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Hey guys, just bought a 1999 Civic Ex(First Honda ever). It has the D16Y8 with a 5 speed with 156xxx miles. So it has this issue with the idle. When I turn it on(Mostly happens on cold starts but sometimes happens when warm) it wants to die but catches itself before dying. Also will sit at 2k+ when driving if I let off the throttle and also when pressing in the clutch. Eventually it'll come back down. Haven't had time to take a look under the hood but curious and hoping you guys can maybe point me in the right direction? No CEL, all stock. I'm thinking I'll look at the vacuum lines and start there, any other suggestions? Thanks guys!
 

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08 H-D Ultra Classic
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I would start with the throttle body checking to make sure all passages are clean and clear.
Make sure the throttle cable is moving in and out of the sleeve freely and not catching.
Check to make sure the return spring isn't broken or binding.
Then I would check the IACV make sure its clean and the gasket looks good.
 

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93 Civic HB SI
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There are many things that can affect your idle. I would personally start by checking for vacuum leaks, as there is nothing that can correct for base engine vacuum leaks other than fixing the leak. Gaskets are also stationary, non-controllable sources of leaks, therefore a computer/device won't have you second guessing IF you have a leak because it's trying to compensate for things. Gaskets can't move, check them first with old fashioned methods.

Check for vacuum leaks around the intake manifold and components. An easy way to do this is to use flammable yet highly evaporative chemicals such as Throttle Body Cleaner or Non-Chlorinated Brake Clean sprays.

DISCLAIMER: DO NOT SPRAY FLAMMABLE LIQUIDS DIRECTLY AT THE DISTRIBUTOR. Ask me how I know not to do this, it's a good story.

1. Open the hood and start the engine and let it idle.

2. Spray around flange surfaces (think TB to Intake, Intake to Head, IAC to Intake, etc.)

3. Also spray around vacuum hoses looking for cracks/leaks.

4. Don't forget to spray around the injector-intake interface O-Rings, the large black rubber seals where the injectors seat against the intake. Over a long enough time these dry rot and crack becoming leak sources.


Whenever you spray near a leak and the engine sucks in the flammable vapors, it causes the engine A/F to richen up which causes a brief change in engine speed. If you get the engine speed to change when spraying in a particular area, question any gaskets/components that seal or use vacuum in that specific area as sources for vacuum leaks.

Another quick place to check for leaks is the brake booster. If the large internal diaphragm has ruptured or cracked, the engine will suck air past the diaphragm from inside the passenger compartment of the car.

Because of all the stuff usually blocking the brake booster part that extends through the firewall at the brake pedal linkage, it can be difficult to hear leaks from this area without getting up close to it. Take a pair of vice grips or hose clamp pliers and pinch the main brake booster hose in an area closest to the intake manifold. If the RPM's drop, you found your leak and will need a booster. Many times the booster diaphragm can fail from a slowly leaking brake master cylinder leaking fluid into the diaphragm chamber. If the booster is the source of the leak, inspect your master cylinder for brake fluid leakage around the area where it butts up against the brake booster.

You can narrow down IAC issues by removing the intake boot from your throttle body and sticking your finger/thumb into the IAC air source hole while the engine is running, cutting off all IAC controlled air flow.

If the car stalls out quickly from plugging this hole, chances are you don't have any vacuum leaks anywhere else and the IAC is simply allowing too much air past the throttle body keeping idle high. If this is the case, verify IAC circuit integrity using a multimeter and schematics as well as ECM control operation of the IAC before opting to replace it. Many times it simply needs to be cleaned, but occasionally the solenoid coil windings do short out prompting replacement of it. Electrical specifications for the IAC should be in the service manual for your engine.

If you plug the IAC air source hole and the engine continues running/idling like normal, you are sucking enough air from some other place to keep the engine alive without the IAC. This is a problem, go back and continue looking for any other intake air leak points.


If the spray method found nothing, cutting off brake booster vacuum found nothing and you found nothing during ruling out IAC mechanical/electrical issues, consider additional items that can affect idle:

-Throttle cable adjustment too tight
-Throttle body main air bleed screw is flowing too much air
-Base ignition timing issues
-Fast Idle Thermo Valve (if equipped) issues
-MAP sensor issues (when a MAP sensor erroneously registers an intake air density value above a certain point, the ECM can force the IAC to open to help with air flow demand. Verify MAP sensor circuit integrity before replacing, because replacing a component that has wiring issues will have the same symptoms as the old part...duh lol)




Hopefully this helps you out. If you feel uncomfortable with any of the advice given above or you don't have the tools required to perform some of these tests, I suggest taking your car to get professionally diagnosed.

I have seen people replace every single sensor/component on their engines only to have the same symptom they started out with. Only after they spent $1000 on parts do they bring their car in for diagnosis at a shop. Then the shop spends 1 hour labor performing a correct diagnosis, and fixes a single wire or adjusts something small and it's fixed and it didn't need any of the components the customer installed.

Just keep this in mind. If you end up spending $500 chasing a high idle issue and it's still not fixed, just bring it to a shop! It will save time and money in the long run.
 

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BATSLOMAN GIVES NO FUCKS.
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air in the coolant?
 

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Discussion Starter #5
I would start with the throttle body checking to make sure all passages are clean and clear.
Make sure the throttle cable is moving in and out of the sleeve freely and not catching.
Check to make sure the return spring isn't broken or binding.
Then I would check the IACV make sure its clean and the gasket looks good.
Awesome, thanks for the suggestions!

There are many things that can affect your idle. I would personally start by checking for vacuum leaks, as there is nothing that can correct for base engine vacuum leaks other than fixing the leak. Gaskets are also stationary, non-controllable sources of leaks, therefore a computer/device won't have you second guessing IF you have a leak because it's trying to compensate for things. Gaskets can't move, check them first with old fashioned methods.

Check for vacuum leaks around the intake manifold and components. An easy way to do this is to use flammable yet highly evaporative chemicals such as Throttle Body Cleaner or Non-Chlorinated Brake Clean sprays.

DISCLAIMER: DO NOT SPRAY FLAMMABLE LIQUIDS DIRECTLY AT THE DISTRIBUTOR. Ask me how I know not to do this, it's a good story.

1. Open the hood and start the engine and let it idle.

2. Spray around flange surfaces (think TB to Intake, Intake to Head, IAC to Intake, etc.)

3. Also spray around vacuum hoses looking for cracks/leaks.

4. Don't forget to spray around the injector-intake interface O-Rings, the large black rubber seals where the injectors seat against the intake. Over a long enough time these dry rot and crack becoming leak sources.


Whenever you spray near a leak and the engine sucks in the flammable vapors, it causes the engine A/F to richen up which causes a brief change in engine speed. If you get the engine speed to change when spraying in a particular area, question any gaskets/components that seal or use vacuum in that specific area as sources for vacuum leaks.

Another quick place to check for leaks is the brake booster. If the large internal diaphragm has ruptured or cracked, the engine will suck air past the diaphragm from inside the passenger compartment of the car.

Because of all the stuff usually blocking the brake booster part that extends through the firewall at the brake pedal linkage, it can be difficult to hear leaks from this area without getting up close to it. Take a pair of vice grips or hose clamp pliers and pinch the main brake booster hose in an area closest to the intake manifold. If the RPM's drop, you found your leak and will need a booster. Many times the booster diaphragm can fail from a slowly leaking brake master cylinder leaking fluid into the diaphragm chamber. If the booster is the source of the leak, inspect your master cylinder for brake fluid leakage around the area where it butts up against the brake booster.

You can narrow down IAC issues by removing the intake boot from your throttle body and sticking your finger/thumb into the IAC air source hole while the engine is running, cutting off all IAC controlled air flow.

If the car stalls out quickly from plugging this hole, chances are you don't have any vacuum leaks anywhere else and the IAC is simply allowing too much air past the throttle body keeping idle high. If this is the case, verify IAC circuit integrity using a multimeter and schematics as well as ECM control operation of the IAC before opting to replace it. Many times it simply needs to be cleaned, but occasionally the solenoid coil windings do short out prompting replacement of it. Electrical specifications for the IAC should be in the service manual for your engine.

If you plug the IAC air source hole and the engine continues running/idling like normal, you are sucking enough air from some other place to keep the engine alive without the IAC. This is a problem, go back and continue looking for any other intake air leak points.


If the spray method found nothing, cutting off brake booster vacuum found nothing and you found nothing during ruling out IAC mechanical/electrical issues, consider additional items that can affect idle:

-Throttle cable adjustment too tight
-Throttle body main air bleed screw is flowing too much air
-Base ignition timing issues
-Fast Idle Thermo Valve (if equipped) issues
-MAP sensor issues (when a MAP sensor erroneously registers an intake air density value above a certain point, the ECM can force the IAC to open to help with air flow demand. Verify MAP sensor circuit integrity before replacing, because replacing a component that has wiring issues will have the same symptoms as the old part...duh lol)




Hopefully this helps you out. If you feel uncomfortable with any of the advice given above or you don't have the tools required to perform some of these tests, I suggest taking your car to get professionally diagnosed.

I have seen people replace every single sensor/component on their engines only to have the same symptom they started out with. Only after they spent $1000 on parts do they bring their car in for diagnosis at a shop. Then the shop spends 1 hour labor performing a correct diagnosis, and fixes a single wire or adjusts something small and it's fixed and it didn't need any of the components the customer installed.

Just keep this in mind. If you end up spending $500 chasing a high idle issue and it's still not fixed, just bring it to a shop! It will save time and money in the long run.
Really appreciate the advice man! I did a bunch of searching and found other threads with similar issues. I'll be diving in the car this weekend and hopefully I can sort it out. I just downloaded the FSM and with your advice and the other threads I'm sure I'll get her fixed. Thanks again man!
air in the coolant?
Thanks for the input, will be checking the car this weekend and maybe even do all fluids including new coolant.
 
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