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1991 560SEC
669 Posts
Discussion Starter #1
Last week I completed the motor swap on my 1987 420SEL, using the motor from a 1990 560SEL. Been driving for a week, everything is fine and the car runs great, so I wanted to share some tips for anyone thinking about doing this. (This won’t be a comprehensive how-to on the swap)

This swap is super, super easy. This is my 2nd motor swap on a w126 car, although the first was like-for-like. I have to say that pulling the motor on these cars is so easy. There are a lot of things to take off, but if you take your time it’s really straightforward. I have to admit, I have access to a lift which helps a lot, but don’t be intimidated to do this at all.

Things to remove. My suggestion is to leave most of the motor intact, but there are a couple things that will make your life easier if you take them off. Obviously the cooling system all has to come out, but I would also suggest removing the front bumper (6 bolts) esp if you have a US bumper just to get it out of the way, as well as the fuel distributor and the Alternator. The FD is debatable, I do it simply because the lines are very fragile. The PS and AC pumps can stay, but the alternator will get hung up in a couple places believe it or not. If you want to take a little more time and give yourself a little more room, the AC pump is easy to get off as well from the bottom but it’s not necessary.

Some heat if your friend. For the old wiring, that has to come out of the way, I suggest a heat gun on the low setting to get things flexible. Most of the wiring will be able to move out of the way easily, but warming it up JUST a bit will prevent problems with old rubber and wires.

Remember: Match the motor to the car. So this may be simple but remember you need to make sure that the new motor is using the senders, harness, etc. from the car the motor is going IN. Don’t try and fit the new donor parts to the existing car. Fortunately for us, Mercedes made this simple since all the sender holes are the same regardless of the electrical connection. This is related to my next point:

Take Photos and DRAWINGS. The biggest hiccup I had in this swap was the vacuum line routing. The ’87 420 had a different routing scheme than the ’90 560. I had taken pictures of both, but I should have done some drawings. It turns out there are some subtle differences in how they are setup that the camera did not capture, and the online diagrams show sort of a universal blended version. In my case, I actually used the 560 routing since in 90 they simplified the EGR system and removed a couple components. But I probably spent more time on this than taking out the motor!

Things to do “while you are there”. This is sort of based on your budget, the condition of your motor, and how much time you have, but nothing on the motor will be easier to do or access than once it’s on the engine stand. Chain and tensioner, rubber parts, intake gasket, etc. In my case, I did everything. Gaskets; injectors, seals and holders; intake donuts and gasket, lower throttle body gasket, all the upper intake runner hoses, etc. I also had a valve job, head gaskets and chain and guides done. None of this is mandatory but it will never be an easier job than right now. There are a couple things that I would consider CRIMINAL not to do at this stage.
  • Water Pump – Just do it. Budget for it. Doing a water pump requires removal and draining of the cooling system, taking off all the belts, removing pulleys (in some cases the crank pulley) and is generally a PITA. You have done 99% of this work for the motor swap, and you’re like 7 bolts away from taking the pump off. Just replace it now and be done with it.
  • Motor mounts – goes without saying but they are literally a 5 min job with the motor out and even the good mounts are still cheap.
  • Air Pump Delete – Unless you’re running factory cats or are legally required, ditch this thing now. There are some bolts that are tricky to get to with the motor in the car, but it’s easy to get it off on the stand. Cap the Air Pump fitting with a large vacuum cap (it’s a 1-way valve so doesn’t leak), replace the 2 bolts in the timing case with shorter ones, and you’ve removed a likely failure point, excess weight, excess drag on the pulleys and cleaned up a bunch of vacuum lines and mushy rubber hoses.
What to Swap and what to keep when upgrading. Ok this is the meat of the swap but basically every accessory on the 420 is in the same place for the 560. I was worried for instance about the exhaust because of the lower heads, but the 560 exhaust outlet is in the exact same place. So any custom exhaust work will be fine. You need to match the sensors to your car, and in my case the ’90 560 had different sensor in the Water Outlet and on the intake manifold. In my case I used the 420 water outlet (instead of removing all the sensors) and replaced one of the coolant sensors into the 560. That made the sensors match the harness. I think this is related more to year of the car than the engine itself.

Things that won’t swap over: There is a difference in the fuel distributor lines and the throttle linkage brackets due to the intake sitting lower on the 420 motor. So you need to use the 560 FD lines, although the FD itself is the same (I’m using the one off my 420 since it was in flawless shape). Throttle linkage brackets are a little longer on the 420 for the same reason. So use the 560 linkage brackets. The starter motor in the 420 uses a spacer, and while the motor works if you put the spacer back car won’t start. But the wiring harness lines connectors are the same.

The only other thing that needs to be swapped (and I just figured this out last night) is the Ignition Module. I was a little disappointed in the performance of the engine after install it didn’t feel quite as crisp as my other 560's. I happened to grab all the computers from the donor car and noticed that the 560 ignition module had a different PN# and had 117 on it, vs. the 460 module had 116. I swapped them over and I think the timing curves are different as I noticed a bit more pep in the motor with the different ignition module.

No other computer or wiring needs to change, and for other things like AC pump, PS pump, pulleys, etc. just use what’s better between the two cars. I mixed and matched stuff to use the cleanest, nicest parts.

560 Motor: Exhaust Manifolds, Throttle Linkage, Ignition Module, Fuel distributor lines, Starter.
420 Motor: Sensors (if applicable), vacuum routing (to match car)
Doesn’t Matter: Fuel distributor, Relays, Computer, pulleys, Alternator, PS pump, AC pump.

Transmission: So there are different PN#’s for 560 and 420 transmissions. I know they will physically swap and work. I understand from other posters that the 560 transmission is beefier, and since I found a donor car with exactly the same mileage, I swapped the transmission over as well. YMMV on this, and cost as well as driving style may factor in. I recommend getting a donor car for this swap in which case you would get a transmission too, but it’s something to consider. I did not run the 420 trans with the 560 and while I’m 100% confident it would work, I’m not sure how much longevity is affected.

Total time for this swap for me was 3 weeks with 2 weeks of that having the heads and timing chain done (note: it’s cheaper now too since you shave hours of labor off the work with the engine on a stand). What’s nice with a donor car is you can prep 99% of the new engine while keeping the current car running. They physical swap itself (motor in/motor out) you can be driving the car on Sunday night if you start Saturday AM and have all the parts ready. That includes the trans swap too and some time cleaning out the engine compartment.

I think you can also do this swap pretty cheap. NOT counting the head and timing work (my choice), the donor car with 130K was $700 (-$50 what I got paid for scrap). Assume if you do all the labor yourself on the rubber parts you’re going to have ~$500 in parts (this is ALL rubber on the motor) including trans parts like filter, gasket, and K1 spring. If you’re a little more frugal, you’re going to be out $200 for Water Pump and Engine mounts, and a few bucks for oil, filter and coolant.

So that’s it. Swap is easy, you only need the ignition module from the old car, and enjoy the extra 55hp and 77lb/ft of torque!

Happy to answer any other questions if I can.
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