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How does the mechanical OMP work?

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Old 12-06-17, 12:13 AM
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How does the mechanical OMP work?

Hey, I just got an 1988 FC S4 N/A, and was wondering how the OMP works?
I understand that it pumps oil into the engine with the fuel and lubricates the apex seals, but how does it scale?
will it still pump when the car is idling?
will it only pump when it hits a certain rpm?
does rpm change the amount it pumps?
And how do I know if my OMP is working? (apart from oil level drop)
Thank you in advance for answering, and sorry if it's already been posted somewhere...
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Old 12-06-17, 11:48 AM
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Originally Posted by Rigrag View Post
Hey, I just got an 1988 FC S4 N/A, and was wondering how the OMP works?
I understand that it pumps oil into the engine with the fuel and lubricates the apex seals, but how does it scale?
will it still pump when the car is idling?
will it only pump when it hits a certain rpm?
does rpm change the amount it pumps?
And how do I know if my OMP is working? (apart from oil level drop)
Thank you in advance for answering, and sorry if it's already been posted somewhere...
Your questions have been answered before already, but here is the cliff notes version:

1. The pump scales based on a lever that is connected by a rod to the throttle linkage on the throttle body. As the throttle increases, the rod pulls up on the lever increasing flow of oil.

2. Yes, it does still pump when the car is idling. Not as much as when actually driving, but it does pump.

3. No, it is always pumping regardless of rpm

4. Not directly. RPM doesn't control the pump, throttle does. For example, if you were to put it in first gear and floor the pedal, you would have full flow immediately upon flooring it, and the flow would stay at maximum until you let off the gas. So in that situation, from idle to 7000 rpm you would have the flow at maximum because the throttle is wide open.

5. If your OMP is not leaking oil, and your oil level decreases as per the FSM specification (or near it) it is probably functioning. That being said, if you are leaking oil from somewhere else it can trick you into thinking that you are burning it. So take a flashlight and carefully examine around the engine bay to make sure there isn't any oil leaking anywhere. If you aren't leaking any oil, but it still decreases in level, it is burning. If you want to be really sure, the pump is really easy to rebuild (although getting it in and out can be a pain unless you put the car on jackstands and get it from below) or premix until you know it is working.
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Old 12-07-17, 10:12 AM
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To really split a hair, the OMP is rpm AND load dependent.
And at least in the S6 world, I think it’s becoming more popular to premix at about half-rate (~.5 oz. per gallon). I mention this because almost every rotary engine I’ve been involved with has at least some fuel dilution of the oil over time... and that can mask a slightly dropping oil level from the OMP.
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