check engine light

Joined
Apr 30, 2018
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pa
have an odd problem , at least to me....my check engine light comes on maybe for a 100 miles then goes off for maybe the same amount ..when i did get to throw the code reader on , it said an evap problem , most likely a leak, shouldn't the light stay on until the problem is fixed ? the car runs fine with or without the light being on by the way a 99 honda accord 2.3
 
Joined
Jun 15, 2003
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38,365
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ME
You're right on the cusp of needing work. The system pressurizes your gas tank, hoses, charcoal canister etc and watches how long it takes to bleed down pressure. If it's too fast you have a leak and a code. Sometimes the system is good enough and sometimes it isn't.
 
Joined
Sep 17, 2012
Messages
2,134
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A Barrier Island
The first thing is to replace the gas cap. If that does not fix it then it most likely needs to go to a shop that can smoke the leak.
+1
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Joined
May 25, 2005
Messages
2,341
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USA
Gas caps are a pain, and can look ok but not seal or hold pressure. Be careful as there are 8 different gas cap "designs ". Some look close but will throw a check engine light unless it's the same type and makes a deal.
 
Joined
Apr 3, 2017
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3,081
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Ontario, Canada
You're right on the cusp of needing work. The system pressurizes your gas tank, hoses, charcoal canister etc and watches how long it takes to bleed down pressure. If it's too fast you have a leak and a code. Sometimes the system is good enough and sometimes it isn't.
Well, it doesn’t pressurize the system, the car has no way of doing that. It puts the fuel system under a vacuum and watches for a pressure increase which indicates a leak.

For OP, by far the most common fault is a failed evap canister vent shut valve. That would require replacement of the vent shut valve and charcoal canister. A fairly easy repair.
 
Joined
Jun 19, 2020
Messages
1,370
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Iowa
Before replacing the gas cap I’d try some Vaseline on the o-ring. I’ve used this to diagnose the evap system before. My guess is on a 99 you have a weathered or cracked hose or what @mightymousetech has suggested.

Just my $0.02
 
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