I have a 1999 prelude with 300k miles, no mods.... new plugs, wires, cap, rotor, air filter, thermostat and timing belt.... car runs perfect (motor is a bit tired, but perfect for 300k) other than at idle... when idling, very rough, sometimes dies and the check engine light flashes for misfire..... as soon as the rpm's climb it's all good..... there are codes for a misfire in 3 ( i think) i am thinking distributor but wanted some other input before i go there. Thank you in advance

shid man 300k miles?

Unless you are the original owner and have done each and every oil change right on time, that motor is tired. Assuming that the motor is still all original, I'm sure those piston rings are worn. Probably has some scoring on the cylinder walls.

If you must keep driving that thing. Make sure oil is perfect always and keep rpms below 6k. Lay off the VTEC lol.

If this is your daily, it's time for a newer car bud.

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Resuscitate.

I am the original owner, bought the car new in 1999.... last year I sold a 2001 prelude that had 30k on it, the only difference between the 2 was the motor.... they drove exactly the same.... when I did the trans in the 1999 @ 275k also did all the front end as well. the 1999 drives almost as good as the day i bought it, no bull****. the motor is a bit tired but other than the idle issue for which i am seeking help that is no reason to get rid of the car, plus I still love driving it. My daily driver is a 2015 Huracan that I put 75k miles on in the first 3 years I had the car...

So back to my question, any thoughts on why the misfire and what to try first on trouble shooting?

Hmmm... interesting.

I would rebuild FITV and IAVC valves. Both are pretty easy to clean. Since you've made it to 300k and are familiar with Preludes, I'm assuming you've already cleaned out the intake manifold at least once.

Have you tried those already? It sounds like you already have ignition covered, I'm assuming you are using quality parts like NGK/Denso...

This might be unrelated but I've replaced my O2 sensors with new NTKs on my Prelude. I've passed 175k.

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Resuscitate.

Yes, the intake was cleaned, can't recall when off the top of my head but it has been done. Have not done the FITV or IAVC, so will give those a look. The o2 sensors sound like a good place to look as well.

Thank you for the input and I will circle back when I have more info.

Cool, sounds good.

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Resuscitate.

Do you have a check engine light on? Never mind, I saw it is on.

One thing about these cars is the engine can start running crappy with no check engine light. Often the vaccum system is out of whack. The combustion engine is a giant vacuum system. But it needs to burp at times. It sounds like your engine has a vacuum build up issue and that leads to crappy idle and motor shutting off at idle. It needs to be able to burp again like when it was new.

It could be as simple as water build up in the exhaust but that is doubtful since it's a daily driver.
If you have never cleaned the EGR ports inside the intake manifold that is long over due. Honda built a ticking bomb of an EGR system in the H22 and several other engines where soot clogs the alluminum ports. That produces vacuum buildup without setting off the CEL in many cases. Later they used stainless sleeves to reduce soot buildup.
The exhaust valves on the cylinder head need adjusting from time to time. If the exhaust valves close too soon, again a vacuum build up that won't set off a CEL.

The CEL eventually lit because the engine is running lousy and not producing the reduced emmiosions the computer is searching for but the misfire is simply a guess on the diagnostic center's part.

I chased this very thing one fall and into winter 2 hours each Saturday and in my case it was a leaking head gasket allowing coolant into the exhaust system without mixing with oil or oil mixing with coolant. Vacuum buildup occured due to the exhaust pipe being loaded with water thus not allowing proper back pressure resleaseBut along the way I also saw others with the same issue solve theirs with valve adjustments or EGR port cleaning.

Go ahead and clean the IACV because it shares the EGR system. If the screen in loaded with soot that is a good sign your EGR ports are too.

Best regards.

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God's Not Dead

Double check the timing marks on the belt!

If you have a misfire cylinder, pull the plug for that cylinder and check it. Just because it’s newer, doesn’t mean it can’t be oil fouled. Check it.

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If you have a misfire cylinder, pull the plug for that cylinder and check it. Just because it’s newer, doesn’t mean it can’t be oil fouled. Check it.

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Worth sayin' twice

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God's Not Dead