Just changed the oil for the first time in my "new" 1979 240D. Prior to the change, oil pressure would read about 10-15 psi at idle then gradually build to 45 psi steady at cruise speed. After changing the oil filter (my car has the single post in the middle housing with two bolts to hold on the top cap piece), the gauge spikes to 45 psi as soon as the engine is on and it's idling. Just stays at 45 psi as the engine speed increases. I haven't driven it out of my driveway because I'm afraid of doing major engine damage.

I'm assuming I'm missing a piece (i.e. spring or spacer or something) in the oil filter housing. According to the manual I have, 1979 had no spring in the oil fiter housing. However, when I removed the old oil fiter element, there was a brittle o-ring with a chunk out of it laying in the bottom of the metal housing. The hardened rubber ring was about 1" in diameter and about 3/4" inside diameter. None was supplied with the Mann filter I purchased, so I just pitched the brittle thing out. I'd sure appreciate any advice on how to set this right. Geez, you'd think you couldn't get into trouble just trying to change your oil!

It should peg when cruising and will likely peg when idling until the oil gets to operating temp. What you're seeing is normal.

Sixto
95 S420
87 300SDL

Thanks, Sixto. So the safe thing to do would be to let it just idle until operating temperature (about 180 degrees or so) is achieved? 10 minutes or so. So no chance of wrecking the thing in that time frame even if something else is actually wrong and causing the high pressure condition? Excuse my paranoia. It's such a great beast, I'd really hate to screw it up!

Sounds like you fished a piece of an ancient filter out of there. What brand of filter did you put in this time?

__________________
Palangi

2004 C240 Wagon 203.261 Baby Benz
2008 ML320 CDI Highway Cruiser
2006 Toyota Prius, Saving the Planet @ 48 mpg
2000 F-150, Destroying the Planet @ 20 mpg



TRUMP .......... WHITEHOUSE
HILLARY .........JAILHOUSE
BERNIE .......... NUTHOUSE
0BAMA .......... OUTHOUSE

It is no more likely to hit 180F in 10 minutes of idling as it will in 10 hours of idling. Diesels in proper tune don't get to operating temperature by idling. Or so I've read. I've never actually tried it.

Sixto
95 S420
87 300SDL

I used a Mann filter. That's one of the acceptable brands isn't it? I definitely want to use the right stuff on this baby. Hope to keep it going for a L-O-N-G time.

Yes, Mann is a good choice.

Welcome aboard. Be sure to check out the Diesel Discussion Forum.

__________________
Palangi

2004 C240 Wagon 203.261 Baby Benz
2008 ML320 CDI Highway Cruiser
2006 Toyota Prius, Saving the Planet @ 48 mpg
2000 F-150, Destroying the Planet @ 20 mpg



TRUMP .......... WHITEHOUSE
HILLARY .........JAILHOUSE
BERNIE .......... NUTHOUSE
0BAMA .......... OUTHOUSE

It could also be a bad (open) sending unit or that it has been inadvertently disconnected.

The guage will peg in both instances.

__________________
Chas

Had a 380SL, 450SL, 380SEL, 420SEL, 500SEL

Got a Euro 500SEC right now.

I reckon it's more likely disconnected somehow when I was rooting around under the hood since it was working fine just BEFORE I did the oil change. Thanks for the welcome. Glad to have found this excellent resource of helpful folks!

It's a mechanic gauge. If you disconnected it, as in pulled it apart, it would have oil all over the place.
Thanks
David

__________________
_____________________________________________

2000 Honda Accord V6 137k miles

1972 300SEL 4.5 98k miles

_____________________________________________

Stranger symptoms now. I started it up again this afternoon. Gauge pegged immediately to 45 psi. I quickly turned it off and the pressure gauge stayed at 45 psi! It very slowly returned to 0 psi. Took about 45 seconds by my watch to return to 0 psi. This can't be normal. I'm getting nervous. Any further ideas on this additional information?

I'm figuring there must be some kind of spacer or spring or something that is missing in the oil filter housing OR I got the wrong filter cartridge?

There is no spring or spacer on that model.
What you described before filter change (oil pressure gradually builds to 45) is NOT normal. Oil pressure should peg as soon as the RPM comes off idle. The debris you found in there was probably the lower grommet off the old filter. If that was not in place on the filter, you could get a symptom like that.
With a proper filter, the oil pressure should take a few moments to come up on the first start, but should come up to the peg and stay there. It should not come off the peg at adle until the temperature gets up to almost normal operating temperature. When fully warmed up after a run, it will probably idle at 15 psi or better. Again, when warmed up, it should peg as soon as ou come off idle.
Bottom line is, what you are seeing is normal. I would start it up and let it idle. I bet the pressure starts to decrease after 5 to 10 minutes.

edit: To re-emphasize another point, the oil pressure gauge is not electrical. An actual oil tube goes from the filter housing to the gauge. That's why pressure stays up for a few moments when you shut the engine off cold.

__________________
Palangi

2004 C240 Wagon 203.261 Baby Benz
2008 ML320 CDI Highway Cruiser
2006 Toyota Prius, Saving the Planet @ 48 mpg
2000 F-150, Destroying the Planet @ 20 mpg



TRUMP .......... WHITEHOUSE
HILLARY .........JAILHOUSE
BERNIE .......... NUTHOUSE
0BAMA .......... OUTHOUSE

Just to reinforce the last post, all the MBs I've driven show pegged oil pressure at anything over idle, whether cold or hot. Behavior at idle varies a little depending on model. If I see the gauge below the peg at anything above idle, even as I first start out of the driveway, I want to know why.

__________________
Craig Bethune

'97 SL500, 40th anniversary edition

'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)


SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)

Is it normal for the oil pressure to drop so slowly once the engine is switched off? Mine, takes about 30 seconds to drop to 0 psi once the engine is off. Is this normal as well as the other behavior mentioned in the responses in this thread? Thanks all.