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This happened twice last summer, and twice this summer. On very hot days ~90F, only after my car has been sitting in the sun all day (so, right after work), it won't start.

The first time I thought it could be a dead battery (which makes no sense, because lights, radio, fan all worked), so I got a jump, which didn't work (of course). I left the hood up -- and after 15 minutes or so, it started.

The second, third, and fourth time, I just lifted the hood, and after 15 minutes or so, it started. I did this thinking that keeping the hood up perhaps helped it cool off.

After it starts, it just keeps starting -- the very first time, I just drove right to my mechanic, but he couldn't make it happen, and he didn't know what was wrong -- they kept it overnight, but didn't find anything wrong -- I only got the heat clue later when it happened again on a hot day.

I am taking it in soon for other repairs, so any ideas of what to suggest would be welcome.

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I look forward to seeing the results. I have the same problem with my '91 Toyota, except that it only happens on really cold days... :-) Our leading suspicion was fuel injectors due to it giving a big black puff of smoke when it finally starts (when it does start), but we ruled those out eventually by testing with the flood start mode and having zero change in behaviour. Current suspect is ignition, but haven't been able to catch it in the act of failing. Only likes to fail when I'm at work and nowhere near tools (have attempted to duplicate those conditions, but it doesn't fall for it). –  Brian Knoblauch Jul 12 '11 at 17:56
    
I was listening to Car Talk today, and a very similar problem came up. They said Hondas develop faulty fuel pump relays, and cause this behavior on hot days. The solution was to replace it, which I think they said could be done behind the dash. –  Lou Franco Aug 14 '11 at 15:48
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3 Answers

up vote 2 down vote accepted

So when your car doesn't start, what happens? does the engine turn over? or you just get some clicking but nothing else?

If the engine does turn over, but doesn't get going, then it could be the injectors. This is just a guess, but its easily verifiable. Attach a fuel pressure gauge to your car (or suggest this to your mechanic), let it run, then stop the engine and monitor the gauge. Is there a slight pressure drop that occurs over time (few hours)?

I had 89 integra with a very similar problem. If you just stop the car it would start fine. If it sits for few days, it would start fine. But if it sat for 6-8 hours, especially on a hot day for whatever reason, it would be very difficult to start it. The cause (based on everything I researched), leaky injectors which over time would flood the cylinders with liquid gas making difficult to start them. After day or so, the gas would leak past the rings into the oil pan, that's why it would start after sitting a long time.

I ended up buying another set of fuel injectors from a junk yard ($7/per). Mailed those in to have them professionally cleaned and calibrated. Swapped them in for the ones that were in there and that problem never came back.

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It's more than clicking, the engine feels like it's just about to start, but doesn't. –  Lou Franco Jul 12 '11 at 12:36
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Another common problem in this generation of Hondas is a faulty main relay. I had a 90 civic that had a very similar problem. The problem is that this little circuit board's solder points have started cracking and are not making proper connection. For several years when this had happened to mine, I would firmly tap on the relay while trying to start it and it would kick right in. This website has detailed information with pictures on how to locate your main relay.

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Starter motor turning the engine but no ignition is very commonly caused by corrosion or loose connection at the main ground terminal on the thermostat assembly. This is where the computer and the ignition system (distributor etc.) get their ground. It's located at the end of the lower radiator hose. Take the bolt out and clean the connection well then put it back on and see if this fixes the problem.

I suspect it's also possible that the whole assembly has poor conduction to it if you've had cooling system problems; corrosion may have formed on the back surface of the thermostat housing (where it mounts) especially if the thermostat gasket is worn and some coolant has escaped there. If you've used radiator stop-leak products, they could have made it worse. I'd recommend using a multimeter to check resistance between the negative battery terminal and this ground terminal, and if it's more than a few hundred ohms (wild guess; anyone know what it should be?) try running another ground from a known-good point (or direct from the negative battery terminal) to this terminal. If that fixes it, you'll want to take out the thermostat and clean the surfaces very well, then put it back together with a new gasket.

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