duthehustle93

I recently installed an FC oil cooler in my '97 NA Miata and ended up having low oil pressure. My oil pressure is fine when cold or when <2k RPMs, but when the oil is at operating temp and RPMs are >3k the oil pressure refuses to go above 32PSI (it was 45-50PSI). It's clearly a restriction issue since it becomes a problem when flow is at it's greatest, when warmed up at higher RPMs. It's different from the pressure drop effect caused by a long hose, since the pressure remains the same when flow is lower, it's more of a "choked flow" sorta thing where it's capped at a certain flow limit by a restrictive orifice.

Long story short, after a couple days of trying multiple things and diagnosing, I decided to bypass the oil cooler in my setup and I got my oil pressure back. Therefore, there's some sort of restriction in either the banjo bolts or the cooler itself. The cooler seems to be flowing as both end tanks and the fins get hot, and it's doing a great job at cooling my oil (I have an oil temp gauge). This is where I'm confused, because I'm thinking it's a blockage but if there was a blockage it also wouldn't be cooling. I believe the two most likely scenarios is that FC oil coolers aren't designed to flow as much oil as a miata, or there is a partial restriction somewhere. I'm not sure if there's anything common in terms or these coolers as far as restriction, any input is greatly appreciated.

Thanks!

Honaker510

what weight oil are you using and are you accommodating for the oil cooler needing more oil? maybe its getting trapped in the cooler and the return hose is clogged? just a couple ideas if you haven't thought of them yet lol make sure if you used Teflon tape or anything it could block those holes easily as well

WANKfactor

iTrader: (1)

Check the cooler isnt plumbed backwards. The tubes are filled with "turbulator strips" that will bunch up in your end tank if flow is reversed. I believe the thermo bypass pellet still works in revers but could be worth popping that out and check it by bringing it to boil in a saucepan of water and a thermometer. Should open at 65c.
Other than that like you say could be a kinked line, crappy fittings, some junk lodged in there somewhere.

Pete_89T2

iTrader: (4)

OP - Did your '97 NA Miata engine come with an engine oil cooler from the factory? I'm going to guess no, or if it did, it would have been a much smaller volume/less restrictive flow rate cooler as compared to an FC cooler. Think about what your modification just did - the FC cooler, plus its associated plumbing, has added at least a quart or so more oil capacity to the lube system's pressurized volume. Could well be that you've exceeded the ability of the engine's oil pump to pressurize & maintain pressure on that greater volume. So what you're seeing with lower oil pressure is to be expected, and not good. Recommend removing the mod ASAP and bring it back to the way it was.

Also just curious, what were the measured oil temps before the mod? Were they so high that you thought an oil cooling mod was really necessary? Rotary engines require oil coolers by design because the oil cooling circuit dissipates about 25~33% of the engine's excess heat (i.e., the water cooling passages in the housings & irons don't do much to cool the rotors deep inside, so oil cooling does the job). Rotaries also have oil pumps that are designed to flow larger volumes of oil at higher pressures than the typical piston engine counterpart. Piston engines don't have the same oil cooling needs (i.e., pistons are pretty much surrounded by water cooling jackets in the block/heads), so oil cooling is usually not necessary, and they can get away with using smaller capacity oil pumps.

duthehustle93

Honaker: I'm using factory weight and yes I've topped off the oil. No I didn't use any teflon tape on the AN fittings...

Wankfactor: Yep, I made sure it's plumbed in the correct direction. The fittings I got are from a reputable retailer and there aren't any lines kinked, but it's hard to say if theres something in the cooler causing a blockage... I have verified that the thermostat works before installing by placing it in an oven at 170F, and it did in fact actuate as it should.

Pete: Nope, they only come with a coolant<-->oil cooler that is more for heating up than anything. As much as I hate to admit it you could be right, it makes sense, but the only thing that's puzzling me is that it's fairly common for turbo miata guys to install FC oil coolers, and I only found one other person complaining about low oil pressure which was compensated with an oversized filter (which I've already tried doing). If it was due to extra plumbing, and the extra length that oil has to travel through the cooler, wouldn't the pressure drop be observed at lower RPMs and when cold as well? As I mentioned, it seems to be a choked flow sort of thing with the limiting factor being a small orifice, AFAIK usually the pressure drop caused by lets say a longer hose would be observed across all flow rates.

I was seeing oil temps over 300F at the track, so it was definitely necessary, but I can definitely get away with a smaller cooler. I'm currently running this one because I got it for free from a friend.

Thanks for all the replies!

arghx

iTrader: (3)

Just FYI, oil to coolant coolers can still make a big difference in oil temps vs no cooling at all under severe conditions. It's like the difference between being 230F and 270F.

Try bypassing the stock cooler and see how hot it gets when you drive it hard.

duthehustle93

arghx: I've never run it without the stock cooler but regardless it's not doing enough since I'll see over 300F at the track.

Just an update: Pete was correct in that in being the oil cooler itself being oversized for my oil pump. Today I removed the thermostat on the cooler and did two runs: one with the thermostat out and the bypass hole open, and another with the bypass hole plugged (using a bolt/nylock nut). With the bypass hole open oil pressure is fine, but when it's plugged and oil is forced to go only through the cooler, oil pressure drops. The cooler temps are roughly the same on each end when the car isn't moving which means that it's unlikely that there is a blockage in the cooler. Anyways, gonna just run it with the bypass hole open and thermostat out until I get a more appropriately sized oil cooler, at least this way I'm not running on low oil pressure and there will be some cooling since some of the oil still passes through the cooler.