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I've searched up and down, and haven't found anyone with my same symptoms.
99 civic LX, 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.
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 a 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.
Ok, well thats good to know at least. Would I see any increase if I switch to a non-ethanol fuel? I know these cars are made to run on 87, but I could get an 89 that is ethanol free.
Are my fuel trims messed up though? I see no reason they should be like that at idle. Also, is it normal for the car to read an engine load of 25.9% while sitting warm at idle? That worried me a little...
Does the downstream O2 sensor voltage seem a bit off for sitting idle? I'm not sure if it should be fluctuating up and down like the upstream sensor does?
Today while driving I checked my fuel trims, and with about 40 minutes of highway speeds, the STFT was perfect at -1 to +3% and the LTFT was beginning to trend that way as well. For some reason, the ECU is requesting a bunch of fuel when the car is at idle in park.
Any ideas? Hope that helps to narrow it down more.
You must check your spark timing. That will kill mpg and hurt performance, but no codes or other symptoms.
Burning that much oil will make soot. Watch for actual black smoke, that would mean running way too rich. The downstream O2 sensor is only to check cat performance. It should move slowly when the cat is good. If the cat were not actually converting anything, the downstream signal would more closely follow the upstream sensor, and the ECU will throw a P0420 code.
Are spark timing and ignition timing the same? If so, my timing is perfect at 12 degrees right now. I don't have soot when I step on the gas, I did record a video and didn't see anything out of the ordinary. I do however have a dried black soot puddle on my garage floor where my car warms up before I go to work that is new since the headgasket job.
Are spark timing and ignition timing the same? If so, my timing is perfect at 12 degrees right now. I don't have soot when I step on the gas, I did record a video and didn't see anything out of the ordinary. I do however have a dried black soot puddle on my garage floor where my car warms up before I go to work that is new since the headgasket job.
You need to set the timing with a timing light. Only way to be sure. A lot of the data you see with a scan tool isn’t actual but just a calculated value.
If your trims are only high at idle, that is a textbook case of a vacuum leak. Just because your MAP sensor reads a certain vacuum doesn’t mean there isn’t a leak. You need to smoke test the intake or spray various areas with brake cleaner and wait for the idle to jump up. Another thing that people always fail to check is the PCV system. If you’re burning that much oil all of a sudden, you need to check the entire PCV system as that’s an easy way to burn a lot of oil. It can also be the cause of your high trims. Any kind of uncalculated air into the intake will cause trims to rise including too much air through the PCV.
You need to set the timing with a timing light. Only way to be sure. A lot of the data you see with a scan tool isn’t actual but just a calculated value.
If your trims are only high at idle, that is a textbook case of a vacuum leak. Just because your MAP sensor reads a certain vacuum doesn’t mean there isn’t a leak. You need to smoke test the intake or spray various areas with brake cleaner and wait for the idle to jump up. Another thing that people always fail to check is the PCV system. If you’re burning that much oil all of a sudden, you need to check the entire PCV system as that’s an easy way to burn a lot of oil. It can also be the cause of your high trims. Any kind of uncalculated air into the intake will cause trims to rise including too much air through the PCV.
Thanks for the quick reply. Are there common vacuum leaks spots on the Y7's I should look to first? How would I go about checking the whole PCV system? I replaced the valve with a brand new one when I did the headgasket, but I'm guessing there's more to it than that. I will definitely look at getting or renting a timing light.
Well as far as I can tell there are no vacuum leaks. I did carb cleaner spray first then used propane with an extension so I could get all over near the throttle body and other connections. Checked all over the injectors, EVAP canister, solenoids, and lines, PCV canister and valve, intake gasket and throttle body gasket, IACV, brake booster and cruise control, and the line from back of intake to the fuel rail.
Any places I missed or should check even further? I will be getting a timing gun this afternoon.
The timing was very close at 10 or 11 degrees. I went through the procedure and it is now set at 13 or 14 degrees. I also have a photo of my spark plugs, as I noticed a slight misfire feeling from revving the engine to 2,000 RPMs and holding it there. No codes were thrown. Is number 4 spark plug, the one all the way on the right in the photo, fouled? I ordered new plug wires and a cap and rotor while I'm at doing all the timing since those were never replaced in the 70,000 miles I've owned the car. Don't worry, they are just laid out from left to right, they were not in the car that way
Still have no idea what is causing all that soot/carbon to be blasted out of the exhaust. I have new plugs, dizzy cap and rotor, ngk wires. Also added some different injectors that were cleaned. Even swapped in another ecu I got for free from a donor car. Checked fuel pressure and its within spec with the vacuum hose attached and disconnected. There was also tons of condensation that comes out with the carbon.
I'm still at a loss as to what's causing the soot blowing out. Is it oil possibly?
I have since my last post replaced my cat/manifold since there was a crack on top, and new O2 sensors. My STFT has come down to 11.7-13% since that. The soot only starts coming out after the car begins to warm up.
my last question is what should my throttle be reading at idle on a obd scanner? Mine is at 11% I did clean my throttle body in a parts cleaner when it was all apart from the head gasket job, but I kept the tps dry the whole time I was cleaning.
Update on this whole thing. I was able to get a few numbers off my brothers '97 Y7, his fuel trims were perfect (LTFT 2.1%) so I tried to adjust a few things to mimick what his was doing. I adjusted my throttle closed a tad more to 9.2%, cranked down my throttle body a tad over spec (16ft lbs), and changed my idle speed to 750 via the screw on the TB. This somehow brought my STFT down to 5-7%. I figured my TB gasket was leaking a tad so I put a new one on, now my STFT is back to 13%.
This is all confusing the hell outta me. On the bright side, no more soot is coming from my exhaust since initially adjusting the throttle plate and screw.
I'm thinking of next trying to disconnect my PCV hose and plug that spot on the intake manifold to see if that changes my STFT back to normal. I'm at a loss as to what else it could be. I blew propane all over evap and vacuum lines with no changes.
Are there EVAP spots known to develop leaks that wont trip a CEL? That's my only other guess.
If you see no external oil leaks, then your engine is burning a lot of oil. Compression test the cylinders (with versus without oil added to cylinders). If the numbers look good, then you may need to replace the valve stem oil seals. If you don't fix the excessive oil burning, you'll likely kill the new cat in short order.
Cylinder 4 appears to be burning the most oil.
Also make sure the throttle cable is adjusted to have some slack so that the throttle plate is not stuck partially open without the throttle pedal pressed down. See service manual.
Thanks for the replies. Still dealing with the high STFT. I changed oil yesterday finally, went to rotella T6 (5w-40) just to see if it will slow the oil burning. (I did not notice any thin oil or gas scent to it at all) I do also have a small leak I noticed. Maybe rear main seal? That area has some oil on it, and I get a single drop on the ground once every other day or so.
Next on my list is to swap injectors and rail with my brothers in the next few days to see if that fixes the STFT.
I found a very small vacuum leak, it was on the IAVC where the magnetic switch attached to the IACV body. I applied some liquid gasket and tightened it back down. I then rechecked with smoke through the brake booster line and did not see any further leaks. I capped the PCV quick to check if that was the problem but it is working properly.
I'm very open to other suggestions on what else could cause this. The extra fuel/soot spray has returned since the ecu got reset with me having to put a new ground wire on my battery.
Last edited by mx5med; 01-10-2019 at 12:31 PM.
Reason: Added
Well, my issue is solved. It turned out to be my fuel pressure regulator even though I was within spec with the car running at idle. I neglected to check how long I held pressure after shutting the car down though.
I pulled my vac line off the regulator while watching my stft, and it went perfect. So yesterday I was able to get a junkyard regulator off a 98 accord and my stft was at 5-6% on an ice cold start, and once the car was fully warm stft was at 1.9-3%.
Only question I now have is are the regulators different between the accord and civic? I know the accord was a 2.3l. While my stft was perfect at idle, while running highway speeds (70mph) my stft was running -7% roughly the whole time. Is that because made to give enough pressure to a 2.3l engine so my ecu was cutting back? I'll try to get my hands on a regulator from a y7 and see what happens.
12-01-2018, 09:50 AM
Poor fuel trim along with bad gas mileage
