Hey there folks! This is my first post, but it’s gonna be a bit of a doozy. My 98 pathfinder has been having some continuous issues for the last 6 or 7 months, and they really need sorting. I have to rely on this car on a daily basis, so I unfortunately don’t have the luxury of keeping it off to the side while I go about my day.

In any case, I have done a lot of major repairs on this vehicle in order to try and clear the P0300 codes that keep popping up. The codes very occasionally fluctuate between P0301 and P0304. I have replaced the distributor, cap, rotor, plugs, wires, both upstream oxygen sensors, the MAF sensor, all the injectors (with brand new ones of varying brands. All tested before installation with a 9 volt battery and an ohm meter), the battery, alternator, fuel filter, and an assortment of other things that could be causing this.

I replaced the injectors under the upper plenum recently thinking that since they were still factory, that they would need replacement. The plastic was degraded and crumbled as it took them off. Most of them were still functioning with a 9 volt battery, but the click was weak. I replaced all those and put on a new upper plenum gasket, as well as replacing some vacuum lines and an air line that was rather old. Before replacing the injectors ,the car was difficult to start, but that had only become a problem within the last month or so. After replacement, took a long time to fire up, and required some finagling with the fast idle adjustment screw (previous owner messed it up a bit). It finally started, but ran like crap with white smoke out of the tail pipe. It eventually cleared up, but I decided to take it to the dealer due to the extensive work I had already done to it.

I took it to a dealer a few weeks ago to have them diagnose since I don’t have the tools to properly test the rest of the system (fuel pressure and other live data tests). They told me my fuel sending unit was bad (fuel leak) and that my upstream oxygen sensor on the passenger side was also bad. I had them replace the fuel sending unit and I replaced the upstream oxygen sensor . After the replacement, the misfire was gone when the engine was cold, but immediately returned when it started warming up. Additionally, the vehicle was hard to start when warm. This has since progressed to the vehicle is hard to start at any temperature.

I’m really stumped on what to do. I have an appointment with a local mechanic in the coming weeks, but I would really appreciate some advice about some possible causes so I can learn and try to fix it myself. Thanks for taking the time to answer!

Trooper329 wrote: ↑

Sat Mar 25, 2023 9:15 am

... They told me my fuel sending unit was bad (fuel leak) ...

A leaking fuel pressure regulator could cause all the symptoms you have described. A fuel sending unit, which is part of the fuel pump assembly inside the fuel tank, is the device that sends a signal to your instrument cluster fuel gauge to tell you how much fuel is in the tank. Scratching my head over how that was thought to be an issue unless the internal fuel lines in the tank were leaking back into the tank.

Wow, you've certainly done a lot of work to sort out the problem so it must be frustrating to still have that issue. The first place I'd start would be testing live data with an appropriate scan tool. I'd bring the engine to operating temperature and look at the short-term fuel trims for each cylinder bank. Would have been nice if you could have gotten the STFT data from the dealer you took it to. Since you wrote that you had a stored P0301 and 304 codes I'd also look at those spark plugs to see the burn condition. White smoke out the tailpipe initially could mean excess fuel being dumped caused by a faulty injector. One concerning issue could be the fact that you used "multiple brands of new injectors." The best injectors for this engine - Hitachi FIJ0006 - are no longer manufactured and unfortunately have been replaced by a bunch of cheap chinese knock-offs typically sold on ebay and amazon -and are poorly calibrated junk. You must have injectors that are flow-matched for each bank or you'll have problems. The flow rates are indicated by a colored dot at the neck of each injector - usually black or blue for your vehicle.

I went through the hassle of doing all my injectors last year and had to settle on reman injectors from GB engineering. Big mistake. Two out of six were initially defective and last week the #4 injector started sticking open and I now I have to pull off the manifold once again. You could have multiple issues at work here, but live data will tell you a lot in terms of identifying the root cause.

mdmellott wrote: ↑

Sat Mar 25, 2023 9:52 am

Trooper329 wrote: ↑

Sat Mar 25, 2023 9:15 am

... They told me my fuel sending unit was bad (fuel leak) ...

A leaking fuel pressure regulator could cause all the symptoms you have described. A fuel sending unit, which is part of the fuel pump assembly inside the fuel tank, is the device that sends a signal to your instrument cluster fuel gauge to tell you how much fuel is in the tank. Scratching my head over how that was thought to be an issue unless the internal fuel lines in the tank were leaking back into the tank.

Thanks for taking the time to respond! The dealership had stated there was a fuel leak of some kind. It was likely leaking into the fuel tank, as you had explained.

Additionally, I replaced the fuel pressure regulator in 2018(?). Lol. The dealership did check all aspects of the fuel system, as far as I’m aware. So I think that’s likely not the issue. I do appreciate you helping me out though!

MisterH wrote: ↑

Sat Mar 25, 2023 10:00 am

Wow, you've certainly done a lot of work to sort out the problem so it must be frustrating to still have that issue. The first place I'd start would be testing live data with an appropriate scan tool. I'd bring the engine to operating temperature and look at the short-term fuel trims for each cylinder bank. Would have been nice if you could have gotten the STFT data from the dealer you took it to. Since you wrote that you had a stored P0301 and 304 codes I'd also look at those spark plugs to see the burn condition. White smoke out the tailpipe initially could mean excess fuel being dumped caused by a faulty injector. One concerning issue could be the fact that you used "multiple brands of new injectors." The best injectors for this engine - Hitachi FIJ0006 - are no longer manufactured and unfortunately have been replaced by a bunch of cheap chinese knock-offs typically sold on ebay and amazon -and are poorly calibrated junk. You must have injectors that are flow-matched for each bank or you'll have problems. The flow rates are indicated by a colored dot at the neck of each injector - usually black or blue for your vehicle.

I went through the hassle of doing all my injectors last year and had to settle on reman injectors from GB engineering. Big mistake. Two out of six were initially defective and last week the #4 injector started sticking open and I now I have to pull off the manifold once again. You could have multiple issues at work here, but live data will tell you a lot in terms of identifying the root cause.

Thank you for responding! While the injectors were from various brands, they all tested fine and the dealership had checked them. I’ll make sure that when I take it to the mechanic, I’m able to see the data you informed me of! Thanks again!

Trooper329 wrote: ↑

Sat Mar 25, 2023 9:32 pm

Additionally, I replaced the fuel pressure regulator in 2018(?). Lol. The dealership did check all aspects of the fuel system, as far as I’m aware. So I think that’s likely not the issue.

You also paid the dealership to fix the issue you are having but they failed to do so. You can check a fuel pressure regulator a couple of ways to verify it does not leak. A pressure gauge test to validate your fuel pump pressure and pressure regulator at the same time is the best option. A borrowed test gauge kit is often available at auto parts stores, like AutoZone and O'Rielly, at no cost. Another possible check on the regulator is to pull the regulator vacuum hose off the intake plenum. Do this when the engine is warm. Not immediately after you turn off the engine but after 5 minutes or so. If there is a regulator leak, the fuel pressure will leak into that vacuum hose and you will see a small amount of fuel drip out of the hose when you pull it off the intake plenum. This is not as assured of a test as using a test gauge, but if the leak is bad enough, there will be fuel dripping from the hose.

Trust and verify is the prudent thing to do.

Sorry for the long delay since last posting. I have a few updates to share! Ended up taking it to another mechanic to see if they could figure out what was going on, and found out that my exhaust manifold on the driver’s side was cracked pretty badly. Ended up replacing that and the spark plugs again. Still didn’t fix the problem. Proceeded to test the MAF, MAP, and entire EVAP system. All green lights and no issues! The previous mechanic checked my fuel pressure, compression, and spark and said that the system was working as it should. He said fuel pressure was a tiny bit low, but nothing to worry about. Had a persistent fuel smell in my car, and found out that the guy at the dealership replaced the O ring between my fuel pump and the gas tank with the wrong size, so fuel was evaporating into the cabin of my car! Replaced it with the proper size and the smell is, for the most part, gone. The car still struggles to start (needs to crank for 4-5 seconds with the gas pedal depressed) but it always starts eventually. Running really rich (white smoke out of the tail pipe) and still getting crappy gas mileage (around 14 in town and on the highway, when I normally get 16 in town and 20 on the highway.). I found another forum that discusses the replacement of an ignition coil condenser, and I’ve never heard of that part before. Does anyone think that might be a possible cause of the troubles? The other guy who posted had very similar problems to mine, and it was the same make and model year! Thanks in advance for your patience!

Edit: forgot to add, we also tested the wiring and harnesses for all the aforementioned sensors! The fuel pump also tested well and is working as the factory service manual says it should. All of my readings were compared with the service manual or a common repair manual for my car’s year.

Sorry you haven't found the cause yet. Have you looked at the short term fuel trim readings when the engine is warm? I ask because whitish exhaust with raw gas smell means you're running REALLY rich and an intake leak will cause it to run lean while triggering the random misfire code. Anyway, having the actual data from the engine during running is the first step in the diagnostic process. Regarding the distributor, that is a failure point on this engine when it gets over 100k, but generally it's due to a failure of the internal cam sensor, or sometimes a mechanical failure in the distributor drive gear. I would tend to think that isn't the source if your problem is a consistently over-rich mixture, but maybe someone else will weigh in.

Hey MisterH,

No worries about not finding the solution. It’s a pain, but it’s a good learning point. I have gone over a recent STFT reading that I got from a loaned OBD2 scanner, and all of the trim levels are negative, which indicates a rich fuel mixture (?) if I understand that correctly. I’m not exactly sure what would cause that. I’m going to look into the fuel pressure regulator, but I would greatly appreciate any advice you might have about what could cause those rich mixtures. I have broken down the short term and long term reading below!

STFT B1: -14.1%
LTDT B1: -10.2%
STFT B2: -25.0%
LTFT B2: 0.0%

Additionally, regarding an intake leak, what kind of leak might that entail? More of a gasket situation or something like a sensor or hoses?

With the distributor, I’ve replaced that several times, unfortunately. I got a lifetime warranty one years ago, and anytime I suspect that something is off with it, I just exchange it for a new one. The most recent distributor has been installed a few months ago. I hesitate to replace it again because I know my timing is bang on right now, and replacing it would risk messing it up.

Thanks for the reply and taking time to help me figure this out!

The parameters you're looking for are called STFT (Short Term Fuel Trim) and LTFT (Long Term). The latter is an offset based on past performance, the former is a realtime value based on what the ECM reads from the MAF and front O2 sensors. What they actually represent is how long the ECM is leaving the injectors open. There are separate STFT's and LTFT's for each cylinder bank. They add together to indicate how much fuel the ECM is adding or subtracting on each bank in order to get a balanced reading (stoichiometry) from the front O2's, indicating all the fuel is being burned. Normal STFT/LTFT is zero, positive numbers mean the ECM is adding fuel (engine is running leaner than normal), negative numbers mean it's subtracting fuel (richer than normal). If you have a vacuum leak, the ECM will be adding fuel because there's more actual air entering the engine than the MAF is indicating. That makes the front O2's read lean, so the ECM bumps up the STFT in response. Small vacuum leaks are easy to identify using STFT in realtime. If you get a high positive number at idle (above +8% or so) but the reading drops substantially when you throttle up to 2000~2500 RPM, it indicates a leak. That's because the percentage of air contributed by the leak becomes much smaller at higher engine speeds, so the reading normalizes.

Those trims do confirm you're running really rich; particularly on bank 2 (cylinders 2,4,6) VSTar's explanation of fuel trims was outstanding of course but I know this means you still have to figure out why, as he indicated a vacuum leak will cause a lean condition; not rich. When you're running that rich it will show up on your spark plugs. The ones most affected will be very black, sooty and even somewhat glossy. You could pull each of your plugs and have a look at them to determine which are most affected. Since your STFT on bank 2 is the worst you could start there (even though I know #6 is a PITA). One of the top reasons for running rich is a leaking or faulty injector. You mentioned at the beginning that you replaced all the injectors, but with different brands. That alone shouldn't be a problem but I refer back to what I wrote earlier:

One concerning issue could be the fact that you used "multiple brands of new injectors." The best injectors for this engine - Hitachi FIJ0006 - are no longer manufactured and unfortunately have been replaced by a bunch of cheap chinese knock-offs typically sold on ebay and amazon -and are poorly calibrated junk. You must have injectors that are flow-matched for each bank or you'll have problems. The flow rates are indicated by a colored dot at the neck of each injector - usually black or blue for your vehicle.

If you have even a single injector with a different flow rate - or one that is leaking - it will cause a rich condition and will probably set a fault code for the affected cylinder(s). However, the engine management system for the VG33e only compensates for fuel delivery on bank 1 or 2 - not by individual cylinders.

MisterH wrote: ↑

Tue Jul 11, 2023 11:29 am

Those trims do confirm you're running really rich; particularly on bank 2 (cylinders 2,4,6) VSTar's explanation of fuel trims was outstanding of course but I know this means you still have to figure out why, as he indicated a vacuum leak will cause a lean condition; not rich. When you're running that rich it will show up on your spark plugs. The ones most affected will be very black, sooty and even somewhat glossy. You could pull each of your plugs and have a look at them to determine which are most affected. Since your STFT on bank 2 is the worst you could start there (even though I know #6 is a PITA). One of the top reasons for running rich is a leaking or faulty injector. You mentioned at the beginning that you replaced all the injectors, but with different brands. That alone shouldn't be a problem but I refer back to what I wrote earlier:

One concerning issue could be the fact that you used "multiple brands of new injectors." The best injectors for this engine - Hitachi FIJ0006 - are no longer manufactured and unfortunately have been replaced by a bunch of cheap chinese knock-offs typically sold on ebay and amazon -and are poorly calibrated junk. You must have injectors that are flow-matched for each bank or you'll have problems. The flow rates are indicated by a colored dot at the neck of each injector - usually black or blue for your vehicle.

If you have even a single injector with a different flow rate - or one that is leaking - it will cause a rich condition and will probably set a fault code for the affected cylinder(s). However, the engine management system for the VG33e only compensates for fuel delivery on bank 1 or 2 - not by individual cylinders.

Hey MisterH and VStar!

Thanks for elaborating on the fuel trim conditions and how that affects my engine running. I really appreciate it! Is it possible for a vacuum leak to cause a rich condition as well? Additionally, I have yet to test the IAC valve, and I wondered if that would have any effect on the running condition of my engine. That’s the only sensor I haven’t been able to test yet because it’s under the intake manifold.

Secondly, I’ll be under the intake manifold to test the AC and the fuel pressure regulator, so I can check out the fuel injectors under the upper Plenum as well while I’m at it. I have some spare injectors laying around. Luckily, they have lifetime warranty so I can return the ones that are faulty. Hopefully this will work!

Thanks again for the advice and help!

" Is it possible for a vacuum leak to cause a rich condition as well?"

With that engine the only type of vacuum leak that can produce a rich condition is when the diaphragm of the fuel pressure regulator is ruptured. Since the location of the FPR is closest to cylinder #6 the excess fuel will often flood that cylinder first and can trigger a misfire for it. Spark plug inspection will bear that out as well.

Hey all! Quick update. Spent a good chunk of time repairing and finding problems on the car this week. Found that the crappy injectors I bought were dumping fuel into the engine, and the one on the sixth cylinder was leaking outside the fuel rail. Bit the bullet and bought nice new Beck and Arnleys. Also discovered a MASSIVE vacuum leak between the EGR and EGRC, so I replaced every vacuum line I could see, both under the upper plenum and in the engine compartment as a whole. Ran better for a small while and misfired again at op temp. Am I missing any vacuum lines? Any clue as to what’s going on? I’m pretty close to just saying screw it and letting it misfire til it dies. I would appreciate any insight. On the plus side, my car no longer has a fuel smell in the vents, so that’s awesome! Still struggling to start though, even after replacing the IACV.

Are you sure your Fuel Pressure Regulator is any good? @MisterH is spot on with that, a blown diaphragm will make a fuel mess exactly where you were getting a mess. See if #6 plug has a black burn, if so the FPR is the prime suspect.

Pretty darn sure since we don’t have any more fuel leaks, and I’ve had the dealership and another mechanic both say that it’s functioning as it should. We did not test it personally, but every spark plug was black. The whole engine has been running rich, not just 6. That’s not to say it’s fool proof, but I feel like that’s decent evidence.

A blown diaphragm isn't a "leak" per se, it allows fuel to pass through into the vacuum line that controls the regulator, and that in turn dumps into the manifold. The regulator can function perfectly and still have a pinhole.

Gotcha. I’ll have to take a look at that then. Thank you! Do you think that would cause the hard start and the misfire?

Positively.

"Do you think that would cause the hard start and the misfire?"


That's exactly what it would do. A ruptured FPR will be most noticeable once the engine reaches operating temperature. If you have properly working fuel injectors and each of them have identical flow rates you've eliminated one common cause for running rich. If you've verified that all your fuel injector seals (there's three of them!) are installed correctly and not pinched, rolled or distorted, you've eliminated another possible cause. Any other component that would cause an over-rich condition (faulty MAF, misrouted vacuum hoses in the evap system) would trigger a specific fault code.

Awesome! I’ll have to check that out! Thank you for the help!

IF you want to buy one or more injectors , I and others on here believe Beck-Arnley to be a relatively consistent and reliable brand ....................IF genuine - knockoffs ARE out there in cheaply printed up boxes.
If this brand mentioned prev. here sorry - heat's got me down so I didnt go back and speed-read all posts again

1 1 8 deg. 'F here

Trooper , by the way curious if you been a LEO ? ........me , just come from backyard , got dogs - I'm pooper scooper , just for one day , I can be scooper ............just for one day ay !....
Gotta have Bowie play'in in background ya

macgiver wrote: ↑

Mon Jul 31, 2023 6:48 pm

Trooper , by the way curious if you been a LEO ? ........me , just come from backyard , got dogs - I'm pooper scooper , just for one day , I can be scooper ............just for one day ay !....
Gotta have Bowie play'in in background ya

Maybe I’m just tired, but mind explaining LEO?

Also, got Beck and Arnley on 4 out of the 6 cylinders, the other two are hitachi! Finally got the best I could brand new. Gonna check the FPR here soon to see if there’s any vacuum issues!

So sorry , Law Enforcement Officer is what you'll hear as shortened A C R O N YM , scuse me , it was a juvenile......... no INFANTILE on my part of the " trooper / pooper " joke , my bad .
6 2 and even over & out ! ...........( d!ck Tracy ? )
Honestly......................................................................so sorry

macgiver wrote: ↑

Wed Aug 02, 2023 6:01 pm

So sorry , Law Enforcement Officer is what you'll hear as shortened A C R O N YM , scuse me , it was a juvenile......... no INFANTILE on my part of the " trooper / pooper " joke , my bad .
6 2 and even over & out ! ...........( d*** Tracy ? )
Honestly......................................................................so sorry

No worries! Just hadn’t heard that one before!

Hey y’all! Checked out the FPR and it’s all good. Both on vacuum and function. The fuel trims are all solid on B2 now with B1 causing problems. Thinking O2 sensor might be going on that side.

Hey all! Just a quick update for you, I replaced the driver side exhaust manifold, which had already been repaired by the previous owner, as evidenced by a really interesting weld on the manifold itself. Additionally, when they put it back together, they didn’t take down the bolts to torque spec so the manifold was loose the entire time and caused an exhaust leak. The fuel trims are reflecting, the change, but they’re still in the -17.62 -10.2 range on bank one. Anybody have any ideas? Maybe the evap canister perhaps? Components? I’m at a loss for what could be going on to make only bank 1 have bad fuel trims but have bank two be exactly where it needs to be.

I'd pull the plugs on cylinder 1,3,5 and take a look at them. You might have a faulty injector even though you said you replaced all of them. With today's quality of parts its not unusual.