: Why Check the Oil When Hot?



kckranz
04-15-09, 09:33 AM
From the Technical Archive:

"The best way to minimize oil consumption in a Northstar is to keep the sump filled slightly low (many are continuously overfilled) by only checking the oil level when hot"

I am troubleshooting poor oil consumption and want to make sure I am checking the oil correctly and getting the most accurate readout. I thought that when the engine is hot the oil is distributed throughout to lubricate all the moving parts, thus not in the sump to which the dip stick "dips". Wouldn't this make one believe an oil level is too low and lead to over filling? I trust the technical archive but would like to know why.

PS - This may have been covered before but the search engine will not search for words of 3 or fewer characters, e.g. "hot", "oil", "low".

dkozloski
04-15-09, 09:39 AM
Oil expands in volume as the temperature increases. The viscosity is at its lowest so it readily drains back to the sump. When the engine is at operating temperature, shut if off for a few minutes like when you are refueling and let the oil drain down. Now check the level with the dipstick for the most accurate results.

Submariner409
04-15-09, 09:46 AM
Yeah that ^^^, and know that the full, or 7.5 quart, mark on a Northstar is halfway up the dipstick hashmark, not to the MAX mark. Fill it to the MAX and it will throw out that half quart in a very few miles. The bottom third of the hashmark is perfect.

I run it that way with Pennzoil Platinum 5W-30 and use a quart of oil every 3,500 - 4000 miles, highway, crazy, or suburban driving included.

kckranz
04-15-09, 10:07 AM
So, in summary, for taking an accurate oil level reading: "Wait 5 minutes after shutting down the engine that has been warmed to operating temperature. It is still considerd 'hot' and yet enough time has passed for the majority of the oil to retreat to the sump."

Thanks, folks.

dkozloski
04-15-09, 10:26 AM
So, in summary, for taking an accurate oil level reading: "Wait 5 minutes after shutting down the engine that has been warmed to operating temperature. It is still considerd 'hot' and yet enough time has passed for the majority of the oil to retreat to the sump."

Thanks, folks.
Don't forget to be parked on level ground.

Ranger
04-15-09, 11:27 AM
I personally think too much is made of "checking it while hot". Heating and expansion is true of coarse, but I did an experiment a few years ago to see the difference between hot & cold. It was barely noticeable on the stick.

Submariner409
04-15-09, 12:27 PM
I'll betcha even money that the "Check while hot after 5 minutes" is a holdover from the days when "Full Service" at a gas station meant something: the grease monkey cleaned the windshield, pumped the gas, checked the oil, got your change ("Gimme a dollar's worth" - 6 gallons) and maybe even did the tire pressures. NOBODY but the most avid wrencher (or his automotive-wise son) lifted the hood in the morning and checked the oil before starting out for the day.

JimD
04-15-09, 08:16 PM
I personally think too much is made of "checking it while hot". Heating and expansion is true of coarse, but I did an experiment a few years ago to see the difference between hot & cold. It was barely noticeable on the stick.

I have to check in on this one, Larry.

The dipstick design is calibrated for "hot oil" after fill-in-the-blank minutes for drain down. I likewise took a scientific look at the potential difference between hot & cold so I could have a reference point when I pulled the dipstick on a cold engine before starting an all-day drive to (somewhere).

The "cold oil" level mark I filed in the thin edge of the dipstick is 1/4" lower than the same volume of oil in a "hot" engine.

That said, I continue to put my faith in the oil level sensor and the CHECK OIL LEVEL message at engine start. I have learned that the message can be flagged at a cold start, but not on a subsequent hot start.

I rarely pull the dipstick.

Ranger
04-15-09, 08:58 PM
1/4" eh Jim. I swear Mine was no more than half that. I may have to try this again.

mtflight
04-15-09, 11:23 PM
know that the full, or 7.5 quart, mark on a Northstar is halfway up the dipstick hashmark, not to the MAX mark.


Wow this is neat to know... is there a reference for this? I would've guessed MAX was full.

Question: is the "Check Oil Level" message triggered at a specific oil deficit? (e.g., 1 qt, 1.5 qt, 2 qt etc) It happened to me this morning, I added 1.5 qts (1 qt had it at MIN, I believe, or slightly higher).

Comment: I had heard that oil at the MIN mark indicated a quart low (e.g., 1 qt difference between MAX and MIN).

Is this so?

JimD
04-16-09, 12:21 AM
....Question: is the "Check Oil Level" message triggered at a specific oil deficit? (e.g., 1 qt, 1.5 qt, 2 qt etc)....That message will trigger at approximately 2 quarts below FULL level. Which should mean there is 5 to 5.5 quarts of oil remaining somewhere in the crankcase. Five point five quarts is more than enough oil to lubricate the engine during legal public roadway driving.
Comment: I had heard that oil at the MIN mark indicated a quart low (e.g., 1 qt difference between MAX and MIN).

Is this so?I have no idea.

What I do know for sure about my '98 engine is that about the time the crankcase oil level falls below the bottom of the dipstick, the CHECK OIL LEVEL will appear.

Submariner409
04-16-09, 08:18 AM
Over the years there have been several TSB's posted concerning engine oils and fill levels. One TSB specifically addresses the oil consumption problems associated with overfilling a Northstar.

When you change your oil and filter, follow the Service Manual recommendation to prefill the filter and use 7.5 quarts of the recommended viscosity oil for your engine. Run the car to check for leaks, and shut down. Allow the oil to drain down for a few minutes. The level now on the dipstick is your FULL level.

dkozloski
04-19-09, 03:28 PM
That message will trigger at approximately 2 quarts below FULL level. Which should mean there is 5 to 5.5 quarts of oil remaining somewhere in the crankcase. Five point five quarts is more than enough oil to lubricate the engine during legal public roadway driving.I have no idea.

What I do know for sure about my '98 engine is that about the time the crankcase oil level falls below the bottom of the dipstick, the CHECK OIL LEVEL will appear.
I did a crash investigation teardown of an aircraft engine for the FAA that ran out of oil and failed with no more than a cup of oil in the system. As I disassembled the engine I accounted for every drop of oil I could find and there was no more than one cup.