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  #1  
Old 09-26-2003, 08:08 PM
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Low hot idle oil pressure 94 420E

My friend the MB mechanic has a problem car in his shop he cannot diagnose, so he asked me to pass this on in case someone else has run into the same problem.

Car is a 1994 420E, always properly serviced (mostly by him, I think) with 80,000 miles.

Symptom is low oil pressure at idle hot. Pressure comes up pretty fast with increasing rpm. No obvious reason (ie good oil, engine runs well, no leaks, no noise from lifters).

This is what he has done/checked so far:

Oil tubes on cams (no leaks -- but what a pain to get to!).

Oil sending unit is good (oil pressure is low on an external gauge, too).

Not a bad cam tower or other internal leak on the upper end.

He was going to pull the oil pressure relief valve on Thursday to check it.

He is concerned that there is a main bearing with excessive clearance that isn't making any noise.

Issue started recently.

Any thoughts out there? He's covered everything I can think of, he's stumped, and he's been working on MBs since 1959!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #2  
Old 09-26-2003, 10:10 PM
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peter

Two obviuos questions come to mind:
1. What weight of oil ?
2. Oil pump ? ( clearances )
3. I'm just guessing.
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  #3  
Old 09-27-2003, 12:44 AM
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Question

How low is "low"? And yes, what weight oil are we talking about? Your mechanic might want to put 20-50 non-synthenic in and see what happens. Oil weight can make a big, big difference in the pressure gauge reading.
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  #4  
Old 09-27-2003, 04:07 PM
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Thanks guys.

This guy knows his business -- it's not the oil (fresh 15W 50 synthetic), it's not a leak, and it's not a visible or detectable failure of any sort that he's seen before (and he's seen most of them by now....).

Low is 5 psi or so at idle hot, way too low for a W119 with only 80,000 miles. Comes up nicely with increased rpm, but 5 psi won't cut it. No way you would have less than 1 bar oil pressure on this thing.

The only things I could think of were a defective bearing or a bad oil pump, neither exactly likely.

Peter
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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #5  
Old 09-27-2003, 04:39 PM
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Mercedes does not recommend synthetic oil for that car.

I'd run conventional motor oil
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2001 E430, Bourdeaux Red, Oyster interior.
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  #6  
Old 09-27-2003, 06:38 PM
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You may be chasing a ghost. The owner's manual for my '88 specifies 0.3 bar as minimum oil pressure, but it usually runs 1.5 in mild weather and I've never seen less than about 1.0. Reducing oil pump volume flow is a trick that many OEMs use to reduce parasitic loss, and 5 psi is sufficient for idle when there is virtually no load on the bearings.

Look in the owner's manual and see what it says about oil pressure.

Duke
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  #7  
Old 09-27-2003, 07:44 PM
Bud
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Quote:
Originally posted by suginami
Mercedes does not recommend synthetic oil for that car.

I'd run conventional motor oil
That's news to me. The only Mercedes that synthetic oils could negatively effect were made decades ago.

Mercedes doesn't approve of Mobil 1 0W-30, 5W-30 and 10W-30 (probably because they aren't true synthetics).

See http://www.whnet.com/4x4/oil.html

Last edited by Bud; 09-27-2003 at 07:51 PM.
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  #8  
Old 09-27-2003, 08:35 PM
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Mercedes did not recommend using synthetics in any of their motors until the M112 and M113 engines were released in 1998.

That's not to say that you can't use synthetic; it's just that the factory never recommended it in any of the engines prior to 1998 (including the M104's that were used in the S class until 2000).

In the Sep / Oct. 1997 issue of the Star Magazine, Stu Ritter and George Murphy (technical editor and vintage editor) discuss this issue and are against switching engines designed to run on conventional oil to synthetic.

While no damage will likely result, they discuss how synthetic oils are hard on seals, and how if left sitting, synthetic oil can lead to hydraulic valve lifter clatter. The valve lifters can de-pump due to the thinner synthetic oil leaking out or bleeding out.
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2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".
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  #9  
Old 09-27-2003, 09:46 PM
Bud
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I began using Mobil 1 back in the early 80's on the advice of the great Frank King who was the technical director of the Mercedes Benz club for decades. I used it in my 190D for 18 years and when I sold it that engine still got over 4,000 miles on a quart of oil.

I've used Mobil 1 in my 300E for 13 years and the engine uses less oil than any car I've owned in the last 50 years.

I've used 15W-50 Mobil 1 in Minnesota winters and in Arizona summers. 0W-40 Mobil 1 can be used in ANY modern Mercedes Benz gasoline engine and 15W-50 can be used as well, even in FSS systems.

Synthetic oils may have lower pour points than Dino oil but they are not *thinner*. I used to be able to pour 15W-50 Mobil 1 in my garage in the dead of a Minnesota winter but it was never *thin* even in an Arizona summer.

The comment attributed to George Murphy doesn't surprise me. When Frank King passed away, I quit the club.
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  #10  
Old 09-27-2003, 10:39 PM
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Mine used to go to 0.5 bar, idling in gear, in summer, in stop and go traffic, until I made the switch to Mobil 1 15W-50. Now it never goes below 1 bar.
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  #11  
Old 09-28-2003, 12:46 PM
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My point exactly -- with synthetics (Mobil 1 of the appropriate grade has ALWAYS been on the MB approved/recommended list, at least since 1980), oil pressure hot should be higher than with dino oil.

My experience (and that of several professional technicians) is that full synthetics oils drain down from the tappets more slowly than dino oils. In fact, several recommend that one not check the oil level with the engine hot, only cold as much more oil stays up inside the engine hot, and there is a risk of overfilling and wasting oil.

An oil pressure gauge bouncing on the 0 peg at hot idle in an engine with low milage is a problem.

Peter
__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #12  
Old 09-28-2003, 03:14 PM
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When I had the 190E 2.6, it always had lower hot oil pressure when using synthetic than dino. It would run right down to .3-.5bar when hot. All my MB's have done this, and all of them seemed to drop a little lower when using synthetic. Also, lower hot pressure was more likely when using lower numerical viscosity oil.

Does it go to 3bar under acceleration?

Does it consume oil?

A spun main bearing might be the culprit, but I wouldn't do a bottom end on the engine just yet.
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  #13  
Old 09-29-2003, 01:35 AM
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Quote:
Originally posted by psfred
My point exactly -- with synthetics (Mobil 1 of the appropriate grade has ALWAYS been on the MB approved/recommended list, at least since 1980), oil pressure hot should be higher than with dino oil.

My experience (and that of several professional technicians) is that full synthetics oils drain down from the tappets more slowly than dino oils. In fact, several recommend that one not check the oil level with the engine hot, only cold as much more oil stays up inside the engine hot, and there is a risk of overfilling and wasting oil.

An oil pressure gauge bouncing on the 0 peg at hot idle in an engine with low milage is a problem.

Peter
I really don't want to turn this into an oil thread.

I don't believe the synthetic oil is the cause of your MB mechanic's problem, either.

However, I've been told differently about synthetics and oil pressure. Synthetic oil is thinner than conventional oil, and slightly lower oil pressures result.

The comment about the synthetic oil draining from the tappets was based on what happened to Stu Ritter's personal 300E. His reasoning was that since synthetic oil is thinner, it bleeds right out.

Once again, this was published in the Sep. / Oct. 1997 The Star Magazine.
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2001 E430, Bourdeaux Red, Oyster interior.
79,200 miles.

1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".
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  #14  
Old 09-29-2003, 02:01 AM
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Synthetic oil is not "thinner" than mineral oil. For multi-viscosity grades, the lower number represents a range of absolute viscosity at 32F and the higher number represents a range of absolute viscosity at 212F. These viscosity ranges for a given "weight" are defined by the SAE. For a mineral-based oil to achieve a 20W-50 rating, a "viscosity index improver" has to be added, and these additives are slowly consummed as the oil ages. Straight mineral base stocks cannot achieve a multiviscosity rating without VI improvers.

A synthetic base can achieve a wider viscosity range of 15W-50 with little or no VI improver , so it will maintain its viscosity index for a longer use period. If anything, a 20W-50 might be a 20W-40 after it's used, but the thinning due to loss of the VI improvers is offset to one degree or another by thickening due to oxidation.

There are two basic advantages of synthetic - greater oxidation resistance, which means they can be used for extended drain intervals, and wider viscosity range, which is an advantage if you want to use a "year round" oil such as 0W-40 in below zero winter temps and 100+F summer temps. That's it!

Beyond these two differences, the two behave the same inside the engine.

Duke
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  #15  
Old 09-30-2003, 12:45 AM
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Engine does not use oil.

This is NOT an oil problem, it is a very low (near zero) oil pressure at idle problem. Oil is fine.

Hans is going to put a new oil pressure relief valve in it this week to eliminate that possiblity, but at that point the owner will have to decide what to do. If the new pressure relief valve doesn't fix the low oil pressure, he will probably sell the car.

Investigating bearings or oil pump will probably require removing the engine, and removing the crank gets into a minor engine rebuild pretty fast....

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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