Oil pressure drop from dirty plugs?

[The Other Red Baron]

We've got an IO-360. Had the oil changed and flew around for 10 minutes. I lean on the ground and go taxi for takeoff. By the time I get to the hold short line the engine starts sounding rough and oil pressure just keeps ticking down, down, in the yellow but further than I've seen it drop at idle.  I'm automatically assuming the worst...wrong oil or worse. Taxi it over to the shop on the field and ask them to check it out.  Next day they run it up a few times, check the oil... only thing they could find is two plugs were dirty so they cleaned them and re-gapped them.  Would the rough running engine alone cause a low oil pressure like that? Just never experienced that before if so.  Going to fly it today just wanting to try and cover my bases.

Edited October 15, 2020 by The Other Red Baron

[cferr59]

It probably caused an RPM drop which would affect the oil pressure.  Any engine monitor data to post?

[DonMuncy]

Very unlikely rough running would affect oil pressure very much.

[Parker_Woodruff]

1 hour ago, The Other Red Baron said:

We've got an IO-360. Had the oil changed and flew around for 10 minutes. I lean on the ground and go taxi for takeoff. By the time I get to the hold short line the engine starts sounding rough and oil pressure just keeps ticking down, down, in the yellow but further than I've seen it drop at idle.  I'm automatically assuming the worst...wrong oil or worse. Taxi it over to the shop on the field and ask them to check it out.  Next day they run it up a few times, check the oil... only thing they could find is two plugs were dirty so they cleaned them and re-gapped them.  Would the rough running engine alone cause a low oil pressure like that? Just never experienced that before if so.  Going to fly it today just wanting to try and cover my bases.

how hot was the oil when you saw it going down further than you're used to into the yellow?

[tmo]

Not really relevant, as the Continental TSIO-360 in my plane is different from your Lycoming 360, but I regularly see oil pressure in the yellow at idle when the oil is warmed up really well - usually when taxiing back after a longer flight. I don't recall it ever happening before take-off - the pressure is usually at max allowed.

[The Other Red Baron]

3 minutes ago, Parker_Woodruff said:

how hot was the oil when you saw it going down further than you're used to into the yellow?

Wish I remember the exact number it was in the green though didn't strike me as out of ordinary.

[Parker_Woodruff]

2 minutes ago, The Other Red Baron said:

Wish I remember the exact number it was in the green though didn't strike me as out of ordinary.

At least the oil was warm then.  I remember my IO-360 would go yellow at idle (~700 RPM) when the oil was warm.  If we were talking 1000 RPM significantly in the yellow then I might be more worried.

[The Other Red Baron]

2 hours ago, cferr59 said:

It probably caused an RPM drop which would affect the oil pressure.  Any engine monitor data to post?

I don't believe there was a significant rpm drop that went along with it.  I'm going to get the data later day.

[cferr59]

1 hour ago, The Other Red Baron said:

I don't believe there was a significant rpm drop that went along with it.  I'm going to get the data later day.

Definitely worth looking and the engine monitor data and also comparing it to previous flights.  Like others have said, the roughness itself shouldn't change the oil pressure.   What was the power setting when you noticed this? 

[N201MKTurbo]

Be very careful of oil pressure changes, it usually means something bad.

[The Other Red Baron]

33 minutes ago, cferr59 said:

Definitely worth looking and the engine monitor data and also comparing it to previous flights.  Like others have said, the roughness itself shouldn't change the oil pressure.   What was the power setting when you noticed this? 

Initially noticed it at idle about 800rpm, we tried richening it up and going to 1200 but still sounded like crap and no positive change in oil pressure. 

[The Other Red Baron]

21 minutes ago, N201MKTurbo said:

Be very careful of oil pressure changes, it usually means something bad.

That was my initial thought   which is why I’m surprised the shop didn’t find anything when they were running it up other than dirty plugs. 

[N201MKTurbo]

5 minutes ago, The Other Red Baron said:

That was my initial thought   which is why I’m surprised the shop didn’t find anything when they were running it up other than dirty plugs. 

They don’t know how your oil pressure used to behave. When oil pressure drops, even just a little, it usually means that oil is leaking past something that it didn’t use to leak by. 
 

I would keep an eye out for metal in your screens and just so you can sleep at night, you might want to have them pull the mags and inspect the gears.

[M20F]

4 hours ago, Parker_Woodruff said:

At least the oil was warm then.  I remember my IO-360 would go yellow at idle (~700 RPM) when the oil was warm.  If we were talking 1000 RPM significantly in the yellow then I might be more worried.

I would echo this.  You can’t troubleshoot a one off incident in a lot of cases.  Use light oil, go fly, your pressure is going to be low taxing.  I would try running the plane a bit and note what the pressure does in various scenarios.  Then you have some data to troubleshoot with. 

[DXB]

If nothing obviously wrong, then maybe some high speed taxis, then look under the cowl for leaks, look at data from engine monitor for reassurance of normal oil pressure before becoming airborne again?

[Guest]

Verify that your suction screen is clear/clean.  Then verify the oil pressure indication by use of a known test gauge.  

Clarence
 

 

[The Other Red Baron]

Well, I got it nice and warm, run up, beat up the pattern and no issues.  I guess it was just dirty plugs. I guess I'm relieved but I just found the timing a heck of a coincidence.

Here's the "bad day"

https://apps.savvyaviation.com/flights/4435193/8de3240b-7457-45e8-a667-8274110a9493

Here's me beating up the pattern and a short cruise

https://apps.savvyaviation.com/flights/4435197/f4611e0a-f3e2-41bf-9a0c-011797de029f

[carusoam]

TORB,

You are doing well... but...

We have things that supply oil pressure... and things that let pressure escape before the reading gets made...

Pressure (+)

• The engine rpm is directly connected to the oil pump rpm...
• Cold OilT will generate higher OilP...
• Tight clearances like a new engine...
• high rpm delivers more oil downstream...
• Fresh oil, has higher viscosity...

Pressure (-)

• With all these +/- variables... the pump delivers a lot of oil... unless something blocks it’s inlet screen.
• pressure builds when the flow meets a restriction...
• The last restriction is the big one... the oil pressure regulator...
• Oil escaping the system... like filling the prop. Or going on the ground...

 

Worst case... (how to get the lowest OilP)

• Hot oil
• low rpm
• high engine time
• Oil missing from the intake screen... (add more oil)
• Oil that has so many hours on it... it has lost viscosity...
• Pulling the prop control back...

Everything working together delivers the required flow to the pressure regulator... that controls the pressure right where the OilP sensor is mounted.

So... where was the dirty plugs on the list... 

PP thoughts about oil pressure only, not a mechanic...
 

Best regards,

-a-

[carusoam]

One of these graphs shows the oil pressure being kept pretty steady... note A

The other one... the oil pressure isn’t being controlled...  The flow probably isn’t enough to build the pressure needed... by the regulator...  Note B
 

 

Great data sets...  go back to the data to see what the rpm was during these two periods, A & B...

If the OilP falls out of the yellow zone... that is not normal... if it is in the yellow zone during taxi that is OK...

If you are flying with oil in the yellow zone, or outside of the green zone... this is time to find a place to land...

Still PP thoughts... trying to err on the conservative side...

Best regards,

-a-

 

[carusoam]

In this graph...

You can see the RPM and oil pressure moving together... as expected...

Depending on conditions... You may have to hold the rpm steady to see steady OilP...

-a-

[The Other Red Baron]

Took me a minute to figure out how to get more data on the screen.  Pretty neat all the stuff you can look at.  Both A&B are cruise portions of flight but B was much shorter and oil pressure drop there does coincide with lower rpm.   It was still running smooth at that time.

I keep it leaned out on the ground as much as I can as the bottom picture hopefully shows

[carusoam]

It is really interesting to see how the OilP runs up to about 75psi... before settling back over time to around 50...

Probably just an artifact of the old mechanical control device...

As far as how much info a JPI can store... all with time stamps... and can include locations as well... connected to your favorite GPS for coordinates...

JPI the box that is so good... nobody knows how well it works... or how much it can do...

Best regards,

-a-