I have a Ph.D in organic chemistry. Cars a hobby. I have an 96 Civic (with B18C1 swap from an Integra GSR) (440,000 miles on the body), an 02 Civic with 1/93,000 miles, the 08 Fit (190,000 miles) and a recently inherited 2000 Toyota Camry V6 (147,000 miles). I find there is a scientific method to troubleshooting although I am guilty of not allows following that. Easier to look stuff up on Youtube first lol. Every problem is different so it keeps my brain active. Some problems took me years to figure out. The Fit, after I changed the compressor (never should have, as it was the AC clutch coil that burned out and nothing was wrong with the compressor). had a refrigerant leak. So always had to top it up every few months. Changed O rings, the condenser. Checked for leaks with UV dye. Seven years later (yes, 7 years), finally found the leak using soapy water. There were three pin hole sized leaks in the AC rubber suction line to the compressor. I have not have had to add refrigerant for a year and a half now and I am very happy about that!
I have not had to do too much on the Fit. A couple of alternators, a starter motor, the primary air fuel ratio sensor twice. They seem to last about 90,000 miles. Had loose spark plugs. Others have had this problem. Could always smell gas when under the hood. Last month changed the original front struts (kept the factory springs). I have replaced the passenger side door lock 3 times. Last year changed the rear drums/shoes for the first time. CV axles and tie rod ends about 2 years ago. Still have the original cat and exhaust. Original ignition coils. Really hope that you have solved your problem and I hope that I can help you in the future. Keep us posted.

 

Well, that explains the detailed chemical specs. Outside of routine maintenance and some minor items my 07 Fit (177K miles) has been very good to me except when I fed her something she didn't like. I do need to work on my suspension as it bottoms out on deep dips with passengers. It seems like a big job so I have been procrastinating. All other parts except for spark plugs, coils, battery, are original. I am guessing that my trim numbers will decay as the tank depletes. Gonna keep my fingers crossed. Very much appreciate your help and time to compare data, I will keep you posted.

 

Your fuel trim numbers will only decay as you continue to fill up the tank unless the culprit is restriction that is going away as the Lucas cleans things up. In that case, the number should continue to drop as you deplete this tank. Again, I think that it is the bad gas so hopefully better numbers after your next fill up.
As for the suspension, replacing the front struts was really not too bad a job. When I had everything apart, the only thing that delayed my progress was that the upper spring seats were quite rusted so I replaced them with genuine Honda parts as the aftermarket does not sell them (Honda part 51688-SAA-003). I read that the factory springs are the best so I opted to keep them. Otherwise, I could have gone with a complete strut assembly with the spring and shock. No need to compress springs this way. In order to get the nut off of the top of the assembly under the hood, you are best to take off the cowling and use an impact wrench. I tried the allen key and a box wrench and that did not work. There is another nut above the strut bearing and that will spin. So you can grip the piston shaft with vice grips and spin the nut off with an impact here too. I replaced the upper strut mount, bump stop and dust cover boot with aftermarket parts. Once you get the upper nut off, the rest of the removal should go quickly. 10-15 minutes if you can easily disconnect the sway bar end links. Mine were fairly new so the nut(s) came off with an impact gun. Otherwise, you can use a pair of vise grips to hold the back end of the link to keep the threaded portion from spinning. As for the rear shocks, that job I have not done yet but it looks easier than doing the front suspension work.

 

For junk yard practice (in California) I used 6mm Allen key braced against the body (with piece of wood under it) and used open end 17mm wrench with cheater extension

 

Broke 2 allen keys trying this method. Again, removal of the cowling (easy) and an impact gun solved the problem. Threads seemed OK even though I live in an area where they heavily salt the roads in the winter. Put antiseize on the threads of the new struts. Car is 15 years next year and hopefully will never need new struts again. I undercoat the car with a mixture of petroleum jelly ("Vaseline") and camping stove naphtha. The naphtha flashes off rapidly (I do this outside) and leaves a coating of petroleum jelly that will not wash off. It also is drip free and therefore does not leave stains on the driveway.

 

Numbers are about the same, fuel tank is just below half so no change. I was thinking, what about the upstream O2 sensor which I believe is called the Air/Fuel ratio sensor. This sensor is not displayed in my scanner and was thinking how I can verify if it is good. If it is faulty, can this cause my high fuel trim? Thoughts?

 

I would run the tank down to 1/4 and then fill it up. Check the LTFT after using up a 1/4 of a tank. But is the check engine light still off? Usually you have to hit +20% LTFT before a code is set. I would live with the LTFT number you listed before from one of your posts from 12-15 where you had 9.4%. The higher LTFT number does not necessarily mean worse fuel economy. The value from the A/F ratio sensor is used by the ECM to maintain a stoichiometric mixture. If it detects excess oxygen in the exhaust, it keeps the injectors open longer. A partially clogged fuel system means less fuel can be delivered so oxygen is seen in the exhaust. It can only correct the problem by keeping the injectors open longer. Point is again, this does not mean you are using more fuel to maintain the perfect air fuel (stoichiometric ratio).

If you are not getting an O2 sensor code, I would not worry about the O2 (in this case, A/F) sensor being bad. I found that the A/F sensor in my Fit lasted for about 90,000 miles before I got a code. This happened again for about the same distance for the second sensor. If the one in your vehicle is high mileage, you can always replace it as preventative maintenance but they are pricey. I bought one from Rockauto (Denso) for an 09 Fit and cut the heat shield off of it as the 09 sensor was cheaper than one for the 08. Incidentally, you can use the 08 sensor in an 09 but not the other way around because of the heat shield on it, unless you cut it off.

Hope some of this helps.

 

You make sense. The fuel trim crept back up again, STFT 21-25%, LTFT 20%. The check engine light is on steady again. Not understanding why it was good then went sideways. However, I don't notice it much as I drive. No O2 sensor code either however, both sensors are the original ones. I'm going to continue driving and treat with fuel tx next couple of fill ups, unless something worse pops up. Thanks for the break down. The mystery continues.

 

Looks like things are pointing towards the inevitable need to check the fuel pressure.

You could also try Techron or Gumout Regane as both have polyetheramines (PEA), which is a really good solvent. I have had good results with both. Takes a while to work but is fairly cheap when you buy it at Walmart.

Not sure why your LTFT has crept back up unless it is because you cleared the check engine light around the time you last filled up. Takes some driving for the LTFT to change. So if you cleared the CEL, you would initially have a 0% LTFT and perhaps when you checked it when it was at 9.4%, it was not too long after that. So you thought it was the additive making the difference.

 

I think you are right about the fuel pressure, but how without the proper tools? I'm considering going to Honda and pay for a deep diagnostic to include fuel pressure. Not sure if they will be able to tell me more than I already know. My Walmart has no Gumout with PEA but already ordered a few days ago. We'll see. Thanks again, Merry Christmas!

 

Any reputable shop should be able to test the fuel pressure. Hopefully you can find one. Some dealers, like independent shops, are better than others. Some will try to sell you more than you need so tell them you just want the fuel pressure tested. You already know that it only has the code for a lean condition.

I see no reason why testing the fuel pressure should not cost more than one hour of labor. You can then decide from there and should not feel under pressure to act on any recommendations that they might have. You can take your car home and decide later if you want them to do any more work.

I think that this one piece of info will be very important. If it is low, you know it is the pump, filter, pressure regulator or a damaged/clogged line. If it normal, it likely points to an injector problem (find it hard to believe however that the reman injectors that you used would all be restricted. However, having said that, each injector should have a very fine filter screen that can easily become plugged. My Civic injectors have them and from what I can see from the pics at Rockauto, the Fit injector has it also. This wire mesh filter is finer than the filter at the fuel pump. So these could all be slightly clogged). Especially if the varnish remover stripped some varnish out of the tank.

Merry Christmas to you too!

 

Update 18JAN2023: So, when I fill up, fuel trim improves from an average 20% down to 10-15%, often in the single digits with the MIL no longer lit. Fuel trims go back to 20% average (depending on driving or idle) after about 30-50 miles driven and of course, the MIL indicator back to on. Most of my commutes are less than 20 miles averaging 12 miles. Today I drove over 30 miles straight and my MIL started flashing (with demand) as before (back in November on a long-distance trip) with multiple cylinder misfires and poor acceleration. This time I was running my live data and the STFT was a constant 0 when the MIL started to flash. It sort of looks like my computer was no longer trimming fuel for my lean condition, as if it was locked up. I turned engine off for a few minutes, restarted and things went back to as before, +20% fuel trim, MIL on but not flashing, sort of like a reset. The new info is the 0 STFT possible no trim condition until a reset was performed. Thoughts?

 

Update 18JAN2023: So, when I fill up, fuel trim improves from an average 20% down to 10-15%, often in the single digits with the MIL no longer lit. Fuel trims go back to 20% average (depending on driving or idle) after about 30-50 miles driven and of course, the MIL indicator back to on. Most of my commutes are less than 20 miles averaging 12 miles. Today I drove over 30 miles straight and my MIL started flashing (with demand) as before (back in November on a long-distance trip) with multiple cylinder misfires and poor acceleration. This time I was running my live data and the STFT was a constant 0 when the MIL started to flash. It sort of looks like my computer was no longer trimming fuel for my lean condition, as if it was locked up. I turned engine off for a few minutes, restarted and things went back to as before, +20% fuel trim, MIL on but not flashing, sort of like a reset. The new info is the 0 STFT possible no trim condition until a reset was performed. Thoughts doctor?

 

Happy New Year!

I was wondering lately what was happening with your Fit.
Once the Long Term Fuel Trim has increased sufficiently, the STFT should be zero. So not suprised by this.
It appears that the fuel pressure is dropping as you drive. Only speculation but as the fuel pump motor heats up, it will not work as well due to increased heat in the coils and therefore more resistance and less current. But you said that you put in a new Honda pump. But if there is a restriction the pump has to work harder. Perhaps when you shut the car off, the pump has some time to cool. Clutching at straws here.
Again, I think you are having to bite the bullet and get the fuel pressure tested. It will give you very valuable information.

 

When I said the STFT was zero, I believe there was no more trim going on. STFT was zero and the LTFT was locked up also. Looked as if the computer was no longer trimming or locked up. Turning off then on seemed to correct this. The fuel pressure /trim stays about +20% when cruising, no decrease. When things went sideways the other day, it was instant, no warning. Your fuel pump theory makes sense but in this instance, i did not give it time to cool down, only a few minutes. Also, my tank is a little more than 3./4 full, so chances of over heating are minimal, if any. Your probably right about the pressure, but the zero STFT and locked up fuel trim over all on my live data has me stumped. This is when the MIL flashed. As long as my computer was trimming, I was rolling pretty good.

 

Weak spark (subtle misfire) can also cause elevated FT due to increased O2 from incomplete combustion;

 

Not sure why the fuel trim would lock up (you mean not read higher than 20%?).

I think that you should get the fuel pressure tested as Dr. J originally suggested and I have suggested several times also.

 

Thanks Doc

 

I plan too, don't want to go to a dealer. Hopefully they can tell me something I already don't know, especially if the pressure is not the problem. The live data trim display were frozen/locked up, yet all other displays were functioning/registering at the same time. This implies, to me, that something was going on with my vehicles computer, for that part at least (fuel trim) . What other explanation could there be for all live data to run except fuel trim? I'll get back after the dealer experience ;(

 

I am wondering what type of scan tool you are using. Could be that it maxes out at 20%, Highly unlikely that it is a computer problem. Honda ecu's rarely have a problem. You said that your issues began after you cleaned the tank with lacquer thinner. This would likely rule out a computer issue.

Don't forget, if the fuel pressure when tested is within spec, it could be clogged injectors. I mentioned before that they each have a very fine screen in them which is the last line of defense to protect them. So they would have to be checked.

I would try to avoid the dealership and find a reputable shop and tell them that you only want the fuel pressure tested.