W126 engine swap 4.2 v8 to 3.0 inline 6
 

The engine in my 89 420sel which is not a daily driver needs more work, I think, than the car is worth. About 153k miles. If I use it almost entirely for easy highway driving, and could get a 300se engine with the same miles for about $450 tested; would that be terribly complicated and/or just a terrible idea. Somehow it just seems the sixes are generally simpler and last longer without overhaul. I'm not concerned with missing the v8 power as I have other v8 cars.

Why not just buy a 300se?


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Swapping the engine would be more work than it's worth. The wiring harness is going to be different, ECU, ignition control, radiator and other bits. You'll also probably want to swap the transmission for the correct one to match the M103 and put in the proper rear diff so the car would be able to move under its own weight. Parts alone you'd be better buying a 300SE or fixing the M117, let's face it, these cars aren't worth a ton anyway.

It really depends on how much you love the car.Is interior good,paint,and rust free.
Spending $10,000 for a newer low mileage engine,and trans,then labor might be the thing.Might even go with a different motor,and five speed auto,and use off brand computer to make it run better than stock. A new car,is a house mortgage,$10000 would be easy to pay off.

Sell it and buy a nice 300SEL. Even if you buy the nicest 300SEL and donate your car, it will be cheaper than trying to do a conversion.

The total cost (parts & labor) of repairs to the present engine (M116 V8) is the most that could be spent (again, parts & labor) to replace with an M103 I6.
The "worth" of the car is not relevant to the comparison, unless, of course, you are in the trade; in which case your question would not arise.

I just wanted to belatedly thank all those who responded. I recently bought a 300sdl, 1987 which I’m bringing back to life (an overstatement but it sat for an extended period). Pretty low mi with172k etc. But when I’d leave it and get back in the 89 420, the 420 just seems more modern and sophisticated between the Seats, suspension, and the difference in condition in the two. But as I’ve continued to improve the diesel it seems to be ‘gaining’ on the other, wonderful road car, and I may just have to donate the 420 and be done with it. I paid $3k for it, drove it for 3 years and that’s not too bad. I think it needs rings, etc, and I’ve been chasing gremlins most of the time I’ve had it. But on a good day... (makes me want to find a way to keep it). Interior is great, exterior looks good from 20 feet, maybe 10 feet on sides and 25 feet on roof - haha. Thanks again; passing on the transplant.

The SDL should be a nicer car to drive than the 300SE or 300SEL since it makes better torque and way easier on fuel, not to mention no issues with CIS or emissions controls eating your lunch. I've been resurrecting an '86 that sat for 10 years, you can read about that in the thread in my signature if you ever need a cure for insomnia!

You can swap interior parts between the SEL and SDL. Hopefully color isn’t an issue. 88-91 seats are more comfortable and the padding lasts longer than 81-87 seats.

Sixto
98 E320s sedan and wagon

If I were a swapping man with a dead gasser w126, it would receive one of the larger GM LS v8's in a relative heartbeat. And have a ton more power and get noticeably better MPG afterwards I strongly suspect. It might actually increase the value of the car which many cross brand swaps do not.

That said, sixto's suggestion is likely the better-er way to go. Later cars do feel more modern and nicer in general despite being so similar.

Assuming it'll clear the steering box anyway, might become a paint in the rear if not.

Installing a 103 would be a bad idea. You would probably have to swap the rear end also to get any sort of decent gearing. The 103 is way underpowered for the 126.

One of the members on our Aussie forum just took his 420SE on a long trip. He has done the same trip in his 300SE. The 420 gets better Gas Mileage. - One reason being the Diff ratio.
My own experience is that an old V8 380SE gets better gas mileage than a straight 6 280SE

if you think that the W126 300 3 litre was under powered - in the ROW there was a 260SE - 2.6 litre M103 W126.