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Honda Accord EX 1998 (Sedan) V6 - After changing radiator, car's RPM too high (2500-3000) when idle

@VDCdriver Sorry, I meant 4-cylinder engine model
I have a 6-cylinder model (V6)

Not the fan necessarily - but have the heat control on Hot. That opens the valve between the engine and the heater core. You may see the valve on or near the firewall, engine side. The ignition may need to be on to make the valve move - I don’t recall.

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Turn the car around so it is facing uphill. That helps air bubbles move up to the open radiator cap while the engine runs. Level or downhill it may never happen.

@shanonia, but like I mentioned before, if I leave the cap off and engine running, a lot of coolant will come off, it will settle for the time the fans kick in, but a lot will start coming off. I can again try leaving the cap off and have coolant overflow, but need to know details, such as for how long, etc.

@Tester, I have car facing uphill with heater full on, radiator cap has been on it has been like this for close to 10 mins, rpm still at 2750 (between 2500 and 3000) No overheating.

So the shift lever is working now ?

You might have a head gasket leak - if so, it’s exhaust gasses that are coming out of the rad cap. That would explain the ongoing nature of the problem. A mechanic’s hydrocarbon sniffer can tell if the gasses are exhaust. There are also chemical test strips that can detect hydrocarbons in the coolant.

Please catch and properly dispose of the coolant that comes out of the radiator. It is poisonous to animals.

Good luck and please keep us informed.


I would leave the radiator cap off and place a catch basin under the car. Some coolant may come out as air is expelled.

@VOLVO_V70 Well, after removing a lid, I gained access to a ‘shift lock release’ slot, so I pressed it and was able to shift to neutral, reverse (when reversing, the rpm went significantly lower), and then Drive (when Driving, wow, a lot of power was felt as RPMs went back up as soon as I shifted to D)
I wonder why the shit lock is kicking in constantly! This occurred after replacing the radiator… it had locked only a couple of times in the past 5 years, for whatever reason… now it locks every time it goes to Park.

@shanonia did this yesterday, I think I let the car run for about 4-8 mins, with the cap off, and coolant was coming off and off`to a point where I believe there may have been a significant portion of coolant off the engine and it started overheating, even after the fans came on. It’s hard to catch the coolant coming off a radiator with an open cap/top as it goes everywhere.

I don’t want to be mean or disparaging, but I fear you are in way over your head with this. You may wish to have the car towed to a good, independent, local mechanic.

@old_mopar_guy I believe you are correct. I may try to do the IACV though… will give that a shot, or may try running the car with full heater on while it’s uphill. I’m learning a lot during this process, which is of benefit, of course… thanks to all of you!

If the coolant is flowing out of the top of the radiator you are making no progress, put the cap on so the coolant stays in the radiator and engine. After the thermostat opens the coolant level in the radiator will drop and you can add more coolant.

Does anyone think it would be ok to drive the vehicle for about 25 minutes on the highway? I noticed when on reverse, the rpm goes down significantly, but when on Drive, it may be 2500-3000 or so and it has a ‘kick’ to it. So far, Ive let it run until the fans come twice and no overheating.

I can’t think that anyone would say Yes to that question . Is an accident worth the 100.00 tow charge you would save ?

I don’t know if you got this sorted out or not. I’d recommend filling it with coolant and driving it a bit to let the coolant circulate and the thermostat open. I wouldn’t want to do the 25 mile highway trip just yet, but it might return to normal idle after the car is driven at operating temp. Most likely you will need to top off the coolant after the car cools down.

I always fill with coolant and drive the vehicle if there’s no bleeder screw. I sometimes find it difficult to purge the air, get the thermostat to open, etc with the engine just idling in the driveway.

You arent driving anywhere with an idle speed of 25-3000rpm… You will be going near 70mph…everywhere and Lord knows how you would get out of park or into drive or reverse without leaving half of the transmission on the ground.

You either have air in your cooling system… Most likely

OR you have something int the way of the throttle plate that is holding it open and at a higher idle.

Usually a Honda will “Hunt” for its proper idle when air pockets are to blame.

You need that vehicle NOSE UPHILL… Get it hot… then let it cool entirely and top off the OVERFLOW CONTAINER… so that the rad can pull in solid coolant into the air pockets by itself… IF your overflow container is empty and or the hose running to it is broken anywhere this cannot function… this is one of the main purposes of the overflow container… It will burp your system for you… upon Hot and Cold cycles.

I cannot stress how important that is…

2500-3k rpm at idle is going to drop quite a bit when the engine is put in gear, no? Have never owned a Honda, but I’m assuming it’s just doing the high idle thing because the temp sensor thinks the engine is cold. If that’s all it is, you can drive a Honda with a cold engine, right? Or is there something else going on here?

I don’t think a 98’ will allow shifter movement at those rpm numbers in Park… Or so it seems by the OP’s message… I would think that was on purpose by the vehicle, but I could be mistaken. Sure sounds about right, since the ecu knows all those parameters etc…

The car is now doing about 2200 RPM I have not taken it out yet for a short drive, but may do so over the weekend.

If you do drive it, you really only need to get it up to normal operating temperature. In other words, get the temp gauge up to where it normally resides after the car is warmed up. Leave the heater control to full hot. Hopefully, driving it up to temp will circulate the coolant and force any air out of the system. Then after it cools down, you can top off the radiator and overflow tank.