I have a 1990 Honda Civic DX, with 123k miles on it, I've had for several years and this summer (and part of last) it won't start if it's too hot.

I know what you're gonna say, "It's the main relay." It's not. I have replaced it twice now with no luck. But the symptoms point to a main relay:

* Starts fine on cool mornings and evenings
* Will not start after it has sat in the sun all day but pushing it to shade and opening the doors for about 10 minutes will allow it to start (I have been going into work early to get one of the few spots that gets shade all day)
* If I turn off the motor and immediately restart it will fire right up. If I wait about 15 minutes it will not start again unless it sits for an hour or two.
* When it does not start I will not hear the 2nd click from the relay nor will I hear the fuel pump running up to the 2nd click. When I get in, turn the key to run and hear the pump and the click, I know I'm good. One funny thing, though: if it's having a bad day and just won't start it will take _2_ tries with the 2nd click before it will actually start (E.g. no click, no click, no click, click (won't start if I tried), click (will start))

It seems to be getting worse and worse (cabin temp threshold for not running is getting lower, I'd say it's around 75F) and bumming rides to work is getting old. It ran fine all winter.

If I didn't just replace the main relay again (just in case the one I bought in the fall was faulty) I would have been 100% sure that was it.

My next culprit is the ignition switch but I don't have any of the symptoms mentioned in this thread. However it is the only thing I can think of that would cause the main relay not to click (and this confirms it to some extent: Starting Problems). I haven't taken the switch apart to check for pitting yet, I'm about to go out and do that now (I have one on it's way in the mail regardless, just want to be done with it), but I've spent _days_ on this and just wanted to see if anyone has had a similar problem and had it turn out to be something other than the main relay and, if so, what that something was.

Any help would be much appreciated. I can go into greater detail about anything that could help, I can listen for other symptoms, I can take pictures, anything, you name it. I'm not a gear head by any means but I am reasonably competent and willing to learn.

Thanks,
Josh

 

If you're sure it's not the main relay, the next part to consider would be the ECU. ECU failure is of course rare, but it is what drives the main relay for the 2 second prime. Does the CEL cycle on and off when it won't start? Do the spark plugs still fire when it won't start? Also check the ground wires under the hood (on thermostat) as those are necessary to power up the ECU.

 

I had a similar problem but mine wouldn't start in the morning and in the afternoon it would start fine. I replaced the the main relay and the fuel pump which didn't fix anything. However, I traced wires for a few weekends and finally came to the true problem. I had a bad ecu. I picked up a used one on ebay for $50. Now it starts and runs all the time.

 

@mk378 No, the Check Engine Light does not flash or do anything out of the ordinary. I have not checked the plugs for spark so I guess that will be my next step. I'm not usually at home around my tools when this issue happens. Though I'm sure I could duplicate it at home.

@lunger Ug, I looked up the pricey for a new ECU and they are not cheap. I'm leery to get a reman but it sounds like you've had good luck with yours?

I think I'll pull the ECU as well and check for corrossion, scoring and that sort of thing. I'll let you know how it turns out. If you have any more tips I'm all ears.

Thanks for your help!
Josh

 

In normal operation, the CEL will come on for 2 seconds then go out when you turn the key on but don't crank. Also the LED on the ECU should give one quick flash then stay out.

 

Correct, the CEL comes on for 2 seconds when the key is turn to "run" and the goes out (if everything is working fine the hum of the fuel pump will stop and 2nd click form the relay will happen then, too). I will pull the ECU tonight and check the LED. Thanks for that tip.

 

A bad ECU is a good guess, but first clean and tighten the G101 ground on the thermostat housing, and also test the two power and several ground wires for the ECU when the engine will NOT start.

 

i thought my main relay was good too, then i opened it and reflowed the solder joints and added more solder. starts strong even in 35 degree Celsius heat. if you replaced it with a brand new store bought one, then that is truly odd.

 

Ok, here's where we stand:

Last night after work I went to pull out the ECU. The garage was warm after sitting in the sun all day. When turning the key, the fuel pump ran and the relay clicked but the car would not start. Repeated this several times with the same results.

I pulled the carpet back to reveal the ECU. When I turned the key to "run" the red LED blinked once and then blinked 16 more times (then a pause and 16 more. I assume it would have kept on going like this). It appears to be ODB0 and according to this post that indicates something with the fuel injectors. Which doesn't make a ton of sense to me since the car runs fine after it gets started.

I unhooked the battery and pulled the ECU out. I brought it inside (where it's cooler) and popped off the the front and back covers. There was no corrosion and no scoring of any kind from what I could see.

About an hour later I took it back out and hooked it up and reconnected the battery. I turned the key to run and the LED flashed once and then remained off. I turned it to start and it started right up. I let it run for 15 minutes (smooth idle, no skips, misses, etc) and then turned it off and then immediately turned it back on. Fired right up. I turned it back off and let it sit for 20 minutes and then turned it back on. Fired right up again. By now it's later in the evening and significantly cooler out. This morning it started up fine again.

@RonJ I couldn't find the G101 ground that you described. There are no wires attached to the thermostat housing. G101 is a new term to me so maybe I missed it somehow. Could it be connected to anything else in the engine bay? I did see a ground connected to the clutch cable and one to the head. Also, testing wires is a bit outside of my skill set and tool set. But I will look into purchasing the necessary equipment and giving it a shot.

In the meantime, I located a scrap yard that may have a matching ECU. If the price is right I'll pick it up today and give it a shot.

Thanks everyone so much for your help. Hopefully, after a year of fiddling, I'll have this licked soon.

 

Also, I read that water and moisture could (obviously) cause issues with the ECU. In the spring I had a bad leak around the windshield that resulted in standing water on the passenger floor a few times. As deep as .25-.50 inches. So that could definitely have been an issue.

At this point I'm thinking my troubles started with a bad relay (year ago) and got compounded by the ECU this year, making me think it was one issue but I really had two separate issues. But again, we'll see as the testing progresses.