Glowplug failure is a very common problem on old Merc Hymers.
For those who don't actually know what they are ... don't confuse glowplugs with sparkplugs. Diesel engines don't have sparkplugs. But a diesel glowplug is not the equivalent of a petrol engine sparkplug. A glowplug is a pre-heater to warm up the cylinder and the fuel for the first few seconds until the engine comes fully to life. When you turn the key, the light should come on, you wait until it goes out, and then start the engine. What has happened is that the glow plug relay box sends power to the plugs for about 5 seconds which makes them glow red hot, which helps start a cold engine. After the engine is started, glowplugs have no function until the next time the engine is started again.
Glowplugs don't last forever - about 10 years in my experience. As soon as a single plug stops working, then the light no longer works as usual - which gives you a warning. However the other plugs will carry on working, and the engine will start OK, especially in summer. Once one plug goes, it is only a matter of time before others go, and ultimately if you ignore the problem you will notice that the engine becomes more difficult to start in cold weather, until one day it won't! If your plugs are acting up, and the weather is cold, best to count to 5 after turning the key to give the best chance to the remaining plugs. If you get stranded - ie the engine won't start because the plugs are not working, try not to flatten the battery. A "get out of jail card" is a can of "Easystart", available in Halfords etc. It is evil stuff and not good for the engine. You spray it into the air intake (you have to take the air filter housing off) and spray it into the filter while turning the engine over. It is a high octane chemical that will explode the engine into life. Breakdown guys use it - or they used to in the old days!
It's best to replace the whole lot as a set as soon as you get a problem. They are not expensive, but I have found that cheap plugs (a fiver or less on ebay) are generally a false economy. The best plugs are from Bosch or NGK and cost around a tenner each. Replacing them is best done by a garage - the front plugs are hard to get at - you need the right type of sockets and extensions to get a good purchase on them. They can be hard to get out, especially if very old. My mechanic tells me that you have to be careful with them - if you apply too much pressure or don't have the right tools it is possible (but rare) to break the plug, leaving the threaded part still in the engine block. It then becomes a nightmare of a job to extract and fix. But as I said this is quite rare and unlucky. A decent garage shouldn't charge more than an hour - less if you are lucky. I just had a new set fitted a month ago and it took 30 minutes.
The relay box is generally quite reliable, but occasionally they do go. I have had to replace one in my old van. It is easy to check - if you take the plug off you will see 5 big pins - you should get 12v on each of these when the key is turned. There is also a 60amp strip fuse mounted on the top of the box - this is easy to see if it is blown. It rarely blows unless there is a good reason. The boxes are available on ebay for about £70 - I have known Merc garages charge a lot more, so shop around. However the £70 ones are generally pattern parts - the original Bosch/Merc ones are dearer. The same box was used in loads of merc cars and vans from the 80s and 90s. Very occasionally you will get a break in the wiring, but this is rare, unless the engine has been worked on and the wiring has been disturbed. It's easy to check with a meter.
For those who don't actually know what they are ... don't confuse glowplugs with sparkplugs. Diesel engines don't have sparkplugs. But a diesel glowplug is not the equivalent of a petrol engine sparkplug. A glowplug is a pre-heater to warm up the cylinder and the fuel for the first few seconds until the engine comes fully to life. When you turn the key, the light should come on, you wait until it goes out, and then start the engine. What has happened is that the glow plug relay box sends power to the plugs for about 5 seconds which makes them glow red hot, which helps start a cold engine. After the engine is started, glowplugs have no function until the next time the engine is started again.
Glowplugs don't last forever - about 10 years in my experience. As soon as a single plug stops working, then the light no longer works as usual - which gives you a warning. However the other plugs will carry on working, and the engine will start OK, especially in summer. Once one plug goes, it is only a matter of time before others go, and ultimately if you ignore the problem you will notice that the engine becomes more difficult to start in cold weather, until one day it won't! If your plugs are acting up, and the weather is cold, best to count to 5 after turning the key to give the best chance to the remaining plugs. If you get stranded - ie the engine won't start because the plugs are not working, try not to flatten the battery. A "get out of jail card" is a can of "Easystart", available in Halfords etc. It is evil stuff and not good for the engine. You spray it into the air intake (you have to take the air filter housing off) and spray it into the filter while turning the engine over. It is a high octane chemical that will explode the engine into life. Breakdown guys use it - or they used to in the old days!
It's best to replace the whole lot as a set as soon as you get a problem. They are not expensive, but I have found that cheap plugs (a fiver or less on ebay) are generally a false economy. The best plugs are from Bosch or NGK and cost around a tenner each. Replacing them is best done by a garage - the front plugs are hard to get at - you need the right type of sockets and extensions to get a good purchase on them. They can be hard to get out, especially if very old. My mechanic tells me that you have to be careful with them - if you apply too much pressure or don't have the right tools it is possible (but rare) to break the plug, leaving the threaded part still in the engine block. It then becomes a nightmare of a job to extract and fix. But as I said this is quite rare and unlucky. A decent garage shouldn't charge more than an hour - less if you are lucky. I just had a new set fitted a month ago and it took 30 minutes.
The relay box is generally quite reliable, but occasionally they do go. I have had to replace one in my old van. It is easy to check - if you take the plug off you will see 5 big pins - you should get 12v on each of these when the key is turned. There is also a 60amp strip fuse mounted on the top of the box - this is easy to see if it is blown. It rarely blows unless there is a good reason. The boxes are available on ebay for about £70 - I have known Merc garages charge a lot more, so shop around. However the £70 ones are generally pattern parts - the original Bosch/Merc ones are dearer. The same box was used in loads of merc cars and vans from the 80s and 90s. Very occasionally you will get a break in the wiring, but this is rare, unless the engine has been worked on and the wiring has been disturbed. It's easy to check with a meter.
Glowplug relay box with the cover removed. Pull off the plugs to expose the pins. The strip fuse is on the left in this picture.
Glowplugs!
