Transient oil temperature concerns

I have doubts that my engine oil temperature sensor is telling me the truth. The reason for my concern is that the oil temperature rate of change, both up and down, seems far too quick for such a fluid.

I'm running a Bosch combo pressure/temperature sensor in an oil filter sandwich plate on a Honda K24 engine. It's specifically placed in a chamber that is post oil cooler (when the oil stat is closed), which is fed from the sump via the pump. So in theory it's oil straight from the sump via the cooler (albeit it travelling through the crankcase).

Below is a rough trace of what's happening - under WOT the oil temperature is increasing as rapidly as the engine speed increases, and then almost instantly recovers to the 'baseline'. In fact I see a 8C increase in oil temp in just 200ms after the throttle is opened. This doesn't feel right to me.

For what it's worth, the sensor is approx. 100mm away from the turbo manifold.

Thoughts?

[img="blob:https://www.hpacademy.com/938e000f-c0ff-494e-9d67-77106683a4a5" alt="" alt="" width="1000" height="1139" ]

I've used hot pink on the top chart as a theory to what my true oil temperature is.. the basis for this is oil should not have faster temperature transients than coolant..

Ok.. some more digging.. this may not be as it seems, correlation between oil temperature and VTEC actuation:

I'm not sure where the oil gets routed beyond the VTEC solenoid valve, but I suspect it'll go back to the sump..

It might be an electrical issue affecting the sensor. Maybe!

And just for shits and giggles, here is oil temperature data excluding data points where VTEC is enabled.....

Ok, I think i've sussed it. The oil temperature sensor has been installed for a while and i've always just look at it from a transfer function perspective, e.g. Deg C, rather than the raw analogue input.

I compared other analogue inputs to see if they were affected by VTEC switching (as in electrical affect/noise), and noticed only one other was impacted. What I realised is that both the oil temperature and other input (cruise control stalk) were operating over a very narrow voltage range.

In fact, oil temperature would show a 9 deg C shift with only a 0.025v change. It appears VTEC actuation is affecting the ground plane by approx 0.020 volts, which is showing a 8-10C shift in oil temperature (depending where you are on the curve).

With the current pull-up on the oil temp sensor, you can see it has low resolution across the range of interest (80-140C):

Changing this pull-up value to a much smaller value (150 ohms in this example) leads to considerably better resolution where it counts, and should be much more immune to noise introduced by the VTEC actuation:

So this is a relatively straight-forward fix (apart from having to take the dash out and do some wiring)..

Moral to the story is.. if the data don't look right, investigate.. it's easy to look at logs/graphs and implicitely trust it.. A clear example of where having at least a basic understanding of how engines work is priceless when coming to tuning!

Excellent detective work there!

I had been thinking it may have been the thermostat cycling, but your discovery is much more likely.

Great job! It's amazing how much noise VTEC puts out. On all the old D and B series stuff I asked people to add a ground strap from the solenoid and it solved a number of issues over the years. How do you have things grounded?

Mike - The wiring is not the strong point on this build, and on the to-do list is to build a complete new wiring loom. At present it's an OEM Honda engine loom, spliced into a Ford Fiesta chassis loom, central junction box, battery junction box, and beyond. I'd actually struggle to answer your question honestly, however generally sensors would be going back to a common sensor ground, as you'd expect.

One thing that did cross my mind is I modified the loom so that the VTEC solenoid would be low-switched rather than highi-switched (to suit my ECU). I stole +12v from the coil feed in the OEM loom and re-used the other solenoid wire to go back to the ECU switched earth.

It's all tightly packed in the loom, and I guess there's a small chance of earth leakage, but fundamentally the pull-up in the ECU is not correct for the operating range of the sensor, and 0.025v is not generally an issue in automotive applications, but it is when you're incorrectly setting up a sensor to be super sensitive.

I use proper Molex perma-seal sealed splices, you can see the modification here in the loom:

However, realistically I suspect even a fresh wiring loom will not cure a 0.025v ground plane shift.

Gotcha. The noise in the signal is what I'd expect to fix, and much of the error with it. Your image shows it getting very erratic when VTEC operates.

It sounds like you know what you need to do, it's just going to be a bit of a project.

Fixing the noise isn't the right approach here; using the sensor correctly is - which requires a change in the ECU pull-up on this analogue input. Easily solved now.