How to change fuel injectors in the M110 engine

Ever felt that the engine is not making as much power as it used to? And that the fuel economy has been slowly decreasing? The first thing you should check is the condition of your injectors. Unless you have a pressure tester for the injectors, you would have to guess out from the age of the injectors. If they are more than 10 years old I would suggest to change them, that said if the injectors are relatively cheap since they vary a lot in price between the models. They are still relatively cheap for the M110 engine. E.g. If you have the rarer M100 engine, it will cost you nearly a new house to buy new injectors!

I will describe how you change the injectors with new ones. Refurbishing the old ones is a bit more cumbersome and it needs a pressure tester which you can clean them with and test for release pressure and spray pattern.

 Theory

Changing the fuel injectors is not difficult and you don’t need much preparation except acquiring new injectors, gaskets and o-rings. There are two gaskets per injector you will have to change or you will end up leaking fuel which is expensive. You can change the injectors easily in an afternoon. I also recommend to change the fuel filter at the same time to not clog up your new injectors.

Practice

 Removal

Start with opening your hood and disconnecting the battery. Sparks can make explosions from gasoline vapor when you are working with the fuel system, and we don’t want that to happen.

mercedes m110 engine without air filter housing
Remove air filter housing to gain access

Remove the air filter housing to get access to the fuel delivery system.

The four front injectors you can now access without removing more stuff, but to gain access for the last two, you have to remove throttle linkages, fuel hard lines and a fuel regulator. We’ll come to that later.

Two and two injectors are held in place by a single metal plate, to remove the metal plate loosen the injector hard lines first. The injector hard lines should not sit on very hard unless they corroded over time or they have been over tightened. Use two spanners, one 14mm to loose the line nut and one 12mm on the injector itself to to keep it from rotating.

fuel injector removal
Loosen fuel injector hard lines with two spanners.

When two hard lines are loosened you can bend them a little to the side to get more access and then remove the injector hold-down plate.

injector removal
Loosen the injector hold down plate

After the plate is removed you can pull out the injectors with you fingers, they sit loose in there. The injector housing will be left, This plastic housing you can just pull out straight too, but be warned, do not use excessive force and pull only the housing straight out with you fingers, don’t use tools or you might damage it. It most likely have become quite brittle by heat over the years. If they are in such a bad condition they are cracked, you have to replace them.

Most likely some of the injector housing gaskets do not come out with the housing, but is still sitting in the injector hole, use a small flat screwdriver and pry it carefully out.

After removing the first four injectors, the two back injectors are left, now the big job starts. See in the picture below that these things blocks access to the fuel injectors.

fuel injector access
Throttle linkage, hard lines, fuel stuff blocking access to the injectors.

Start with removing the throttle linkage. Remove the pin and the c-clip along the ball joint. I use a wide flat screwdriver to pry the ball joints off. Remove some more linkages and then you are ready to remove the regulator.

throttle link removal
Remove the throttle linkage.

Warning:
Get a small collecting can of some sorts and tilt it towards the injector line shown in the picture below. Loosen it and fuel with shoot out with high pressure. Wait until the fuel stops flowing, then loosen the second fuel line connected to the regulator.

fuel line disassembly
Fuel will shoot out when you loosen this line so be prepared with a can of some sort.

Also loosen the vacuum lines before removing the fuel regulator, it is fastened to a bracket with two small bolts.

removing fuel regulator m110
Unbolt the fuel regulator.

Now you should have clear access to the last two injectors.

fuel injector removal M110
Clear access for the last two injectors.
Inspection

Now is the time to inspect the old injectors and look at those old brittle gaskets. It is a reason why your car was using excessive fuel…

injector o-rings
New vs. old, looks can deceive.

My old gaskets were super hard, especially the round outer ones. No wonder why there was a lot of dirt around the injector area by the engine, they have probably been leaking a while. This job should have been done years ago. I didn’t notice it when the engine performance still was very good, but obviously not optimal when looking at this leaking issue. The old injectors I don’t even bother testing, I will just throw the old ones and put in new ones.

brittle old injector o-ring
Just look how brittle the old injector o-rings were. They must have given little sealing when they are hard as rock!
Assembly

This is how you should assemble the injector with the gaskets:

  1. Pop on the big rubber gasket on the injector itself.
  2. Pop on the little rubber ring on the injector plastic holder.
  3. Pop the injector holder into the engine block until you hear and feel a nice pop.
  4. Slide all the injectors into the holders. Put the hold-down plates over them and tighten.
injector assembly M110 engine
Injector assembly

The injectors will now have a good seal and no fuel will leak. It is also a good idea to clean up the area around the injector holes in the engine before assembly. Also try to be super clean with the new injectors, keeping the plastic cups on as long as possible before fitting them.

No you also have the perfect opportunity to lubricate all the throttle linkages, such as the pivot points and ball joints, I know they have been neglected for years and we are all guilty as charged. However now is the perfect opportunity for redemption. Remove all the linkages and pivot plates, clean them and apply new synthetic grease that will last a while and hold up to water and heat from the engine.

When everything is bolted back together, there is no such thing as dreadful bleeding like in a diesel. Just crank the engine over a few times and the engine should start, it will run rough for a little while until the pressure have settled, and you might experience hard start the first couple of times before the first drive. This is the time when you realize if the electric fuel pump needs replacement or not, mine is some years old and still going strong, although it have been replaced a couple of times the last 15 years.

Cheers, Robs out!