Oil Question 1984 500SEL - PeachParts Mercedes-Benz Forum

pmckechnie · #1 05-29-2013, 10:27 AM

This is only a question about viscosity, not brand. As some of you may remember, I put an alarm for low oil pressure in my 84 500SEL. It was set for the following levels to activate the alarm.

Less than 45 psi above 2000 RPM.
Less than 30 psi above 1500 rpm
Less than 15 psi above 1000 rpm
Less than 10 psi above 300 rpm

It has been like this for several years now with never an alarm.
I have used 20W-50 SYN since the alarm was installed.
About 250 miles ago I changed the oil using 15W-50 Syn.
I have been getting oil pressure errors of 8 psi at 500 RPM since I changed the oil to the 15W-50, only when the engine temp is 190 to 200 deg F. I don't remember exactly what it was before, but I know it was above 10 psi at 500 RPM.
I thought that the 15W meant that the oil will never be thicker than a 15W oil when it is cold and would never be thinner than a 50W oil when it is hot. So since we are talking about viscosity when the oil is hot, why would the pressure be lower then when I was using the 20W-50?
Or, do I have another problem?

Any Ideas?
Thanks
Paul

Edit: I'm sorry I put this in the wrong place but It could apply to a diesel as well.
Edit #2: The alarm has only been in for a little over a year, not several. These things happen when you have been around as long as I have.

Jeremy5848 · #2 05-29-2013, 10:52 AM

Synthetic oils have smaller molecules (carbon chains) so it's entirely possible that the low-RPM pressure will drop just a smidgen -- the synthetic oil can get through tight spaces better than the larger "natural" oil molecules. I don't think you have a problem. Perhaps you could tweak the alarm just a little? It sounds interesting, obviously much more sophisticated than my simple alarm. Can you share a diagram of the design?

Jeremy

pmckechnie · #3 05-29-2013, 01:02 PM

Jeremy Thanks for the response.
This car has had nothing but SYN oil since I have owned it.
The alarm system is microprocessor based. It uses a ZBasic ZX-24A processor and about 600 lines of code. Each input has to have some signal conditioning but not much. It checks Oil,Temperature, Voltage, Fuel remaining, Oxygen sensor, RPM. Other things could easly be added. It has start up delays, It saves any errors until they are cleared. It uses a 16X2 display that displays the 4 important readings and will point to an error with a flashing arrow. I use 2 p/b switches to change to other screens for error codes, etc. As long as the error remains it will flash a led and sound a beep. If it is an intermittent error, it turns off the led and buzzer. The error is stored even after the ign is turned off.
It uses all the original sensors so the original gauges have to work. Most of the code is reading the sensors and calculating the values.
I would be happy to send diagrams and code but I'm not sure I would know how to do it. The code is an 18K text file and the diagrams would have to be scanned or I could just send the files (75K) and you could install the software I used to create them (it is free). The hardest part is generating the code for the calculations, not impossible, just time consuming.
The cost is around $100 for all the parts, and a lot of time. I would be very happy to see someone come up with a printed circuit board and a parts kit, but I am just not able to that, but would be glad to help.
Let me know If you are interested in getting the information.

Paul

WINGAS · #4 05-29-2013, 01:18 PM

Do you have any idea what safe is? With pushrod V-8s we talk in terms of 10 psi per 1000 rpm. The 15W will be lighter at normal operating temps.

I swapped a pushrod V-8 from 20-50 to 10-30 semi synth and gained nearly 200 rpm in idle speed. I didnt notice a huge drop in PSI, but was only monitoring it +/- 5 psi. Incredible result however. I dont touch 50W even for the race car anymore.

pmckechnie · #5 05-29-2013, 02:00 PM

I use 20W-50 because that is the MB recommended oil for the 117 engines for climates that get up to 100 deg F, which it does here in the summer and very rarely gets down to 32 deg. In a SBC I would never run 20-50. On a SBC 5 psi is safe at idle if the engine is stock. Don't know about a MB engine. Anyone know what the safe minimum oil pressure is for MB engines?

Paul

WINGAS · #6 05-29-2013, 02:05 PM

This is a BBC with solid lifters, but same basic deal ( even better oem oiling than sbc they say) . It doesnt sound as if I'd worry about it, but having the "safe zone" data would be important. Again, the lower number will indicate a slightly thinner mix, as the base is thinner.

BTW, I've never seen such a geyser of oil gushing under a valve cover as I did in a running 606. 5 psi at idle wont move that volume of oil, no doubt in my military mind!

panZZer · #7 05-29-2013, 02:09 PM

Quote:

Originally Posted by pmckechnie

This is only a question about viscosity, not brand. As some of you may remember, I put an alarm for low oil pressure in my 84 500SEL. It was set for the following levels to activate the alarm.

Less than 45 psi above 2000 RPM.
Less than 30 psi above 1500 rpm
Less than 15 psi above 1000 rpm
Less than 10 psi above 300 rpm

It has been like this for several years now with never an alarm.
I have used 20W-50 SYN since the alarm was installed.
About 250 miles ago I changed the oil using 15W-50 Syn.
I have been getting oil pressure errors of 8 psi at 500 RPM since I changed the oil to the 15W-50, only when the engine temp is 190 to 200 deg F. I don't remember exactly what it was before, but I know it was above 10 psi at 500 RPM.
I thought that the 15W meant that the oil will never be thicker than a 15W oil when it is cold and would never be thinner than a 50W oil when it is hot. So since we are talking about viscosity when the oil is hot, why would the pressure be lower then when I was using the 20W-50?
Or, do I have another problem?

Any Ideas?
Thanks
Paul

Edit: I'm sorry I put this in the wrong place but It could apply to a diesel as well.
Edit #2: The alarm has only been in for a little over a year, not several. These things happen when you have been around as long as I have.

Get ready for the oil leaks.

whunter · #8 05-29-2013, 02:47 PM

Quote:

Originally Posted by pmckechnie

This is only a question about viscosity, not brand. As some of you may remember, I put an alarm for low oil pressure in my 84 500SEL. It was set for the following levels to activate the alarm.

Less than 45 psi above 2000 RPM.
Less than 30 psi above 1500 rpm
Less than 15 psi above 1000 rpm
Less than 10 psi above 300 rpm

It has been like this for several years now with never an alarm.
I have used 20W-50 SYN since the alarm was installed.
About 250 miles ago I changed the oil using 15W-50 Syn.
I have been getting oil pressure errors of 8 psi at 500 RPM since I changed the oil to the 15W-50, only when the engine temp is 190 to 200 deg F. I don't remember exactly what it was before, but I know it was above 10 psi at 500 RPM.
I thought that the 15W meant that the oil will never be thicker than a 15W oil when it is cold and would never be thinner than a 50W oil when it is hot. So since we are talking about viscosity when the oil is hot, why would the pressure be lower then when I was using the 20W-50?
Or, do I have another problem?

Any Ideas?
Thanks
Paul

Edit: I'm sorry I put this in the wrong place but It could apply to a diesel as well.
Edit #2: The alarm has only been in for a little over a year, not several. These things happen when you have been around as long as I have.

pmckechnie · #9 05-29-2013, 02:54 PM

I keep getting replies like I have done something drastic like going from conventional oil to synthetic or a lot thinner oil. I only went from 20W-50 to 15W-50, both fully synthetic. Why should I expect oil leaks. I didn't have any leaks with 20W-50 fully synthetic,why should I have any with 15W-50 fully synthetic? It is not that different. What am I missing.

Paul

pmckechnie · #10 05-29-2013, 03:16 PM

Whunter,
About 60K miles ago, I replaced all chain guides, timing chain, oil pump, strainer, oil pump chain and tensioner ,Rebuilt heads, all engine seals except rear main seal. cam oilers (then again about 10K miles ago). As far as the other things you mentioned, probably not all of them but anything I thought it would need. Oil pressure is still good above 1000 rpm, it is just a little lower at idle (500 rpm). My question is if 8 psi at 500 rpm is too low when the engine is at about 200 deg F?
When oil pressure drops any at all with nothing but an oil change, I get nervous.

Paul

whunter · #11 05-29-2013, 03:43 PM

Quote:

Originally Posted by pmckechnie

Whunter,
About 60K miles ago, I replaced all chain guides, timing chain, oil pump, strainer, oil pump chain and tensioner ,Rebuilt heads, all engine seals except rear main seal. cam oilers (then again about 10K miles ago). As far as the other things you mentioned, probably not all of them but anything I thought it would need. Oil pressure is still good above 1000 rpm, it is just a little lower at idle (500 rpm). My question is if 8 psi at 500 rpm is too low when the engine is at about 200 deg F?
When oil pressure drops any at all with nothing but an oil change, I get nervous.

Paul

Most of that was simply FYI for future members.

I will move the thread to the correct forum.

Per your data; The idle is below SPEC.

Your engine is designed for 600 - 750 RPM idle speed.

.

pmckechnie · #12 05-29-2013, 03:50 PM

whunter,
Thanks, I will see what I can do to increase Idle speed and see what the oil pressure is at 600-750.

Thanks
Paul

pmckechnie · #13 05-29-2013, 05:59 PM

whunter, As a test, I raised the idle speed to around 600 rpm and the oil pressure went to 16 psi. I can't believe 100 rpm would make that much difference but it did. Now I just need to figure out how to make the IAC increase the idle to 600 rpm.
Paul

whunter · #14 05-29-2013, 08:07 PM

Quote:

Originally Posted by pmckechnie

whunter, As a test, I raised the idle speed to around 600 rpm and the oil pressure went to 16 psi. I can't believe 100 rpm would make that much difference but it did. Now I just need to figure out how to make the IAC increase the idle to 600 rpm.
Paul

You have e-mail.

.

Hirnbeiss · #15 05-29-2013, 09:18 PM

No one here will be able to answer this question. It doesn't make a lot of sense, as you point out, but without running a lab test on the old and new oils, we're just guessing. Note that 50W oil can have viscosity in the range of 16.3-21.9 centistokes at 100 C, so a sizeable range.