Hissing Sound in Engine Bay? Vacuum Leak? But Where?
 

After my last track day about 3 weeks ago, I noticed a rather loud hissing sound in the upper left quadrant of the engine compartment after finding my power to be down from where it should be in the higher RPM band (felt like I need to install oars on the car and row at times). I did all the obvious things like tracing every hose looking for pinholes, loose connections, etc., but 3 weeks on, I have not been able to locate the problem. I can change the pitch of the hiss by wiggling the hose that goes from the oil filler neck to the lower intake manifold, so I replaced that hose. No change. Otherwise, I cannot find any evidence of a leak in any other hose or connection or around the LIM itself.

Another symptom of the problem is that the engine seems to be running rich. Idle is fine.

Is this a known problem? Am I missing something obvious here? What else should I be looking at?

Here is a YouTube video of the problem area and the sound:
My next track day is this Saturday, and I very much need to sort this out before then or lose my $225 fee. Any help will be appreciated.

Well shit we aint got much time, coming from the symptoms it is definitely an air leak. Hissing sound, leaking air, running rich, not getting enough air in the engine. Rather than the hose, have you checked what the hose is attached to? Try moving your hand around feeling for leaking air, keeping away from belts, and try to find it.

smoke the intake

Never would have though of that, going to keep that in my bag of tricks. Tell us how it goes scott!

Other things I have done...

I installed a BHR coil set and disconnected the negative battery cable for a few days to reset the ECU afterward before driving the car (it is not my daily driver). The problem showed up at the next track day after this, but I am not blaming the kit, obviously.

Checked the new plugs and verified they look good, are in the right holes, and are torqued properly.

Checked the wire sequence to make sure it is correct. It is.

Used brake cleaner to try to narrow down the location of the hissing sound, but did not get any results.

Traced the length of every hose, and felt around every connection, nipple, gasket, etc.

Cleaned the MAF and ESS.

Disconnected the negative battery cable for a week while I was waiting for brake parts to arrive.

Unfortunately, I did not log AFR or fuel trims at the track. I can turn those on and do some logging tonight and post the results. Anything else anyone wants me to log?

Thanks again for any help with this one.

Hissing sound? Down on power? I would check for clogged cat/plugged exhaust.

Have you checked air filter?
Have you cleaned e-shaft?

So do a log for AFR, also do one for fuel pressure if you can, other than that don't know. And yes clogged cat could cause high end power loss but I don't think it would cause the hissing, or should say don't know how it would cause that.

Here is a link to a data log in which I drive to the gas station. I warmed the car up before starting the data log. The trip seemed normal. The hissing sound is still there. I'll try the old vacuum hose to the ear trick here in a few minutes. Another thought is that the car started acting up after adding the track's Sunoco 93 octane. I wonder if there was something funny in the gas that screwed with the engine computer, the rich condition resulted from that, and the hissing sound is wholly unrelated?


The log has at least one WOT in 1st and 2nd gear and a lot of cruising at about 50mph in 6th gear along with a lot of other conditions in between. Anyone see anything interesting?

Zip version

CSV version

Plugged exhaust causes hissing by having the exhaust trying to escape any way it can.

True but there is exhaust gas coming from that hose he showed in the video? I have no idea what that hose is for but I doubt it has something to do with exhaust gas. Seems more like a vacuum leak but he doesn't know where the leak is coming from.

The hose in the video eliminates the milky oil condition by drawing vapor from the crank case into the LIM to be burned in the engine. I knew it was a long shot when I replaced it, but it needed to be ruled out. It has been.

I am planning to bleed the brakes tomorrow night and will remove the shields from the undercarriage and check around for leaks while the car is on jack stands. The plan right now is to do the track day regardless since the drive train is still under warranty.

rule #1 to proper analysis, never assume, had it happen to me and it's how the linked thread below came to be, same procedure also applies to OE coils or any "smart" coil:

https://www.rx8club.com/series-i-do-...-wires-222641/

^ Is it absolutely necessary to drag your differences with BHR into every thread?

While I had the car on jack stands to put the track brakes on, I removed the shields from the bottom and checked around underneath with a gloved hand. I could not locate the source of the hissing sound, though I certainly could hear it down there. There are no exhaust leaks, but I do suspect my cat is less than perfectly healthy.

Track day was fine. All the old gas was purged out by the time I arrived at the track, and I used Texaco 93 with 1oz per gallon of Lucas 2 cycle as premix. The car pulled like it should, and for the sake of comparison, I managed 98mph on the longest straight in 90 degree weather, whereas I made it to 102mph last November in much cooler temps. That seems about right.

I suppose I will just watch it at this point and see what happens. I am out of other ideas, and my local Mazda dealers are not very good with RX-8s, so they would be of no help other than to help drain my wallet.

http://cdn-2-service.phanfare.com/im...92f25ceaa912_1