Engine Swap Question - Mercedes-Benz Forum
 
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post #1 of 10 (permalink) Old 05-21-2019, 11:07 AM Thread Starter
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Engine Swap Question

I recently bought a 1991 300sl that has a bad motor. Its a really clean car but the engine is apparently shot (low compression in 2 cylinders). I've been looking at used motors on ebay, and I've noticed that early m104s are twice the price as later ones. I've read that there are some changes between the years with fuel system, etc, but I am wondering just about the engine components.

So my question: Can I take the 3.2 liter block and heads, and move all my other components over? I don't want to do a custom swap or anything ( I would have to do an LS1 if I did that haha).

I understand that I will have to deal with Air Fuel Raitios from the ECU fuel map, but I'm mostly wondering if there will be any hardware issues.

Does anyone know how much work it would be to take a later model 3.2 liter and put it in my early model sl300?

Or....is there a known swap with a different motor that I could easily do? I wouldn't mind spending more money to get something cool in there if it was straight forward.
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post #2 of 10 (permalink) Old 05-21-2019, 02:59 PM
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Have you ruled out the common head gasket issue the 3.2L has ??? If it doesn't have a zillion miles, if it were me, I'd pull a head before I went any further.
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post #3 of 10 (permalink) Old 05-21-2019, 03:24 PM
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Quote:
Originally Posted by Eric Youssefi View Post
So my question: Can I take the 3.2 liter block and heads, and move all my other components over?
No. The 300SL is mechanical fuel injection with electronic mixture correction, and the 3.2-litre motor is electronic.

I'd repair the existing motor or replace it with a like motor.
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post #4 of 10 (permalink) Old 05-21-2019, 04:03 PM
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Originally Posted by bobterry99 View Post
No. The 300SL is mechanical fuel injection with electronic mixture correction, and the 3.2-litre motor is electronic.

I'd repair the existing motor or replace it with a like motor.


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post #5 of 10 (permalink) Old 05-23-2019, 04:37 AM
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Or go the Chevy LSX route.
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post #6 of 10 (permalink) Old 05-25-2019, 06:29 AM
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How many miles are on the engine? Which 2 cylinders are low? what are the compression numbers of all the cylinders for relative comparison? I found on my '90 300SL the 2 middle ones (3,4) were low the rest were fine. spec is 10-14 bar(145-204 psi) see attachment. I think my low ones were in the 125 range. I did head gasket and replaced all the soft parts in the intake that were petrified (idle control hoses, breather tubing, injector holders, injector o-rings). Your engine is 25 yrs old and vacuum leaks on intake side "MAY" be reason for low compression. A leak down test could id other causes.
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post #7 of 10 (permalink) Old 06-01-2019, 06:32 PM Thread Starter
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Thanks for the tips.

1. I would love to do an LSX swap, but I have a pretty full plate with car projects right now. I did a google search and I saw it has been done, but I think I would do a turbo on the m104 before I went that route. I thought the M104 AMG motor would be a cool swap, but probably not worth the cost/HP/Hassle.


2. I did a leak down test on cylinder 1 today (the one the seller told me was bad). The first time I tried the test I had 80% and I couldn't find where the air was going ( no air coming from the dipstick, oil filler cap, or radiator) so I assumed I was on the exhaust stroke so I rotated the engine until TDC again.

This time I got the same result...about 80%. Still couldn't hear where the air was going so I pulled spark plug 2 and 3, and air was pushing fast out of both of those.


That seems crazy that it was 2 and 3, so I'm wondering if I'm just doing the leak down test wrong.

I did the leak down test on # 2. It was showing 80% but than after 10 seconds or so it climbed up to 20%.

I'm not really proficient with a leak down test though and I'm wondering if this gives any clues, or if I need to go back and do a compression + leak down on all of them. Also, should I pull the exhaust manifold to make it easier to see where the air is going?

Thanks for the help
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post #8 of 10 (permalink) Old 06-17-2019, 09:35 PM Thread Starter
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I repeated the test tonight. When doing the leak down on cylinder 1 I either had air blowing out of cylinder 4, or cylinder 2. I'm guessing 1 and 4 fire together? If that is the case, I am really starting to think there is a blown head gasket between the cylinders. Does this sound possible?
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post #9 of 10 (permalink) Old 06-18-2019, 06:18 AM
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Quote:
Originally Posted by Eric Youssefi View Post
When doing the leak down on cylinder 1 I either had air blowing out of cylinder 4, or cylinder 2.
You seem to be saying that at any given moment air would blow out of 2 or 4 but not both and presumably not out of 3. That makes no sense at all to me.

Quote:
Originally Posted by Eric Youssefi View Post
I'm guessing 1 and 4 fire together?
No cylinders fire together, though #6 piston is in the same position as #1 piston.

It may be prudent to do more testing or post this over on the "Tech Help" forum at peachparts.com where a professional Mercedes tech will likely see it and perhaps reply. But I'd just go ahead as Chip suggested and remove the cylinder head.
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post #10 of 10 (permalink) Old 06-19-2019, 08:29 AM
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Have you found a replacement Engine? I have a 1990 300SL, that only had 67,000 miles on it. Removed engine to sale to a third party and he never responded. If you are interested, send me an e-mail [email protected]. Thanks.
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