Loping idle ... fixed on my 2001 Prelude by Honda World in Huntington Beach

[myhui]

I had / still have the same problem on my NSX, so maybe both are caused by the same thing.

ECU shows misfire but no other codes. Yet these all check out fine:

• compression
• leak down
• manifold vacuum (via MAP sensor read by ECU, and via vacuum guage)
• ignition (spark test)
• timing
• fuel injectors clean (rebuilt by R C Engineering a block away from AHM in Torrance)
• fast idle thermal valve
• idle air control (IAC) valve

Problem was caused by the exhaust gas recirculating (EGR) valve. The ECU can't see this fault separate from other problems, since it can only sense a sticking valve when the plunger is not at the expected position, but it cannot sense a leaking valve.

This only occurs at low rpm (i.e. at idle) because there is so little fresh air in the intake runners, so any small amount of exhaust gas mixed in there will foul up the ignition in a major way. The exhaust gas is not evenly sent in to all cylinders, so that's why it causes a different amount of power loss for each cylinder.

This is yet another good reason to switch to individual throttle bodies (ITBs), besides the fact that they look great.

This 2001 Prelude has 150000 miles, and it still goes like a bat out of hell. I love it, maybe even more than my NSX, since I can do these silly driving antics (I purposely sanitized that phrase) and have fun with it, instead of always worrying about what'll happen if I crash it.

 

[pgilliam1]

There is an ECU error code for "EGR insufficient flow", number P0401 or 80 (MIL).
I know what you mean by having more fun in the Prelude. I had a BMW before the NSX that I ran balls out with. But the NSX keeps me a little more concerned in the turns.
Happy Motoring!

 

[ENT]

So, basically, it was a vacuum leak.

Cheers,
Ian

 

[myhui]

So, basically, it was a vacuum leak.

Yes, and to make it worst, the gas leaking in isn't atmospheric gas but exhaust gas. But I wonder why the MAP sensor and vacuum gauges don't show anything wrong. Maybe the small amount of leak is hard to detect.

 

[myhui]

There is an ECU error code for "EGR insufficient flow", number P0401 or 80 (MIL).

That code is in the Prelude's manual too, but I presume that means the valve didn't open wide enough, instead of not closing tightly enough when it should? Interesting I never saw that code through many miles of driving with the ECU monitor attached and logging.

NSX keeps me a little more concerned in the turns.

I love how you phrase that.

The Prelude's exhaust is all original, with the original factory catalytic converters still working after 150000 miles. Can you imagine the modification possibilities? Maybe I'll start talking to Bisimoto Engineering for ideas.