Piston Slap: Condensing Honda's Hot Air?

Sajeev Mehta
by Sajeev Mehta
piston slap condensing hondas hot air

TTAC commentator Land Ark writes:


I recently acquired a 2007 Honda Civic EX sedan from a neighbor who moved out of the country. I got a really good deal on it and for the most part it’s in good shape. It has 80k miles, 5 speed, and one major flaw. The air conditioning is a little bi-polar; sometimes if blows cold and sometimes not.

It always feels cool to cold coming out of the vents but it doesn’t cool the cabin down. It seems to get warm most consistently while sitting still, like at a red light. My first thought was that there wasn’t enough R gas and I would just trundle down to my local big box retailer and pick up some Cold in a Can. But my thinking was that 2007 is too new for it to be out of gas, could it be leaking? I determined that was not too likely since the air coming out was still cold and because the temperature fluctuates and I would think a leak would cause the temperature to remain constant as the gas leaks out until there is no cold air blowing at all. I definitely could be wrong on that though, I’ve never had an A/C go bad on me before.


  • AC is cool but not cold.
  • At red lights the inside of the cars gets noticeably warmer. On the highway it gets noticeably cooler most of the time and warmer on occasion.
  • At idle the temp just outside the vents ranges from 68 – 72
  • If I get the RPMs up to 2500 the temp just outside the vents will drop to 65.
  • Clutch on the AC audibly engages when turning it on
  • AC cycles on and off as I would expect. The vent temp rises about 5 degrees before kicking back on.
  • The fuses are good.
  • I swapped the relays and noticed no change in behavior. I left the compressor relay out and it stopped functioning as I would expect.
  • The clutch spins when the AC is turned on and does not spin when it turns off.
  • Both fans spin when the AC is turned on.
  • I ran the self diagnostic check and the recirc light did not blink.
  • I measured the pressure with both front doors open at ~80 degrees and got 40 low and 160 high.

I took to the Civic forums and it appears it is a wildly common problem with this and the previous generation. There doesn’t appear to be one common cause or solution. Lots of people trying things and having that fix it only to post a few days later that it stopped working again. I have checked and both electric fans are functioning, I can’t say that they spin all the time but when I checked both were working. So that is the only symptom I have ruled out. I read that sometimes something in the compressor can break off and block the lines. My fear of trying to add some R-134 would be that most cans come with leak sealer and if there is a blockage and no leak I wouldn’t want to send sealer through the system.

The most likely candidate for the problem seems to be the compressor clutch. I don’t know why I think that other than it was mentioned and seems like it would be impossible to fix in your driveway so no one has tried to replace it and reported back that it didn’t work.

I haven’t dug too deeply into the car but I bought it to get used to driving a stick and it spends lots of time sitting when it’s hot out so I am missing some good opportunities to drive. And because I plan on selling this car in a few months, I am not interested in putting a whole new A/C system in it. So what should I be checking/testing? I’ll try anything that doesn’t require releasing the pressure on the system, otherwise I’ll have to take it in.

Sajeev answers:

I’ve personally experienced poor air conditioning in a co-worker’s Civic on a wonderfully hot and humid Houston afternoon. And when A/C systems don’t work but pass your (comprehensive) diagnosis, thank goodness for forums that agree with us. In particular, commentator Rayspitcher41:

“Went and had my A/C recharged and a dye test done for $50. Turns out my compressor was starting to go and so was my condenser. It cost about $650 to repair with labor, but my extended warranty covered it all so I had no out of pocket. I have a 07 with 60K on it. Now the A/C is running Ice Cold within a mile of starting and driving and I’m in south florida with a heat index of 103 every day.”

In theory, air conditioners operate below peak efficiency when the engine is idling (below 1000-ish rpm) because the compressor isn’t effective at those speeds. But that’s Problem #1 in this two-pronged quandary. I suspect one of the HVAC’s radiator lookin-thingies is also to blame: the condenser or evaporator.

I suspect the condenser. Look at which HVAC component takes the most abuse: the condenser is a big net that catches anything in front of the vehicle. (The evaporator safely nests inside the dash.) Assuming that every 8th Gen Civic owner is happy with the HVAC when new, odds are the condenser loses efficiency over time. Combine that with a compressor’s ineffectiveness at idle and you have a hot mess of a problem.

So what do I suggest? In this order:

1. Check the pressures you find against a Honda service manual, add or remove refrigerant safely (don’t vent to the atmosphere, obviously).

2. Keep the idle above 1000 rpm when you need cold air.

3. Clean the condenser “fins” and inspect for physical blockage. Use a condenser fin comb ( yes, really) to fix bent fins.

4. Replace condenser, look for an upgrade from a revised design or newer model.

5. Replace compressor.

6. Give up and sell it.

[Image: Shutterstock user BACHTUB DMITRII]

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

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2 of 37 comments
  • Scoutdude Scoutdude on Aug 11, 2014

    Visit a professional and have them properly charge the system and while they are at it replace the valve cores in the service ports. Hondas are know for leaky valves and with the small amount of refrigerant it needs to be spot on for proper cooling. Do not mess with trying to charge it yourself there is no way to get it properly charged w/o recovering what is in there and recharging with the proper amount. The poor at idle is a symptom of the low charge. Once you get up to speed there is enough air flow for the condenser to supply liquid to the evaporator.

  • Mnm4ever Mnm4ever on Aug 12, 2014

    I live in Florida, we have been having the hottest summer I can remember this year, and me and my family have a bunch of Hondas of varying ages. Two 2013 Civics, a 2003 Civic, a 2002 CRV, plus my 2008 GTI, 2001 MR2, and my brother in law has a 2003 Dodge minivan. Everything except the Dodge has a black interior. All of the Hondas have stellar AC, cools the cabin off quickly, even after sitting in the sun all day, maintains temperature to the point where we almost always have to move the temp dial slightly off full "blue" to not have such cold air pumping in. My CRV as I commented above has an issue if I leave it sitting idling for a while, but that is a new problem, never happened before and we do that quite often for a random reason I dont want to try to explain right now. The MR2 AC is phenomenal, even with the thin convertible top, but I suspect that is more because its compressor is sized for a Corolla sedan and not a tiny 2-seater with half the cabin volume. The VW was the worst of my cars, it would take 15-20 mins to get the interior cool on a really hot day, you could just feel the heat emanating from the dash sometimes. But the heat was amazing, actually too hot to use even in winter in Florida and hard to regulate for slightly cooler temperatures. Clearly the Germans value heat over AC! The Dodge has terrible AC, but that thing is all windows and has no tint so its to be expected. Most of my cars have tint which helps, but not as big a difference as you would think. The MR2 and 2002 Civic are not tinted, the newer Civics and GTI have full tint, the CRV has factory tint only.

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