1991 350SD - New to me - PeachParts Mercedes-Benz Forum

Lynrd · #1 07-23-2008, 12:11 AM

Hi everyone. I'm new to this forum, but have spent some time lurking after I made the classic mistake of making a large purchase before doing enough research. I now own a very pretty 1991 350SD. She has 167k Miles on her and a Mercedes mechanic claims she's mechanically very sound. My personal pre-purchase inspection and 2 months worth of driving her around agree.

When I got her, she had a few problems...Fuse for the clock and other "Always on" stuff in the cab would blow imediately (Traced down to a hack wiring job for a old cell phone hands free device, now fixed by removing the non-stock wire) and AC wouldn't blow cold. I fixed the AC by completing a PO partial conversion to R-134...the car still had the old evaperator/dryer in it along with old seals. 2 Months later the AC is great, everything works, she's beautiful, comfortable, the absolute greatest car I have ever owned...

Except, I start reading about the "Rod Bender" problem a few weeks ago...I spent the last hour going through several threads on the subject, and haven't come to much of a conclusion at all. In the case of the Triple B (my Big Black Benz) she will burn a quart of oil in about 1500 miles, and doesn't smoke at all except if I really stomp on it on the freeway. So, at the moment, the engine is good.

The body is near-perfect ...she could use paint or maybe just a hazy clear coat rubbed out, but no rust anywhere. She has a few small parking lot dings but you have to really look for them, and the interior is like new. I got the car at a good enough price that replacing the engine, while expensive, is an option...I intend to keep her around for a long while. So, I wonder about some options here:

Bite the bullet, just pony up for a replacement long block and do a pre-emptive replacement...trusting that the replacement engines have *whatever* causes the rod failures rectified.
Bite a lower displacement bullet, change the block out for a 3.0L and therefore avoid the whole "Trust" thing. Have head inspected at the same time for signs of previous abuse and rectify as necessary. If I went this route, what is involved in getting the older engine to work in the newer chassis?
Get kooky, and just replace the rods/pistons, and cylinder liners, along with accompanying bearings and what not. With labor prices, would this be any less expensive or give a better chance at sucsess than option 1 or 2?

I guess I could just shrug my shoulders and enjoy her as is, but I would rather deal with a major hassle like an engine swap on m schedule rather than hers...I should have time to consider the best way to keep her on the road for the half million miles I intend to.

Thoughts? Opinions? No fair telling me to sell her at once on eBay or Craigslist and buy a 300D...I'm already attached to the big beast!

compu_85 · #2 07-23-2008, 12:27 AM

The 3.0 engine would be plug and play. You'd want to get the trans from the donor car, the 3.0's redline is 1000 rpm higher.

-Jason

Jeremy5848 · #3 07-23-2008, 12:28 AM

I'm a little concerned about the oil usage; 1500 miles per quart is high -- reminds me of my 603 with its bad #2 cylinder. You need to hear from other rod-bender owners.

Other than the oil usage concern, I would just keep the car and drive it. You have no idea whether the engine will let go or not and it may last years longer. The time value of the money not spent now would pay for a rental car while you have the engine rebuilt several years from now.

If you really insist on replacing the engine "on your schedule" (really, not a bad idea), a 3 liter 603.960 with a later head would be a good choice, provided that you can find such an engine without buying one from Metric Motors ($$$).

Jeremy

Lynrd · #4 07-23-2008, 12:42 AM

Now, If I found said 603.960 short block and rebuilt the bottom end as necessary , would the head/intake/turbo from my current engine all be transferable, or should I plan on locating those parts as well?

Bio300TDTdriver · #5 07-23-2008, 01:37 AM

You can use the head you have. It is updated from the 14 that was original on the OM603.

TMAllison · #6 07-23-2008, 01:45 AM

Some 3.5's were OK. Why jump the gun until you have a known problem???

If you go down the 3.0 road in the future you need to "meet" SIXTO and pick his brain first.

JimmyL · #7 07-23-2008, 01:52 AM

I would drive and enjoy the car as it is for now. Monitor oil consumption very closely and look for increased usage.
Whilst enjoying your very luxurious ride, I would be forever looking for an OM603 that can go in when the time comes. {and it will} When you find a really good deal on a wrecked W124 or an SDL, or somebody selling maybe an OM603 with a bad head, hop on it. As stated, you can use the head from your 3.5, and most agree that it is a much better head anyway.
You will have an intervention to take care of some time in the future, but sounds like you have time to do your due diligence prior to any failure.
Enjoy that BBB.......

sixto · #8 07-23-2008, 03:27 AM

I second, third and fourth the advise to keep driving it. 1500 miles to a quart of oil is far from ideal but even MB literature says to just keep an eye until you're below 1000 miles to a quart.

You haven't described external oil leaks or how much oil is blowing into the engine through the turbo.

If there is a significant trail of oil from the area of the #1 exhaust runner, you probably have a failed head gasket. The failure sends oil down the front passenger side of the block and into the #1 cylinder as well. If your engine hasn't advanced in bent rod-ness, you can get away with a new head gasket. In fact that's all I'd plan to do if/when oil consumption gets below 1000 miles per quart. The extent of cylinder ovalness, which you can only confirm with the head off, will determine whether you press on with that block or resort to plan B.

There's no magic to 1000 miles per quart. The car will likely run well with a bent rod or two. It's the inconveninece of adding oil and the constant cloud you leave in your wake that will push you to do something about it. Maybe you can tolerate it down to 300 miles to a quart.

Keep your eye out for a deal on a 603.96 short block so you're ready to move if/when the time comes. I would look for one with up to 200K miles with clear and uniform hone marks on the cylinders, minimal or preferably no ridge, and crank and rod bearings within installation spec. I wouldn't trust a rebuild I could afford over a good used short block. My benchmark is a Metric Motors short block and I certainly can't afford one of those. I'd even keep the original oil pump if the alternative is anything less than an OE (not even OEM) replacement. Do change the timing chains, guides and tensioner.

Don't worry about the rods. With 167K miles on the clock, it's the transmission that will keep me up at night

Sixto
87 300D

Lynrd · #9 07-23-2008, 05:18 PM

Even using a mirror...no sign of oil on the outside of the engine at all, and she leaks nary a drop. THis is why I figured she must be burning the oil, because it isn't leaking...she doesn't leak at all, any fluid.

I theorize that the oil must be blowing by the rings and burning along with the diesel fuel, but she doesn't really smoke at all. But she's definately using some oil.

I appreciate the responses. Regarding Metric Motors, yes, expensive, but has anyone used them? Would dropping in one of their short blocks be equivilent to returning the engine to zero miles (Acknowledging that the wear parts around it, like the head and turbo, should be freshened up as well). Or are they overpriced and doing a good job at marketing?

babymog · #10 07-23-2008, 09:43 PM

The car's roughly half-way to 300k, if the car exibits bad oil-management it's time to tear down, if not, drive and enjoy. I like the '91 SDL more than any other 126.

If you need it, you'll want everything from your 3.5 on the 3.0 block: Turbo, IP, transmission, front cover, vacuum pump, head, etc., as they're all compatible.

If you use the 3.0L injection pump you'll want to change the transmission to take advantage of the higher RPM & power the 3.0 configuration has.

BTW, does your '91 have the 603 xxx 22 xx head? Contrary to what I've been reading here, I believe that the original head on the '91 is the 22 head.

Hatterasguy · #11 07-23-2008, 09:44 PM

Oh don't worry if a rod starts to bend you will get a ton of warning. As the oil burn goes up, you'll know whats happening. But it doesn't happen overnight, it takes thousands of miles.

Until than just keep enjoying probably one of the best W126 models made.