Vacuum leak issue

[owequitit]

Here is a good one.

I have a vacuum leak somewhere, apparently. But, using all of the traditional tricks, I can't find it. I felt, listened and sprayed carb cleaner everywhere in along the intake tract and nothing noticeable.

I KNOW it has a vacuum leak because if I change the vacuum load on it, it stops momentarily, and is surges like 25-50 RPM up and down up and down. You can barely see it on the tach, but you can HEAR it. It does not surge in open loop mode.

Also, if I completely cover the IACV port on the TB, it still runs. The IACV is drawing air, but not as much as is typical of an engine idling, as that port usually has a very strong vacuum on it. Now it is there, but it isn't very strong. If I completely cover the TB, so that air can't get in, it still runs. The confusing part is that any vacuum leak bad enough to be able to idle the engine, ought to be pretty detectable, and yet, I get nothing. Checked the IM gasket, the TB gasket, the IACV gasket, the plenum gasket, and all lines and fittings. I blocked off the FITV, with no luck either.

Here is the only other clue I have. If you pull the PCV valve and block it with your finger, the idle returns to normal. The valve has been replaced twice recently, with no positive or negative effect.

Any ideas?

[HondaFan81]

Have you done a leakdown test? How well are your rings sealing?a

[nostringz]

check your intake manifold gaskets, you may need to replace them, and make sure the bolts you have properly hold it together, if not add washers.

[owequitit]

check your intake manifold gaskets, you may need to replace them, and make sure the bolts you have properly hold it together, if not add washers.

Gaskets seem to be fine. They don't respond to carb cleaner.

[baracuda]

Have you checked the adjuster screw on top of the tb? it may have come loose that would cause it, i had the same issue and i was reading a release by honda on how to fix the erratic idle and that was one of the thing that had to checked. it was the last thing i did and of course was the route of the problem. i had also sprayed the hell out of my intake area and it never did a surge at all. hope that helps out some what.

[owequitit]

Have you checked the adjuster screw on top of the tb? it may have come loose that would cause it, i had the same issue and i was reading a release by honda on how to fix the erratic idle and that was one of the thing that had to checked. it was the last thing i did and of course was the route of the problem. i had also sprayed the hell out of my intake area and it never did a surge at all. hope that helps out some what.

The idle adjustment screw is fine. I did check that. I also reset the idle several times, and that did not cure it.

[HondaFan81]

You reseal the IACV yet and recheck yet? Keep me posted, I'm interested in your resolution. If it's not a small leak, I really wonder on it being electrically sensitive and fluctuates 20-50 rpm because of ECU input from other sensors throwing the ECU off.