Vacuum leak causes HC high in smog
01-04-2007, 12:34 AM
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Car: Firebird
Engine: 383
Transmission: 700R4, lots of Mods
Vacuum leak causes HC high in smog
Hey all
I have a 89 bird with Accel DFI and super ram, it passed the calif pre-test smog 6 months ago, but today the HC's in idle were good, but at 15 MPH was very high about 400 or so, and the guru who did my test told me that the problem is most likely a vacuum leak.
What I need is to be educated on this, how can a vacuum leak casue the car's HC's to be very high.
Timing is good,
cat is good
wires are good.
Thanks in advance
Brad
I have a 89 bird with Accel DFI and super ram, it passed the calif pre-test smog 6 months ago, but today the HC's in idle were good, but at 15 MPH was very high about 400 or so, and the guru who did my test told me that the problem is most likely a vacuum leak.
What I need is to be educated on this, how can a vacuum leak casue the car's HC's to be very high.
Timing is good,
cat is good
wires are good.
Thanks in advance
Brad
01-04-2007, 01:09 PM
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Car: 82 Berlinetta/57 Bel Air
Engine: LS1/LQ4
Transmission: 4L60E/4L80E
Axle/Gears: 12B-3.73/9"-3.89
If the air leak is feeding the side that has the O2 sensor, it will cause a lean reading which the computer responds to by richening up everything. The extra fuel where there is no leak causes the overall to be rich.
01-06-2007, 06:01 PM
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Car: 1987 IROC-Z
Engine: 350 TPI
Transmission: 700r4
Axle/Gears: 3.23
sorry, I just like to read posts, but what are HC's? I'm stumped I can't seem to figure it out, and I'm sure its going to end up being something pretty obvious. Thanks John
01-06-2007, 06:10 PM
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Car: 1987 IROC-Z
Engine: 350 TPI
Transmission: 700r4
Axle/Gears: 3.23
01-06-2007, 06:12 PM
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Car: 83 POS monte carlo 2015 chevy P/U
Engine: 92 5.7 tpi 5.3
Transmission: 700r4 6L60E
Axle/Gears: 2.42 too high
five7kid is right about a leak on the O2 sensor side. you should also see higher than normal CO in that case.
but if you have a vacuum leak on the side of the motor the O2 sensor is not on, you get into lean misfire on that side because there isn't enough fuel for a good burn, HCs go up but CO goes down.
what was the CO, CO2 & O2 readings?
jcisthechamp05, HCs is hydrocarbons, or unburned fuel.
but if you have a vacuum leak on the side of the motor the O2 sensor is not on, you get into lean misfire on that side because there isn't enough fuel for a good burn, HCs go up but CO goes down.
what was the CO, CO2 & O2 readings?
jcisthechamp05, HCs is hydrocarbons, or unburned fuel.
01-08-2007, 10:02 PM
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Car: Firebird
Engine: 383
Transmission: 700R4, lots of Mods
well, to make a long story short, I had two small vacuum leaks and two spark plug wires that had a very small crack therefore I believe my HC was from the spark plug wires, (misfire) problem was the crack was just behind the headers and I believe this would mis-fire every so often. Anyways got that fixed and car passed smog with flying colors, however on way home something went wierd and now I have raw fuel coming from header donut, but motor runs ok, and tonight while car was running there is oil coming from the header donut, 1,3,5,7 cylinder head. So I guess I have a bad head gasket!!! this really sucks for me!!!
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