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Issues with evaporation / ac?


I am having issues with my 2002 chevy cavalier.

  1. The car occasionally surges forward while idle and revs up to 1500 rpm approximately occasionally when the air conditioner is on… if the brake is not applied I move forward… the lurches are continuous every 3 seconds or so.

  2. I am getting a code for an evap leak… I have replaced the purge solenoid valve, gas cap, and checked all connections with no luck… slightly decreased gas mileage has occurred.

  3. A little bit of a slick substance has been leaking from the car… it has the consistency of water, but leaves a slight brown tinge on concrete.

I know that there are quite a few problems here, but any advice is appreciated!

I had an issue in a gm where the rpm would surge for four seconds or so, go back to normal and maybe 10 seconds or so do it again, no cel light for me though. Disconnecting the battery for a minute or so reset the brains of the car and the problem went away. The fix was suggested by a mechanic for a dealer that works on my car on the side. Your cel will probably come back it is hopefully not related.


I would clean the IAC and Throttle body first.

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Above ideas by Barky and knfen good place to start. Some other ideas … the engine computer may increase the idle speed when the A/C compressor is engaged, to prevent stalling. The idea is that the compressor loads the engine enough to reduce the rpm, and the bump in rpm just compensates for that, keeping the rpm steady. Maybe the computer thinks the compressor is engaging when it actually isn’t. Due to a faulty “clutch-engage” sensor or something. That could cause the engine idle speed to increase intermittently when the AC was being used.

If the compressor was starting to fail , binding up, and causing extra load on the engine beyond normal when engaged, that’s a possibility too.

On the evap leak, the engine computer may occasionally pressurize the fuel tank , purposely, then monitor the pressure vs time, to test for evap leaks. So if the pressure sensor was defective, that could cause the computer to think there’s a problem when there isn’t. There’s an evap-related valve at the top of the gas tank in many cars that can fail and cause this problem too I think. Of course if any vent hoses are leaking, that’s another possibility.

On the leak, my guess it is coolant, which indeed feels slick. The brownish stain is probably just rust getting into the drip as it slowly leaks over the iron engine components. This needs to be diagnosed by a shop to know for sure, but it might be the water pump is starting to leak a little from its seal, or just a hose connector isn’t quite tight enough, thermostat gasket starting to fail, etc. Probalby something minor but needs to be properly diagnosed at least. You may decide after figuring out what it is and knowing it isn’t serious to let it go awhile and just keep the coolant topped off.