Don101

I have a 2004 RX8. My wife ran it out of fuel. She thought it was broke down and had it towed home. I am a 40 year car, truck, and heavy equipment mechanic. I put fuel in it and when it started it is only running on #2 rotor. The car ran great before. The throttle is fly by wire and is hard to hold it open. While running there is no peddle response. I did stick a screwdriver through opening and could move the throttle plate and the RPM's would raise to about 3K and then bounce RPM. I feel that it is in limp mode. Any ideas?

OnebaddRx8

I had a rx7 my girlfriend ran out of gas and it had a lean spike and trashed the apex seals on the front rotor. Would still start but only on one rotor(ran very rough).
It is possible to trash a motor just by running it out of gas!
Hope this is not your results but it can happen.

Posted From RX8Club.com Android App

Don101

Well I am back on it. I feel like if I could get it out of limp mode it might be ok.

Don101

Well I am back on it. I feel like if I could get it out of limp mode it might be ok. How do I get it out of limp mode? I have already cleared the codes with a scanner and even unhooked the battery.

RIWWP

iTrader: (2)

There is no limp mode for the RX-8 that can be 'cleared' by any electronic reset. All limp modes for the RX-8 are specifically engaged as long as the ECU sees a failure condition that trips the limp mode. If the failure that tripped the limp mode is fixed, then all you need to do is turn the car off and then back on again to disengage the limp mode.

For example, there is a limp mode if the ECU sees a throttle position sensor mismatch, which can happen if the throttle pedal gets so cold that air moisture in the pedal freezes and the potentiometers get stuck (happened to me once). This particular limp mode overrides control of the throttle plate to hold 3,000rpm, regardless of what you do with the gas pedal. Once you pump enough heat into the floor to thaw out the sensors, then all it takes is turning the car off, then back on again, and everything is normal.

So if you DO actually have one of the limp modes trigger (there are several), then you will have to fix the problem that triggered it before it will clear. If the limp mode remains engaged, it means that the ECU still sees the problem.

I recommend getting an OBD2 tool that can read all of the raw sensor feed PIDs, and then seeing if you can figure out if the pedal position sensors are malfunctioning. Most OBD2 smartphone apps can do this, if you have a $15 bluetooth/wifi OBD2 adapter. I don't know what physical OBD2 tools do, and I suspect they are a lot more expensive given the prices i've seen for basic code clearing tools out there. I believe all limp modes will also fire a DTC code that can be used to help solve the problem.


A throttle position problem will not be related to only running on one rotor however, and i can't think of any circumstance in which running out of fuel would affect throttle operation.

BigCajun

iTrader: (1)

Hey, RIWWP, for clarification in this case, could running it out of gas cause one or both of the rotors to be damaged?

RIWWP

iTrader: (2)

Yes.

A sudden drop in fuel pressure is a sudden drop in fuel injection, and it entirely possible to have that generate a lean spike that produces a detonation/knock that damages engine seals.

I know one owner who lost her first engine to a fuel pump failure, and the dealer lost the 2nd one due to the same failure within a couple miles of the dealership during the shakedown drive because they didn't recognize the failed fuel pump.

There are a few ways to have a sudden starvation of fuel that could all produce the same conditions inside the engine, running out of gas is one of them. The risk is greater with higher RPM and higher load, but the risk is there at all loads and all RPMs.

BigCajun

iTrader: (1)

Thank you.

RIWWP

iTrader: (2)

Take the upper plug out of each housing, and have someone crank the engine over with the gas pedal to the floor (to cut the fuel) while you place your hand near the open plug holes. You should feel even compression pulses pushing out air on each rotor. If you don't feel them even front vs rear, and a steady even series of pulses within a rotor (and not strong-strong-weak or strong-weak-weak), it's pretty conclusive that you lost one or more seals.

Don101

Well, it feels uniform on the compression "poofs". I can make the engine run smooth by feathering fuel in the #1 lower vacuum hose port in the intake manifold. I really do appreciate all your replies and input. I have been a car, truck and heavy equipment mechanic for 40 years and this car makes me feel stupid.

Don101

IT JUST RAN OUT OF FUEL. What a weird engine

RIWWP

iTrader: (2)

Most engines suffer damage from a lean spike, it just is harder to have an ill timed lean spike from a fueling failure on most piston engines, and they are more resistant to a single detonation. The rotary has a much higher rate per second of combustion, so 'landing on' a fueling cycle is much greater, and rotaries are very very sensitive to detonation.


So if you add fuel in the front rotor service port it will run smooth, but will run rough if you don't do that, or add fuel in the rear rotor?

Don101

I have only changed the rear plugs. I have a temp gun and I checked EX pipe and felt there is no problem with the #2. So I have only worked with #1. Thanks for the rundown on detonation.

RIWWP

iTrader: (2)

Both front and rear rotors feed into the same header, and the center of the 3 exhaust ports is shared by both. Even the end two that are specific to only that rotor are only a few inches apart. You won't get a noticeable heat difference between them unless you swap to a header that has independant runners for each port.

Could you answer the prior question? I am working toward a theory, but I need you to help me help you

Don101

I have put all 4 spark plugs in it. The engine is a little slow getting started up. The engine runs rough until I feather fuel in it. When I stop fueling it the engine starts running rough. The exhaust is considerably hotter towards the back of the pipe, or #2 side.

RIWWP

iTrader: (2)

Is the engine behavior only changing if you add fuel to the front rotor? Or does it change if you add fuel to the rear rotor too? What about both rotors?

Don101

I couldn't tell you as I have disabled the engine now. It ran so smooth when I juiced the front rotor that I did not try the back rotor

RIWWP

iTrader: (2)

Ok.

One theory might be that when the gas tank ran dry, something unintended from the bottom of the tank got sucked through the system and is blocking a fuel injector. It would be asomewhat freak occurance, but technically possible. It would produce a rough running engine that would smooth out if fuel is provided to the rotor that the blocked injector is firing for.

Just a theory, and it might not be the problem. If you re-fire the engine, testing to see what supplying more fuel to the rear rotor does.

04Green

iTrader: (1)

I thought that was where RIWWP was going, and concur.

Toss a bottle of fuel injector cleaner in. If you have any flow, it may help loosen things up.

RIWWP

iTrader: (2)

Or pull the fuel injectors and have them cleaned/flow tested.

04Green

iTrader: (1)

that would be the post Hail Mary injector cleaner. The hope is that the clog is soft stuff, AND that the injector cleaner will get to it, AND that it can dissolve it.

I really think you will be looking at getting them cleaned. But, with luck, maybe not.