bbundy

Ok now that my high oil pressure after the engine rebuild seems to have settled down. I have another weird issue.

Under a full throttle pull the oil pressure gage will suddenly jerks from 80 psi down to 30 or less. Searches on Miata.net come up that this is caused by knock. The OEM electro mechanical gage it appears will behave like a knock sensor due to how it works, seems plausible any way.

I have plenty of oil and an Accusump on it so I don’t think the pressure drop I’m seeing on the gage is real. Other people with similar symptoms have verified a fully mechanical gage not showing the drop that the stock one does.

I recently added a Deatcshwerks fuel pump and an Areomotive regulator and It seems like it doesn’t always return seem to the same base pressure and I have been fiddling with it. So I thought there might be a possibility of a fuel supply issue.

I hooked up the laptop drove around with it and drove around with a friend watching it and I’m not seeing knock on the knock sensor. It seems to give a small signal indicating it’s working but not knock. I’m not sure if my ears are calibrated to here knock but I don’t hear any. AFR tracks right on target under load a little fat near redline but seems fine.

Any body else have any similar experience with the stock oil pressure gage?

Bob

miatauser884

iTrader: (11)

I have this with my stock gauge. I have searched as well for more data supporting the OP sender operating as a knock sensor, but there isn't a lot.

It happens under full throttle during high boost pulls. I removed some timing and it went away. I added timing back in, and the needle started wildly oscillating from ~60psi to 30psi again. I pulled the timing out again and it went away. I think it is detecting knock, or some resonance change that occurs just before knock occurs.

Unfortunately I didn't have my electronic det cans hooked up a the time.

Keep me updated if you come to any conclusions. I opted for safety.

fooger03

iTrader: (2)

Also in for having this problem, but I run 93 octane on 8.7:1 and still running on a base-map (Read: conservative tune) so I would hope it's not detonation related. Only happens as I approach the rev limiter. Drops from 60 down to 40 for a moment, and then down to 20. I've searched and asked around and haven't gotten an answer.

hustler

iTrader: (6)

The oil pressure gauge in my car is completely whacked out, did the same thing yours did...and as of about 6-months ago it quit working. Every now and then it decides to read 20psi or so when I'm on the track, and if that were the case the motor would have died a long time ago. It's probably time to man up and buy one but that takes effort.

bbundy

I will say one change I did was Energy suspension Diff mounts and a 4 puck un-sprung clutch. There is a lot more driveline vibration. I wonder if it could be related to the extra vibration. I couldn’t take the Goodwin AWR engine mounts so I went back to Mazda Comps. The diff mounts are more buzzy than I hoped especially around 3500 rpm but the engine mounts were just more vibration than I could handle on a regular basis.

I have a set of old Toyo T1-s tires on it just for getting around right now. A third gear pull results in wheel spin at ~4500 rpm. Between 50 and 60 mph I can spin the tires in third gear. It feels impressively quick.

Bob

miatauser884

iTrader: (11)

So what OP gauge do people recommend. Are aftermarket gauges subject to this type of interference? The reason why I would guess that mine isn't knock is because we tuned it on a dyno and didn't hear anything. I would imagine that knock sever enough to make the gauge oscillate wildly would be audible. On the other hand, Why did pulling timing fix my problem?

I just need to get my detcans on and listen.

hustler

iTrader: (6)

My sender is mounted to the body, not the engine.

fooger03

iTrader: (2)

I found that the ground strap running from the dipstick tube to the chassis will cause the OPG to read low, become intermittent, then eventually cease functioning as it progressively loosens, but mine is nice and tight right now.

bbundy

Interesting I don’t have a ground strap to the dipstick tube. I have two grounds going to the head.

Bob

miatauser884

iTrader: (11)

I am not running a ground strap to the dipstick, I have one strap going to the head.

fooger03

iTrader: (2)

Runs from where the dipstick tube bolts to the trans plate, and then immediately to a bolt (10mm head) on the horizontal chassis piece ("frame rail") probably an inch away from where the horizontal piece meets the firewall, and probably two inches away from the inboard termination of the horizontal piece. (This is the horizontal piece that runs beneath the BMC, and is an extension of the part that gets "notched-out" when installing an FM turbo) The FM heat shield's lower inboard mounting point uses this bolt.

thasac

iTrader: (3)

Thread jack. Were you running the 70 or 95 durometer mounts?

I ask because I was considering picking these up to mitigate the risk of part interference due to engine movement - I'm a little tight in some spots (<.5").

I find the MS mounts pretty comfy so I could deal with a little more NVH, however, not to the point where I need to wire tie my rear view mirror on.

-Zach

bbundy

I remember that ground but it hasn't been on my car in a long time a lot of my wiring has been re-done.

Mabe I will add another ground there.

The other thing I am wondering if replacing the sender would change anything. I don't have a spare though.

I’m not even sure how the stock sender works. Its got one wire to it does it output a voltage or is voltage applied to it. Must be magic.

Bob

fooger03

iTrader: (2)

Hmm.....

I do.

miatauser884

iTrader: (11)

I think if I went through the trouble of changing the sender, I would upgrade to an aftermarket gauge and sender.

bbundy

The 70 durometer mounts seem like an order of magnitude stiffer than Mazda Comp mounts. Everything on the car vibrates and buzzes at all sorts of RPM's. I’m pretty tolerant of driving a beastly track oriented car on the street but those were too much for me. The 70's feel like track use only to me. Maybe it wouldn’t be as bad if I didn’t put in Energy suspension Diff mounts at the same time though.

Bob

JasonC SBB

It goes from circa 15-80 ohms and the gauge is basically a 2 phase motor.
Current passes through the gauge circuit, then to the sender, to ground.

One-wire senders only work if they're low impedance (i.e. < 200 ohms ish).

If fooger's gauge is one wire and has a nominal resistance at say 30 psi, of say, 1 k-ohm, that's bad juju. The offset voltage between the gauge cluster ground, and the sender ground (block), will affect it.

bbundy

So your saying the sypmtoms cause is most likely a grounding issue?

I will add a ground strap back in the OEM location and see if it does anything.

Bob

miatauser884

iTrader: (11)

post back with results

hustler

iTrader: (6)

Hmmmmm, I wonder if the sender's super-special mount needs better GND conductivity?