14 October 2023
V6 glow plugs are notoriously difficult to remove. Ed gets away with a relatively easy fix on his Range Rover, then shows how to get into big trouble, and out of it again
Need to know
Time: 2 hours plus
Difficulty: 3 out of 5
Models: Discovery 3 and 4, Range Rover Sport 1.
Tools needed: General workshop tools, multimeter, deep socket and extension. Specialist tools and/or services if things go wrong.
Parts used: Bosch glow plugs (six required), part number 1354289G (for this engine, but check model specification). Around £15 each.
• Use barrier cream or gloves to protect hands, as preferred.
• Wear eye protection and a particle filter when clearing dust and dirt from the top of the engine.
• Wear thick gloves to protect against abrasion, and always when using power tools.
• Beware of hot components on a warmed engine.
Thanks to: Steve Grant and Britpart.
The trouble with the glow plugs on the V6 diesel engines fitted to Discovery 3 and 4 and Range Rover Sport 1 is that they last such a long time that when it’s finally necessary to change them, they can be well and truly seized in. They are also difficult to reach, and they’re easy to break, and when that happens it’s a specialist’s job to extract them after a fair amount of dismantling.
Why glow plugs?
At warm times of year, a good diesel engine will fire up without the need for the glow plugs to operate. When the engine is turned over on the starter, the pistons compress the air in the cylinders making it hot enough to ignite the incoming diesel fuel from the injectors, and away you go. In winter though, especially in sub-zero conditions, it’s essential that the glow plugs (also known as heater plugs) operate to initially heat the air inside the engine cylinders, otherwise, the compression alone will not generate sufficient heat to ignite the fuel, and the engine won’t start.
Of course, the glow plugs don’t all fail at once. If two or three aren’t working, the engine might still start up on the other three cylinders, but probably running less smoothly for a few seconds until the cylinders with the defective plugs warm up and begin firing properly. These V6 diesels are naturally quiet engines, well soundproofed and well insulated from the chassis but, even if it’s not possible to hear this initial rough running, it can usually be felt as slight body rock or vibration. That’s the first clue that the glow plugs are failing, and it will become noticeable as the weather gets colder.
The glow plugs are automatically activated (and a start-up fuelling regime is invoked) by the Engine Control Module (ECM) when the engine temperature is low. Electrical power is supplied to each engine bank of three glow plugs via a relay and a fuse. The plugs on the later 3.0-litre V6 diesel engine work on pulse width modulation in which the ECM pulses the electrical supply causing the cylinders to heat more quickly. The pulsing continues for a short period after engine start-up to burn the fuel more efficiently, and thereby reduce emissions.
Testing the plugs
Simply finding that the engine won’t start on a frosty morning is something of a disaster because the cure is not quick, leaving the vehicle out of action. But it’s easy to check glow plug operation right now, to identify those that aren’t working, and then plan repair time accordingly. Plug voltage varies between models, so we’re showing a simple continuity test on this 2.7-litre Range Rover Sport, and this test is carried out with the engine cold. It’s important to write down the positions of failed plug/circuits: the engine cylinders are numbered 1, 2, 3 from the front of the RH bank; 4, 5, 6 are numbered from the front of the LH bank. On this engine, plugs 1, 4, 5 and 6 are inoperative, which was sufficient to prevent the engine starting when the outside temperature was zero or below.
Pull off, push on: The acoustic engine cover simply pulls off after removing the oil filler cap – grasp at both sides and tug vertically. Refit the oil cap to prevent the chance of dirt ingress.
Circuit access: On the engine top, two electrical connectors (brown feeding the RH bank, black feeding the LH) are disconnected to access the cable contacts inside to test the circuits.
Three circuits: Each of the three pins in the male (forward) connector blocks relate to one glow plug supply. From here, we can gain an indication of each glowplug’s functionality.
Simple check: The meter probe is connected to each pin in turn, the other probe to earth. Thus, all pins and respective heater plugs are checked for circuit continuity.
Cold test: Supply voltage can be checked at the female connectors with ignition on in cold conditions. If failed, which is unlikely, identify and check the relays and fuses first.
Economical repair option: Removing the glow plugs
Having identified the failed plugs on our Range Rover, preparation to remove them starts a day or two before by applying easing oil onto the plug threads at the cylinder heads. That’s easier said than done, especially in the case of the less accessible rear plugs, but it is possible to poke a nozzle tube down to them. Any components removed for access must then be replaced because plug removal must be done while the engine is warm, so it’s necessary to run the engine up to temperature before attempting removal. This can incur a lot of time and expense when a garage is doing the job because, if the engine cools too much while extracting the first plugs, the components will need to be reassembled to run the engine up to temperature again before attempting the remaining plugs. This may need to be done more than once.
On this job we spent two hours gaining access and replacing the plugs that were accessible without risking the potential problems of significant dismantling. The time varies according to how many and which plugs need replacing. The difficult to access rear plugs need engine parts and other components to be removed, and if any of these plugs shear off, then it’s time to call in the professionals, which we’ll talk about later.
Steve Grant did this job in the Britpart workshop and, after showing me on a scrap engine just how easily this job can go wrong, I could understand why he approached it with some degree of caution.
Cleanliness will be essential to prevent dirt and dust entering where components have been opened or removed, especially the oil filter housing whose casing and filter needed to be taken off during this job. Ensure the top of the engine is clean and dust-free before starting work, and cover all openings with clean cloths.
Tricky access: Easing oil applied to the glow plug threads a couple of days earlier may help release them. Here, Steve parts the pipework to get the nozzle tube in.
Dust trap: Foam from the underside of the engine cover falls as dust onto the engine. It must be vacuumed off or removed by blowing with compressed air, as shown here.
Gaining access: The air cleaner outlet pipe that runs across the front of the engine bay, and the fan cowling, are removed to gain access to the engine inlet system.
Intake: The intake chamber is removed after disconnecting the sensor plug, front fixings and rear clip. Two EGR pipes (see bottom right) are then detached from the throttle body.
Throttle body: After disconnecting the sensor plugs, this section can now be removed by carefully manoeuvring the bifurcated outlet tubes from the individual inlet manifold on each cylinder bank.
Further back: We can now reach the front glow plugs. To access the centre plugs the oil filter and casing needs to be removed and stored in a clean location.
Avoid drips: The filter casing is upended on removal to avoid oil spillage. The opening in the engine is carefully covered with a clean cloth to prevent the ingress of contaminants.
Uncovered: That’s enough parts off for now. At the front we now see plugs 1 and 4 (arrowed) at the front of the right and left cylinder heads respectively.
First attempt: Number 1 plug (front, RH bank) is easiest to try first. The orange harness is disconnected and a deep socket (8mm for this engine) fitted on the plug.
No problem: We’re surprised as the plug releases easily. But the thread may still tighten as it comes out, so it’s gently unscrewed, feeling the load on the socket wrench.
Extraction: It comes out cleanly, with no carbon deposits restricting withdrawal. On older vehicles, the plastic cable sheaths fracture into small pieces (arrowed), don’t let them enter the engine.
Good so far: The old plug (bottom) has stayed clean in the engine after 162,000 miles – a benefit of good maintenance and not being over-tightened when originally fitted, all helping removal.
Plugs 4 and 5: These were tight. Steve eased them a little, re-tightened and eased again until feeling free, then eased a little more, and so on (photo is of different engine).
Number 2 plug on the RH head began to unscrew, but quickly went tight. As this plug was working okay, we didn’t risk breaking it with further attempts, and left it in place. Another working plug, number 3, was also left in because to access this requires the oil separator/breather to be removed before it can even be seen, let alone get a spanner onto it.
On the left bank, it’s possible to get a socket onto number 6 plug, but it’s too restricted to risk unscrewing without removing the oil separator, the EGR coolant and fuel pipes (at rear), and wiring.
Too risky: Given this vehicle is in daily use, the risk of breaking plug 6 and subsequent dismantling for machine removal, which might also fail, was a risk too far. The dud stays in, at least until a top-end strip down is ever needed for other reasons in the future. Five out of six plugs working will fire the beast up in the coldest of weather.
Rear plugs: This view from the rear of our spare engine shows the two rear plugs (foreground). Access here requires considerable dismantling with risk of damage to parts and wiring.
Acceptable cold weather starting
So, after trying carefully and replacing what’s reasonably possible, the Range Rover has enough good plugs to reliably start the engine in sub-zero temperatures. Naturally, if not all cylinders are heated by working plugs, the engine is best not revved for a few seconds until the unheated cylinders warm up and the engine is idling in balance. Of course, it’s very bad practice anyway, to immediately rev any cold engine if a decent lifespan is expected from it. And, in reality, many engines will be running with the odd failed plug without the owner even noticing, so always start and run the motor sympathetically.
New plugs: The new plugs are fitted without anti-seize grease, being careful to avoid dirt getting into the holes or on the threads. They’re tightened to 10Nm.
Getting into trouble
Although two of the plugs came out easily from this Range Rover Sport engine, they are nonetheless notorious for breaking off during removal and then being extremely difficult to extract, in rare cases needing the cylinder heads to be taken off. Land Rover was aware of the problem back in 2007 when it recommended a torque of no more than 20Nm should be applied when trying to unscrew them. Failing that, it suggested running the engine to full temperature, applying easing oil to the threads, and trying again, still with a maximum of 20Nm torque. Hence, Steve’s very careful approach, including working the threads gently back and forth.
At this time, Steve also demonstrated a glow plug removal kit, using it on a spare scrap engine where failure would not be an issue.
Anatomy: The good glow plug (top) has two main parts, seen in the sectioned plug below. Middle shows the outer sheath with thread and hexagon thread. Bottom shows the heating element which passes through the sheath, and the connector post.
Big trouble… This is what can happen when unscrewing a tight plug. The top of the plug (below the hexagon) has sheared from the threaded portion in the head, leaving no means of unscrewing the remains of the plug.
Extraction kit: We’ll try to remove the sheared plug using this glow plug extraction kit comprising drills, thread taps and pullers. It will need much patience.
Drill and tap: After twisting off the broken section, this stepped drill removes the plug’s body while drilling concentrically into the sheath still inside the head.
Pulling: A thread is tapped into the sheath and the stud of the extractor tool screwed into it. Force is applied to pull the sheath out of the cylinder head.
Failed: But the delicate process fails, and although the body thread is removed, the internal sheath section stays put, still held by a build-up of carbon deposits.
Unsurprising: Part of the problem is the long narrow port through the head to where the plug tip projects into the combustion chamber. Carbon in here just jams it.
This problem of plugs snapping during removal is not solely a Land Rover concern, it happens to numerous other vehicle makes. Hence, there are plenty of businesses that specialise in glow plug removal, often with the ability to do this without removing the cylinder heads (they can also remove injectors, fractured bolts and other metallic parts). They are located in various parts of the country and some offer a mobile service. Which is just as well because many garages are unwilling to replace glow plugs because of the potential complications. But it’s important to note that most glow plug extraction specialists expect the engine components to be removed first to give space for them to install their equipment, and most will simply remove the broken plug and not get involved with necessary dismantling or rebuilding of engine components. So it’s important to discuss and fully understand their requirements before committing.
It makes sense also to ask a trusted garage to remove the engine parts and bring in a specialist they regularly work with. Indeed, a garage that is prepared to take on glow plug replacement may bring in their own preferred specialist to remove any seized plugs while the engine is dismantled, which can be more cost-effective. The following is a selection of plug removal specialists. It’s not an exhaustive list, so check your local area or your garage:
• midlanddieselsolutions.co.uk, Hinckley, Leicestershire (work done on the premises).
• ukinjectorremoval.co.uk, north-east based, but mobile service nationally.
• thethreadmaster.co.uk, south-east.
• injector-removal-service.co.uk, London.
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