Has anyone removed the fuel temperature sensor and replaced with a pressure sensor? Recognize that new wiring to the datalogit would be required. I have really not seen any hard data that the fuel speed system is needed. Fuel pressure is a missing piece of data that I need.

 

You just need a fuel pressure sensor that returns a linear 0-5v signal. Then you can log the voltage using Sensor/Sw Fuel, or as an aux input for the Datalogit.

 

Thanks arghx
I will probably use a PLX pressure module and integrate into both the Datalogit (0-5V) and the PLX DM200 (IMFD data stream) I have.

Going back - is there an impending case to keep the fuel speed system? The best pressure data source is right at the fuel rails.

 

the two speed fuel pump system (fuel pump resistor)? what does that have to do with the fuel temp sensor in this context? or are you talking about the fuel pressure/hot start solenoid?

 

The resistor bypass system to raise the fuel pump speed.
From what I understand the ECU monitors the fuel temperature via the temperature sensor on the secondary rail and if temperature is too high it raises the fuel pump speed and fuel flow to prevent vapor lock.

I also understand that this system is really not required even in the Florida summer weather as the nominal fuel flow (pump voltage through the resistor) is enouth to prevent vapor lock.

So, if the sensor can be removed and replaced with a pressure sender it would be of a benefit to me.

 

The short answer is, the FD is not prone to vapor lock or hard starting. You can remove the hot start and fuel pump resistor system without any problems even in a very hot climate. The more detailed answer is:

1) On the subject of vapor lock: the FD is very "overbuilt" to prevent this. Look through the service highlights manual (page F-30 and F-31) and you'll see what I mean. The side feed injectors (also mentioned in the service highlights) were specifically used because they carry away fuel vapor better than the top feeds used in series 3 - series 5. The hot start solenoid is switched based on fuel temp as opposed to air temp like on the older engines.

2) The two speed fuel pump system does work in conjunction with the hot start solenoid (helps maintain higher fuel pressure), but you have to think about it backwards to understand its true function. No series 4 non turbo engines were equipped with a 2-speed system (turbos did have it), although the non turbo engines did have a hot start solenoid. The two speed system was added partly to reduce wear on the pump, but also because it reduces fuel evaporative emissions. Less hot fuel is pumped back into the tank when the pump is running slower.

Without a variable speed pump, more hot fuel will return back to the tank and produce a lot of vapor (HC emissions). And that vapor is going to escape when the gas cap is opened, or it will have to be metered back into the engine using the evap purge control solenoid (something the FC never had). If you look at trends in fuel systems over the past 10 years, all the OEM's are trying to reduce the amount of hot fuel in the gas tank for this reason. Returnless fuel systems (Rx-8) are one way of attacking the problem. On a modern return system you may have a 3 speed (like Subaru uses) or a completely variable duty cycle pump. You usually won't see a hot start solenoid on a newer car either.

So the 2 speed fuel pump system is first and foremost for emissions and pump life. The hot start solenoid is a holdover from the old engines (where they were hardly needed anyway) and is overkill considering the use of side feed injectors. The trigger fuel temp is pretty darn high, 80C on 93 models and 50C on 94 models according to their respective FSM's.

 

Thanks arghx

That is my take on the system also. Thanks for the emissions reasons also.

I plan to retain all of the features and components but replace the fuel temp sensor with a pressure sensor. The fuel pump will continue to operate through the resistor as it normally does. I'll even leave the electrical connector and wiring to the PFC so it can all be returned to "stock" in the future.

Last question: as the fuel temperature sensor is 0V cold and 5V hot (more or less) the PFC should ignore if the connector is removed from the sensor - right?

 

just "zero out" the INJ vs fuel temp setting and you could even leave it plugged in. You can also unplug it. My FC does not have the sensor hooked up, and all values in INJ vs fuel temp are set to 1.00

If you are having trouble holding fuel pressure up top you will have to either rewire the two speed system or bypass it altogether.

 

Right - thanks!

 

It is recommended to rewire the fuel pump for high hp applications right? I also plan to do this mod. Does rewiring the pump eliminate the 2 speed pump system? And if it does, does that mean the fuel temp sensor is no longer needed?

I am just trying to decide whether to buy a fuel rail with fuel temp sensor port or a rail with no fts port

 

in regards to a linear 0-5V pressure sensor, would these be a suitable item?

http://www.ebay.com/itm/170737540589...84.m1423.l2649

whats everyone's thoughts on the quality of these?

Brad.

 

that's a 3 wire sensor whereas as the stock fuel temp temperature sensor is 2 wire. It probably wires up like a MAP sensor... ground, 5v in, signal out . You would need to get the wiring diagrams out to make it work.

I don't know about the quality but the ad says it's made by Texas instruments, which it claims is the supplier for Defi. That's certainly believable.

 

Yes that is a ground bias transducer which will transmit a superior linear output. The ground and +5V must be stable and in The case of the OMP position sender and MAP sensor the ground and plus voltage come from the ECM. There is no reason the same +5V and ground used on the map and OMP can't be used for this sender.