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Old 05-25-2015, 04:40 AM   #1
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Join Date: Jun 2011
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repair piston rings/ valve seal or replace engine?

Currently driving a 02 325xi auto. Pretty sure the execessive oil being burnt is caused by bad piston rings; going to run a compression test to ensure.

That being the case, since I wont have time to do the test till Tuesday,, if the rings need to be replaced at this point is it better to replace the engine then to actually just have them repaired?

Ive been told parts wise just for rings alone is 200+, that being said I would replace the valve stem seals, springs etc as well. I cant do this work by myself so I would have to get a mechanic to do so. Would it be better to just switch in another engine or have the parts replaced? Essentially looking for the more cost effective root. Price difference large?


As well; just for curiousity; will the m54b30 swap in directly? Any problems since its an auto? Just asking as ive seen more m54b30s available then the b25.
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Old 05-25-2015, 08:18 AM   #2
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How much oil are you adding? Why are you sure it is bad piston rings? M54s can burn oil in many many ways. If you haven't changed your CCV (and added the extra vacuum line), do that first.

A compression test will not tell you whether you are leaking past your rings or not; you need a leakdown test for that.

Junkyard M54s tend to run around $1000-$1500. I'd bet that every one of them burns some amount of oil, so you'd just be playing the lottery and hoping that the replacement isn't as bad as your current motor.

The problem with replacing rings is that to replace the $200 in parts, you're going to have lots of disassembly/reassembly time, and you'll need to replace a lot of other parts along the way, including rod bolts, rod bearings, HG, VCG, oil pan gasket, valve seals and springs. Dual VANOS engines require special tools to time, so most non-specialized shops would not be able to handle it.

An M54B30 will swap in directly. It will run, but it will run better if you flash the B30 software to your existing DME.

You're looking for the price difference, so let's look at it this way:
-For both jobs, a shop needs to re/re the motor, so that cost is the same
-To swap a junkyard M54, add ~$1000-$1500 for the "new" motor.
-For the engine rebuild, the shop then needs to put the motor on a stand, tear it down, replace parts, and reassemble. It's nearly certain this will be larger than the $1500 listed above.

Either way, you're looking at a job that is probably going to be around 50% of the value of the entire vehicle. This brings me back to my first question: How much oil are you adding? Is solving this worth pursuing?
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Old 05-25-2015, 02:16 PM   #3
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I've never seen a 3.0 M54 that doesn't burn a lot of oil. My E46 needed a quart every 3-4k, my old man's needed some at longer intervals, as did another family members. They all consume a lot of oil.

How much are you adding and how frequently (km, not time)?
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Old 05-25-2015, 03:00 PM   #4
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Just got my oil done not to long ago; drove less then 800km and the oil check light has turned on already. I've changed the ccv already earlier this year; as well my mechanic has said its not leaking anything as far as he can find. The oil consumption is large enough that the entire backside of my car is greased from the fumes from burning oil.
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Old 05-26-2015, 11:53 AM   #5
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There are many reasons an E46 will consume oil. Even if it is your pistons, it'll be cheaper in the long run to add oil every 1000km.
Have you tried switching to a thicker oil? you changed your valve cover gasket?
Cleaned your ICV?
Checked to see if your oil filter housing gasket is slowly leaking oil?
Did you replace entire CCV system? Did you clean out dipstick tube?

I just changed my CCV as well and replaced spark plugs and pre-cat O2 sensors which were failing (due to age and CCV failure likely). Just rolled over 200k KM on my car.

Some good links to read up on.

2002 E46 325ci // 5spd // ISC N1 Coilovers
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Old 05-26-2015, 01:53 PM   #6
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Haven't tried switching to a thicker oil but I will definitely attempt that. My mechanic had stated there weren't any apparent leaks but I will go over that list. Thanks!

Ccv, vcg, front O2 sensor have all been recently replaced. The car is sitting at about 270km right now
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Old 05-26-2015, 02:01 PM   #7
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If you've tried what all others have suggested, then perhaps a leak down test will tell you if cylinder pressure is dropping quickly.

At that mileage, I wouldn't recommend tearing down the motor unless you were doing it yourself. You'd probably be better off looking for a low mileage motor and swapping it in. I wouldn't go for a B30 as they have a greater risk of burning oil. Usually 325s don't burn oil as bad, but yours does sound a bit excessive.
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