Hey everyone, I'm having pretty severe fuel dilution into my oil. Somewhere around 1000 miles would probably put 1/2 of a quart of fuel into my oil. The dipstick would fill up and smell strongly of fuel.

I'm trying to diagnose if it's coming from leaky injectors, blow-by, rich fueling, or crankcase ventilation.

The car is a 94 rx7 with a street port, bnr twins, power fc, and stock injectors for a bit longer.
The car fires up right away whether it's cold or hot. Although when cold it blows a bit of excess blue/black smoke for a minute or so. Rest of the time I'm driving it doesn't blow any excess smoke. Everything drives smoothly and runs excellent.

My compression is around 100-105 psi across all of my faces.
I cut my side seals a hair too short when I rebuilt my motor, so my biggest worry is that I'm getting excessive blow-by. Unfortunately I can't remember what my clearances were since that was a couple years ago (this dilution started this year).
I normally associate dilution caused from blow-by with really low compression numbers. Do you think my numbers could rule out blow-by, or could go either way?

For a quick injector test, I put in a fuel pressure gauge where the line comes up from the firewall. It idles cold around 30-35psi, then drops all pressure as soon as I turn the engine off. I pinched the lines as I shut the engine off and it held pressure that time(dropped 10psi in 1.5hrs), so probably not a significant injector leak after shut off. I figure my next step will be to take out my injectors and have them tested, but I won't have time to get to that right away.

My tune was a bit rich on the rich side (13-14AFRS cruising--low 10s WOT-- and12 Idling). Is that enough to cause that much dilution?
I finally got FC tweak dialing it in and it's running about(14.5AFR cruise--mid 10s AFR WOT-- and13 Idling). I haven't gotten too many miles on this yet, but so far it's looking like the oil level is still rising.

My crankcase ventilation system is just the oil neck port > sealed catch can > pre turbo inlet. I don't have a PCV valve.

Lastly, I primarily drive the car on the hwy. Trips are usually 20 min each way to town, or long highway cruises several hundred miles. Hardly any short drives cold around town.

Any thoughts on other tests I can take, or if I can rule any of my suspects out? Thanks!!

 

Oil should only be filled to halfway up the hashmarks on the dipstick. Interesting you have catch can inserted between the filler neck and the intake elbow. Is the catch can filling up with oil?

If you overfill with oil, you could be sucking crankcase oil into the front turbo and hence into your UIM.

Check the cross pipe from the y-pipe to see if there is oil residue there.

Is there any oil in your coolant, or coolant in your exhaust?

 

But it more likely your side seals are leaking, eh?


https://cimg2.ibsrv.net/gimg/www.rx7...63ffa59de6.jpg

 

You can rule out the injectors by getting them cleaned and benchmarked.

 

From what you shared, here's my thoughts:

- Compression isn't ideal but it's perfectly acceptable; if your side seals were fitted a bit short, it will reduce the compression a bit on the rotor faces with the shorter side seals, and will increase blowby. That's likely why you're getting the blue/black smoke on cold starts. Was the cold start smoking symptom always like this since the rebuild a few years ago, or something that started recently?

- Based on your injector test results, I'd still be concerned with one or more leaky injectors, and would advise sending them out for servicing. The 1st test result where you didn't pinch off the return & feed hoses after shutting the fuel pump are normal results - pressure should drop to zero fairly quickly since pressure just vents right back to the tank via the fuel lines. But in the 2nd test result, assuming that whatever you used to pinch off the lines did its job (100% prevented fuel flow back to the tank from supply & return paths) tells me that one or more injectors is leaking down more than it should. Considering that the size of the injector nozzle is orders of magnitude smaller than the fuel lines themselves, I think it should have held pressure a lot longer than the result you shared (lost 10psi in 1.5 hours).

- Your AFRs at idle are a bit on the rich side, but probably not the root cause of the fuel dilution since it doesn't sound like your FD spends much time at idle from your use case description. Were those AFR numbers posted current, (i.e., monitoring AFRs currently, WBO2 sensor/gauge fitted), or were those numbers taken from when it was tuned after the rebuild? Reason I ask is if the tune were somehow buggered up recently, it might be running a lot richer than you think it is now.

- On this statement "My crankcase ventilation system is just the oil neck port > sealed catch can > pre turbo inlet. I don't have a PCV valve", are you saying that your catch can is completely sealed, no open air venting whatsoever? If so, you need to fix that to allow blowby vapors to escape from the catch can to the atmosphere, OR sucked back into the engine via a PCV valve. Otherwise any blowby gasses have nowhere to go except into the crankcase.

 

Typically this issue has more to do with reusing oil control rings, side housings with some wear or porting that violates the oil seal track.

Cleaned/flow tested injectors, not running excessively rich and letting car warm all the way up will all help decrease the amount of fuel in the oil.

Id have injectors cleaned, get car tuned and send it with 20-50 oil. These engines can handle fuel dilluted oil well withouy any damage in my racing experience.

 

In endurance racing it was one of Mazda's pit crew job to remove excess oil from the engine. The oil became more and more gasoline as the races progress

 

Agree with a lot of the comments above. If you can yanked and have the injectors cleaned and balanced, that would be ideal place to start.

How many miles are on the motor? I typically run a few bottles of Techron Fuel system cleaner through every 1000 miles to keep the system cleaner. Also, I use boost juice/water injection to help clean out the motor.

 

Oh really, I didn't know oil should be filled up halfway on the hashmarks, I've always filled it to the top of those.

From what I read, having a catch can in that placement is pretty standard, or just a catch can from the fill neck vented to atmoshphere.
Do you have a different catch can method you use or recommend?
​​​​​​​The catch can is pretty new as it used to just route from fill neck straight to the pre-turbo intake.But in the last 2-300 miles it built up maybe 2.5oz of liquid in there. It wasn't oil, more of that typical watery mix of stuff that smelled lightly of fuel and oil.

I'm pretty sure i'm not sucking crankcase oil into the intake now since that residue is dropped in the catch can. But it could be getting extra oil vapor in there for sure.
I'll check the cross pipe when i have a moment.

No oil in coolant, smell of burning coolant, or signs of coolant seal issues.

 

Yep that's what I would like to do next. I'm hoping to rule out other things before I have the chance to get to that.

 

Thanks so much for the detailed response!

- Compression isn't ideal but it's perfectly acceptable; if your side seals were fitted a bit short, it will reduce the compression a bit on the rotor faces with the shorter side seals, and will increase blowby. That's likely why you're getting the blue/black smoke on cold starts. Was the cold start smoking symptom always like this since the rebuild a few years ago, or something that started recently?
So I take it than the increased blow-by could be contributing, but likely isn't the main culprit in your opinion? My memory is terrible, I can't remember for sure how it was after the rebuild. But I don't think it smoked as much until just this last year.

- Based on your injector test results, I'd still be concerned with one or more leaky injectors, and would advise sending them out for servicing. The 1st test result where you didn't pinch off the return & feed hoses after shutting the fuel pump are normal results - pressure should drop to zero fairly quickly since pressure just vents right back to the tank via the fuel lines. But in the 2nd test result, assuming that whatever you used to pinch off the lines did its job (100% prevented fuel flow back to the tank from supply & return paths) tells me that one or more injectors is leaking down more than it should. Considering that the size of the injector nozzle is orders of magnitude smaller than the fuel lines themselves, I think it should have held pressure a lot longer than the result you shared (lost 10psi in 1.5 hours).
Ok good to know! If it's leaking a bit then that's something I can look further into. Definitely hard to say if I got a 100% seal. I guess the only way for sure to know is to get them tested, which I do plan on doing as long as something else doesn't solve it.

- Your AFRs at idle are a bit on the rich side, but probably not the root cause of the fuel dilution since it doesn't sound like your FD spends much time at idle from your use case description. Were those AFR numbers posted current, (i.e., monitoring AFRs currently, WBO2 sensor/gauge fitted), or were those numbers taken from when it was tuned after the rebuild? Reason I ask is if the tune were somehow buggered up recently, it might be running a lot richer than you think it is now.
Yep that is correct, I do not spend much time idling. Those AFRs are current, I do have a new wideband installed.

- On this statement "My crankcase ventilation system is just the oil neck port > sealed catch can > pre turbo inlet. I don't have a PCV valve", are you saying that your catch can is completely sealed, no open air venting whatsoever? If so, you need to fix that to allow blowby vapors to escape from the catch can to the atmosphere, OR sucked back into the engine via a PCV valve. Otherwise any blowby gasses have nowhere to go except into the crankcase
Sorry for the confusion there. I just meant that the catch can isn't vented. It's routed from the fill neck, to the can, and then out to the pre-turbo inlet so it can at least get a little bit of vaccuum and help draw some extra vapor out of the crankcase.

Thanks again for all your input!

 

Hmm I got new oil control rings. Side housings were within spec, but old, so possible that's a contributor for sure.

Hopefully now that I got the tune a little more dialed, that might help a little. I pretty much always get the car up to temp whenever I drive it. And I do run 20-50. Certainly looking like injector cleaning and flow testing is my next move.

I'm glad to hear that these engines run pretty well with dilluted oil. They much really pump some fuel in there while racing huh?

 

Yep I definitely want to get the injectors tested and cleaned. Sounds like that's the next step.

The motor has 79k miles on it. Maybe 5-8k since the rebuild. Fuel system cleaner is a great idea, I've never run fuel cleaner through it. Boost juice injection would definitely be nice!

 

If you fill it all the way, oil will get sucked into the catch can when you take a fast righthand corner.

From experience, if you accidentially overfill the oil pan, you will end up with a full catch catch very quickly during a track day.

 

Oh yeah, that totally makes sense. I noticed that when the oil dilution caused the oil level to rise, it got sucked right into the intake and smoked out the highway haha!

 

Another note to add. I did do a compression test right after the engine rebuild a few years back, it was right around 100-105. So it has not been losing compression at all.

 

I would maybe try a different catch can vented to atmosphere. The turbo inlet won't really "draw" vacuum the way the UIM would. Use at least a -10 line from the filler to the vented can.

The other is, I'm not sure of your OMP or engine management setup, but if you have a good working oil metering system, and apex seals that don't have special need, check the engine management to see if the decel fuel cut is turned off (and turn it on). I remember a friend having this issue when he tried to run OMP-less and had the fuel not cut on decel for extra lube.

 

I'd add a catch can vent and use a -10 line, or the largest line you can, off of the oil filler neck. With that said- what's your purge control setup- stock?

 

Alot of great advice in here.

OP - We have pretty much identical setups, besides my short block was new. Same catch can setup ( sealed - routed back the the primary elbow with -6 lines ). I even run the stock filler neck with the pcv deleted. I have seen stock injectors bleed down, I always get them cleaned and flow tested before installing. I would agree with having all of them cleaned and tested. It’s a bit of a chore on a twin Turbo car tho. After you get that done, do an oil change and use 20/50. Fill it till it touches the bottom of the stick. Start the car and let it warm up. Top off till it’s 1/4 way up the stick. Drive it 250-500 miles. If your catch can is empty, it was a bleeding down injector, if it has a bunch of oil, then you have to keep digging. I have basically nothing in mine after 2500 miles. Do you have the vapor purge from the fuel tank hooked up with the metal check valve and solenoid? Are you using the OMP or premixing?


~ GW

 

Yep, I know the pre-turbo inlet won't draw vacuum anything like a true vacuum source like in the UIM. But I figured it might be better than venting to atmosphere. If that's not the case, then I'll probably look into getting an aftermarket fill neck with -10AN fitting and go that route. I believe my lines are 3/8" if I recall. So that set up could certainly handle a larger volume of vapor.

My OMP has been deleted and I always run pre-mix. The decel fuel cut is still active, I just have to be careful to not let it cut fuel when driving. But I can totally see how turning off that fuel cut can cause this problem, great advice!

 

I actually haven't ever paid attention, I'll need to look into the purge setup.

And thanks for the input, sounds like a vented catch can with -10 line is the way to go.

 

After rebuilding my engine and setting up a highly simplified parallelled twin set-up, the local tuner shop simply left the drain tube open ended dangling about 12 inches down in the engine bay.

When I pointed this out, they said that it was fine and the minimum oil blow out would help against corrosion of the underbody (somewhat tongue in cheek). I connected the hose to a catch can (I used a simple aftermarket ast) and it now vents to the engine bay. I check from time to time and it appears any oil blowing out is minimal.

When I first got the car, the PO had simply left the filler neck nipple open. Not knowing,I put a cap on it. The engine stalled out pretty quick, teaching me the venting of the oil pan was important.

 

That's good to know! Really sounds like the injectors can be problematic! I have a feeling they haven't been cleaned and tested in a very long time, if ever. Do you usually send them out in the mail for cleaning and testing? I see Injector-Rehab online can do side feed. Not a lot of places seem to.
After the oil change, do you generally leave it at 1/4 up the hashmarks(after warmed up) all the time? Or add a bit more after monitoring it?

I haven't ever looked into the purge system, I'll need to dig into that. And I premix, OMP is deleted.

Thanks so much for the great advice!

 

Be mindful if injectors have not been removed for a while, that the machine screws on the clamps holding them in place might have fused in place. So care is needed in removing, to avoid bigger problems.

 

If you still have the stock FPR check to see if it is still getting vacuum assist. If not it could be adding back pressure to the fuel line and increase any issues with leaking injectors. In the stock set-up the FPR vacuum feed has a solenoid that is set up to close off the vacuum feed in hot-start situations. The solenoid can fail.