87SiGuy

1987 Prelude 2.0 si
This intermittent starting problem used to happen 5% of the time now it is more like 20%.
Starter turns over motor just fine.
Fuel pressure checks OK at about 39 psi (port on top of filter).
Spark test indicates it has spark.
Occasionally it throws a code of 6 blinks (ECU window under drivers rear seat ashtray), but this may be unrelated (only thing I can see online is "coolant sensor"?).

Since it usually runs fine, this is hard to diagnose, however it is currently in the "failed state", so I can try things but don't know where else to look.

tamboo

bad coolant temp sensor would cause a no start when warmed up.

Starting Problems

87SiGuy

OK, I see that a bad coolant sensor could cause a no-start on another thread also, but that looks like it is for a newer Prelude.
Where is this sensor? I can see 3 electrical coolant sensors in the metal part just before the large coolant return hose goes back into the top of the radiator. I disconnected both of the 2 pin connectors, and it still wouldn't start. In that other thread someone said to try that. Once it cooled off it did start, but if you let it die (rough idle), then it wouldn't start again.

Are there any more than those 3 sensors for coolant temperature? One is a single electrical pin, and the others are two pins each. Is there somewhere that describes what each sensor does?

tamboo

pm'ed you a link to fsm at h**ndahookup. have fun!8)

87SiGuy

I did get that supposedly "Full" 1987 Prelude shop manual. However, it has no wiring diagrams or mention of the coolant temp sensors. The manual stops at page 27-23, leaving out the Wiring Diagrams (probably because they had to fold out and therefore difficult to scan and pdf).

So, I am still trying to figure out if the no-start is related to the ECT (engine coolant temp), since the 6 blink code on the ECU is the only code ( this may be wrong, or unrelated).

There are 3 electrical coolant temp sensors in the part where the thermostat is mounted, right before the large return hose to the top of the radiator. The sensors #1 and #3 drop in resistance as the motor heats up, and #2 is just a switch that is still open even after heating up to normal operating temp. The #3 just goes to the dash temp gauge, but the #1 goes somewhere else (maybe the ECU). Measuring resistance of the sensors in start and no-start mode shows no major difference. Unplugging sensors in both start and no-start modes makes no difference. (BTW - The two cooling fans operate off a sensor in the radiator, so they aren't involved.)

Relating to the no-start, the PGM-FI light can come on sometimes, but sometimes not. It seems like the normally fine idle can start to miss and falter a little when the motor is headed to a no-start condition. Then if the motor dies, it won't start (ie. no-start mode). If I keep it revved up (higher idle) at a stop sign, then I can usually keep it running until I get home. But, if it dies, or is shut off, then there is a no-start condition. I thought it might be heat related, but took it out on a very cool day and it still went into a no-start mode.

So, I am looking for:
1) What does a 6 blink code from the ECU really mean on this generation motor? (with no CEL)
2) If the code is the ECT, does that really relate to a no-start in this year Prelude, then how?
3) What can I check related to the intermittent PGM_FI light?
4) What other tests can be performed when in the no-start mode?

87SiGuy

UPDATE

I was finally able to get a tech at a Honda dealership that had worked on these older Hondas 25 years ago. Temperature sensor #1 goes to the ECU so it can make fuel injection corrections based upon engine temperature. What apparently happens is that an errant WT (Water Temp) ECT sensor indicates it is VERY cold and ends up flooding the motor with fuel. So the no-start just has too much fuel. He said they used to joke that the sensor should be on a routine maintenance list since they went out so often. I thought that the ECU just decided not to start if that ECT sensor was bad, but it is really trying to start, just giving too much fuel.

The dealership part would be about $72, but is no longer even available. I was able to locate a "new" used part from a junkyard (pull your own), and it is installed and currently working fine. So we will see...

Here is a link to this forum ( https://honda-tech.com/honda-accord-...stion-3008918/) talking about the sensor. I measured about 2500 ohms on this sensor before startup, then it drops to about 100+ ohms at operating temp. When it died and would not restart, I measured it at only about 4000 ohms, but apparently even that extra amount of fuel was to much since the motor was not cold. It is interesting from the resistance versus temp plot, that at -4F the sensor has about 20,000 ohms resistance.

I am hoping my old ECT (WT) sensor just behaved fine "most" of the time, but went errant at times causing too much fuel, and this "new" sensor is better. I am surprised that a relatively slight change in perceived temp can cause enough extra fuel to not just run rough, but actually not start. Also, that you can totally unplug the sensor and it will still run and even start. So, it may take some time (or average) for the ECU to give too much fuel?

I am hoping this is the cure, so I will respond back if not.

Thanks for your help tamboo!

1funryd

I did not see if this is a second gen or 3rd gen lude?

87SiGuy

Since it is a 1987 Prelude 2.0 si, I understand it is 2nd Generation.

tamboo

good to see this (hopefully) resolved.
rockauto sells the aftermarket sensors for $10-20,
couldnt hurt to keep a spare handy.