Datsun Roadster Forum-311s.org

Greetings gang, had another question... the past couple times out driving I've noted that my oil pressure gauge is reading down near the zero at high sustained rpm (above 4000). I know the early (pre- '68) electric senders tend to read low at idle but actually have 7-8lbs and mine has always read this way, but it jumps to between the 0 and 40 quickly.... until I start hitting high speed. 1) I'm not sure I trust this sender as the outside canister part seems to move some around the base. I have not tested it vs a mechanical gauge yet. Thoughts? 2) If I am in fact losing oil pressure, what would cause this at high speed? I run 20/50 oil, 7 quart pan, Hastings filter. The pump was disassembled a few years ago and found to be in good shape. Chain tensioner is shimmed of course. 3) the senders are not cheap. I tried a 240z one in the past and it was way wrong... are there alternatives? My plan is to see if it goes low at 4000 rpms no load and then install a mechanical one temporarily for comparison if it does. I also understand somewhat that the pressure is mitigated by a spring inside the pump? I will check my manual but putting it out there to those with more knowledge...
Thanks as always.

Standard Motor Products PS 202, exact replacement, made in Japan usually. Mine are.

You run 20w50 in winter ? Why.

Oil pressure in my U20 seems like a on/off switch most of the time. Shows practically nothing at idle and about 60 when the Rpms are up. Not very reliable before and after rebuild. Hi windshield roadster.

Thank you Curtis for the info, I will see about securing one.
The reason why the motor has 20/50 oil in it is because I typically don't drive it in the winter (no heater). However, this year I added seat warmers and they make a huge difference so we've been taking advantage of the sunny but cold days when we can. We drove up to a ski resort on Thursday but unfortunately the parking lot was cleared of snow, so no hand brake emergency "Bat-turn" practice...

Can anyone answer the question as to what might cause the pressure to drop at high RPM, if I confirm that this is actually happening?

Maybe just the vibration of it makes the sensor short itself or maybe the actual oil pressure gets to a certain level and that makes it fail? Not sure how you would test either of those scenarios though.

Install a quality mechanical gauge and see what really happens.

Stan
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Gregs672000

Take a look at Engine masters S3:E3, How Oil Kills Power. They talk about too much oil will kill pressure at
higher rpms. Hope this helps.

Nissantoy

Thank you Nissantoy, looks like I would need to subscribe or wait until it's on Motor Trend next month. I've seen windage or crank oil "scrapers" before for other motors, and I think someone made one for a Roadster, what are thoughts on that (not specifically about my problem but in general)?
I've ordered a new sender that should arrive next week.

Is there anyone else out there, particularly those who run their engines hard, who have either noted changes on their gauges with the same sensors (didn't later years go with a mechanical gauge?) OR actually confirmed oil pressure changes at high RPM?

Thanks all.

So I purchased and installed a new Standard Products PS202. Only difference was it came with a spade connector. Cut off the old connector and crimped on a spade. Fired it up and it read even lower, barely breaking past the zero line. Great, new part, worse reading. Went to the autoparts store and purchased a mechanical gauge. It reads 40 psi at idle, and 75psi at 5000rpm. So, no low oil problem, but why does my stock gauge read so low? I assume the gauge and the sender work in unison and must be matched, but these should be the correct parts. Is there something I can check, clean or adjust to get my electric gauge to reflect reality?

I was just thinking... I tried a sender for a 240z awhile back, and I think I thought it was off because it read so high. Now that I know from the mechanical gauge that it does have somewhere around 35-40psi at idle, maybe I'll try that sender again (hope my memory is correct!)

Is there some way to test these things? I was thinking I could rig up a rubber tube to the end and put some air pressure to it, then measure what, ohms? Me not so smart when it comes electrical...

First maybe test the sensor: Measure resistance from the connector to the block with the engine off, then again with it running at 2000 RPM or something.
Record these two values. Maybe tomorrow I'll go check my values. It might be something like:
0-2 PSI - 75 Ohms min
5.5 PSI - 44-60 Ohms
30 PSI - 27-41 Ohms
80 PSI - 7-20 Ohms

If the sender checks out then it's the gauge.
To test the gauge you could get a resistor and wire through the gauge (battery-resistor-gauge-battery).
If you can find the resistance value that it is supposed to be then it should have the correct reading....

I run both the original gauge and a mechanical (more reliable) gauge now under the dash.

Good luck
Rustbucket

Ok, so I confirmed the 240z sender is incorrect... pressure went to nearly 90psi so I know it doesn't match the gauge. So for now I have two PS202 senders that don't seem to read correctly vs the mechanical gauge. I've always known the gauge was more of an idiot light, but what started all this was very low pressure indicated at high rpm (freeway driving). So, for now I'll test things with the mechanical gauge to see what's what... as soon as we can go driving again!

Gregs672000

The show I referred to Engine Masters S3:E3 is playing tonight on Motortrend TV @ 9:30, hope you can get
this.

Nissantoy

Just to finalize this, the mechanical gauge showed no pressure drop at high rpms on the freeway yesterday, so it's clearly a sender or gauge problem. Once warm, it had about 30lbs at idle and would hold 65lbs or so otherwise. I went ahead and reinstalled the original sender, have yet to try it but I won't be worried about lower pressure readings.

I also was able to install a new temp sender as well (different post) and found my engine is actually running much cooler than the old sensor told me.