The oil pressure sender has been reported by many people to fail. This typically shows up as low oil pressure, but could also show no pressure or high pressure.
Mine failed at 8K miles, and that is sadly not that unusual. Several people have reported going through 2 or 3 of the senders by 50K miles.
This should cause no other problems, but could be a risk in terms of a person getting used to it showing low and not paying attention to the oil level. Since the rotary injects oil as part of normal operation, it consumes oil. When the gauge works properly, it is easy to note small changes in level by looking at the pressure. If the gauge isn't working properly, then it may not be easy to identify these changes in level.
So if you choose to live with a bad pressure sender, check the dipstick REALLY often, at least once a week.
PLEASE NOTE: I am not recommending that you do not fix the problem. It is possible that something else has failed and is truly causing low pressure. No way of knowing that unless you know you have a good sender.
The sender could be OK, and you could have a serious problem such as a bad oil metering pump.
--Steve June 07, 2000
Mine went from showing ~90 - 100 PSI (normal) on the way to get an oil change to showing maybe 50 PSI on the drive home. After checking the level on the dipstick, I decided the sender had just went bad. I even tried adding a half quart extra to see if it made a difference. Nope.
My car has 8K miles on it, so I am guessing it is the sender. I really need to get it replaced to make sure it is a simple sender problem.
--Steve June 07, 2000
Date: Tue, 30 Jan 2001 19:14:12
From: "Mike Putnam" (firstname.lastname@example.org)
The oil pressure sending unit is notoriously unreliable typically lasting 2-4 years before going on the fritz. There appears to be three basic failure modes:
>It got me started on and idea of a better replacement. I had an old >van that had an oil pressure sensor that lasted through 2 motors >(200,000 miles) with no sign of failure. Couldn't there be a replacement >that is less likely to fail?
One would think so. Derek Vanditmars and I are looking at various options which would retain the stock gauge ala our temperature gauge modification. There are several technical hurtles to over come. First, the 3rd gen has a very high normal oil pressure. Most stock oil pressure gauges on cars peak out at 90-100 psi, the third gen goes to 120. Second, the oil pressure sensor is located in a bad spot for logevity (lots of heat, vibration and spilled oil when the filter is replaced). And finally, there is no such thing as a standard oil sensor. >Something that will work with the same resistance(as stock) for >stock gauge to read correctly? There has to be something. And while >I was on that thought, I remembered the gauge hit zero psi with no >lights, buzzers, or any other type of warning that your engine will >shortly be toasted. Who always watches their oil pressure under full >boost? The red light in the oil pressure gauge should go off with no >pressure. Why not remove the oil level warning and make it oil >pressure >warning? Before I start in on this little >project, has anyone tackled these subjects yet?
After about a year's worth of searching I think I can confidently state that, there is no sensor that is an exact replacement of the existing sensor although, you can have one custom made. But, I am now starting to think that this really isn't a sensor problem but an environmental problem. Here is what I base my current thinking on. 1994 and earlier Miatas have the same sensor type as the third gen. Well, they look the same and are probably from the same supplier (the gauges are also of similar construction and are from the same supplier). The resistances and range of the Miata oil sensor is different of course. Now my Miata friends tell me that they don't know of anyone who has had an oil sensor malfunction with many Miatas pushing 100,000 miles on the odometer. Hummm, same design same manufacture, and no failures.
So, here are three approaches currently in the works:
Now, the biggest drawback to all these approaches is that there is no way to test their reliability other than trying each one on a dozen cars and reporting the results in 5 years. So, there you have it. If you have other ideas or approaches let me know.
Date: Tue, 6 Jun 2000 07:57:04 -0500
From: "Westbrook, Chuck E." (CWestbrook@tmh.tmc.edu)
Subject: RE: (rx7)  Excessively high oil pressure. Help
> Depending on how many miles are on the car, if they are high, I would > look first at the sending unit. But there is something that some people > don't know about. It's called a thermal pellet, in the front of the > engine. After so many miles, it may become bad and not allow proper > flow.
I thought the purpose of the thermal pellet was to bleed off pressure when the engine was cold, and when it failed pressure would be low when the engine warmed up.
Who knows for sure about this?
Date: Wed, 7 Jun 2000 19:02:02 -0700 (PDT)
From: RETed (email@example.com)
The oil bypasses the rotor bearings (well, very little gets to the rotor bearings) when cold.
When the front eccentric shaft thermo pellet fails, it fails in this bypass mode, and your rotor bearings see very little oil...
Date: Tue, 6 Jun 2000 08:27:14 -0700
From: "Ehrlich, Daniel " (firstname.lastname@example.org)
Subject: RE: (rx7)  Excessively high oil pressure. Help
Changing the oil and filter was the first thing I did. I have spoken with several speed shops including Tripoint Engineering, Mazdatrix, and PFS and none of them have ever experienced this problem either.
I have had some intermitent problems with my factory oil pressure sending unit reading low as others have in the past but have never heard of it reading high. My mechanical oil pressure gauge also shows high pressure.
Today I am going to check my mechanical gauge calibration on a dead weight tester to make sure that it is accurate.
It seems unlikely to me also that either of the oil bypass regulators have gotten stuck but I haven't been able to find anything else so far. Another thing that makes a stuck bypass seem unlikely is that idle oil pressure is also higher than normal. I wouldn't expect a stuck bypass valve to effect idle oil pressure unless for some reason Mazda designed the system to bypass at idle.
Someone on the list mentioned something about what they called the "thermal pellet" which somehow regulates oil pressure. Do you know what this is, how it works, and where exactly it is located? I'm running out of ideas and will have to drop the oil pan soon to check the bypass valve and we all know what a pain that is.
Date: Tue, 6 Jun 2000 23:41:09 -0500
From: "Robert Anderson" (email@example.com)
I thought it purpose is to allow the engine to warm up faster by limiting oil going to the oil coolers. I didn't think 3rd gens had them, it is not in the lubrication section in the manual. It is in the overhaul section, C-59. In the 2nd gen manual the inspection is in the lubrication section. I've been meaning to replace this in my '90 with the plug Mazdatrix sells. Do we need to replace this in our 3rd gens as well? I've not heard much about them failing in 3rd gens.
Date: Wed, 07 Jun 2000 18:57:46 -0500
From: marc (firstname.lastname@example.org)
>I thought the purpose of the thermal pellet was >to bleed off pressure when the engine was cold, >and when it failed pressure would be low when >the engine warmed up.
OK, it seems you have confirmed that it is not the gauge/sensors as you have 2 units (as do I).
You are possibly right about the eccentric shaft oil bypass valve (or "thermo-wax pellet cylinder" as some call it). It's function is to bypass the oil which would normally flow throught the rotor cooling jets when the engine is cold. Rotor temperature is not necessarily a function of oil temp.
If oil temp stays low due to cold weather, thick old oil, no oil cooler bypass system, high oil capacity, etc, the rotors can overheat, damaging the oil seal o rings, and that leads to big time engine damage. This cylindrical device holds back oil from the rotors so that they heat up fast (giving higher than normal initial oil pressure), then it opens as oil temp rises (I think 158F if memory serves me right) - it's there only for lowering start up emissions.
They are easily disabled if you care to do so (as I have done in my aircraft 20B engine build up, and on other 13 engines I have rebuilt). BUT YOU CANNOT LEAVE THE BYPASS OUT, as that will do as much damage as a malfunctioning unit. Racing Beat and Mazdatrix I think sell the pellet mod, but it is easy to do.
If you need instruction on how to disable the thermo-wax pellet, I have 2 methods I have used (one from Bruce Terrentine's excellent 13B rebuild video, the second edition), both are satisfactory. Let me know if you need one or both.