W114 oil pressure and A/C - Mercedes-Benz Forum

 
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post #1 of 10 (permalink) Old 07-16-2013, 05:03 PM Thread Starter
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W114 oil pressure and A/C

Hi,

I took this video of the car after being driven for an hour and with the a/c on.
As you can see the oil pressure sits at 15 and with the a/c on drops even lower and almost stalls.

I can always increase the idle speed but it doesn't affect the oil pressure (at least not by a lot).


I don't know if you can hear but when I rev the engine you can also hear the knocking which I was told it's crankshaft bearings.

Cold start has full oil pressure to the top and as it gets warmer it just keeps dropping.

My question is:
Can worn crankshaft bearings affect the oil pressure this much and if not what else can be the reason for such a low oil pressure?


Thank you.


M130 with a/c on - YouTube
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post #2 of 10 (permalink) Old 07-16-2013, 07:38 PM
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Here's how it works. Oil pump picks up oil using vacuum via the sump tube. Pump compresses the oil. Oil seeks escape route and finds it through oil galleys. First level of oil galleys lead to crankshaft. Crankshaft bearing clearances are restrictive (in a normal unworn situation) which leads the oil to the next exit point. Rod bearings, which once again, in a good state, are restrictive due to close tolerances so the oil (still under pressure) seeks another exit. Last place is the cam oil galleys which release the final bit of pressure onto the cam bearings first and whatever pressure is left though the squirters onto the cam lobes.

Essentially this is how the lubrication system works on all over head cam engines. There are a few differences in engines but this is the basic set-up.

Oil pressure is highest at the crank. Sloppy crank results in decreased pressure at rods, which leads to reduced flow to the upper end. As the engine block warms up it expands which enlarges the circumference of the main bearings. The machine steel of the crank journals does not grow at the same rate as the casting of the block so you end up with a gap which is an escape for the oil pressure. This process is carried on through the rest of the oil system.

In case you were wondering, the pump feeds the oil filter housing first which has a pressure relief valve to sustain the pressure before the oil galleys.

Four paragraphs later the answer is, yes.
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post #3 of 10 (permalink) Old 07-16-2013, 09:14 PM Thread Starter
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Here's how it works. Oil pump picks up oil using vacuum via the sump tube. Pump compresses the oil. Oil seeks escape route and finds it through oil galleys. First level of oil galleys lead to crankshaft. Crankshaft bearing clearances are restrictive (in a normal unworn situation) which leads the oil to the next exit point. Rod bearings, which once again, in a good state, are restrictive due to close tolerances so the oil (still under pressure) seeks another exit. Last place is the cam oil galleys which release the final bit of pressure onto the cam bearings first and whatever pressure is left though the squirters onto the cam lobes.

Essentially this is how the lubrication system works on all over head cam engines. There are a few differences in engines but this is the basic set-up.

Oil pressure is highest at the crank. Sloppy crank results in decreased pressure at rods, which leads to reduced flow to the upper end. As the engine block warms up it expands which enlarges the circumference of the main bearings. The machine steel of the crank journals does not grow at the same rate as the casting of the block so you end up with a gap which is an escape for the oil pressure. This process is carried on through the rest of the oil system.

In case you were wondering, the pump feeds the oil filter housing first which has a pressure relief valve to sustain the pressure before the oil galleys.

Four paragraphs later the answer is, yes.

Mike,
That is fantastic explanation.
Thank you very much for taking the time to explain, honestly I didn't know it does that.
I can do a lot of things around cars but sure I can learn a lot too.

In your opinion, once I fix the crankshaft, should that fix the noise and the oil pressure issue?

While I have the engine out, is it recommended to change the piston rings for better seal or if it's not broken don't touch it?
The piston pressure was about 100 to 130 for all six.
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post #4 of 10 (permalink) Old 07-17-2013, 02:15 AM
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When the engine oil is warm, it is less viscous, that also reduces the oil pressure because of the easier flow. The viscosity rating of the oil can also affect oil pressure.

15 psi isn't that low for a warm engine at idle and within specs. The idle speed is too low when the A/C is on, normally there should be a system that increases the idle speed to prevent stalling.

If you take the engine out, you can overhaul it completely, there is almost no limit to what you can, but it depends on the size of your wallet, how long you want to own it and how many miles it has already done. Engine overhauling is done either because the engine has done a certain amount of miles (you can more or less predict when an engine is worn out) or when there is a problem.

Piston rings renewal means removing the engine head. If you have the engine head off, you can inspect the pistons and the cylinders too and measure if they are within specs or need renewal.

If the knocking is indeed a worn-out crankshaft bearing, renewing it can solve it.

Last edited by Govert70227; 07-17-2013 at 02:18 AM.
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post #5 of 10 (permalink) Old 07-17-2013, 06:10 AM
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Crank and rod bearings at a minimum. Anything further depends on your wallet.
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post #6 of 10 (permalink) Old 07-17-2013, 09:19 AM Thread Starter
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Thank you.

The engine has 149.000 kms, so not that many thus my surprise on the crankshaft bearings being gone.
On the other hand it's 42 year old engine so maybe it is normal.

If the engine goes out I will be doing the crankshaft and the main/rod bearings for sure.

On the piston rings I was asking more if putting new will cause more issues than benefits as they are not expensive to replace. The rest will cost some money.
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post #7 of 10 (permalink) Old 07-17-2013, 01:13 PM
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what kind of oil do you have in there [ viscosity ]
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post #8 of 10 (permalink) Old 07-17-2013, 04:02 PM
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And are you sure it's only 149 and not 249?

But if you're removing it you might consider either a complete rebuild yourself (that should take you into fall and no need for AC) or buy a professionally rebuilt long block (5k? 10?). But if you've worn out the crank shaft then you got to think there's significant wear everywhere else. Do you really want to pull the engine again in a year or two?

Good luck


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post #9 of 10 (permalink) Old 07-17-2013, 06:45 PM Thread Starter
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From all the records over the years that came with the car and the overall condition of the car I have every reason to believe its 149000km


I have added Lucas oil conditioner and the oil pressure did go up a little bit but nothing significant
I do believe the knocking is not as loud.

I used rottela
15w 40

Should I try another oil
W114 oil pressure and A/C-imageuploadedbyag-free1374108293.943223.jpg


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post #10 of 10 (permalink) Old 07-17-2013, 10:45 PM
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The Rotella is fine. I use Chevron Delo 400 which is essentially the same stuff. I use 15W-40 here in the Southwest where the temperatures are quite a bit higher with no problems. M110, M115 and a M117, all using the same oil.
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