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Interpret fuel trim values

2006 Nissan Pathinder, 163K miles.
Engine runs great in general, zero oil burn, MPG is 14-16 in city, around 18 highway.

The question I wanted to ask: I think my highway MPG is on low side, it has to be around 20-22, wanted to ask for ideas.

Some related car history.
Spark plugs are new/OEM, fuel injectors were recently removed/cleaned with a solvent, checked for a flow pattern to be consistent. Air/Fuel ratio sensors are new/OEM, same for downstream 02 sensors. Both catalysts are “fresh” (one is a shiny OEM, replaced by prior owner, one aftermarket, replaced by me), so restriction is out of question.
I also tried replacing MAF sensor (just had a compatible OEM unit laying around) and after re-calibration it did not change a thing, so I’m back to original MAF.
Timing chain tensioners were recently replaced (dreaded VQ40DE engine “whine”), valves were adjusted in the same time.
I know for sure that fuel pump back-flow valve is dead, as sometimes I have to “prime the pump” if vehicle sits long with nearly empty tank.
I checked the fuel trims, and this is something of suspect.
I have long-term trims around 9.0 - 9.5 (two banks) and short-term trims were 11.0-11.5 at idle.

I was always under impression that trims are supposed to be much closer to zero, are they not?

Once again: car drives fine, no power loss what I can observe.

Any ideas?

You are close to correct. LTFT +STFT should add up to 0 plus or minus 2 to 3. If your pos neg signs are typed correctly, you are +20. So the computer agrees you are using more fuel than designed.

Now you have to figure out why. Vacuum leak testing is easy and cheap so that’s where I like to start.

Yes, both LTFT and STTF are positive, around +20 in total.

When I was redoing the upper side of the engine, I had all the rubber hoses and gaskets replaced, but even before that I was doing a smoke test and it showed no leaks, both intake and exhaust sides are solid.

Or it is possible the fuel injection components are not delivering as much fuel as THEY were designed to do. LTFT and STFT would and should be relatively equal if that was the case. The injectors were cleaned and spray tested so you can probably eliminate those. Maybe the fuel pressure regulator is weak or the pump is worn. You admitted the fuel pump is not quite up to snuff. Is pressure within spec - or towards the low side?

If the O2 sensors are not showing rich, then overall there is no excess fuel being delivered even if there was a vacuum leak.

Full disclosure; I have been working on a similar problem with my Mustang. Lost mpg’s, LTFT high one bank. and a cylinder head temp 10-12 degrees higher than the other. Compression tested OK. 4 rebuilt injectors on the high LTFT side cleaned up the readings. Mileage has come mostly back.

Is your pathfinder AWD? In my experience, real world highway mileage is always lower that the EPA estimate with AWD. I usually do better than the EPA estimates with either FWD or RWD, but I don’t even come close with 4WD or AWD on the highway. I do around town though.

The computer has a program that uses the input from the MAF or MAP to control the amount of fuel to inject during open loop (cold engine) operation. When it goes into closed loop after warm up, the O2 sensors adjust the parameters for the fuel injection based on the O2 levels in the exhaust. The fuel trim is the difference between actual and pre-programmed values. It is normal for these to vary from the factory program as a vehicle ages and especially as components are changed.

You are only 11 to 11.5% off right now. You won’t get check engine light until they are about 22% off. Since your long term and short term are close to each other, you don’t have an issue. Your engine is running efficiently. The STFT and LTFT do not indicate rich or lean, it just means to achieve the correct A/F ratio, the measured has varied from the predicted fuel required.

One last thing, what the computer is actually measuring is the pulse width of the signal that opens the valve on the injectors. The pulse width is either less or greater than predicted. One cause of this variation is the fuel pressure in the fuel manifold. Higher pressure would cause a shorter pulse width and visa versa.

Why only do 4 injectors?

Only one bank had high LTFT. Buying 4 allowed me to eliminate the injectors as the source of the problem without the cost of 8 nor the hassle of changing them out one by one. I had time, and I’m a bit cheap.

I think that qualifies as frugal


I’m also leaning toward the fuel pressure as a primary suspect.
It is no fuel regulator as a separate unit here, it is one unit of pump/regulator/vale/level sensor.
The question is how to get it properly tested.
My injector rail port has shape like the lower connector here:
My fuel line is terminated with a female adapter like the top part on that image.

This is the closest I’ve got to the tool to put under my Christmas tree:

It does have an adapter of the kind I need, but for some reason they decided to do it dual-ended (?!?)
Unless I find something more appropriate, gonna buy one and shut the second port down with a piece of rubber tube bent 180 degrees.

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It is 4WD and by EPA it is supposed to get 14/21 or so, while RWD is supposed to get 16/22.
My lower/city side is very close, moreover, I’m getting toward 16 when I have a chance to drive more than my usual 7-12 miles in one go, so it’s OK when I’m on the lower part of the power range.
The strangely lowered MPG happens on highway only, under the load, and it is not about my driving style: I’m getting 1-2 MPG above ETA rated on highway on my other cars.
Looks like my toolkit is to add a pressure gauge soon :slight_smile:

Your 13-year old 4WD Pathfinder mileage seems to be spot on the current EPA estimates for that model. Not under.

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Somehow I was under impression I read about 20-ish numbers at some point when I was buying this SUV, but I bothered to go and check it myself.

So, all good then, so probably fuel pump is slowly degrading, but ECU is so far compensating for that.
I will need to replace the pump unit at some point anyways, as this “priming the pump” thing is not working well with a remote start.

You probably did read it and did remember it correctly. The EPA periodically updates its estimates and then it goes back and adjusts them. In the past, they may have been a bit optimistic. The EPA has a page at which you can compare the original and current EPA estimates. Try this.

John, you are right.
That’s exactly what I remember from the time I was buying it:

Yep, me, too as my current gauge only has the 45 degree threaded fittings to a schraeder valve. Works for my GM truck, not for the same quick connects you have. I may try and make some fittings to adapt.

OK 4WD, those are the exact experiences I’ve had with 4WD/AWD, highway is always lower than the EPA, city OK and all non 4/AWD highway is better than EPA.

thank you, guys

at this point, I’m taking a “lazy road” and do nothing

a little bit unexpected [for me] twist on my fuel economy topic

I’ve redone all brakes yesterday - it was the last thing I did not yet touch on this SUV and they were SUPPOSEDLY redone PROFESSIONALLY by prior owner shortly before sale, so I did not consider it high on priorities list

the reason for me to redo brakes was in front rotors glazing, even with my relatively gentle use, so I suspect that brake pads were of low quality

what I found immediately explained my fuel economy issue: both front calipers were “tight”, on both the rubber insert on the sliding pin was improperly installed by “the professional” (local shop actually, not shade tree guy!), on both rubber piece was pinched between the pin and the caliper

after careful reassembly and replacing pads (half-worn in front after mere 15K miles), I’ve got +1.5MPG on ~30 miles I made on this SUV so far, my regular commute route.

so, forget fuel trims and go check your brakes :slight_smile:

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