93 Honda Civic - will only start after sitting all night when its cool in the morning, but not when its hot outside (its been in the 90s) What could this be????

 

main relay!

answer of the week!

 

ya, it very well could be your main relay. the joints inside the main relay box are connected with solder and when it gets really hot outside the solder has a tendency to melt and leave the joints disconnected. they are relatively easy to replace, and are located on the driver side of the cabin usually right above the hood release lever.

 

yep main relay i had this same issue with my 90 civic, replaced the relay and now it runs great

 

It could be the main relay - check that first.

Or it could be the ECT sensor.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by EE_Chris &raquo;</TD></TR><TR><TD CLASS="quote">

Or it could be the ECT sensor.</TD></TR></TABLE>

ummmm......wtf does the ECT sensor have to do with the car starting???

/yrself

 

main relay is located at driver side kick panel.

 

This happend to me once. Turned out to be the the ignition coil. Good luck.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by DirtySol &raquo;</TD></TR><TR><TD CLASS="quote">ummmm......wtf does the ECT sensor have to do with the car starting???

/yrself</TD></TR></TABLE>

When the Honda ECU sees a cold engine (~32*F), it widens start pulse width to about 60ms. On an engine that is already fully warmed up, this will flood it causing it to not start.

Educate yrself.

 

it will not flood it, it is almost physically impossible to have a 190cc injector flood out an engine larger than 1l. It will have a extended crank mtime but IT WILL start. I have seen a vehicle with 360cc injectors start up and run with pig rich fuel trim (which later caused us to have to re-ring the motor).

You do not want to argue me on this, I can and will prove you wrong. I make my living off of technical knowledge of automobiles.

 

an ECT is nothing but a Thermistor, the resistance values vary depending upon temperatures.

If the ECU were to open injectors for 60ms for a -32*f startup that means it just dumped approximately 114cc of fuel in a combustion chamber that is approximately 400cc in size. Yes, it would be considered to be a VERY rich mixture but it would still run.

don't believe me, unplug your ECT sensor and see how rich your car runs, This is simulating a -32*f startup.

 

I'm pretty sure I do, actually. Please, prove me wrong. It's the only way I learn past my own experiences...and I've fixed one car that had this problem. Just one though, which is why I said "Or it could be the ECT." - this was said after I agreed to check the Main Relay first. Might be a long shot, but it's still something to keep in mind.



There's my info. Had I never read that (came from Tech2Tech), I would have never thought to check the ECT when diagnosing that particular car. Maybe it was just sheer coincidence that this literature exists AND it turned out that this was the problem for the car I fixed. But that's enough of a coincidence to at least keep it in mind and present it as a possible suggestion when symptoms like this pop up.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 92_civicsi &raquo;</TD></TR><TR><TD CLASS="quote">ya, it very well could be your main relay. the joints inside the main relay box are connected with solder and when it gets really hot outside the solder has a tendency to melt and leave the joints disconnected. they are relatively easy to replace, and are located on the driver side of the cabin usually right above the hood release lever. </TD></TR></TABLE>
Okay you have the end result right but the cause wrong. I does not get hot enough inside the car to melt solder. Most solders will start to melt around 500*f. What happens is the solder is a connection between relay pin and wire. As the connection is heated and cooled the connection expands and contracts. This flexing of the joint causes it to crack. Things like dirt in the solder, cold solder connection, not enough solder,vibration of the car all play a part in how long it takes before that connection fails. The reason the relay tends to work when the car is cool is the broken connection is tight and making contact. As the interior of the car heats up the connection expands and brakes the contact between the 2 surfaces. Sorry a bit long winded maybe this will help everyone understand the reason why these relays fail

 

its the coil

 

^I agree. One of my friends Civic's does the same thing. When it gets hot his car doesn't start, or after the engine has been run up and is hot. We're waiting on a new coil to replace it out and fix the problem.

 

Main Relay was the problem. DONE! Thanks!!