I happened to noticed fresh oil on the backside of my engine ( 98 Mazda 626 GF 2L ), right under the oil filter cooler recently, right after starting up the vehicle after not driving it for a few weeks:

enter image description here

Looking around, I saw what looks like oil leaking from inside the knock sensor, which is above and left of center to the oil filter / cooler assembly:

enter image description here

I'm wondering if that fresh oil below the filter is attributable to the knock sensor leak, and how exactly could the knock sensor be leaking, and lastly how to fix the leak. I assume that screw on the sensor is unscrewed to release the connector wire, and then the whole sensor is removed with a socket and ratchet.

Can I fix this, or do I have to just replace the sensor? How is the sensor built that it would leak like that from it's center?

I've never seen oil on the ground below the car.


According to a link provided by @HandyHowie it doesn't seem like it's the knock sensor. If I zoom in on the OPSU in the first picture, it looks like oil is leaking from it also:

enter image description here

However, I don't think that would account for fresh oil to the bottom right of the oil filter cooler, since the OPSU is to it's left. There had previously been a serious leak from the valve cover gasket, which had turned to stone and was basically almost not sealing at all, but I replaced that a few months ago.

So that leaves me with the possibility that the oil is actually coming from the base of the oil filter cooler it self? Has anyone ever heard of something like that?

EDIT May 26th, 2016

I'm going to take FredWilson's advice and just replace the oil cooler seal as it looks pretty simple. Here's the instructions from the WSM, maybe it'll help someone:

enter image description here

  • 3
    I couldn't understand why a knock sensor would thread into an oil way. This link - mazda626.net/topic/43390-what-is-this explains that it is not engine oil. I think the oil is leaking from somewhere else.
    – HandyHowie
    Commented May 25, 2016 at 12:03
  • @HandyHowie Thanks for that link! I modified my question. Commented May 25, 2016 at 16:36

2 Answers 2


The oil cooler design usually has an O ring seal at its base and leaks are common. That seal is likely as hardened as the valve cover gasket was.

The oil pressure sensor (OPSU) leaking through the sensor is also common. This one can get quite large.

The leak at the Knock Sensor is odd. Never seen that before.

If you want to know for sure what is leaking and what is not. Clean the area thoroughly. Then spay some powder on it; I have used foot powder. Then run the engine for as long as it takes for the leaks to return. The powder will show the leaks.

  • Apparently, according to the link provided by HandyHowie that's not oil, it's a gasket that's part of the sensor and has kind of dissolved and turned gooey over time. Regarding the potential oil cooler leak, is this one of those thing that I can just observe my oil level and safely ignore it if there isn't noticeable oil loss? I've never seen oil on the ground under the car. Regarding the OPSU, it sounds like you're saying it could easily turn into a serious leak and that I should go ahead and replace it, correct? Commented May 26, 2016 at 10:35
  • @RobertS.Barnes The oil cooler seal is cheap and easy to replace. As is the OPSU. So I would replace them both. One can always monitor leaks with oil level. As a professional tech I never recommend this method because my customers will forget to check and fill. Commented May 26, 2016 at 16:44

I actually had an oil pressure sender leak in a similar fashion. Not from a leak around the sensor thread, but actually through the sensor itself.

Your question has a simple answer, replace the sensor.

The internals of the sensor has degraded due to pressure/temperature/age or whatever enough so that the sensor is no longer sealing the inside of the engine from the outside. The sensor is not a solid lump of metal, it contains components housed inside some other material, possibly ceramic and that has failed. It's really that simple.

Just replace the sensor.

  • It is not an oil pressure sensor.
    – HandyHowie
    Commented May 25, 2016 at 14:09
  • Not saying it is an oil pressure sensor, Im saying that sensors like these are similar in the fact that they can degrade and fail to provide an adequate seal, no matter what they measure. Commented May 25, 2016 at 14:13
  • 1
    Why would a knock sensor screw into an oil gallery?
    – HandyHowie
    Commented May 25, 2016 at 14:24
  • Good point, perhaps OP has assumed incorrectly it is a knock sensor. It is after all installed right near the oil filter. Commented May 26, 2016 at 1:17

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