recent engine rebuilt - troubling oil pressure (3 liter big six)

[mabbonizio]

Hi All,

I've recently had the M186 in my 54 300B cabriolet rebuilt to the tune of a hefty sum

The motor runs great with one exception that I've noticed as the weather gets warmer.

On HOT days the oil pressure fluctuates while driving. Here is what happens after a good 30-45 min drive with the outside temps being 80+

idle oil pressure is around 20PSI depending on where the idle ends up settling. Sometimes it is higher and sometimes lower. If I tap the accelerator the oil pressure pegs.

When I accelerate the oil pressure pegs - depending on my shifting pattern.
Winding the engine out through the gears the oil pressure stays pegged.
shifting leisurely, especially if its very hot the oil pressure will drop on the gauge to say 30 - 35 PSI in fourth gear at 30-40 mph and linger there for a few seconds until the engine builds up speed even if I punch the throttle it will stay dipped and rise slowly.

At 35pmh in 4th gear the engine is at approx 1,900 rpm.


If I'm cruising around 40 MPH in 4th gear with a hot motor and punch the throttle, the oil pressure will be pegged with light throttle and dip slightly to 35 PSI and riser slowly til it pegs.

is this cause for concern?

[Sugar Bear]

It sounds normal to me. What weight oil is being used? What is the engine temp? What is the ambient temp? What is the color of the car? Last question is because we want pictures, it sounds nice.

Not pressure related; but, use an oil that has sufficient levels of ZDDP/zinc to protect the sliding surface cams.

Good luck!!!

[mabbonizio]

^^ Interesting. Neither of my diesels with well worn in engines have any dips in oil pressure when punching the throttle at low rpm in high gear.

Oil is 20w 50. I'll be doing its yearly oil change soon!

outside temps are 75/80 + when this happens.

Engine temps are on the cooler side - 170 degrees.

Pics:

[Sugar Bear]

I just re-read your original post and understand what you are encountering. I'm guessing here, but thinking the gauge is on the edge of being pinned and punching the throttle gives a slight dip in rpm's with a higher rebound. That slight dip in rpm may be just enough to dip the gauge.

My gut is that it is still ok, pegged at cold idle and at approx 2,000 rpm's.

Absolutely stunning car. Best of luck with it and be selective about oils on that motor. Consider a diesel 15w40, they usually have slightly higher ZDDP. Motorcraft adverts 1,000 ppm.

[Tony H]

Might you consult the party that rebuilt the engine?

[Tomguy]

20w50, is there a reason that you'd use oil that thick, especially on a freshly rebuilt engine?

You should want 10w30, or maybe 5w30. Modern oils aren't your granddad's oil, especially synthetics. If your engine has newer-style seals, a 10w30 or 10w40 synthetic (maybe 5w40 or even 0w40) is the route I would go with.

[rbtoj]

I think 20w50 on a freshly rebuilt engine is a bit thick.

Also, perhaps the oil gauge itself is wrong? I'd say it's worth double checking the pressure with an actual pressure gauge tool hooked up directly to the engine.

[Whatsup]

Mabbonizio, my first consideration would be to ask the rebuilder if it wasn't you, seek a owners manual or repair manual, something that would state the suggested motor oil to use for that time.

Remember multi-viscosity oils and synthetic oils were hi-tech back then and the zinc content has been reduced since then. Was NOS gaskets and seals used? Was a rope seal used for the rear main.

Match the oil with the type of seals used. As mentioned in other posts, diesel oil has a higher content of zinc than the common oils of today. Checking/cleaning the oil gauge itself might be helpful.


Your oil pressures do not seem out of line though. Cool car to enjoy an afternoon drive.

[Usaguy]

Originally Posted by Sugar Bear

...Consider a diesel 15w40, they usually have slightly higher ZDDP. Motorcraft adverts 1,000 ppm.

It has to be rated for byproducts of gasoline combustion. I don't remember the name of that rating but you could be doing damage running a diesel oil in a gas engine

[Diesel911]

To find out if it was the gauge the generic thing would be to hook a test gauge and verify that the Cars gauge is working OK.

I also agree with the person who suggested you contact the party that rebuilt the Engine. If something needs to be warranted the sooner you tell them you might have a problem the the better.

[Benz Dr.]

Originally Posted by christuna

It has to be rated for byproducts of gasoline combustion. I don't remember the name of that rating but you could be doing damage running a diesel oil in a gas engine

Since when? Diesel rated oil will have more anti-wear additives and a higher level of the detergent package but it won't hurt older MB engines.

[Usaguy]

Originally Posted by Benz Dr.

Since when? Diesel rated oil will have more anti-wear additives and a higher level of the detergent package but it won't hurt older MB engines.

It needs to have the SN rating (I know that chevron delo does). I'm going to quote directly from the amsoil website:

The American Petroleum Institute (API) publishes its “C” category of specification for diesel oil (currently CK-4) and its “S” category for gasoline motor oil (currently SN). It’s common for today’s diesel engine oils to carry both the API CK-4 and SN specifications.
Though they’re truly designed for diesel applications, they can be used in gasoline applications for those drivers who want to use one oil in all applications. If the diesel oil carries both specs, feel assured the oil is safe to use in diesel and gas applications. If the diesel oil does not carry the “S” category, I strongly recommend against using it in gasoline applications.

[Tomguy]

Most of the reason isn't because of additives that won't withstand gasoline burning byproducts, but because the additives in Diesel oil (such as ZDDP) can cause buildup or even failure in modern catalytic converters.

[Benz Dr.]

Originally Posted by Tomguy

Most of the reason isn't because of additives that won't withstand gasoline burning byproducts, but because the additives in Diesel oil (such as ZDDP) can cause buildup or even failure in modern catalytic converters.

Which pre 1975 MB cars won't have.

[Tomguy]

Yes, which is why I pointed that out

I still think on a fresh rebuild, even of a 1950s clearanced engine, 20w50 oil is a bad choice, unless there's a reason to do it. 5w30 or 10w30 would be better choices.