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291 Posts
Discussion Starter · #1 ·
I realize the forum is pretty dead now, but with everyone migrating to FB and whatnot, how are people going to get info through a google search anymore?

That's going to be an unfortunate consequence of the migration. So I will go ahead and document my journey here, so one day someone may have the same problem and google it, and find my post and maybe it will help them out.

The other day I came back from a 50 mile all highway trip and filled up gas. Got 35 MPG, which is one of the highest I've ever gotten. I filled it up all the way, but the needle didn't go all the way to the top, even after the usual delay. I thought that was odd, but whatever.

I drove a few miles around town, and once to work which is mostly highway. I noticed that my gas gauge was dropping pretty fast. I filled up after 66 miles and got 19 MPG!

I scanned the car and my long term and short term fuel trims were about +7 or +8 each, which doesn't set off a code, but is high. The car drives perfectly fine, but the gas mileage is horrible. I've scanned the car with over 200K miles before and my fuel trims are usually like -1 short term and +2 long term. Really close to 0 like they're suppose to be.

Thought maybe it was a fluke, so I drove only to work the next time. Usually I get around 29 MPG with a mix highway/city. I got about 24 something the next fillup, which still isn't good because there was all highway and no traffic that day.

In case it was something clogged, I did a steam clean with water, and 1/3 bottle of B12 down the vacuum line. It didn't help.

I drove it the same 50 mile trip all highway again. When I got to my destination, I actually got a pending code. P1456. Google search basically told me it's the gas cap every time. I replaced it with a brand new Stant gas cap I had laying around. That code hasn't come back after a week. When I got home from the 50 mile trip, I got a P0420, which I know is pretty much always a cat code. I've replaced the cat twice already, I'm skeptical that the cat is actually bad, I think it's something else throwing off the computer. Code hasn't come back in a week either.

After I replaced the gas cap, it actually did change something. When I'm driving and coasting, the STFT actually does go to 0 now. But it jumps to like 12 or 18 sometimes when i put the slightest pressure on the pedal. If I come to a traffic light, or park it in the garage the STFT is usually around +8-12. LTFT is about +7.

I thought maybe it was the purge solenoid because that's related to the gas cap code. So I bought a used throttle body assembly and cleaned the IAC up and installed it. I got it cheap. So it's the new throttle body, IAC, and Purge Valve. Installed that today with no improvement.

I have a fuel pump assembly I ordered earlier this week, but it's delayed due to weather. I'll be putting that in next. The assembly wasn't that expensive, and i figure worst case scenario I get my filter changed.

If that doesn't work. I'm gonna stumped. I'll list the stuff I've done so far, and other items for reference.

2001 Honda CIvic EX
320K miles (engine changed at 140K, Head gasket changed at 180K, Head job and head gasket replaced again 5 months ago at 314K)
Automatic transmission replaced at 40K, 80K, 220K. Fluid drain and fill 15K miles ago.
Throttle Body swapped out around 210K, and again today
Fuel Pump Assembly swapped out around 210K
Both O2 sensors replaced around 217K (Denso for Downstream, NTK for Upstream)
Spark plugs last replaced last spring I think. NGK Coppers. I took them out last week. A little browning to the white porcelain part but it's been like that for a while.

Recent Maintenance:
New Gas Cap Last Week
Replaced throttle body with newer IAC and Purge Valves
Steam clean plus B12 Chemtool through brake booster line
Cleaned EGR Valve
Marvel Mystery Oil 4oz in the gas tank
Tire pressure set at 34 PSI two weeks ago right before my 35 MPG trip

Checked for vacuum leaks and couldn't find anything, but it's very very hard to find leaks. My Dad's car almost assuredly has a vacuum leak and I tried for 3 weeks to find it but couldn't. I did the brake cleaner test, but I don't even know where all the vacuum lines are. I just sprayed everything around the throttle body and the brake booster line. In my Dad's car, the fuel trims actually do go down when you're accelerating, which is a classic symptom of a vacuum leak. My car doesn't do that. The fuel trims increase very quickly when you accelerate.

Yeah, the car drives fine, and it has a ton of miles on it. But if you're on this forum you're probably the type that really takes car of their car and wants it to run optimally. And I have an emissions test next month I don't want to fail, and I don't want to do damage to my cat. And if I'm burning an extra $7 worth of gas per week, that'll add up.

I'll update use this thread for updates. I've attached some relevant pics.
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Telephony Mobile phone Portable communications device Communication Device Mobile device
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· Registered
291 Posts
Discussion Starter · #2 ·
First update.

I got my fuel pump today. It was a used one from a salvage yard that supposedly had 88K miles on it. Cost me a little more than a brand new aftermarket one would, but I felt better about getting an OEM one knowing they very rarely go completely bad. EricTheCarGuy said he's been working on Hondas for like 20 years and he can count on one hand how many fuel pumps that have gone bad and still have fingers leftover.

I've had plenty of problems with this car, but one thing it has never done was not started unless it was a dead battery.

Took me about 30 minutes since that tightening ring was on there pretty tight. Easier to do stuff when you've done it before. Install was straight forward but I put down all my windows first, and ended up priming the pump a couple of times, so gas spewed out the hose a lot more than last time. I also found a second fuel pump gasket floating around (oops).

I got a check engine light when I turned the scanner on. P1456 for that fuel cap thing again, but shouldn't be surprising considering I had the tank wide open exposed for a couple minutes. May have been P1457 but I had the tank open so probably nothing.

I started the car and drove it a couple of miles and came back. I was excited at first cuz the long term fuel trim was 0, but there's not really any change to the car. It still has elevated short term fuel trims that go to 0 if I'm coasting without my foot on the accelerator. The LTFT was adjusting upwards as I put more miles on it.

It's puzzling cuz even while coasting at low speed, the STFT starts to creep up when I hit the brake and come to a stop. When I got back home I parked it and the STFT was fluctuating between 4.2 and 8.6 for the most part. Out of curiosity, I stepped on the brake repeatedly, and when I did that I was able to make it go up to 10-11. That does seem like it could be normal though. When you press the brake, you're activating the brake booster, which should cause the engine to run somewhat differently. Am I wrong about that?

A vacuum leak does seem like the most likely scenario at this point, but like I said, I did spray brake cleaner everywhere. And the STFT does not go down when I accelerate like in my Dad's car that probably has a vacuum leak. And in my car the STFT actually does go to 0 when coasting.

Next step for now is the primary upstream O2 sensor. I ordered one from rock auto yesterday and used the 5% off code that works indefinitely (just google rock auto coupon code). It cost me $34.99 all in shipped for an NTK OEM replacement. I used the same exact one last time 90K miles ago. A note on that is to be careful of fakes. I think I saw a couple of fakes on Ebay, and that's a real thing now. So I went with RockAuto even though it cost a few dollars more.

If the O2 sensor doesn't fix it, I may just accept the fact that I'm gonna have to pay $6 extra per tank of gas indefinitely while I have this car. Not the end of the world. And I think my mileage was decent this current tank, but it's hard to tell cuz I've been putting in 2-3 gallons at a time waiting for this fuel pump to arrive.

I'll update again after the O2 sensor has been replaced.

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291 Posts
Discussion Starter · #3 ·
Second Update:

Installed the upstream oxygen sensor today. The OEM NTK part #24291. If you've never done it before, it's probably on par with the difficulty of changing your oil, but a lot less messy. My car doesn't have the bottom cover part anymore, so all you have to do is jack the car up (if you're lowered), and use an adjustable wrench and it comes off easily from underneath. No oxygen sensor wrench required for either of the two O2 sensors on this vehicle.

The replacement did absolutely nothing. The sensor was charred just like the original one that I took off around 230K miles, but it was fine. This one had about 90K miles. So probably a waste of money.

Somebody said I should just take it to the dealer for a diagnostic, but I'm pretty sure they wouldn't be interested in really helping me. The fuel trims aren't tripping a check engine light, and they say as long as the fuel trims are under 10 that it's considered normal. Probably just tell me it's a worn engine. Vacuum leaks are very hard to find if it is indeed that, and if it's a just a hose or something, it's not a big money maker for the dealer who's goal is to charge a minimum of $500 a ticket (my friend used to work at a Honda dealer, it was kinda rule).

Maybe I'll mess with it this weekend, but I'm interested in doing another tank of gas and seeing how the mileage goes this time around. I don't think the fuel gauge dropping like crazy anymore, so maybe it's at an acceptable level now. If not, I probably just have to deal with it anyways.

I've been constantly checking my fuel trims while driving. Long term is usually about +6 or +7 at idle at a traffic light. Short term is usually between +4.2 and +8.6 at the same time. When I first turn on the car, the short term is usually like +12 or +14 if the car is cold. That's not normal.

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291 Posts
Discussion Starter · #4 ·
The other day my STFT went all the way to +21 at one point when I was at a traffic light.

I just wanted to post one thing that I just read off a web site that is very useful info, because I've never heard anyone else say it before.

"On engines that use a manifold absolute pressure (MAP) sensor to determine airflow, a vacuum leak does not affect fuel trim because the extra air (pressure) in the manifold is still measured by the MAP sensor."

Our cars have a MAP sensor, not a MAF, so it's not a vacuum leak if that statement is true. And if it was a vacuum leak I'd probably have driveability issues or misfires I think.

I haven't done anything with the car since my last post. I just added 4 ounces of Marvel Mystery Oil hoping to clean the injectors out a little bit. I drove about 80 miles and filled up. I think I got like 24 MPG, but that was mostly local I think. Dunno what's considered "city." The roads I drive on were mostly 45 MPH speed limits, but they're not highways.

I have a tank right now that has like 90% highway on it, and the fuel gauge isn't dropping as quickly as the 19 MPG tank.

Right after I topped off the gas for the first time since the fuel pump, I actually had like 1 mile where the STFT was -3 and LTFT was +2 and I thought it was fixed. But it went back up again. LTFT is set at about +7-8, STFT averages about the same +7-8.

I ordered a used fuel cap today off ebay. I just can't swing $55 for a new OEM Honda fuel cap right now. My old one had 320K miles on it before I got that gas cap code, so these things last a long time. I'm just weary about aftermarket caps, even if Stant is supposedly a good brand.
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