil. If your engine is low or out of oil, it can heat up very quickly and catastrophically.

C. What to do now?

First, if you suspect a coolent leak, you usually need to look carefully to find the source. Here's what I do:

1. With the engine off, top off the system with coolant and carefully examine the engine bay for evidence of leaks, either in the form of coolant or water deposit stains. Examine the radiator, all hoses, the engine block (around head, water pump, freeze plugs) and on the ground. Squeeze the large hoses to see if they are damaged.

2. With the engine still off, find a way to pressurize the coolent system without compromising the function of any pressure relief caps. There are a number of ways to do this with inline T's for an air or water hose. Once the system is pressureized, look and listen for any escaping air or coolent.

3. If nothing is found, carry out the same examination with the engine running. Please be very careful to keep hair and clothes away from the turning fan and belts. Once the engine has warmed up, the coolant system will develop pressure, making it easier to detect leaks. But, with the fan running, leaking coolant can get blown all over the place, making detecting the source harder.

4. Remember that there may be multiple problems and fixing an obvious leak may not entirely clear up the problem. Try not to get frustrated, keep watching the temp gauge and coolant level until you are sure the problem is fixed.

If no evidence of leaks are found, you can take the shotgun approach. This is a list of some things that a backyard mechanic can easily do to address an overheating problem. It assumes that you have common auto tools like a timing light.

1. Check the front of the radiator for debris.
2. Hose the radiator off with water to clean it.
3. Check that the mechanical fan doesn't spin freely.
4. Check the condition/tightness of all belts.
5 Set the ignition timing properly (remove vacuum advance line).
6. Check that both the centrifugal and vacuum advances work.
7. Set valve clearance then adjust the carbs.
8. Replace the radiator cap(s).
9. Replace thermostat.
10. Flush and refresh the coolant.
11. Remove and clean the radiator.
12. Replace every hose and belt.
13. Replace waterpump.
14. Replace radiator if necessary.
15. If you are still overheating, consult a professional mechanic.

D. Coolant system pressure discussion.

The idea behind checking carefully for coolant leaks is that these allow your system to boil over much easier by lowering your cooling system pressure to 0psi. As I'm sure you know, the boiling pt of water is directly related to pressure (at sea level, water boils at a much higher temperature than it does here at 5500ft above sea level where the air pressure is much lower). The same goes for your cooling system. When your system operates at it's rated 15psi (SIII XJ6) pressure, the boiling pt of the coolant circulating inside of your head and block is much higher than if your system is operating at ambient pressure. If you've got a bad hose or radiator cap, the internal pressure of your cooling system falls from it's rated 15psi pressure down to 0psi. Once this happens, your coolant can start boiling in the block and the head on a large scale and bad things start to happen as the expansion of the water in the block into steam forces coolant out the overflow tank and then the new steam now in the block is very inefficient at dissipating the combustion heat, so you really start to
overheat. I'm sure that somewhere along the way you start to get reduced coolant circulation due to cavitation in the water pump too.
I've heard that the real killer is that your temperature gauge sensor may not give you a clue about what's going on because it's now trying to read the temperature of the steam (which it can't do) rather than the coolant temperature.

E. Ignition timing discussion

It has been my impression that faulty ignition timing is a major contributor to Jaguar's reputation for overheating. If the timing of the V12 is retarded, it will overheat BIG TIME. This is supposedly true of most cars, but I have never seen it happen the way it happens on the HE.

Unfortunately, many problems result in retarded timing:

1) If the centrifugal advance seizes, it will usually seize in the retarded position.
2) If the vacuum advance is NOT disconnected when setting the timing, it will be retarded -- ALWAYS disconnect when timing!
3) If any of the myriad vacuum lines fails and vacuum is lost to the distributor, guess what? Retarded again.

Folks, if you get a tank of lame gasoline in your HE, it will knock gently at almost any speed or throttle condition above idle. This is a sign of an EXCELLENT arrangement of advance curves, since the objective is to keep the timing just OUT of the range of knocking. The engine is designed for 89 octane (usually referred to as Plus here in Florida,
midrange between Regular and Super), and apparently will knock on 88-1/2 octane. Now, some of you use Super anyway, but if you use Plus from different stations and have NEVER heard your engine knock, you
might want to look into your ignition timing -- especially if it runs hot.