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I’ve searched up and down, and haven’t found anyone with my same symptoms. I have also posted this over at honda-tech, but I’m looking to get this repaired as quickly as possible, so I’d like to get more eyes on the problem.
99 civic LX automatic, d16y7 w/ 208,000 miles, bone stock
Long story short:
Head gasket must have been failing for some time, only noted symptoms were a very slow loss of coolant. Had a bypass hose blow out on me and as soon as I saw the temp rising, I pulled off the highway and shut down the car.Headgasket replaced, new intake/exhaust manifold gaskets, new injector seals, new TB gasket (along with cleaning TB and all IAC valve), new TB/WP, new spark plugs (NGK Iridum), all new exhaust besides cat and muffler due to old leaks/rust. This was all completed around 205,000. First startup went perfectly beside throwing a P0420 code (Catalyst System Efficiency Below Threshold Bank 1) which I erased and it has not returned since.
Since all of that, I have been getting a black carbon gunk that is coming out of my tailpipe, and my gas mileage has dropped. My O2 sensors appear to be working, but my fuel trims seem off for idle. With a OBDll reader hooked up after the car has warmed up, my STFT is at +18% and LTFT is at about +17%. When is rev the engine to about 2,500 rpm the STFT drops to a normal range of -.2%-+3%. I can’t find any vacuum leaks, and my vacuum sits at 22.5 at idle. Upstream O2 sensor seems to be fluctuating like it should, but the downstream seems sluggish in it’s response sitting between .7-.8 volts at idle unless I really rev it, then I see a change in volts.
I also had a P1457 code which ended up being a huge crack in my purge line from the fuel tank to the canister under the hood. That has been repaired and I haven’t had any CEL since. I’ve done the idle relearn procedure twice now hoping to fix the gas mileage issue. Car runs perfect besides that. Everything is very smooth, idles at 700rpm. Timing advance is at 12 degrees. I run 87 octane with 10%ethanol and 5w30 synthetic blend oil. Car does burn about a quart every 600 or so miles, but I’m guessing that is because of the age of the engine and also that I had an overheat which could have warped a sleeve? I also replaced the fuel filter with a K&N within the last 100 miles.
Getting 28mpg combined, but mostly freeway trips running 75-80mph. Maybe it’s just the winter blend of fuel here in WI, but I think I should be seeing mpg is the 30’s at least.
Hoping someone can help out with this! Sorry if I forgot to include any further needed info.
Here’s a shot of my scanner while at idle in P.
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Any ideas? I have checked with propane all over for vacuum leaks and checked timing with a light. No luck finding a cause so far.
Bump. Can’t believe no one has any answers for me. Other than what was previously stated, I have replaced the cat/exhaust manifold due to the crack in it growing large, and new o2 sensors. STFT has come down to 11.7-13% range, but I’m still blowing out a ton of carbon with condensation from the tailpipe.
With a OBDll reader hooked up after the car has warmed up, my STFT is at +18% and LTFT is at about +17%. When is rev the engine to about 2,500 rpm the STFT drops to a normal range of -.2%-+3%.
This is the classic pattern for a vacuum leak. Your title indicates “rich at idle” but positive fuel trim actually indicates a “lean” condition. This excellent two-part video series explains the principles in detail:
Later you note:
I have checked with propane all over for vacuum leaks and checked timing with a light. No luck finding a cause so far
This is a good way to detect vacuum leaks. Carefully point an open, unlit propane torch in the vicinity of various parts of the engine compartment. When you reach the point of a vacuum leak, you’ll notice short term fuel trim leveling out toward 0% (and possibly RPM changes).
You also note:
new intake/exhaust manifold gaskets
The full contact area of the intake manifold isn’t easy to reach in the D16Y7 because this gasket sits underneath the throttle body. Ditto for throttle body gasket. I wonder if you may have missed this area in your vacuum leak tests.
A lean condition under load but not idle can also be caused by a vacuum leak. In this case, however, the engine adds fuel despite there being no need to do so. The air excess air is introduced after combustion and so fuel is added unnecessarily. This may be the reason you’re seeing sooty exhaust. For this reason, it seems worth another try to find a vacuum leak in the vicinity of the manifold gasket (before the O2 sensor).
Take your time with the propane and be thorough. I can’t think of any other cause for what you’re seeing.
Thank you for the explanation. I will definitely check again for vacuum leaks. I did not hear rpm changes when I checked with the propane, but I will watch my fuel trim when I check again.
Do you have an idea of what my STFT should be at idle? And I’m guessing ambient air temps affect fuel trims?
[quote=”civvy” post=200832]
With a OBDll reader hooked up after the car has warmed up, my STFT is at +18% and LTFT is at about +17%. When is rev the engine to about 2,500 rpm the STFT drops to a normal range of -.2%-+3%.
This is the classic pattern for a vacuum leak. Your title indicates “rich at idle” but positive fuel trim actually indicates a “lean” condition. This excellent two-part video series explains the principles in detail:
https://www.youtube.com/watch?v=5WnM_NsOtd8%5B/quote%5D
This was very informative, I finally got a chance to watch it this morning. I did get my STFT down to 4-7% by tightening down my throttle body a little more, re-adjusting my idle via the screw, and setting my throttle at idle to 9.5% yesterday. I put a new throttle body gasket on this morning thinking that was the issue, now my STFT went up to 12% again, ugh. No clue why. I had my propane on full blast with a tube going all over while watching the STFT with no changes.
Any other ideas you could point me towards?
What do some of your other specs look like? For example, fuel pressure. What fuel pressure do you get with the regulator vacuum hose connected and disconnected (as per service manual)?
Something is causing the lean condition. This means that the computer thinks that too little fuel (or too much air) is being received. Thinking beyond vacuum leaks about things that cause lean conditions:
– restricted fuel flow (regulator, pump, leak)
– vacuum leak
– defective throttle bodySome of these fit better than others given what you describe. So far, the best explanation I can think of is a vacuum leak of some kind.
My fuel pressure was 37 with vacuum line attached and 42 with it disconnected. I’m thinking it may be a fuel related problem, or possibly a problem with my injectors. I was able to swap on my brothers fuel rail with his injectors and pressure regulator (approx 150k on his vehicle). My STFT went to 7-9% after warmed up, and once I put mine back on again, I was up to 13%. I neglected to check the fuel pressure after cracking the throttle to see what happened then before I returned the rental tester.
I am currently running a fuel pressure regulator from a ’98 accord since it’s helping the issue. I run 0-1% STFT at idle, but a little rich, -3% at cruising speeds. I know that’s not a perfect fix at all and I don’t plan to keep it on.
I noticed my return line did not have fuel in it when I swapped parts with my brother’s ’97. His did have fuel in the return line. Not sure if that is significant. I also never hear the vacuum release when I open my fuel cap to fill up.
Maybe my injector spray is becoming weak and not delivering what the engine wants? I have cleaned them myself with a battery and carb cleaner pulsing through them. I also know that’s not the same cleaning they’d receive from a professional machine.
I have checked for vacuum leaks with smoke and found none. I checked all intake spots and what I could reach under the hood for the EVAP system.
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