Antihero47

A buddy of mine has a 2001 Honda Accord Coupe 4Cyl. He's been trying to get it smogged. He had a problem where he was getting a P0128 and we've tried a few things in order to get it back in line.

His coolant temperature gauge will just randomly go down to the very bottom while on the freeway and then bounce right back up. I've hooked up my torque app and monitored the temperature when we were on the freeway. Its stays around 181F and then drops to 117F to 158*F and the needle drops seemingly at random.Then it jumps right back up again after a few minutes of being pegged down.

I've tried...
1. Replacing the thermostat twice with new gaskets (First. Autozone Second.Genuine Honda). After each replacement symptom remained.
2. Replaced the temperature sensor (Genuine Honda) located on top of the thermostat housing. Symptom remained.
3. Drained coolant, vacuumed the system and observed no leaks then filled system using vacuum re-filler and heater engaged. (to keep air out of system.) Symptom Remained.
4. Sprayed electrical cleaner on sensor and connector. Symptom Remained

After all of these the car is still doing this. The P0128 comes and goes as a pending code but doesn't hard fault. The heater works fine when this is happening, no loss in heat and touching the radiator hoses when we park after getting on the freeway indicate there is no way its not hot enough. Not sure what else to check. I'm just a guy who dabbles on the weekends. Any help would be appreciated.

When I had originally went to autozone we also bought this sensor, says on top of water pump housing, does this vehicle use this and will this fix the issue? Coolant temperature Sensor (located on top of water pump housing)

professorman

haha, I had this EXACT issue. It took me 8 months to find the problem. I think you might have the EXACT same solution as mine. Check my tread: Instantaneous temperature HOT spike - Random. It has TONS of diagnostics steps to take.

Here was my solution finally though:

Antihero47

I'll ask him to give a few knocks to the cluster window when it happens to him next to see what happens. However I am not sure that would be it as like I said the coolant temperature PID on the scan tool also see's a dip in temperature when the gauge drops down to low. Anywhere from 117*F to 158*F. Could it be a clogged radiator?

He recently replaced the thermostat with a genuine Honda but didn't bleed the system. I am going to drain it and then vacuum check and refill it again. I just want to be crystal sure that its not air in the system.

MAD_MIKE

Check the wiring harness from the sensor to the PCM for any damage, corosion or loose connections.
If it was just the gauge that was faulty there would be no codes set, but since there is an intermittant code the temp gauge is showing you what the PCM is 'seeing'.
Check the harness between the PCM and thermo sensor.

Antihero47

How many sensors does this model year Accord have? I am seeing in my research that the large sensor on the top of the thermostat housing is actually a fan sensor and not the actual temperature sensor? Is that true?

MAD_MIKE

IIRC on the later models the temp gauge sender was deleted and the temp sensor now gets its data directly from the PCM. These sensors are screwed directly into the head below the distributor.
If the later models are like the earlier units there will be two thermo switches, one on the water outlet facing the radiator, and another mounted to the thermostat housing. These are fan controls.

The only sensor you should be concerned for is the one located under the distributor directly screwed into the head.
The image is from an older model, your friends far will ONLY have the TW sensor screwed into the head, that is the sensor/wire harness to inspect. His car will not have the 'coolant temperature gauge sender' as that circuit is no longer used on the engine in the later models.

Thermoswitchs on the water outlet(radiator side) and thermostat housing(firewall side) are for the fan controls only. Do not concern yourself with those unless there are fan control issues.

Antihero47

My Man, Mad Mike. Well, he came on over last week and we instantly found that sensor from the diagram you posted. The first thing we did was move the wiring harness that covers it and I had him go to disconnect it. Next step was going to drain it one more time, replace the sensor and refill.

When I pointed the sensor out to him he exclaimed "oh yeah, I've replaced this sensor about 3 years ago, i remembered coolant came out" So he goes to take the sensor off and slides right off. We look at eachother and confirm it wasnt clipped. So we clipped it back in, hooked everything up and took it for a drive. Temp has stayed perfect and no more errors. He couldn't believe he didn't snap it in.

bw77

Mad Mike - where did you get that diagram of the cooling system sensors?

MAD_MIKE

Google image search usually brings it up under 'honda accord temp sensor' or 'honda accod thermwitch'. If you click on the 'techauto' link in my sig that will bring you to a site that has a good bit of info on 90s Honda diagnostics.