Hi,

Has anyone monitered their fuel trims (Both Long and Short term)?

I noticed that the ECU reduces the fuel trim(Detecting rich air/fuel ratio) when the fuel level gets to 3 bars or less. After fuelling up, the fuel trim goes back to normal values. It does not matter on the octane rating as i get the same results regardless of the fuel i use.

Anyone has the same experience or have any idea what's going on here?

Car has no problems and has 28,000km on the clock.

 

Maybe it’s related to pressure.

With the pump near the bottom and a full tank, you get the weight of all that gas pressing down. That pressure helps add onto the pressure generated by the pump. As you use up gas, the weight becomes less.

The thing is, the car’s ecu doesn’t measure based on pressure. This pressure affects the amount of fuel being sprayed through the injector. Higher Pressure means it doesn’t have to open as long to get the needed fuel for the AFR/O2 sensors to report back in range for the ECU.

Another reason I think this is because early on, after I installed my Sprintex unit and still used factory fuel pump with the piggyback, I noticed that it was easier to trigger limp mode via cylinder misfire (p0300-p0304) as my tank got lower. It was usually much harder with a full tank. That got remedied by both removing the piggyback and using an aftermarket fuel pump that produced higher pressure and flow rate over stock.

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edit: actually, even if the car did measure and track pressure, it would probably behave the same way.

The piggyback was faulty. And even when not faulty, it controlled a 5th injector that would reduce the pressure to the four that are controlled by the engine. And that most likely is how it contributed the misfires.

 

Interesting. Fuel trims should not be changing with fuel level.

I was thinking it might be an air pressurisation issue (e.g. bad fuel cap, EVAP plumbing). As the gas tank empties and fills with air, the EVAP dynamics change a bit.

To @Goobers point, there is a remote chance of fuel filter clog but I would consider that if normal alternatives show no improvement.

 

Thanks for the replies. I still do not know what causes the change in fuel trim but i do have a theory :

At 3 bars of fuel, the fuel level in the tank is probably so low that part of the fuel pump becomes exposed. This exposed area heats up as there's no fuel to cool it. This heat increases the temperature in the tank, causing more fuel vapour to be produced and captured by the charcoal canister. The vapour is then purged by the evaporative control valve. As more than the usual amount of fuel vapour is being purged and burned in the combustion process, the O2 sensor senses a rich condition and reduces the fuel trim accordingly.

After topping up fuel, the temperature in the fuel tank goes back to normal and the amount of fuel vapour goes back to normal. Fuel trim also goes back to as before.

This is the only explanation i can yhink of right now.
Anyone with another theory is more than welcomed to post and offer your knowledge. It will be greatly appreciated.

 

Interesting. That would indicate a fuel pump going wonky so might need to be replaced sooner or later. You could start by checking fuel pressure with a full tank vs with a low tank. I don't have much experience with fuel systems so can't help much. I would avoid throwing parts at this.

With our #2 fuel oil home heater, the very old tank has a lot of sludge on the bottom from natural biological reactions of fuel sitting around for years and fact that fuel oil is "dirty". So when tank goes empty, the fuel filter, lines, and injector get clogged and require a full cleaning.

Gasoline is cleaner and your car is not so old so unlikely to have much sludge as our home oil tank does.

Your car has super low mileage. Do you have it sitting for several months at a time? Maybe the gas went stale and caused problems. Our tiny boat has big problems with fuel system and injectors if we don't fully drain or treat fuel for the winter. Or maybe some bad fuel has beaten up your fuel filter and / or fuel pump. If the filter is filthy I suppose the top of the filter might be cleaner if it is used less frequently.

I don't know if a few bottles of fuel system treatment & injector cleaner might help but they are cheap and easy step one.

 

Been away forever, not fully back, but will chime in here.. can attest to power loss with low fuel. No math, no science, sorry. Tell me somethin, do you hear notable fuel slosh at times?

 

Hi,

I don't hear any fuel sloshing.
Power loss : Yes. slight loss.

 

You ofcourse get how fuel movement could potentially impact pressure, so thats.. why I asked that.
You sound smart, so maybe you could make use of this Link to a page discussing the mechanics of pumping fluid. Its fairly headache-enducing says one sports fan, but should get the gears turning in regards to the operation theory v. Component function and limitations.

Really interested in your feedback.

The end result could be

• Pump needs improvement
• (As you mentioned) ambient pressure in the tank/vacuum ex: restrictive emissions.
• Baffling

Cant think of any others besides some kinda electrical communication. It might be handy to yank the pump and inspect wiring for discoloration, you know, browning or whatever, indicative of excess resistance.
I wonder if trim could be adjusted by the computer based upon recognized quantity of fuel. Not familiar, but it would make sense for the thing to read lower than actual level to account for fluid shiftin' about.

The heat theory is an interesting one!
This is a pretty cool fault you found. Nice job!

 

Fuel pressure(Low) is pretty much the same regardless of the fuel level.

Initially, i was thinking it might be due to low fuel pressure but the low fuel level and pressure should cause the ECU to increase the trim(to compensate for low fuel pressure)

In my case, the ECU is decreasing the fuel trim when my fuel level gets to below 4 bars