Oil level keeps rising gradually

Hey everyone,

I’m a new Nsx owner, recently got a jdm1991 na1 that has Been sat in a garage for about 20 years. I did a full engine seal overhaul and exhaust sensors, serviced injectors and they had no leaks under pressure test, The car is up and running, driving it for a few weeks.

I notice the oil was above the full mark after the first oil change so did a full oil change again and after 2 weeks and a few hundred km, the level has gone up again!

The oil does have a bit of a fuel scent to it. Is it possible that the engine can be running too rich causing this?

Has anyone experienced this? If so can you please share your findings and possible solution?

I don’t want to drive the car until this issue is sorted.

Maybe you can send the oil for analysis to try to find out what is in it.

I'm glad you noticed this and considered it important.
I'd take a look at each spark plug - to see what color it is - a black oily one would mean that piston is not firing.
You'd also smell unburnt gas in the exhaust. A compression test would also be helpful. Oil can smell a bit burned but it shouldn't smell of fresh gas. Unburned gas in a cylinder can wash away protective oil.
I'll be interested in other's thoughts, too.

Hello and welcome to NSX Prime!

Maybe a dumb question, but do you let the car sit for 10-15 minutes after driving before you check the oil?

The NSX oil system is a closed loop and maintains a consistent oil level, where crankcase oil drips back down into the pan and oil from the heads drains via the drain channels into the pan. There are four ways I can think of that would result in more oil in the pan. First, if the anti-drainback valve in the oil filter fails, it could allow some of the oil that would normally stay in the engine block after the car shuts off to drain back into the pan. The total oil capacity of the system is 6.7 quarts, so there is a considerable amount of oil that stays in the engine block and heads. Did you use a Honda genuine oil filter? Second, if an oil passage is blocked somewhere inside the engine, it's possible that the engine block will not hold the full amount of oil and more will stay in the pan. Third, if the bottom of your oil pan is deformed, it could push the oil level higher so that the dipstick is reading high. Finally, if the drain passages from the heads are blocked somehow, it could be reducing the amount of drain back into the pan.

silver2002 said:

I'm glad you noticed this and considered it important.
I'd take a look at each spark plug - to see what color it is - a black oily one would mean that piston is not firing.
You'd also smell unburnt gas in the exhaust. A compression test would also be helpful. Oil can smell a bit burned but it shouldn't smell of fresh gas. Unburned gas in a cylinder can wash away protective oil.
I'll be interested in other's thoughts, too.

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So I just pulled the plugs. None are wet and the tips don’t have any build up, just son soot on the crown. I know if compression was an issue or the car was running super rich, the plugs may have been wet which they are not. Here are some pics

Honcho said:

Hello and welcome to NSX Prime!

Maybe a dumb question, but do you let the car sit for 10-15 minutes after driving before you check the oil?

The NSX oil system is a closed loop and maintains a consistent oil level, where crankcase oil drips back down into the pan and oil from the heads drains via the drain channels into the pan. There are four ways I can think of that would result in more oil in the pan. First, if the anti-drainback valve in the oil filter fails, it could allow some of the oil that would normally stay in the engine block after the car shuts off to drain back into the pan. The total oil capacity of the system is 6.7 quarts, so there is a considerable amount of oil that stays in the engine block and heads. Did you use a Honda genuine oil filter? Second, if an oil passage is blocked somewhere inside the engine, it's possible that the engine block will not hold the full amount of oil and more will stay in the pan. Third, if the bottom of your oil pan is deformed, it could push the oil level higher so that the dipstick is reading high. Finally, if the drain passages from the heads are blocked somehow, it could be reducing the amount of drain back into the pan.

Click to expand...

Thank you for the welcome. Happy to be here, trying to get my hands on one since childhood

So every time I check the oil is when the car was either parked overnight so it’s all cool. And also maybe an hour or so after driving.

The oil pan isn’t deformed and yes I used a genuine oil filter.

Plugs don’t look too bad and they are all firing. I guess I should just change plugs, do an oil change and monitor again…

If you change the oil, make sure you are draining it hot- after a 20-30 minute drive. Give it about 5-10 minutes to settle in the pan (for me it's usually about the same amount of time it takes to put the car on jack stands and gather tools for the change) and then drain fully. Even though the manual calls for "5.3 quarts", you should fill 5 and then gradually add oil until the level is at the top of the indicator on the dipstick. Depending on how much you drained, this can vary. That way, you'll know you have the exact correct amount. Drive the car, try to get into VTEC a few times (to get the oil into the rocker pins) park it and check in the morning. Let us know how it does.

Honcho said:

If you change the oil, make sure you are draining it hot- after a 20-30 minute drive. Give it about 5-10 minutes to settle in the pan (for me it's usually about the same amount of time it takes to put the car on jack stands and gather tools for the change) and then drain fully. Even though the manual calls for "5.3 quarts", you should fill 5 and then gradually add oil until the level is at the top of the indicator on the dipstick. Depending on how much you drained, this can vary. That way, you'll know you have the exact correct amount. Drive the car, try to get into VTEC a few times (to get the oil into the rocker pins) park it and check in the morning. Let us know how it does.

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Thank you. Ok I will give it a try as I had changed the oil last time when the car was cold, just parked in the garage for a few days, sounds a bit odd to me but im new to the nsx game and here to learn.

Im not revving the car a lot as yet, as the car is a JDM 1991 I ordered a new gearbox case half to replace the one on the car in an effort to avoid snap ring failure. Want to take all precautions in moving forward.

I think it most likely lines up with what Honcho describes which is mostly a measurement problem.

The primary way fuel would get into the oil is via a leaking injector during shut down and presumably the injector service rules out leaking injectors. When the engine is running the fuel mix would have to be massively rich for liquid fuel to survive in the combustion chamber and make its way past the rings. Your spark plugs indicate that the fuel mix is not massively rich. They do look a little more tan than I am used to seeing (almost no color) which could be a feature of the ECU tune for the JDM market or perhaps a spark plug heat range issue.

If you want to try and confirm whether your oil level is increasing I suggest that you carry out a series of measurements over a period of days. Always do the measurement first thing before starting the car after the car has sat overnight which allows for more complete drain back. It can help to take a photo of the dipstick against a ruler so that you have a quantitative record that does not rely on memory. You have to be quick about that because the oil level on the dip stick changes pretty quickly once extracted. If that oil level measurement increases over time then you might have an interesting problem.

What oil viscosity are you using? Oils with a higher viscosity may have higher surface tension that causes the oil to climb up the dip stick particularly when cold making it look like you have an over fill. This can also be a hot / cold issue which is why measuring under identical conditions is important.

As an observation, the dip stick is not a very good tool for monitoring small changes in oil level. I use it mainly to confirm that there is oil in the engine and it is at or near the fill line which indicates no major change. Don't obsess about where 'at or near' is.

@Old Guy

I did notice that there is a bit of a tan color on the injector. I don’t expect that for a car that’s rarely driven, so maybe if could be preliminary to build up seen on cars that were running rich and driven a lot?

The plug that I pulled are Iridium BKR6EIX-11.
Is this plug out of the heat range?

Oil being use : amsoil synthetic 10w-30.

If these plugs are out of range I can change them out, do an oil change then monitor the level quantitatively as you described.

I did notice that there is a bit of a tan color on the injector.

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I assume you mean spark plugs, not injector. Those are the correct plug / heat range for the car.

Your oil selection should be just fine.

The color on the plugs is interesting. During start up from cold the ECU is running open loop and will run with a slightly rich fuel mixture which typically tapers until the engine reaches a temperature where the ECU goes into closed loop operation in which case the AFR averages out at 14.7 (bounces up and down around that point). If you are doing a lot of start ups then that could cause some color on the plugs.

When I replaced the plugs on my 2000 after 8 years (but only about 12,000 miles), I set a new plug next to a used plug and had a 'why I am I doing this moment' because they looked almost identical. However, my car is a US market OBDII compliant car. Your car is JDM and it may have a different fuel map in the ECU and what you are seeing may be perfectly normal for a JDM car. You would need somebody with a JDM car to advise as to what might be normal.

The plug insulator color is not a huge issue. For the present, I would focus more on the oil level issue which I expect is a measurement error problem rather than a real problem. Sort the oil level issue and then circle around to determine if you really have a fuel mixture issue.