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Failed Smog Test

1987 Chevrolet Corvette 5.7L Failed LPFET Fuel Test

Hi. My 1987 Corvette passed everything except the fuel part of the smog test. The tech removed the fuel cap and hooked up equipment to measure fuel leaks and it failed. I don't remember having to undergo this test in the past. My Vette has 62k original miles and is in great shape, engine and mechanical wise. The smog tech says that in order to repair this problem he may need to drop the fuel tank, at a cost of around $300.00. If you could provide me insight on LPFET and repair advise I would appreciate it. Thank you!


If the smog tech traces the evap system all the way to the gas tank and is still unable to find the leak source, he/she may need to drop the gas tank; maybe not entirely out of the vehicle, but at least low enough to inspect and if need be replace the hose(s) leading into the tank. If the tank itself is defective, then obviously it'll need removal and replacement. This would be worst case scenario and is not common.

The Low Pressure Fuel Evaporative Test (commonly known as the EVAP test) ensures there are no leaks in-between the hoses leading from the gas tank to charcoal canister, and really anywhere gasoline and fuel vapors may escape. During the LPFET the smog technician clamps the end of the fuel vapor hose leading to the charcoal canister and pressurizes the gas tank with Nitrous Oxide via the LPFET test tool. The tool will then measure and calculate the drop in pressure due to any leaks. Technically there should be very little drop in pressure once the EVAP system (gas tank) is pressurized. If in fact your Chevy Corvette has a leak somewhere in EVAP system it will be necessary to locate the leak and repair it prior to being able to pass the LPFET. Depending on where the leak is located, this repair may be cheap or expensive. The first step however is to find the leak. Look for brittle, broken, loose, and/or disconnected fuel vapor lines. Be sure not to confuse fuel pressure lines with vapor hoses. Pressure lines deliver fuel only. Vapor line transfer and store fuel tank vapors to the charcoal canister and then vent them to your Corvette's intake manifold.

posted by SmogTips Support 07-06-2017 11:55 AM

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