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Old 06-18-2001, 04:30 AM
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I was wondering about the difference in oil pressure behavior between a gas engined car and the turbodiesels. (I know we are talking apples & oranges here, but bear with me).

On the 300D (1984, 198,000 miles) the pressure gauge is pegged when at a cold idle, just like the V-8. Once fully warmed up, the oil pressure will be at about 1.5 Ė 1.7 bar at idle, again just like the V-8. On a fully warmed up engine, my 450 engine will peg the oil pressure at just barely above idle. However, it takes about 1300-1400 RPMs to get the needle all the way up to 3 on the turbodiesel when it is good & hot.

My hypothesis is that the additional flow requirements from the oil jets under the pistons, as well as the oil feed for the turbo take up a lot of flow, and the oil pump just needs a little more speed to increase the volumetric displacement enough to compensate.

Am I on the right track here, or am I just rationalizing an inherent problem with my new baby? (YIKES!)

Thanks Everyone!
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Old 06-18-2001, 06:50 PM
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Unfortunately it sounds a lot like "yikes" to me. The turbodiesels do have a higher flow requirement than the non-turbodiesels. The pumps were totally redesigned and chain driven on the turbos and the almost always have higher pressure at idle than do the V8s.

They all should instantly peg the gauge when reved. I would be sure about the electrical on the gauge and If the pressures are correct I would live with it till you see a change or send in your oil for testing to see if you are into the brass portion of any bearing.
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Old 06-18-2001, 09:11 PM
WDurrance
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The oil jets for the pistons are only open above 1.0 bar of oil pressure. But anyway, the minimum requirements are: No lower than 0.3 bar@ idle
And 3.0 bar by at least 3000 rpm
Either way, you're okay. Could be the gauge...could be temperature differences...who knows? The important part is, don't worry.
Regards,
Randy D.
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Old 06-19-2001, 03:37 AM
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Thanks guys! Talk about a quandary, two completely opposite answers, both from people that I have come to <GREATLY> respect based upon their copious knowledge & previous postings to others on this forum.

Just to clarify, I am wondering if perhaps Steve may have mis-interpreted my original post. The oil pressure does peg at 3 when ďrevvedĒ, but if the engine is slowly brought up from idle (it idles at about 1.5-1.7 bar when HOT) it takes about 1300-1400 RPMs to peg the needle. This is with me using 15-40 Rotella T, at an ambient temperature of 80-100 degrees, after a long freeway run with the A/C on. (I did say HOT .)

Also, I didnít realize that these 123s used an electric oil pressure gauge. I am used to my trusty old 107s mechanical gauge. Iíll have to check this out.

And Bill, I love your approach; I wished that I could just sing that old Bobby McFerran song ďdonít worry, be happyĒ but Iím just too damn obsessive (but not necessarily compulsive, so no meds. At least not yet ) And if my motor actually needed 3000 RPMs to develop full pressure, I probably would need meds!

And now on to my next problem Ė adjusting the wastegate on the Garret turbo. I looked & felt with a dental probe all around the circumference of the retaining spring on the cover, and I could not find a gap anywhere. And I mean Anywhere. Iím going to ask about this in a separate posting.

Thanks Guys!

P.S. If Steve is right, and this engine is on the edge, Iíll probably find out pretty soon, although I donít plan on taking the boost past .75bar. The graph on the 123 service CD shows a peak boost of about 750 mbar at 3200 RPMs.
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Old 06-19-2001, 08:53 AM
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My reply was based on one of those touchy/feely type impressions. My overall impression is that the gauge readings on any diesel oil pressure will be higher than a V8 during idling and up to the top of the gauge.

I also pointed out not to worry unless something changes. That was based on the fact that you are a long ways from the wear limits on pressure.
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