Fuel temperature sensor

I have a 96 subaru legacy. I get a code saying that my fuel temp sensor is a problem. Dealers want 500 for a thermal resistor as big as the end of your pinky-you must buy the whole fuel pump assembly for a tiny sensor-not sold separately. Can a thermal resistor be bought elsewhere,- for this car at a supply house or just an electronics store? If not, since fuel temp is irrelevant-if it freezes you can’t run anyway and if too hot-you are dead. Can the circuit be bypassed and leave the ecm out ot it?

You could use a resistor to substitute the sensor and connect it up to the wiring on the outside of the tank. That should fool the ECU. I have done that to temporarily clear a knock sensor problem. I’m not sure what the resistance or power spec of resistor would work though. You could also try getting a sensor from a salvage yard but you are then taking chances it is really good.

I can’t tell what the resistance or the current/amps to the ecu should be. Subaru specs are kept under wraps somewhere. I can’t access the numbers-so I don’t know what values to shoot for at the ecu. I’ve tried a few at the scrap yard-but still getting message about the sensor. They may only work correctly on certain years. 96 may need just stuff from 96. It is a very particular year. Much changed in 97. Anybody know what the current/resistance/amps to the ecu from the fuel temp sensor is?

Pay the dealer, or an independent Subaru specialist, who might be less expensive. The size of the resistor has no correlation to its importance.

If you need it to make the car run, you need it. Otherwise the car won’t run. Right? How does that help you?

I own a '96 Legacy. I hope I never need a fuel temp sensor. I didn’t know there was a fuel temp sensor.

Yeah, I was wondering about that. Why would anything care what temperature the fuel is?

The car runs fine. Engine lght is on. That is it. Performance suffers not one lick. If you have no answer, don’t share it with the world. There are plenty of extraneous items on cars now-totally pointless-ie battery temperature guage- what is going to do, heat up the battery when the temp drops? Wake up or go away.

If I may I would like to continue in what I think is the spirit of Greasyjacks comment. Maybe someone could explain the significance of fuel temperatures?

At first some cars used a thermistor in the fuel tank to light the low fuel lamp on the dashboard.
The idea was to have a separate system from the regular fuel level sender for redundancy.
It doesn’t measure the temperature of the fuel. A small amount of current heats it slightly.
It’s simply placed low in the tank and when it’s not immersed in the fuel it gets warmer.
The '86-89 Accord had this.

Now they also send that signal to the computer, because running out of fuel can damage a cat converter.

Sounds likely the OP has a wiring problem since multiple replacements from the salvage yard didn’t fix it.

I was following along until I got to the last thread that said “running out of fuel can damage the cat” I am wondering, how can running out of fuel damage a cat? That is a new one for me.

Colder fuel is more dense, so less of it is needed. Knowing fuel temperature just helps the computer fine tune the mixture.

how can running out of fuel damage a cat?

Going too lean and misfiring.

Colder fuel is more dense

I doubt much correlation between temp of the fuel in the tank and at the injector.

I do too, but that was the only explanation that I could find. I challenge you to find better information.

Maybe, if the car’s owner did a lot of running out of gas. But how would a low fuel light prevent that? (I can see that it would slightly reduce the number of people who would let the car run out of gas, but it no way would it prevent it.) The car I drive has a warning chime that sounds every few minutes once it gets to 50 miles to go. It also has a display of remaining miles to empty. The newer version of it also automatically switches to map mode and displays the nearest gas stations, complete with gas prices. Even with all that, I still hear of people running out of gas.

Returning to the scene of the crime. I found a garage with a clever mechanic. Check out iatn.com He said this is a common subaru problem. The sensor shorts out in the fuel. So he simple took the hot coming in from the computer and put a 500 ohm resistor(a $.99 item) and cut out the sensor altogether. Computer is fooled, check engine light is out,car still runs fine and a 500 dollar fuel pump isn’t needed-esp since the other one already works. He said after 96 or 7 the design was changed. The sensor can be installed outside of the tank in that the temp is effectively the same anyway and no short. Got to venture to the salvage yard and see the difference tomorrow.

How did he figure out 500 ohms was a value that would satisfy the computer? Do we fully understand the reason why Subaru engineers wanted the computer to know this info? perhaps a little to quick to discount the value of this system.

I believe (I can’t access the site inside) he accessed needed info at iatn.com where he found that subaru was having this problem. Being a thermal resister, he calculated the avg temp at about 50 degrees and used that to determine the ohms. I can’t as yet verify that this will run perfectly, but he said that this design was abandoned almost immediately–next year. I will check 97-99 to see if any diff in sensor design exists. If easy to swap, I’ll do it. He (knows more than me on this matter) says that the design where the temp sensor is really used to calculate the volatility of the fuel was in its early stages at this point and that the entire design really wasn’t complete in this year car so the effect will be nill. At any rate, after waiting quite a while to have somebody-including the guys at subaru-tell me the real deal, with a 260,000 mile car, I am not investing 500 bucks in a new pump assembly when only a 3 dollar resister is bad. You can’t buy it alone. I don’t plan on going for a million miles. If subaru would give out the secret info as to the specs, I could buy a thermal resister and mount in against the tank on the outside where it wouldn’t short.