Fuel dilution of oil?
 

Back when I was getting my car running, a combination of constant flooding (probably due to a loose main ground) and leaky injectors (cleaned and benchmarked now) meant my oil got diluted very quickly, so I changed it.

I arrived for a second track day (the first one was in the rain so there was very little WOT time) after only a few hundred miles of driving, and discovered the oil was thin again. It just drips straight off the dipstick like it was just slightly thickened fuel.

According to an *ancient* thread I found, doing an oil-only change on an FD with dual oil coolers only replaces 65% of the oil. So I'm hoping it's just leftover diluted oil.

Is it dangerous to do a track day with diluted oil, especially if you're already using thinner 10w30? I used low boost and shifted at 6000 as a precaution, and the engine performed just fine (had good oil pressure and low oil temperatures... almost too low). Or is diluted oil just something that happens to FDs? Or should you change it if it gets thin?

Would I be better off using 20w50 (or maybe even something like 10w60) if I'm only doing track days?

Thicker oil would be a bandaid, I had this issue when I had out of spec side housings. Blow by is going to happen, when the crankcase is getting compressed it will probably also fill up a catch can in a hurry. If you don't have a catch can it's blowing it all into the intake.

I've also had excessive heat from too large an exhaust port where it scavenged too much and the hot exhaust getting back into the compression stroke and getting too much heat on apex seals, collapsing apex seal springs and allowing loss of compression which creates a very rich mix as the fuel needs are less as the compression drops.

Have you done a compression test? Are you running enough premix and or an OMP?

I drained my catch can and it hardly had anything in it.

I suspect it's just left-over from when I was trying to get my car to run.

Haven't done compression test, but it pulls hard.

I'm only using the OMP.

Not sure if it's available where you're at, but I've had good success with Shell Rotella T6 with regard to standing up to oil dilution. Even though, I still change the oil every other track day or 1,000 miles whichever comes first....it's been years since I made 1,000 miles..haha....errr I should drive the car on the street more.

TL;DR it's a good oil that's cheap (at least here) so I have no reservations about changing it often as that's always going to be the best bet against dilution. I change the filter every other oil change.

All rotaries suffer from some degree of blowby and resulting oil dilution. The biggest culprit is usually loose or worn side seals. If your motor is solid, eliminating the stock pcv valve and running a vented catch can will help. Also going to a thicker oil is a good idea. I recommend 20w50 in any rotary that sees aggressive driving and the track as well as 2000 mile max oil change intervals or after every track event. You should change the filter every oil change. Its cheap. The oil coolers themselves can be drained by popping off the lines or unscrewing the thermostat.

I have a vented catch can.

So the engine will be OK so long as the oil is changed enough to not get *too* thin?

Maybe the fuel sits on top and makes the oil on the dipstick look thinner than it actually is...

i get the same thing. i run 20W50 for street and on the track. my oil level will actually go up over time, but this is with premixing. when it gets so high i know its time for an oil change lol. your oil level probably doesnt go up because of the omp

I think my oil level may have gone up, but I'm not sure what level it was when I changed the oil.

Maybe I should drain the oil coolers just once to get rid of the tainted gas. At least that way I'll know for sure if my oil is still being contaminated.

But for the record, if you use 20/50, will the engine still be safe so long as you haven't burned a bunch of oil and had it replaced with fuel?

I mean, if you filled the oil pan with oil and then poured in fuel until the sump overflows, the oil should still lubricate the engine sufficiently, no?

If it's still super thin due to dillution you could also just do another oil change with the cheapest oil you can find, run it a few minutes then change it again with your normal oil. That should get most of the fuel out.

A thicker oil such as 15w40 or 20w50 provides superior protection to the bearings and irons. Mazda, as many other manufacturers do, recommended the thinnest oil they could get away with to reduce friction and parasitic losses and get that last fraction of fuel economy. This doesn't mean its the best for long term protection. In the real world you won't notice a difference. For the rx8 Mazda recommends 5w20. Even with larger oil coolers than the fd, the rx8s exhibit significantly more wear to the bearings and irons compared to a rx7 of similar mileage. Unless you live in a very cold climate, I never recommend 10w30 in any rotary. It sounds like you have a loose clearance motor or are running too rich. What are your afrs at idle, cruise, and wot?

Beat me to it re: his tune. When I was on the stock ECU it was also worse in cool weather during longer rich warm-up. PFC has helped that.

It seems everyone agrees that a thicker oil is "better" overall and thus, for an engine suffering fuel dilution problems, the heavier weight oil provides an additional margin of protection but...in Valkyrie's case, i.e. running hard at the track, is it even recommended he do so...like, at all?
Assuming of course that he DOES have an ongoing problem and wasn't just a one-time thing as described in the o.p.
I know that my engine suffered severe fuel dilution issues prior to being rebuilt due to leaking injectors...my oil level would actually INCREASE between oil changes! So I got to where I was changing the oil at 1500 - 2000 mile intervals.

I suspect I have poor compression or bad side seals on my track car which is also filling the catch in a hurry and was filling the oil with gas to the point of it literally raising the level on the dip stick considerably in one track session.

Ray (PFS) tuned the car to inject some gas while off throttle because I don't have an OMP and he wanted the apex seals to cool more. That's a great idea but this was the main reason I was filling the oil pan. Once I adjusted the PFC to zero or close to zero for fuel injection off throttle the pan stopped filling with gas. That said I still fill up a catch can in a hurry but I'm hoping it's related to other things but maintaining my skepticism because it also smokes heavily under idle. We will see next time I'm at the track.

My car has more issues than I can count on my hands so it will be a long winter LOL


If the oil pan isn't filling you don't have a serious blow by issue :icon_tup:

It's cold but I'm not going to be driving it in the winter.

No clue on AFRs... I don't have an AFR gauge. It was dyno tuned with a PFC, though.

I'm debating buying a 20L can of 20w50 dino juice or trying some 15w50 synthetic.

I may have actually filled it with 20w50 to begin with. I don't remember now. It wasn't a synthetic though.