Car is a '93 Eunos with manual gearbox. I have started to notice that when the engine is really hot at idle the oil pressure gauge falls to about 1Kg/cm.
Driving around it stays within the 2-4Kg/cm range, but the needle position can be erratic and jumps around a bit.
When cold it behaves as expected and jumps straight to the 4Kg/cm mark and stays there until the engine warms up.
Anyone care to guess if I should be concerned about this low reading, or is possibly the sender unit on the way out?
Mazda don’t specify a pressure at idle but with a fully warmed up engine at 1000rpm the pressure should be 2.0-3.0 kg/cm2 and at 3000rpm it should be 3.0-4.0 kg/cm2. If you’re getting these readings I wouldn’t be too worried about the idle pressure as long as it’s reading something.
The gauge should move around with engine speed but at a constant engine speed it should be stable so it depends on what you mean by erratic as to whether there is a problem or not.
It’s up to you if you trust the gauge on the dash to be accurate, if you really want piece of mind I’d get the pressure checked with a proper workshop oil pressure gauge. As richardn has hinted at, the oil does get thinner as it gets older so this causes the pressure to reduce for any given engine condition.
My 93 is the same, with the idle oil pressure when hot changing according to the mood. 170k mile engine, uses a little oil, but starts on the button, no smoke, no tappet chat, pulls strongly. Two oil pressure readings on the same day, so oil age has nothing to do with this. Its not a “real” oil pressure gauge, in that there is no pickup of oil to a dash gauge, but instead a sender unit thats exposed to all sorts of detritus, connected to the dash by wiring thats 21 years old and probably deteriorated. These aren’t young cars anymore.
In 17 years, I’ve never had a 1.6 MX5 reading at hot idle 2-3 kg/cm^2. My M2-1002 didn’t even bother with a gauge, just a light from the factory. In fact there is an old post of mine from 1997, when I had a 5 year old 40k km roadster, that read 1.8 kg/cm^2 at hot idle.
I’ve just aquired a 248k kilo mk 1 import starts on the button a little tappety for about a minute hot idle 2 k-gm 2 3000rpm around 4 dos’nt use oil runs absolutly fine , get on and enjoy it ,if you want to,change oil and filter every 3k and stop whittling ,lifes to short!
Looks my oil pressure sender is about t die; oil pressure at idle hardly registers now, but the engine is rattle free. This is the inside of a 1.6 sender unit
Lots of moving parts. MX5parts want £100 for this. Used places want £30-40 for 20 year+ used parts that are probably about to go pop. Anyone know if pattern MX5 sender units actually exist. Not oil pressure switch; thats just for the later car.
So there’s a diaphragm that operates a bellcrank and a wiper on a resistance coil to make a variable rheostat and a resistor. The Mazda published Manual says that
0 = 52 Ohms
30 = 41 Ohms
90 = 16 Ohms
Unfortunately the transducers in modern Eurpoean senders seem to use a resistance range of 10 - 180 Ohms for this pressure range although there is an American type with a range of
0 = 240 Ohm
50 = 103 Ohm
100 = 33.5 Ohm
Perhaps there is an electronics wizard in the club who could make a circuit to allow a modern sender to be used.
Just fit ana aftermarket gauge and sender. Whack some thicker oil in for summer and the standard stuff for winter. It doesn’t really matter what the pressure is at idle so long as there is some pressure. I had a rover sd1 which had 5lbs at idle but ran fine. My old 911 had 200,000 miles on the clock and gave 1 bar at idle but 4/5 at 4,000 revs plus. So long as the bearing is getting lubricated it will be ok at idle. Some people- me included - swear by flushing oil. It. Does more good than harm in my opinion. We used to used sewing machine lubricating oil when we were kids - thirty odd years ago because everyone’s parents worked in hosiery in leicester and it was free! Worked fine. Cheap fix too .
