P38 Spark Plug Replacement

Where are the plugs?
Tools Needed
Gaining Access to the Plugs
Removal of the Old Plugs
Installing the New Plugs
Finishing the Job
Parts Needed

Picture at Right:  Not much chance of getting at these plugs!


Spark plug replacement is recommended in the official service schedule every 30,000 miles. In the old days the plugs were some of the easiest engine components to get at and remove, but not any more. This page offers several tips to make the job easier on the 4.0/4.6. As a preliminary step the shop manual recommends disconnecting the battery negative lead -- not a bad idea as it is quite easy to cause a short accidentally while wrenching around the engine with steel tools.

Spark Plug Socket and Ratchet
A selection of socket extensions
Flexible/Universal Joint for sockets
Torque Wrench
Phillips Screwdriver
Regular Screwdriver
Gaining Access
On the Left Hand Drive (LHD) Range Rover 4.0/4.6, the plugs on the right hand side of the engine are easily accessible. On the Right Hand Drive (RHD) models which have the brake booster and related apparatus on that side (see photo at right of Ron Beckett's Australian 4.6), the rear two plugs are a bit tricky but not too bad. (Note: Ron's 4.6 has an LPG conversion so the
odd brass T-pieces in the heater hoses are a feed to/from the LPG converter which stops it icing up from the change of state of the LPG from liquid to gas).

On the left hand side of the engine, it is another story. As the photo at the top of this page shows, the spark plugs are deeply buried behind the air intake apparatus. On LHD vehicles, the situation is made worse by the presence on this side of the brake booster and servo (photo at top of page). The shop manual makes no mention of any access problem, merely telling the reader to "remove the plugs".

I have found the whole operation is much easier if you remove the Mass Air Flow (MAF) sensor and its associated flexible air intake tube that connects between the air filter and the aluminum air intake plenum on top of the engine. This makes the job much easier on RHD models as well. Just unscrew the hose clip securing the flexible ducting to the aluminum intake plenum, and snap open the clips securing it to the air filter box. Then compress the flexible tube slightly to remove it from the filter box, and lift the assembly out, disconnecting the MAF sensor multiplug and the air intake temperature sensor plug (the latter is on the side of the air filter box).

Spark Plug Access


MAF Sensor and flexible intake assembly removed for access to plugs (compare with photo at top of page). The bright glow at the right of the picture is an inspection lamp.

Similar view on a RHD model with intake air ducting removed for plug access (courtesy of Ron Beckett). The alloy device at right is Ron's LPG converter).

The result (see photos above) is a much improved view of the left hand side of the engine, including excellent access to the plugs, whether yours is a LHD or RHD model. Optionally, you can even remove the evaporative canister purge valve solenoid (positioned below the MAF sensor and connected by vacuum tube to the air intake manifold) from its moorings so it and its associated tubes and wires float about freely, further easing access when it comes to getting socket extensions in at the right angles.

 Removing the Old Plugs
After pulling off the leads but before removing the plugs, squirt some air into their mounting holes to get rid of any dirt and debris that might fall into the cylinders when the plugs are out. If  you don't have a shop air supply, you can use a can of compressed air -- e.g. the type sold for cleaning photographic lenses (see picture below left).

Air Supply


Removing the plugs using a plug socket and universal joint poses few problems until you get to the 2nd one from the front on the left side; in another brilliant stroke of Land Rover design genius the oil dipstick tube is carefully mounted so it interferes with the socket. Removing the Phillips screw that holds it in place on the side of the engine (see arrow in picture above right) lets it bend out of the way enough to get the necessary clearance.

Installing the New Plugs
I like to get the ready gapped plugs from the dealer -- if you don't, remember to gap them before inserting them. Factory specified gaps are as follows:

GEMS: Champion RN11YCC, 0.90 - 1.00 mm ... 0.035 - 0.040 in
Bosch: RC11PYB4 plugs -- the gap should be 1.00 ± 0.05 mm ... 0.040 ± 0.002 in,  BUT do not attempt to adjust the gaps on these plugs.

Remember you are screwing the plugs into aluminum which is not very strong, so the thread is easy to ruin. Also for this reason remember to use a torque wrench so you don't under- or over-tighten the plugs. The specified torque setting is 15 lb-ft (20 Nm).  Although not mentioned in the shop manual, I coat the threads with anti-seize compound before putting the plugs in. It helps when removing them next time without binding. Ron Beckett points out there seems to be There seems to be conflicting advice on this, with some sources saying using anti-seize might make it easier to overtighten the plug, ruining the aluminum thread. Some plugs (such as Champion's Copper Plus plugs) are supposedly specially coated so they do not require anti-seize, but Champion's vehicle applications data includes this "Special Tip: Anti-seize compound is useful when replacing spark plugs in aluminum cylinder heads." One source even advises reducing the specified torque by 40% when using anti-seize. Regardless of all this advice, I have used the stuff for years in my Range Rovers, tightening the plugs to the factory specs, with no ill effects. 

Probably the most important thing is that the plugs be able to be screwed in by hand. If they won't turn, something is wrong, e.g. a cross-threaded hole. Ron Beckett found out that the rearmost plug on his 4.6 had been cross-threaded by some heavy handed non-LR mechanics. "It is now extremely difficult to get that plug to catch the right threads. I have to eyeball the angle of the plug against the plug in the cylinder in front to make sure the angles are the same before attempting to screw the plug in. It will, eventually, have to have the thread reformed or a helicoil fitted". This is yet another warning against sending your Range Rover to seemingly bargain auto repair shops whose hourly rates may look cheaper than the dealer but whose ignorance can end up doing damage to the vehicle.

Finishing the Job
Replace the spark plug leads, re-install the air intake assembly, re-connect the battery and you are in business. Don't forget to reset the windows and enter the radio code after reconnecting the battery.

Parts Needed
The correct OEM spark plug for GEMS is the Champion RN11YCC with RC11PYB4 for Bosch engines.  I usually get mine at the dealer for convenience but lower cost sources are available. 



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Page revised February 9, 2012