i got a 95 accord ex 5sp. i noticed today that when i have the car in gear and i floor it and accelerate that the temp needle drops all the way down past the cold mark then when i let up off the gas, or when im just driving normal, the temp stays where its supposed to be. anyone ever heard of this sort of problem and have any suggestions. thanks

 

The thermostat could be stuck open causing an influx of cool(er) coolant from the radiator, rather than closing a little bit to regulate the temperature.

I would check/change the thermostat.

How quickly does it come back down?

 

Possible problem with one of the following:

1. Gauge.
2. Wire to the ECT sending unit.
3. ECT sending unit.

 

it drops pretty quick, example: if im going down the road at 45 mph in 5th gear, then down shift to 4th and floor it, the temp needle goes from warm to below the cold mark in about 1-2 seconds. then i will let up off the gas and it goes back up. also the other night my car hit the rev limiter at 5500 rpms. but now it will go up to 6000 rpms but still hits the rev limiter at 6. i know accords can rev past 6k. i will replace the thermostat today and see what happens.

 

have you checked the coolant level, if its low u could get a false reading and when you accelerate the coolant flows over the sensor and show the actual temp, if the car is not warm the redline will be lower, same if you have a CEL

 

First thing's first. Make sure you have full coolant. Second, if your coolant is low you have a leak somewhere. If your coolant is low then what happens is there is not enough coolant to properly cool the engine and the coolant that you do have does not have enough surface area on the radiator to cool so when you take off the air that hits the radiator has enough strength to cool the small amount of coolant you do have.

If you have full coolant then.... Next, turn on your ac, do both fans turn on? If yes, then your fans are working fine. Next, idle the car. Does the fan on the passenger side come on? If no, then the problem is probably the fan switch on the thermostat housing. towards the back of the engine. It is not turning the fan on when it gets hot enough.

If you do not have enough coolant your fans won't work as well since the coolant has to be high enough in the system to touch the fan switch on the thermostat housing.

 

I'm going to go with thermostat

 

check the single wire running to #15. this is the sensor that controls the gauge. over tie the connection gets brittle and cracks or the wire gets frayed and is grounding out. in the past i have made a clean cut and crimped a red butt connector to get a fresh connection.

 

If the thermostat was stuck closed the needle wouldn't go down immediately on accelerating since the coolant wouldn't be able to circulate properly.

 

Who said stuck closed? A fail safe t-stat fails in the open position. IIRC most of the new ones are fail safe. Even if it's not a fail safe, a t-stat could get stuck in the open position as well.

 

If he's suggesting the thermostat is the problem he's suggesting it's stuck closed since stuck open would help the cooling of the engine rather than hurting it.

Also, most aftermarket thermostats aren't designed with fail-safe. They will fail closed.

 

I dont think he said its hurting the system.. Its getting COOLER from what he is saying..

 

It's also possible that somebody somewhere in the cars past just removed the thermostat entirely. It's super common here in the southwest, because most of the time vehicles run with the thermostat wide open due to the heat anyways.

 

Check the connector - it is easy.

If not that, then thermostat. You don't want to run one even stuck open - cooling the water jacket too much will cause increased wear and decreased fuel economy.

 

I just changed from yes to no above. Didn't realize I said that backwards.

 

k so coolant level is full in both radiator and over flow tank. i checked it when i bought the car last saturday and i re-checked it today and its still full. And yes BOTH fans are working properly. no C.E.L is on. also all wires and plugs look fine. i ordered the thermostat today at my work and it will be in tomorrow. Side note: the needle also goes down when the car is just parked at idle and then i rev it up. and also i did let the engine warm up before trying to take it to red line. ill let u know and thanks

 

I have a similar situation. I just got the transmission rebuilt because it was shifting hard from first into second. A week after I got the car back I noticed that when driving at 70 mph my RPMs will sometimes rev from 2500 to 2900 and the temp gauge will drop all the way down. A few seconds later the RPMs will drop back down to 2500 and the temp gauge starts going up to normal temp.

I've noticed that this happens when I don't let the car warm up.

I drive a 2001 Honda Accord LS Sedan Automatic w/ 143k miles.

 

just to finish this (better late than never i guess) i replaced my thermostat, and still had the same problem. Then my brother showed me a little sensor on the front right side of the engine, and he unpluged it, cleaned it, and pluged it back in and what do ya know it worked. i left the new thermostat in, cause hey why not, so still kinda dont know for sure if it was a bad thermostat or just dirty sensor, maybe both. but i will post a pic up tomorrow to show exactly what im talking about. thanks for the help

 

What was the sensor you cleaned? I am having the same issue

 

Doubt he'll be back, hasn't been on in 3 years.

There are two sensors screwed into the head under the distributor.
One is for the ECU, the other is for the temp gauge.