mx5med

99 lx, auto, 208,000 miles

I'm getting a decent amount of black soot/carbon blowing from my muffler. This covers my garage floor and anything within 3 feet of the back of the car. Car is all stock, and this is a new problem since changing my head gasket 2,000 miles ago. I am also burning oil, about a quart every 600 miles which is also new. I seem to be running high fuel trims at idle (+13%),so I'm not sure if I'm seeing unburned fuel or oil coming from the muffler.

The car idles perfect, timing is 12 degrees confirmed with a light, no power loss at all.

Anyone able to point me in the right direction to get this fixed?


If anyone is curious, here's a list of what else I did when I replaced the HG:
intake/exhaust manifold gaskets
Ngk plugs, ngk wires, oem distributor cap/rotor
new cat/exhaust manifold due to a crack in the old one, along with new o2 sensors
new pcv valve
cleaned throttle body and intake manifold very well, along with IACV
timing belt/water pump
coolant hoses all replaced, along with new radiator and oem thermostat

Colin42

Probably burning oil, pop out the spark plugs and see what they look like

mx5med

Anything that could have caused an increase in burning oil like that after a head gasket? Before the gasket, I burned maybe a quart every 1,000-1,500 miles. Plugs are just dry black around the outer edge, otherwise a pale white all over the insulator. #4 cylinder plug was a little darker than the rest

ezone

Was the head warped, and sent to a machine shop for resurfacing?
Most drivers don't discover their engine needs a head gasket job until long after it's been overheated severely and multiple times (several unsuccessful lower priced repair attempts is typical).
Severe overheating is then blamed for all new excessive oil consumption, due to (take your pick) collapsed rings, stuck oil rings, and/or bad valve stem seals.....This of course assumes workmanship on the repair job was good.

Antifreeze passing into the exhaust can and will contaminate the O2 sensor(s) and can damage the catalytic converter. Skewed sensor output can cause overfueling, among other possible issues.


Black water dribbles (soot plus water) is normal from any engine exhaust
Black clouds from the exhaust might be ok while the engine is cold, but once the engine is hot and the cat converter is hot and active the black clouds should no longer be present--- IF fuel is controlled correctly by the computer and sensors.
BLUE smoke is oil. A working catalytic converter can hide a lot of oil smoke.

Suggestions: Compression test (however this cannot tell you ANYTHING about stuck oil control rings)
Disconnect both O2 sensors and reset the ECM, then see if it still blows black clouds of smoke.

mx5med

Thank you for the reply. I did take the head into a machine shop and they checked it for any warpage, and they said the head was in great shape yet. My radiator is still full to the neck and my overflow is right at full so I don't think my head gasket is seeping at all.

I actually dont have any black smoke at all, it's just the soot that comes out at idle when I let my car warm up in the morning (more than I've seen from any car) I have a cardboard box over my bumper and looks like someone spray painted it black inside. I don't get any puff of smoke at all when I first start it, so I'm guessing my stem seals are ok yet. When the car is warm, the exhaust looks like a normal gray color but it does have an "odd" smell that I cant describe. Maybe it's the new cat still cooking stuff out from the manufacturing process?

Would you know what a normal fuel trim is for a stock Y7 at idle? I have another car I can check, but not for a week yet.

ezone

This sounds normal and expected.

Not sure I would be worried about it yet, not enough info

Is that STFT, or LTFT?
What do trims do at 2000 rpm? 3000 rpm?

Do you have any idea what STFT and LTFT were under the same conditions before any work was done?

It can vary a lot, and it varies constantly depending on conditions. Those older cars probably won't code rich or lean until trim stays beyond around (+ or -) 20% for a certain amount of time.

brags

Im curious to see if a good Long drive out on the open HWY migh help burn any of that soot away?

ezone

Sure.
Until the next cold startup

If OP would start it up and immediately drive it instead of letting it sit still idling in the garage he wouldn't notice the soot much LOL

mx5med

That +13% is my STFT, when I start driving or hold my throttle at 2,000 rpm my trims are "normal" at -1 to +4%. I've checked for vacuum leaks all over with carb cleaner, and then propane but found none.

I have no idea what the STFT was at prior to doing the work.

mx5med

I usually run all highway miles, 50 miles to work one way and I'm going 75-80mph during most of it. Ezone is correct, the soot is shooting out of my exhaust 3 feet the next morning.

mx5med

So my car sat outside overnight (20 degrees) and I would say about 20 seconds after I started it, some gray-ish smoke came out for maybe 10 seconds and then disappeared. This normal? My car rarely is out overnight in winter so I'm not sure.

I thought if I did have bad stem seals, I'd see the smoke instantly on start up and not 20 seconds in.

mx5med

Here's what comes out in the morning. This is approximately 20 minutes of idle over 3 mornings

ezone

So it may have been this way all along and not causing problems?

Again, I would not be concerned about soot, I expect it from just about any car because an engine requires a lot of additional fuel in order to be stable while it's cold.

How about your oil consumption?

mx5med

True that the STFT could have been that way all along without me knowing.

The soot is still odd to me though since the car has never done this. My brother has a '97 LX, same motor and 10,000 less miles, and his doesn't spit any soot out.

My oil is still going down quickly I would say. I just went out and checked the dipstick. I topped it off about 200 miles ago, and I have lost a half quart so far.

ezone

Maybe you're seeing soot as a result of high oil consumption?

mx5med

Sounds like it's getting narrowed down to that. The only other thing I can think of is that when I had the head off, I did pressure wash the under side of it to get all the old carbon off. Maybe taking off all the buildup is allowing for more oil burning now?

mx5med

Well, I'm almost positive I found out what was causing the high fuel trim, and soot. I was able to check out my brothers car today with my scanner and his STFT was perfect, so I looked over a few other things his car was measuring.

All I did was adjust my throttle from 10.9% down to 9.4%
Took out my idle adjustment screw and cleaned that up very well since I missed that when I cleaned the TB. There was quite a bit of carbon/junk built up in there.
I then re-adjusted the screw since I turned the throttle down a bit. Set my idle to 750. I also noticed that I do not have that black rubber plug that is supposed to cover the idle adjustment screw, so I covered it with electrical tape in the mean time.
And finally, I loosened and re-torqued my TB gasket. I did order a replacement gasket. I figure I may have had a small vacuum leak there from the cheap gasket that came in a tune up kit.

One or all of those things brought my STFT down to +5 to +7.2%. Not perfect, but much better than the high teen's I was seeing. I also re-adjusted my timing then to make sure it was still on. The car idled for at least 30 minutes and no soot at all! Hoping the new TB gasket will make my numbers better still.

mx5med

Well, I'm back to square one, minus the soot thankfully. At least I know that was excess fuel being shot out.

I replaced my throttle body gasket yesterday and my STFT is back up to 13%. Everything is torqued to spec and I didn't touch anything else.

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.

mx5med

Thanks for the replies so far. 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.

ezone

Clamp off or disconnect and block the hose or port for the brake booster vacuum line?

mx5med

I will give that a try when I get home in the morning. A side question on that booster line, should it be pretty difficult to blow the valve inside open when blowing from the booster toward the intake? I noticed yesterday it took almost all the pressure from my lungs to get it to open up. (I was the smoke machine yesterday using a cheap swisher sweet for the smoke). Unless it opens easily when a vacuum is created by the intake which is probably how it works?

ezone

It's a spring loaded check valve, so yeah

mx5med

No change at all when I plugged the brake booster inlet on the manifold. I'll report back on here once I try the fuel injectors.

mac25

I had a similar problem with oil consumption after doing my head gasket myself and installing internals.

I had two cylinders who's piston rings where not clocked to the correct orientation. One of which had both piston ring gaps directly over the same wrist pin.

mx5med

Did you replace your rings and pistons when you did your headgasket? I didn't touch my rings or pistons besides spraying carb cleaner and wiping the tops of them off to clean up the carbon. I made sure they were at tdc when I did that because I didn't wanna wipe any small pieces onto the cylinder wall by accident.

mac25

Good point, if you just did the head gasket, you should be fine.

I replaced the rings/pistons/rods.

mac25

Perhaps the wrong head gasket was used. Not sure but I think there are different holes for oil passengers between vtec and non-vtec head gaskets.

Do you have any records for the gasket?

mx5med

I triple checked the holes in the gasket to the passage ways in the block and head before I set the head on. Everything lined up as it should. The kit also stated it was for a Y7, non vtec motor.

mx5med

For those wondering about compression numbers, I finally got a tester. I ran the starter for 5 total "crank/turn over" sounds. The cylinder that I was worried most about turned out to have the highest compression.

1: 175 dry, 180 wet
2: 180 dry, 185 wet
3: 180 dry, 185 wet
4: 185 dry, 190 wet

ezone

that looks pretty even and respectable