01 R50 horrific fuel mileage

[tuckers240]

I have a 2001 pathfinder r50 that I am getting horrific fuel mileage with. When I say bad I mean 30 L/100kms which converts to between 6 - 7 mpg. 240,000kms. I have 31" tires and its -20 C here right no now but my question is what to be contributing to such poor mileage. I've changed air filter, fuel filter and spark plugs. I'll clean the maf here soon but I'm wondering if there is a more serious issue like leaking injectors or bad 02 sensors? I replaced valve cover and spark plug grommets a few months back - oil on plug - but the problem persisted prior to that.

The confusing thing is with mileage like that I still have no engine codes.

I appreciate any help or suggestions!

Thanks

[MisterH]

What are the primary driving conditions under which you're getting 6-7 mpg? With 31" tires (and I assume 4WD) I'd guess you were driving through muddy riverbeds or across sand dunes. Is that accurate? The estimated fuel economy with in-town driving was only 13-15 mpg, so getting half that under less than ideal road conditions wouldn't be unusual.

[tuckers240]

Thats driving in town, paved roads, typical commute is 4-5 kms to and from work. Engine warmed for 5-10 mins. Driving in AUTO 4wd. I live in northern canada so its usually around -4 to -25 f

[VStar650CL]

Like a lot of Nissans from way back when until the early 00's, your temperature gauge on an '01 has a different sending unit from the ECM. The ECM sender will cause horrendous fuel mileage if there's resistance in the circuit or it's lying about the block temperature. It's way at the back right of the engine, behind the manifold. See EC-20 here:
https://www.nicoclub.com/service-manual ... 1%2Fec.pdf

You can recognize it by the wiring, it will be two wires with one Light Green/Red (signal) and one Black (ground). See DTC P0125 for a temperature-resistance chart. The IAT (Intake Air Temperature) sensor can cause similar problems if it's lying. That's down behind the left fender, see EC-21. It also has two wires, Yellow/Purple (signal) and Black (ground). See P0110 for a temperature-resistance chart.

[tuckers240]

Awesome, thanks very much. I'll check this tonight and report back.

[tuckers240]

I couldnt find those sensors...i assume there situated well velow the top of the engine. Pretty rough to get at with the valve covers on. I tested my fuel trims tonight and am averging a negative at all rpms so something ia causing excessive fuel getting in. Ill check o2 sensors, maf, fuel regulator tomorrow. Im hoping it isnt my injectors.....

[VStar650CL]

Unfortunately, negative FT's are almost always leaky injectors or injector seals. Lots of things cause plus-shift, but the list of things which cause minus-shift is very brief: Injectors, seals, high fuel pressure.

[tuckers240]

Whats the best way to test injectors? Pressurize fuel rail and see if it holds? Check plugs for fuel

[VStar650CL]

Whats the best way to test injectors? Pressurize fuel rail and see if it holds?

If the problem is seals or a stuck injector, pinching off the return line on the rail side of the fuel pressure regulator and seeing if it holds will tell you. If that's okay, pinch off the return on the tank side of the FPR and see if that holds. If not, the regulator diaphragm is leaking. If that checks okay, then one or more of the injectors are probably balky.

[tuckers240]

Thanks for the help Vstar but I'm not sure I understand. I see the FPR and the Fuel damper and each have a vacuum line going to intake manifold. I'd like to put a fuel pressure gauge on one of these lines to see if fuel pressure drops off after engine is shut off. A decrease in pressure would indicate a leak somewhere.....injectors, seals, regulator or damper. I see nissan has a quick connect connector (J44321) to attach to the fuel damper but there must be an easier way than buy a 80$ adapter.

[VStar650CL]

The check valve isn't leak proof, fuel pressure will drop anyway but it should drop very slowly. Checking the rail, the idea is to pinch off both sides so that you're only checking the integrity of the rail and injectors, nothing else. Those should be tight, with both sides blocked the pressure should hold forever. The problem with the FPR's is that they sometimes work but get pinholes in the diaphragm, which under pressure lets fuel leak into the vacuum hose and straight into the manifold. Usually that will result in the hose being full of fuel after shutoff and is easy to diagnose, but a bleed-down test with the return blocked is a surer method.

[tuckers240]

I've attached clips of the excel spreadsheet from the fuel trim test, My values are all rich and at several points reaching -20.I took my pathy to a local garage where they checked MAF, fuel press regulator, and read o2 sensors. They reported all to be within spec. My gauge doesn't show up till next week where ill install it after the fuel damper so see if injectors are leaking or not. I'd appreciate any advice or tips to try.

[tuckers240]

Another result from today with O2 sensor voltages

[VStar650CL]

That chart is weird, your FT shouldn't be bouncing around like that with the RPM steady-and-rising. That might be an anomaly of whatever your foot is doing, try either adding APP to the graph or get steady state readings at idle and a higher RPM.

Nissan doesn't make any provision for a fuel pressure hookup on most vehicles. To check the rail on one with an FPR, I usually pinch off the flexible hose for the gauge attachment on the inlet side and get at the return hoses wherever I can. If the inlet side of the FPR is hard line, I pinch the return back at the tank and then also pinch off the vacuum line in case the diaphragm is f#cked.

[rgk]

Just how negative are the fuel trims? -2% is a lot different than -20%.

After 20+ years, the injectors, seals, fuel damper, and fuel regulator most certainly need replacement.

[Mike W.]

Thats driving in town, paved roads, typical commute is 4-5 kms to and from work. Engine warmed for 5-10 mins. Driving in AUTO 4wd. I live in northern canada so its usually around -4 to -25 f

You know, under those conditions 6-7 doesn't sound unreasonable. Very short trips, idling more than driving, very cold temps. Try this, fill it up, take a ride on the highway somewhere and fill it up again when you get back after say an hour or so. I bet MPG doubles or close to it.

[BullsEye]

@MikeW: Did you ever figure out what is causing your QX4s drinking problem? Dealing with the same issue, I'm planing on working on it next weekend.

[BullsEye]

@MikeW: Did you ever figure out what is causing your QX4s drinking problem? Dealing with the same issue, I'm planing on working on it next weekend.

[Mike W.]

@MikeW: Did you ever figure out what is causing your QX4s drinking problem? Dealing with the same issue, I'm planing on working on it next weekend.

If you're referring to the oil consumption/smoking, yes, for the most part. Still seems to use oil, but smoking is fixed. I replaced the valve covers. See the writeup
smoke-at-start-solved-t633784.html

[tuckers240]

Well now thats its warmer out ive concluded ita due to my cold weather temps. Above zero im averaging 400-500 kms per tank. Its essentially cut in half due to the colder temps....not an ideal cold weather vehicles....

[Greeno237]

I wonder if the change in behaviour based on ambient temps provides any clues. This is the 3.5L right? Were you ever able to pressure test the rail/injectors?

What do your fuel trims look like? Are they still hitting -20?

You're running slightly bigger tires than I am, but I can't imagine that would make such a difference in the cold. My 04 has just over 400,000kms and I don't see a noticeable drop in fuel economy during the winters in Calgary. Of course, if by north you mean way the hell north, we could have very different winter conditions.

Did you check the intake air temp sensor? It's right behind the driver's side fog light, I'd check it there and at the ECM to rule out a faulty sensor or bad connection. It looks like you should expect 2-3k ohms of resistance at 20 degrees C up to about 6k ohms at 0 degrees. It feels like a long shot, but I'd check that in order to rule out one of the few problems that might not be on the fuel rail. The info you need is on page EC-166

[tuckers240]

I have chexked fuel prssurebat the rails. I also havent checked fuel trims since it warmed up. Ill do that and make sure its back within acceptable range. By north i meant Yellowknife so its consistantly -30 c from jan-mar.

Ill check the air temp sensor, my scan tool could read the ambient air temp last time and seemes to be working properly

[Greeno237]

I guess if your intake temps on the scan tool were within expectation it's probably not the sensor, I must have missed that part or just forgot.

Yellowknife! That's definitely colder than here. Maybe fuel economy falls off a cliff at -30 and I spend a lot more time in the -20s? 400km in the city and 500km on the highway is consistently what I get out of an 80L tank of fuel though. My fuel gauge hasn't worked since I got the thing, so I've been quite diligent about resetting the trip odometer when I fill up.

If your fuel trims are still rich and you've confirmed it's not the FPR, injectors and seals are the only culprits left. At least, that's if I understand Vstar's earlier comment about negative fuel trims having a short list of causes. I've never diagnosed a fuel rail before.