Author Topic: Low oil pressure... what causes it? 450SE  (Read 6213 times)

13B

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Low oil pressure... what causes it? 450SE
« on: 08 March 2008, 07:42 PM »
Finally have the team car running on all 8-cylinders and got to take it for a but of a drive.  Goes pretty well I must say, gearbox is tight, diff is silent, brakes and steering work well (steering slightly groany but the fluid is low).

Trouble is that oil pressure is reading low...  I'm after some ideas.

Cold start it rises rapidly to 2.5 or 3bar but comes down fairly rapidly.

Fully warmed up and at an idle speed of 750rpm the reading is 0.5bar

Driving normaly fully warmed up it doesn't read past 1.5bar

Since I didn't know if I was doing engine damage I didn't take it on the freeway to test how the car runs at 100-120km/hr (all indications are that it will be happy).

Do you reckon it might be just the OEL guage is buggered?

Here's a video of it (not) in action: http://www.youtube.com/watch?v=Yd7JjuGSpqQ

I.
« Last Edit: 08 March 2008, 09:42 PM by 13B »
450SEL 6.9 #5440 = V MB 690 , 450SE # 43094 = 02010 H , 190E/turbo # 31548 = AOH 68K

carl888

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Re: Low oil pressure... what causes it? 450SE
« Reply #1 on: 08 March 2008, 09:54 PM »
Sounds like the gauge Ian, if only because if you had 1.5 bar whilst driving, the engine would have lunched itself by now  ???

Regards,

Carl.



oscar

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Re: Low oil pressure... what causes it? 450SE
« Reply #2 on: 09 March 2008, 12:29 AM »
Vroom vroooom geez even without oel pressure that exhaust sounds cool ;D 8)  Wish I had a sub woofer connected to this work computer ;D

That gauge does seem screwey especially the way it jumped up by itself during the idle after it intially pressurised.  Also the way it didn't respond to the RPM.  For piece of mind, take your oil cap off during warm idle, check to see if there's a nice stream of oil coming from the oiler tube on to the cam shaft.

From what I remember of others asking and comparing about oel gauge operation, the needle should move similarly to your 6.9. ie Shoot up to 3 on a cold start and then drop to around 1.5-2 when the oil has warmed and at idle.  The pressure goes right back up to 3 again when the revs move up from idle.

So why is the gauge not working?  I reckon the line's full of sludge.  I don't know but as a guess I reckon a good start apart from checking for leaks is to disconnect the line and flush it.  Perhaps undo it near the gauge, place a container under the end and start the enigne.  Observe the oil coming out.  It dribbled out of mine at idle.  Not a powerful squirt or anything like that but it was free flowing. 

Didn't have a problem with my gauge, I just thought it would be a good idea for fresh oil in there after accidentally discovering one day that oil does come out of that line and onto carpets when the engine is running ::)  So next oil change I purged the line.  No lumps as such but it was black as.  I wonder if thinners or petrol should be squirted into the guage side as well.  I didn't bother with that but perhaps should've.
1973 350SE, my first & fave

13B

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Re: Low oil pressure... what causes it? 450SE
« Reply #3 on: 09 March 2008, 01:57 AM »
Thanks Oscar,

Had a look down the oil filler while the engine was running and the cam is being nicely sprayed with oil.  The engine appears to be overfilled, the level about 1-1.5" higher than the high mark on the dipstick.  The oil appears clean and doesn't smell too fuelly.

Probably best to remove the oil line and give it a good clean out.  Can you tell me where it connects to on the engine?  (I bet its some impossible-to-reach place.)

 From any of your wrecks do you have a Petrol/Temp/Oel guage spare?

Ian.
450SEL 6.9 #5440 = V MB 690 , 450SE # 43094 = 02010 H , 190E/turbo # 31548 = AOH 68K

torana68

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Re: Low oil pressure... what causes it? 450SE
« Reply #4 on: 09 March 2008, 04:27 AM »
rear of the head left side I think, or you could pull the instruments and disconnect the pipe and run some hose over it to a bucket outside? try using much thicker oil if your going to flog it.
Roger

oscar

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Re: Low oil pressure... what causes it? 450SE
« Reply #5 on: 09 March 2008, 04:36 AM »
Yeah I'll get a gauge for you tomorrow.

The oil line - what roger says, see pic, right above the engine number.  Surprisingly, not that hard to get to, for a change. ;D  I haven't undone mine as all I did was let the engine pump the oil through.  But I reckon in your case, pumping WD40 or similar through the line would be a good idea.  Also, before you reattach i reckon you might have to let the engine pump oil through again to a) wash out whatever solvent you clean out the line with and b) prime the line so no air is inside it.  Just a guess.

Click for a bigger pic.

1973 350SE, my first & fave

13B

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Re: Low oil pressure... what causes it? 450SE
« Reply #6 on: 10 March 2008, 12:55 AM »
Disconnected the hose at the instrument cluster and ran the engine for a few mins... black drops with no pressure to speak of...

Will disconnect it from the engine soon and try to clean it out.  Maybe the guage is OK...

Hot day today - didn't do much else.  Could something inside the left cylinder bank bleed off the oil pressure so that what is being pushed thru the oil-pressure adaptor is bugger-all?  I guess I need to take off the LH cam cover and look for signs of oil starvation?

Ian.
450SEL 6.9 #5440 = V MB 690 , 450SE # 43094 = 02010 H , 190E/turbo # 31548 = AOH 68K

13B

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Re: Low oil pressure... what causes it? 450SE
« Reply #7 on: 15 March 2008, 02:22 AM »
Finally got some sensible readings from the OEL gauge after disconnecting it, flushing it out with carb cleaner and compressed air, and re-assembling.

With a hose fitted to the back of the engine into a measuring cup it takes a couple of minutes on fast idle to fill a 500ml vessel.  The restrition in the fitting is very small, maybe 0.5mm to limit pressure coming into the cabin instrument cluster. 

Reconnected the original hose to the engine and observed oil coming out not in a stream but a series of timed drops, maybe 2 per second at fast idle.  This makes sense as you wouldn't want 60+ PSI oil pressure at the oil pressure gauge.

Reconnected the oil pressure guage and started the engine, oil pressure shot straight up to 3bar, and settled down to 1bar with the engine fully warm and sitting in Drive at about 700rpm.  It responds as it should when I step on the accelerator however doesn't respond as quickly as the OEL meter on my 6.9

Maybe someone who owns both a 450 and a 6.9 can tell me whether the oil pressure on the smaller engine is just lazier than that of the big block.

Tomorrow: remove cam covers for a look-see and then a freeway drive at 100-120km/hr

Ian.
450SEL 6.9 #5440 = V MB 690 , 450SE # 43094 = 02010 H , 190E/turbo # 31548 = AOH 68K

13B

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Re: Low oil pressure... what causes it? 450SE
« Reply #8 on: 23 March 2008, 03:38 AM »
Looking under the car today I noticed the sump is bent - pushed upwards somewhat.

I reckon this will have some impact in the ability of the oil pump to pick up the oil, esp. if the pickup is partly obscured.

What do you think?

I.
450SEL 6.9 #5440 = V MB 690 , 450SE # 43094 = 02010 H , 190E/turbo # 31548 = AOH 68K

s class

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Re: Low oil pressure... what causes it? 450SE
« Reply #9 on: 23 March 2008, 04:00 AM »
Where is the dent - at the front?

On my 280SE I once had a heavy landing on a dirt road that smashed up the sump guard about 2 inches, and the sump about one inch.  It smashed up the rubber puck-up bung on the bottom of the oil pump feeder (it is rubber but age and heat had made it hard and brittle)  This bung thing houses a mesh as well.  The base of the rubber has 4 'legs' that normally sit on the oil pan, and oil is drawn in through the spaces between the legs, then through the mesh and into the pump.  I guess if your pan is bashed in, it could have compressed the rubber legs of the pickup and obscured the apertures between them. 

'76 6.9 Euro, '78 6.9 AMG, '80 280SE, '74 350SE, '82 500SEL euro full hydro, '83 500SEL euro full hydro , '81 500SL

TJ 450

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Re: Low oil pressure... what causes it? 450SE
« Reply #10 on: 23 March 2008, 06:35 AM »
I had a bent sump and a piece of broken timing chain guide ended up wedged it the pickup too, potentially resisting the flow... something to look out for, particularly on an m117. Yes, the castellated rubber insert is very easily rearranged if the sump is bent. 
Tim
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13B

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Re: Low oil pressure... what causes it? 450SE
« Reply #11 on: 24 March 2008, 02:29 AM »
This is the bent sump.  Due to the angle the photo was taken at it doesn't look bad but I'd say its bent somewhere between a 0.5" and an 1".  I can't see how it was done but maybe some doofus P.O. put a jack under it to lift the car??

Anyway, do you think this would cause a restriction in the oil pickup ability of the oil pump?  Heaven knows what else is damaged in the engine in that case.




While we're at it, what tool do I need to remove the sump plug thanks...

Ian.
450SEL 6.9 #5440 = V MB 690 , 450SE # 43094 = 02010 H , 190E/turbo # 31548 = AOH 68K

oscar

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Re: Low oil pressure... what causes it? 450SE
« Reply #12 on: 26 March 2008, 12:47 AM »
Anyway, do you think this would cause a restriction in the oil pickup ability of the oil pump? 

While we're at it, what tool do I need to remove the sump plug thanks...

Hi Ian,

My sump used to be a concave shape too after a life on dirt roads and deep ruts.  It was bashed flat again by my mechanic a few years ago without my asking and he said that it would affect the pick up of oil to the pump if I left it like that.  I didn't really take notice of before and after oil gauge response but I'm glad it now looks normal and takes the right capacity.  It looked worse than yours though.

Sump drain plug is a 14mm hex socket like below.  I use this impact type, a bit of overkill but it works.

1973 350SE, my first & fave

13B

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Re: Low oil pressure... what causes it? 450SE
« Reply #13 on: 31 March 2008, 05:40 PM »
Now that its running on all 8 cylinders and EFI woes seem to have been whipped into a drivable state I've taken the beastie for a decent drive for 25min.

Good points:

Accelerates well
Doesn't overheat
nice idle at 800rpm in drive
1.0bar oel pressure at idle
Revs out
Drives nice manually shifting up and down thru the 3 gears
Long drive burns most of the crap out of the cylinders/exhaust - 1st couple of km were a little smokey

Bad points:

speedo doesn't work
2.5bar oel pressure at 70-80km/h - seems a bit low to me

Got a good used sump and oil pump lined up for later in the week, will give it the best chance of getting decent oil pressure in the engine.

2.5 bar equates to 36PSI at 70-80km hr.  Didn't get up to 100km/hr at any time but assuming 100 brings up 3bar (43.5PSI) does that sound like enough for the engine to survive?  I guess it must be as the PO would have driven it like this before selling it and the engine didn't self-destruct.

On the wankel engines I work on the oil pressure reg limits oil pessure to 71PSI - admittedly thats a long way from 43.5 PSI - but doesn anyone know what the upper regulated pressure limit is on an MB V8 engine?  I've done a search but have come up with no definitive answer.

I.
450SEL 6.9 #5440 = V MB 690 , 450SE # 43094 = 02010 H , 190E/turbo # 31548 = AOH 68K

koan

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Re: Low oil pressure... what causes it? 450SE
« Reply #14 on: 31 March 2008, 07:54 PM »

1.0bar oel pressure at idle

2.5bar oel pressure at 70-80km/h - seems a bit low to me

does that sound like enough for the engine to survive?
.

1 bar at hot idle is good - provided idle RPM is not too high. The manual says 0.3 bar is acceptable, it goes onto say minimum 3 bar at 3000 RPM.

The manual mentions a 5 bar relief valve "accessible upon opening of a closing plug in timing housing cover", maybe this valve is stuck open limiting oil pressure, it goes onto say "The pressure relief valve itself cannot disassembled".

As for the engine surviving, I'd say it would but I'd investigate further if the sump doesn't fix it.

koan
 

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