Oil Pressure Guage stays at Max

[awolff280sl]

Hi all. I have searched previous posts and I couldn't find this problem mentioned before. I am in the process of doing some major work on my 69 280sl 4-speed. Prior to this it was hardly driven for 3 years. Amazingly it appears to run well now, and aside from some annoying "minor" issues, most of the work I will be doing is renovation rather than repair (hopefully).
When I start up the car the oil pressure guage climbs to 45 and stays there without budging the entire time the car is running, idling, or accelerating. This was not a problem 3 years ago, and the guage fluctuated appropriately then.
Please give me some troubleshooting steps.
I am a new member and I am extremely grateful to have this forum as a resource. Thank you!

[al_lieffring]

This is a puzzling question, because the pressure running at the peg is actually the normal condition for these cars, so the question is why was the pressure lower before? Has the oil been changed to a heavier 15w40 oil? Does the guage come up slowly like there might be an obstruction in the tubing that runs from the oil filter housing to the guage? Is the idle speed higher now than before?

Al Lieffring
66 230SL

[rogerh113]

When the engine is cold, the pressue will be pegged whether at speed or at idle.  If the engine is fully warmed up, the idle oil pressure should drop to the midway (30# ?) level.  If you have warmed the engine up fully and the oil pressure does not drop at idle, then you do have an issue!!  It usually takes at least 5 to 10 minutes of driving for my '66 230SL to warm up sufficiently to see any drop in pressure at idle.

Regards -- Roger

[Bob G MN]

This sounds like it may be the gauge and not rteally an oil pressure issue.

[awolff280sl]

I do not see a drop to 30 or so even once fully warmed up. I guess I'll need to start by checking the pressure line for blockage.

[waltklatt]

Does the oil pressure needle drop back to 0 when the engine is shut off?  If it does, how long does it take to drop down?
Walter
1967 220SL diesel-new driveshaft ready for installation.

[awolff280sl]

The needle drops down to zero immediately as soon as the engine is cut.

[GTMSJ]

I recently changed the oil on my 70 280sl and have observed the same thing.  The oil level remains within the normal range on the dip stick, the gauge returns to 0 when the engine is turned off. Because I don't drive the vehicle daily, perhaps I've just not payed close enough attention until now.

1970 280 sl silver/red
1992 NSX
1998 GS400

[J. Huber]

Hey GT, I think things sound normal for you. The oil level and the oil pressure are two different things. I believe normal pressure is: always fully pegged when under throttle (and usually at idle until very warm). And dropping to 0 instantly upon turning off engine.

James
63 230SL

[ja17]

Hello,

I would not react to hastily. The oil pressure at idle will depend on idle rpm, coolant temperature and driving conditions.

It is normal for the oil pressure to fall off the peg after the engine is fully warmed 179 degrees F and at a normal idle  of 700rpms.  Depending on the condition of the engine, the driving conditions the type and condition of the oil, you might see more or less radical fluctuations in your oil pressure at idle.

There are two oil pressure relief valves in the 280 series engine to prevent excessively high oil pressure. One screws into the front of the block (under a plug next to the timing pointer). The other is on the  oil pump itself. It is very unlikely that both of these are stuck. It is more likely that your gauge has become inacurate or your oil is not being warmed enough to make a differenc.

Check the oil presssure after a brisk highway drive then an idle at 700rpm, make sure the engine temperature has risen to around 179 degrees for a period of time.

Joe Alexander
Blacklick, Ohio

[awolff280sl]

The issue with the pressure guage remaining at the peg is hopefully no longer an issue. I took the car on the interstate, running at about 4500rpm for a few miles. Along the way, with some bursts of acceration, brown/black smoke was discharged several times. When I exited and came to a stop, the oil pressure floated down to about 30, and went to the peg again with aceleration. It's been behaving appropriately ever since. I guess this is just another example of why these cars need to be driven. From a mechanic standpoint, I guess I cleaned my pipes.

[gdpmv152]

My guage stays pegged at full pressure also when cold and only drops when very warm and at idle. It will drop to about the half way point and go full on again once under way. I believe this is normal operation.



1968 250SL

[coletre]

Hi,

This is how It goes on a my new rebuilt engine.It always stays at 45 psi and only when  warmed up (179 degrees F)the needle tends to be between 30 and 45 but at a low idle speed (500 rpms) above 750 rpms it returns to 45.I am using semisynthetic oil 10-40 W.

[Benz Dr.]

I drove a car once and it was at full peg when cold. As it warmed up the needle started to drop, and drop, and drop, until it was just about zero. Checked the oil.... full..... not full of gas.
Drove it a short ways back to the shop and it had zero oil pressure at idle yet it still ran. When I shut it off it stopped dead! Turned over but wouldn't restsart.
Did a compression test. Less than 100 PSI on all cylinders. Hummmmm..... not too good.

When I got it apart there was no bearing material left and only the copper shells on everything including the rod bearings and yet it didn't knock. There was over .75mm wear in the cylinders. My machinist couldn't understand how or why it would even run and said that nothing runs with that much wear in the cylinders.

I said, ''Yeah, well this one did. Probably 'cause it's a benz.''

These are very strong engines and will take an increadable amount of pounding abuse. Can't overheat them or run them out of coolant though.....

[J. Huber]

Hey Doc, I thought you were going to say that you then proceeded to fish out a pair of ladies' undergarments...[:0] Must have been a different car?

See. I do pay attention here...

[mdsalemi]

quote:Originally posted by Benz Dr.

I drove a car once and it was at full peg when cold. As it warmed up the needle started to drop, and drop, and drop, until it was just about zero. Checked the oil.... full..... not full of gas.
Drove it a short ways back to the shop and it had zero oil pressure at idle yet it still ran. When I shut it off it stopped dead! Turned over but wouldn't restsart.
Did a compression test. Less than 100 PSI on all cylinders. Hummmmm..... not too good.

When I got it apart there was no bearing material left and only the copper shells on everything including the rod bearings and yet it didn't knock. There was over .75mm wear in the cylinders. My machinist couldn't understand how or why it would even run and said that nothing runs with that much wear in the cylinders.

I said, ''Yeah, well this one did. Probably 'cause it's a benz.''

These are very strong engines and will take an increadable amount of pounding abuse. Can't overheat them or run them out of coolant though.....

Isn't it a mantra of pilots--"Trust your instruments?"

[seattle_Jerry]

cars are no different than people. You had clogging of the arteries which caused high "blood" pressure even while at rest. The drive must have cleared out some of the "plaque".