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Is this normal IDLE RPM or too low?

1 December 2023
Hello everybody so I just purchased this car last month and was unaware of the issue when washing the car.I washed my car at a self serve and noticed the exhaust was making a pop noise and wasn’t running right after .I read that water can mess with coils(no gasket on coil cover from previous owner removing them…)so I replaced them and still get slight pop noise.looks like my car idles very low now and wondering if normal?it stays consistent and needle doesn’t jump around .if this is not normal what is next step?maybe water in intake or clean throttle body .just weird was running better before car wash


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I also noticed sometimes when going into neutral it will sometimes go to very low rpm like car is going to shut off.
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For a MT it should idle a little higher, around 850ish. Did you check for moisture or water on the rear coils or in the rear bank spark plug wells? Is the EACV plug under the throttle body on good ?
My psychic abilities are clearly not working so a model year would be a useful detail as there are production changes in the cars. In particular late cars do drive by wire idle control. Whether or not it is OBDII is also a useful detail.

Did you replace the coils or just the gaskets (you did not specify). If you just replaced the gaskets it may be too late if water has caused corrosion on the coils. Pull the coils out and examine the metal base of the coil body for corrosion and look for signs of flash-over tracking on the very end of the long coil nose. If either is present, sometimes you can clean it up; but, safest to replace.

That said, coil failures result in misfires which can cause an erratic idle and can sometimes cause that pop you hear. However, misfires do not result in consistently low idle speed unless you are misfiring like crazy which will generate error codes on an OBDII car. Note that a slightly erratic exhaust note seems to be a normal feature of the NSX - at least on my 2000 C32 even with new spark plugs and pristine coils.

I also noticed sometimes when going into neutral it will sometimes go to very low rpm like car is going to shut off.

That might be a problem with the ECU failing to go into idle control mode. The ECU gets inputs from the brake switch, clutch switch and transmission telling it when to implement idle speed control. If those inputs fail the ECU does not enter idle control and idle speed may be out of range or the engine may die. Come to a quick stop at a stop sign and having the engine speed dip way down or die after you release the clutch is a common sign of failed idle control. If you find the service manual for your car you can find details on checking idle control.

Remedies will depend on whether your car is cable or drive by wire. Cable throttle bodies have the EACV idle control and the little thermal warm up valve and if dirt / crud gets into the passages that can cause idle control problems.

So much of where to start and things to check depends on the model year of your car
Sorry left put that info It is the 92 with no OB and wire throttle.my covers have no gaskets (already orders for both covers) and I had replaced coil/spark plugs after the car wash.I will check the idle control it maybe something got in there and is clogging it?also just cleaned the throttle body (was pretty dirty ) with no improvement.could it be o2 sensors or fuel related?
Could this be affecting it not sure what this is and couldn’t find where it would be connected to?newbie to this car….that hose is coming from under the throttle body piece on the right


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No, that tube doesn't go to anything. For context: When did you change all your coils and plugs and clean the throttle body? After the problem started? And did the symptoms change at all? Was all good BEFORE you washed your car?
That’s good news thought I cut something taking off the intake .I changed coil,spark plugs,and cleaned throttle body all within last couple days.I did notice car doesn’t shut off when going to stop but still has low rpm .yes before the car wash it was running fine rpm’s were higher at idle from what I remember.there was definitely no pops before also
Welcome to NSX Prime and congrats on the NSX!

The black tube is the throttle body breather tube. It doesn't connect to anything. It should face straight down.

Your symptoms appear to be related to the throttle body idle air passage. On the older OBD-I cars (91-94) it gets clogged with PCV blowby and carbon and causes low idle, stumbling and idle surging. Here is a thread that describes how to clean it (you need to remove the throttle body):


You should clean the screw passage and the little passage that connects to it on the side. After removing the screw, just blow carb cleaner through it until you see clear liquid.
Cleaned the throttle body - did you remove the throttle body to clean it or do the cleaning with the TB still mounted? I don't have hands on experience (I have a 2000 with DBW); but, my understanding is that the idle air passages in the pre DBW TB are a bugger to get clean with the TB mounted. At the very least you probably have to insert the spray can straw in the idle air openings and make sure cleaner comes out the other end.

The fast idle valve and the electronic Idle air control valve (EACV) can get gummed up leading to idle control problems. There is a DTC (1-4) for the EACV; but, I suspect that is just for a completely dead EACV You should use the service check connector to confirm that you do not have this DTC or any other relevant DTCs. If your fast idle (idle during warm up) is OK then the fast idle valve is probably OK. For low idle speed conditions when warm the EACV is typically the #1 culprit along with crap in the passage for the idle speed setting screw. However, it would be unusual for these idle control problems to show up all of a sudden. Idle control problems due to gummed up stuff kind of creeps up on you until it reaches the point that the EACV can no longer correct the problem. Did washing the car include washing the engine directly? If so, maybe check for a disturbed electrical connection at the EACV.

According to the service manual, the EACV and the idle setting screw (which includes dirt in the passages) are the prime culprits for warm idle problems. Search Prime and you can find threads covering cleaning of the EACV. I suggest that you down load a copy of the 1991 service manual (go to NSX Prime Library in the top red banner and you will find links). Starting on page 11-66 is the description and trouble shooting procedure for the idle control system. Familiarize yourself with the system. Make sure the TB passages really are clean and then consider cleaning the EACV. The service manual also describes the clutch, air conditioner, brake and neutral switch signals which put the ECU into idle control. If those signals are not getting to the ECU then the EACV is irrelevant because the ECU will not switch the EACV on until it is told that the car is stopped or coming to a stop.
Do you have an after market intake?
Appreciate you guys turns out it was most likely the throttle body idle passage.when I cleaned the throttle body before I never removed it and this time I took your guys advice to take it off .there was so much gunk back there was all black.cleaned the entire throttle body and is idling solid now thanks !!!
Still weird that it took a car wash to let me know.guess i gotta hand wash it til i get my gaskets but at least it’s idle is smooth now.


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I'd recommend some DDF coil guards if you don't have them already. I wonder if there wasn't some sort of synergy between the two issues.