Rep Power: 0

Working on a 05 Honda Civic cars runs great has 185,000 on it automatic transmission. I can drive all day on the highway wont overheat, as soon as I get off the highway it goes right up and overheats. If I sit in park and rev it 2-3 grand the tempature will start to go on. That led me to believe it was the water pump. Took that off today water pump is perfectly fine. I have changed the coolant temp sensor, radiator fan switch, now going to be water pump and timing belt. I block tested the car to see if exhaust gases were getting in the coolant (blown head gasket) and it came up negative. No excessive water in the exhaust, no sweet smell nothing. Is there another test I should do to see if its a blown head gasket? Leak down test? Compression test?? Could the radiator be clogged?? Please help? No coolant in the oil also. should I just go ahead and replace head gasket? I havent put the timing belt/water pump together yet.

 
Rep Power: 191

I'd guess it really is the HG, and I'd do the block test again. Make sure you are using fresh, sealed fluid (buy the NAPA jug... I hear that the AutoZone fluid might be garbage, though you can use the NAPA fluid in the free AutoZone loan-a-tool).

Start with a cool engine, suck some coolant out of the radiator (or drain some out of the bottom), then start the car and rev between 2k-3k, for about a minute... then let it idle, and go check it with the tester.

FWIW: My first test was OK, but given a few months of time and a bunch of head scratching to figure out the problem, it came up positive the last time a few days ago... my new gasket kit and bolts are on their way .

Tip: Verify the tester is sucking air through the bottom nozzle and not through a crack in the seal between the clear tube and rubber stopper... that will give a false result by sucking fresh air through the fluid.

 
Rep Power: 313

Have you bled the air properly from the system? Change your rad cap yet?

 
Rep Power: 0

Start off with the compression check and look at your spark plugs for excessive cleanliness or moisture before you jump into the head gasket.

Also, have you utilized the dye test kit in your coolant system to check for leaks? Do you hear any hissing coming from the engine? Is your exhaust have any white color to it?