Oil pressure increasing while in boost, drops after throttle lift
 

Normal? Or boost pressure getting by the rings?

Neither.

Boost leaking past the rings would not increase oil pressure measurably. The interior of the crankcase is reasonably well ventilated.

And it's not normal, either. Oil pressure tracks RPM, not load.


How bad is the detonation you're experiencing?

None that I know of, car runs extremely well under boost, pulls hard with no misses, stutters, or otherwise.

You can kind of see it if you pay attention, it goes up about 10-15 psi, then once I let off it drops back to around 30 psi

Joe asks about detonation because it is a known phenomenon that detonation can cause real gauges to fluctuate. He is giving you a nice hint, listen to him.

Oil pressure fluctuating during boost. [Archive] - MX-5 Miata Forum

Yeah, I seen that thread after I searched on google. I went ahead and lowered my timing across the board by 4 degrees until I can get some det cans made. I need to re check my base timing as well, that could have an impact.

If you aren't running any boost do you still get the rise in pressure? I idle at 30psi and am at 60psi+ at revlimit.

Yeah if I just rev it out at say 20% throttle the pressure will rise to around 45-50. Cold starts it sees 60psi. I'm still running straight 30w in it though. Think that may have something to do with it?

Sure looks like normal oil pressure rise with RPM to me. All Miatas (with real OPGs) do this. Not related to boost.

Of course, the tach wasn't in the picture. So I was just going off sound.

Yeah tach doesn't work anyway, need to get that resistor installed. It just seems like it's related since, when I lift off throttle the oil pressure drops after about a second by 10-15 psi, while still in gear. Will have to get an aftermarket gauge just to verify I'm not chasing a bad sender

That was going to be my next guess.

In short: the thing you are describing (with RPM constant, oil pressure goes up and down by a large amount according to throttle) is impossible. I don't mean "impossible" in the sense of "wow, that's really improbable," I mean there's actually no chance at all of it actually happening.

So the question becomes why your gauge is telling you that it is.

I can think of a few wildly improbable but theoretically possible causes.

- Oil level is low enough that at WOT the weight transfer starts to starve the pickup.

- Some kind of crazy resonance between engine vibrations and the pressure relief valve.

- Throttle-based control of VVT cam with VVT actuator mechanism that's mechanically damaged such that it represents much lower resistance to flow in some positions than others.

I'm not actually proposing any of these in this case, just taking issue with the word "impossible". I agree it's more likely to be an instrumentation error. :)

--Ian

In the B engine, the pickup is roughly centered within the square sump portion of the oil pan. Acceleration on flat ground, with an engine that's low on oil, would drive oil towards the back of the sump and therefore away from the pickup, not towards it.

And the OP doesn't have VVT.

I'll agree that it's possible the ghost of John Muir may be reaching down and pressing on the oil control plunger a bit when the driver is at WOT, however this seems improbable.

My oil pressure sender was doing something similar when it was fucked. I thought it was like the engine movement tugging on a wire changing the resistance, but instead I determined the oil pressure sender was fucked by using the multi meter. I only really noticed it when I was doing practise starts at prosolo nationals and noticed I had zero oil pressure till like 3k rpms or full throttle but no bad noises or holes in the block.

Yeah, no bad noises or anything. I have thu-sun off so I'm headed to radio shack as well as a parts supplier to grab an extra sending unit. Hopefully its something silly like the sending unit.

Oil was about 1/2 quart low. Got the old sending unit off and tried installing my aftermarket one, figures the thread is npt not bspt. Will have to get an adapter tomorrow.

Even with the damn adapter the starter bracket is in the way. The sending unit can't sit flush enough to thread in. I will have to either get a 45 degree angled one or block that port off and get a oil filter plate. More waiting, yay.

Take off the starter bracket. I have not ran one in years.

Took off bracket, threaded right in. No more oil pressure rise like it was before, so bad sending unit. Runs about 50psi at 6k. Idles around 10psi :confused: Should be a little higher once I get rid of the straight 30w that's in there tomorrow and put 5w40 in it.