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Old 04-12-2002, 10:26 AM
Registered User
Join Date: Jan 2001
Location: Naperville, IL
Posts: 32
E420 bizarre oil gauge behavior (no really)

Here's the situation..94 E 420 with 103K. I bought it with 92K in 1/01.. When warmed up, the gauge used to read 1.4 bar or so. After last summers big heat, the pressure dropped to 1 bar (by "dropped to" I mean I would drive it on a medium heat day..45-65 degrees..say a long hiway ride..then get stuck in traffic, where it would do this). After changing the oil, the pressure would be ok, back to 1.2 or so after this sort of driving- but 500 miles after the change (10/40 w) it would continue to be low. Now it is below 1 bar, around .7 or so (I can't really be to exact here) after a long drive, then at idle in D.

So far, seems like the average "You are worried about nothin" V-8 mercedes newbie post, right? (and that description would describe me...) -the gauge pins at around 15-1800 rpm and stays there on the hiway, though after reading all the stuff here about what that doesn't seem to "pin" as much as hesitate a bit then get up there...

But- here's the strange thing: when I am idling at a stoplight, car in D, pressure at the 1 to .8 mark or so..and I turn the wheel a hard right or left..without moving, just turning the wheel..the oil pressure gauge dips..then pops back up.

my friend, a merc mech at Laural here in Chicago, said he has never heard of this- and that it sounds "like an electrical problem"..

so, are there 2 problems here..a mechanical /oil pressure thing, and an electrical/harness problem too? allright!

I have read the 1,298 posts here about oil pressure gauges, oil weight, synthetics vs non, oil pressure sensor, etc etc and not found any mention of this.

So, assembled Merc Massive, how to go about this in a logical order? my mech (Master class Mercedes guy for 12 years) says Synthetic is a waste of money, just try 20/50.

does a plan like first trying 20/50, then synthetic, then changing the oil pressure sensor..then taking out a second mortgage to pay for Merc shop diagnostics seem the right way to go? i guess I just don't understand why the idling pressure would change so much after only driving the car 10,000 miles..

thanks for any help.
John Pazdan
94 E420
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Old 04-12-2002, 10:41 AM
Ali Al-Chalabi's Avatar
Senior Member
Join Date: Nov 2000
Location: Knoxville, TN
Posts: 1,837
20w50 should result in a higher oil pressure, I would recommend that you try that and see what the oil pressure reads if it is approved for the climate you are operating in.

When you turn the steering wheel at a stop, do you also notice a slight drop in RPM? With my C280 with the M104, if I turn the wheel at a stop, there is so much drag on the engine from the power steering pump that the RPM drops. The ECU then corrects the slight RPM drop, but after a small delay. If the RPM does drop with the V8 engine as well, there is the possibility that it is due to the momentarily lower engine RPM and thus lower RPM of the oil pump.
Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record
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Old 04-12-2002, 11:54 AM
Super Moderator
Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,315
It sounds to me like you are worrying about normal oil pressure behavior. Oil gets hot, thins out, pressure drops. Power steering engages, engine RPM's drop for a second, oil pressure drops. What you need to watch for is a sudden drop in pressure at equivalent conditions and of course listen for any engine noise.

20/50 is probably a good bet for the warmer months. At least it will make you feel better. I have never been able to get an answer to whether or not it improves lubrication.

I would stay away from synthetics because of expense and potential leaks.
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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Old 04-12-2002, 10:00 PM
Registered User
Join Date: Jun 2001
Location: Omaha, NE
Posts: 962
I agree about the RPM drop from the PS pump leading to oil pressure drop. Probably nothing to worry about. However, turning your steering wheel while parked isn't particularly good for your tires, nor I suspect the PS pump, especially at the extremes.

I personally would HIGHLY recommend synthetic oil in this engine. The M119 engine runs hot and I like the extra insurance synthetics provide at high temps.

If you switch to Mobil 1 15W-50, I assure you your idling oil pressure will rise. I don't know if that actually matters in the overall scheme of things (the owner's manual tells you idle oil pressure as low as 0.3 is OK and "will not jeopardize operational characteristics") but it certainly makes me happier to see a higher pressure. Mine used to drop to 0.5 at idle in summer, and after the switch to Mobil 1, always stays about 1 bar. No leaks either, and I switched at 80,000 miles.
'93 400E
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