Air leak on the back of the plenum

Hi all,
I am working on a friends 99 6cyl Windstar after he got both 171/174 codes. When I reach in and rev the throttle I can hear a whiff or sniff of an air leak behind the plenum. I am going to rip into it this weekend, but I am wondering what the general cause is that most of you find. I don't think any of those vac lines back there have come off but couldn't get my hand back there to tell. Then I found that great webpage dedicated to replacing the plenum lower gaskets(6 of them under the plenum) and some screws. Is this a common fix above and beyond the replacing of the valve cover gasket? Are you guys 99% sure one of those 6 lower plenum gaskets have failed (due to the bolts failing in the bottom of the plenum?) or should I assume there is a cracked vac line back there and just rip into it and locate the busted line (if it exists). I guess I should just go get the screws and gaskets and if it is a busted hose then just take them back? Did I just answer my own question?

Here is that webpage as i am sure someone will say "what webpage?" :-)

http://leckemby.net/windstar/windstar01.html

I think, in general, the 8 isolator bolts are the most important to replace. I doubt if any of the other "upper" seals are leaking ... and if not damaged, can probably be reused with a little petroleum jelly lubricant. I replaced all of mine new... not sure it was really necessary... but who knows?

I did this job on my '99 some 65k mi ago... and so long ago that the new upgraded (green) bolts were not available. I also did the "lower" seals and gaskets ... not mentioned on the link you provided ... and most likely not necessary, but mine was repaired before it was realized that nearly all leaks occur in the "upper". So, I did some unnecessary work.

My car stills runs good, but I would feel better if I had the newer green isolator bolts.

I highly recommend you find a small torque wrench for this job. The bolts don't require much torque ... some folks translate this to mean "no accuracy is required". NOT SO!

Some 3/8" beam (not the presettable "click" type) torque wrenchs will do the job. I bought one from Sears.

Replaced Isoloar bolts and gaskets but the "sniff" sound I hear when I hit the throttle is still there. Do those vacuum regulators fail and cause the 02 sensor codes too? There seems to be 2 of the vac regulators back there. One on the back left of the engine and one against the firewall. I had the thing fired up without the cowl on it and am 90% sure its that back regulator that is making the sniff. Obviously the 02 sensor codes are gone for now, and I don;t know if that fixed it yet, but is that sound normal? All the vac lines back there are snug, the new bolts and 6 plenum gaskets were done so I know it's not those leaking air. Just wondering if anyone has heard this mystery sniff and knows if it is something different...

Thanks!

I am not good at tracking down sounds......but some sounds are normal.
I have a '96....so I am not an authority on sounds in your '99.

The 2 vaccum regulators on the back.....if they are the ones that I am thinking of....have an electrical connection as well as a vaccum connection.

One is the EGR vaccum regulator solonoid, it converts the electrical signal from the computer to a vaccum that is applied to the EGR valve.....
If it is not working, you will get a CEL and code relative to the EGR system. Although the most common cause of a EGR flow code is the DPFE.

The one on the firewall, may be the fuel system canister purge solonoid. It takes the electrical signal from the computer, and purges the fumes from the charcoal canistor. There are failure codes for that, if it is not working.

Another possible vaccum regulator is for the IMRC, which opens the "butterfly" for each cylinder above 2900 rpm. The butterflies are open with the engine OFF......and you most likely could see them when you were replacing the isolator bolts....they are in the aluminum lower intake manifold. When you start the engine, they close...to give you better gas mileage.....and then they open when you are above 2900 rpm, to give you more power.
There are fault codes for those, if they are not working as they should.

My suggestion....would be to see how the car does for the next while...and see if any fault codes come up. If you have a significant vaccum leak, it will throw a code and CEL.

Have you tried using a short length of hose as a stethoscope to better locate the noise?

Wiswind mentioned the EGR regulator valve. This valve has an intentional "leak" ... it leaks in air to help control a vacuum signal. This valve may need to be replaced ... or it may be missing the round filter piece that sits on top. This filter, if missing, would allow some dirt to leak into the system ... and it might allow some noise.

The '99's IMRC actuator (only a single motor) is all-electric.