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Old 10-21-2010, 07:08 PM   #1
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Exclamation Why is my car dumping black smoke?

It is a Z28 with gutted cat, CAI, aluminum intake elbow, and a magnaflow muffler. When i floor it a really noticible abount of black smoke comes out of the exhaust. I know that it is prob a fuel mixture problem but could someone point me in the right direction?
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Old 10-21-2010, 07:44 PM   #2
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black smoke is either a really rich mix or piston-ring-blowby. blowby is usually a blue tint though
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Old 10-21-2010, 08:10 PM   #3
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i dont think this is blow by, because it leaves carbon on the bottom of my bumber after a while
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magnaflow exhaust
K&N cold air intake
aluminum intake elbow
UMI lca's
UMI panhard bar
drilled and slotted front discs
silver C5 corvette wheels
Kicker 10" sub with 230 watt amp
Blacked out tail lights
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Old 10-21-2010, 08:11 PM   #4
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any ideas on how to solve the mixture problem?
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1994 Z28
magnaflow exhaust
K&N cold air intake
aluminum intake elbow
UMI lca's
UMI panhard bar
drilled and slotted front discs
silver C5 corvette wheels
Kicker 10" sub with 230 watt amp
Blacked out tail lights
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Old 10-21-2010, 08:40 PM   #5
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.....scan your PCM, it's possibly a faulty coolant temp sensor, or air intake temp sensor. It's best to start with a scan first.
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Old 10-21-2010, 09:16 PM   #6
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if its something like that, wouldnt my check engine light be on?
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1994 Z28
magnaflow exhaust
K&N cold air intake
aluminum intake elbow
UMI lca's
UMI panhard bar
drilled and slotted front discs
silver C5 corvette wheels
Kicker 10" sub with 230 watt amp
Blacked out tail lights
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Old 10-21-2010, 09:37 PM   #7
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If you have a good scanner, check the fuel tank pressure (given in inches of H2O). Lacking the scanner, take it for a ride, then take off the gas cap. A big rush of pressure from the tank means you probably have an evap. system problem. Pretty common if you always try to squeeze in that last drop during a fill up.

Another possible problem is the pressure regulator.
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Old 10-21-2010, 10:20 PM   #8
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my car does have the rush of pressure that you are talking about. how complicated is fixing the evap system?
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1994 Z28
magnaflow exhaust
K&N cold air intake
aluminum intake elbow
UMI lca's
UMI panhard bar
drilled and slotted front discs
silver C5 corvette wheels
Kicker 10" sub with 230 watt amp
Blacked out tail lights
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Old 10-22-2010, 08:48 PM   #9
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Was going to take some pics to add to my text, but guess I wasn't feeling as well as I thought I wasn't.

There's 3 things you really have to pay attention to:
The rubber purge hoses: Make sure they're all firm and not collasped. These lines weren't meant for raw fuel, and if you have a habit of overfilling the tank, you might have got fuel into the lines.

The carbon canister which hides some where in the front end. Expanding fuel vapors from the tank are purged through carbon granuels to atmosphere. There used to be a replaceabe filter on this, knowing GM you probably have to replace the thing now. Mine is a rectangular plastic unit. The tank vents through this. It could be clogged with dirt or contaminated with raw fuel. The old ones (metal) you were able to blow air through to clean out for a bit.

The last item is the purge valve. The purge valve is next in line after the throttle housing in the intake syatem. It's job is to draw the purged hydrocarbons from the carbon can. It's possible the manufacturer of your aftermarket intake could have not drilled through into the intake tract causing the valve to not work.

Basically you have to make sure the lines from the tank to the carbon can are clear and the can is allowing excess pressure to vent off. In the early days of emissions, the tank cap would vent, but not now.
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Old 10-22-2010, 10:39 PM   #10
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Quote:
The carbon canister which hides some where in the front end.
No it isn't it's above the DS rear wheel well by the gas filer.
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Old 10-23-2010, 12:22 AM   #11
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Quote:
Originally Posted by NavyVet View Post
No it isn't it's above the DS rear wheel well by the gas filer.
Seems a rather "dirty" place to put something that's going to be sucked into the intake system. Good place to put a vapor/liquid seperator.

On my '01 V6 it IS up front.
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Old 10-23-2010, 10:12 AM   #12
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Quote:
Originally Posted by 01 Camaro cp View Post
Seems a rather "dirty" place to put something that's going to be sucked into the intake system. Good place to put a vapor/liquid seperator.

On my '01 V6 it IS up front.

Must have changed the location in later years.
However, the evap system isn't the OPs problem.
The litttle bit of pressure in the tank is nothing compared to the 40+ psi in the fuel and injector system provided by the fuel pump.
It's a MAF or O2 sensor problem.
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96 T/A, Black/Graphite Leather, T-Tops, 6 speed, 10 speaker Delco/Bose "Monsoon"
Original owner, 188k miles and counting.
Total engine rebuild at 187,020 miles.
AIR delete. Bored .030 over.
Comp cams Xtreme Energy LT1 266HR-14 street cam and springs..
lift I .500 E .510 duration I 266 E 276
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Old 10-23-2010, 11:51 AM   #13
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do you guys think the rich mixture is hurting my car? because it stills runs fine. me and some friends went to a local track last night and u ran a 9.491 ET at 77.05 mph and its basically stock. so if its not hurting anything i might just leave it alone.
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1994 Z28
magnaflow exhaust
K&N cold air intake
aluminum intake elbow
UMI lca's
UMI panhard bar
drilled and slotted front discs
silver C5 corvette wheels
Kicker 10" sub with 230 watt amp
Blacked out tail lights
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Old 10-23-2010, 12:10 PM   #14
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Quote:
Originally Posted by mharp12 View Post
do you guys think the rich mixture is hurting my car? because it stills runs fine. me and some friends went to a local track last night and u ran a 9.491 ET at 77.05 mph and its basically stock. so if its not hurting anything i might just leave it alone.
Only thing it hurts is your gas mileage.
It's nromal to see some black smoke at wide open throttle, however if you see it at all times, it isn't a good thing as constant excessive fuel is bad for the piston rings and causes excessive cylinder wear.
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96 T/A, Black/Graphite Leather, T-Tops, 6 speed, 10 speaker Delco/Bose "Monsoon"
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Total engine rebuild at 187,020 miles.
AIR delete. Bored .030 over.
Comp cams Xtreme Energy LT1 266HR-14 street cam and springs..
lift I .500 E .510 duration I 266 E 276
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Old 10-23-2010, 04:23 PM   #15
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it's not constant. pretty much 3/4 to wide open throttle, but only when i get on it from a slow speed or standing start. The newest thing is that a guy flagged me down today because i floored it in a straight stretch and fire came out of the tail pipes when i lifted LOL
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1994 Z28
magnaflow exhaust
K&N cold air intake
aluminum intake elbow
UMI lca's
UMI panhard bar
drilled and slotted front discs
silver C5 corvette wheels
Kicker 10" sub with 230 watt amp
Blacked out tail lights
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Old 10-24-2010, 10:56 AM   #16
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Quote:
Originally Posted by NavyVet View Post
The litttle bit of pressure in the tank is nothing compared to the 40+ psi in the fuel and injector system provided by the fuel pump.
I think you're underestimating the amount of pressure an unvented fuel tank can develop. Enough to blow the old style caps right out of your hand and cover you with raw fuel. The reason we have screw caps now.

The fuel pressure regulator for proper operation depends on min. back pressure. The older low pressure systems could easily be over powered by the excess pressure. The reason I asked if he had access to a good scanner that could monitor the tank pressure.

Quote:
Originally Posted by NavyVet View Post
It's a MAF or O2 sensor problem.
Either condition would throw a code and cause the ECU to go into 'limp in' mode. Something that, again, can be confirmed with a good scanner. Injector pulse width would be =< 3ms. and wouldn't vary much while being driven.

Quote:
Originally Posted by 96firephoenix
black smoke is either a really rich mix or piston-ring-blowby. blowby is usually a blue tint though
That's what the PCV system is for. To recycle blowby. You would really have to have a LOT of blowby to get any smoke out the tailpipe.

Quote:
Originally Posted by mharp12
The newest thing is that a guy flagged me down today because i floored it in a straight stretch and fire came out of the tail pipes when i lifted LOL
You're pushing WAY too much fuel through that engine. And yes, excessive fuel can cause engine damage. Besides the scuffing earlier mentioned, you have to worry about oil dillution and acid buildup. Excess hydrocarbons from the fuel and nitrogen from the air mix with condensation (normal during warm up) and form hydrochloric and sulfuric acids. Reason they reccomend against short drives.

A new question for mharp12: With your CAT gutted (which is part of the soot problem), how do you keep your service light from throwing a code? Also, is the EGR valve operative?
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Old 10-24-2010, 11:46 AM   #17
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Either condition would throw a code and cause the ECU to go into 'limp in' mode
There are several failure modes where the MAF will Not throw a code.

Quote:
You're pushing WAY too much fuel through that engine.
Ah but, mharp12 posted later that it only blows black smoke above 3/4 thottle which is fairly normal for an LT1.
Remember, a stock LT1 has 24 pound ph injectors, 8 * 24 = 192 pounds per hour which is a rate of 30 gallons per hour at WOT.
You're gonna see some black smoke at WOT.
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Original owner, 188k miles and counting.
Total engine rebuild at 187,020 miles.
AIR delete. Bored .030 over.
Comp cams Xtreme Energy LT1 266HR-14 street cam and springs..
lift I .500 E .510 duration I 266 E 276
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Old 10-24-2010, 12:27 PM   #18
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It is most likely a tune issue, nothing more. Any competent tuner can lean it out to cure it. All GM cars will run right around 11.8-1 afr at WOT, not an ideal ratio at all. It's a safe ratio which is why it is done that way. I've neevr heard of a faulty evap system causing problems like the OP has. His car has a few mods and when he floors it, there's black smoke. It's somewhat common. His ET is right in line with a stock LT1 f-bod. A good tune from a vendor or pcmforless would do wonders.
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Old 10-24-2010, 04:28 PM   #19
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Quote:
Originally Posted by NavyVet View Post
There are several failure modes where the MAF will Not throw a code.


Ah but, mharp12 posted later that it only blows black smoke above 3/4 thottle which is fairly normal for an LT1.
Remember, a stock LT1 has 24 pound ph injectors, 8 * 24 = 192 pounds per hour which is a rate of 30 gallons per hour at WOT.
You're gonna see some black smoke at WOT.
Actually closer to 26 gallons (7.34 lb/gal @ .739 sg) and 33,308.6 cu/ft air or 555 cfm assuming 100% volumetric effic.

mharp12 also said he's shooting flames out his exhaust .... Ok for race track Turbo 911 Porches and RX-8's but not normal for LT1's.
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Old 10-24-2010, 04:38 PM   #20
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Except gasoline weighs 6.2 to 6.3 pounds per gallon depending on temp,
according to what I find on the web.
JP4 jet fuel is only 6.66 pounds per gallon, which I know is a fact.
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96 T/A, Black/Graphite Leather, T-Tops, 6 speed, 10 speaker Delco/Bose "Monsoon"
Original owner, 188k miles and counting.
Total engine rebuild at 187,020 miles.
AIR delete. Bored .030 over.
Comp cams Xtreme Energy LT1 266HR-14 street cam and springs..
lift I .500 E .510 duration I 266 E 276
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