Need help understanding OB Smog

MPDano · MPDano's Garage

I need help understanding the Smog System on an OB. I have a Smog Pump, which pumps air to a Diverter Valve, which under constant vacuum dumps the air towards the back of the Cylinder Heads and into the Exhaust Ports Inside the heads. The Diverter Valve is connected by a vacuum line coming off a Tee which also ports to the Decel Valve. I got this vacuum hook ups from various posts of pics in the forums and it also shows the same in my 928 Chiltons Manual.

Ok, now before I replaced my Diverter Valve, ALL air was being dumped to the Air Filter. My A/F was adjusted to this and it seemed semi smooth running. As soon as I replaced the Diverter Valve, I had to really Lean the Mixture and play with the Idle adjustment screw. The screw is almost flush to the bottom. The exhaust sound really changed and sounds a lot louder with a lopey sound (not smooth). Although during a test drive, I noticed a lot more power.

Can someone explain why this all changed by dumping air into the exhaust ports? Is the symptoms telling me that something else is not functioning correctly? Please some advice since I really don't think I can take this to a Smog Check station until I know this maybe normal or ...

Danglerb · 01-28-2009, 07:09 PM

Air going into the air filter box does, "nothing", as in nothing at all.

Air going into the exhaust, does something to help smog, but should have zero other effects.

Something else changed.

MPDano · MPDano's Garage

Nope, it affected the way it runs and mixture was off once I changed the Diverter Valve. I can take the vacuum line off and it will run the way it used to if I richen the mixture. There is a sign here that seems hidden.

rhjames · rhjames's Garage

what you may have had going on was a vacuum leak at the diverter valve.

as Mike says, air into the filter box is of no consequence, as well as air into the exhaust ports (this only helps with improving emissions).

was this a new diverter valve or a used one (that you just installed)? if used, did you test it with a mighty vac?

is the vacuum hose connected to a full vacuum port or throttle vacuum port? this will make a difference as well.

you should also test the air injection lines as well. is there air being pumped into them (and into the engine)? you'll need this to function properly for passing smog cops test. if it's putting air into the engine all the time, regardless of vacuum conditions, this is OK. it makes your smog test pass easier.

just asking questions- as I'm not there to see what you've done.

--Russ

rjm65 · 01-28-2009, 11:09 PM

Quote:

Originally Posted by MPDano

As soon as I replaced the Diverter Valve, I had to really Lean the Mixture and play with the Idle adjustment screw. The screw is almost flush to the bottom....

Makes total sense. You compensated for the vacuum leak on the diverter valve by richening the mixture. Fixed the vacuum leak and then you had to lean it out to get it where it needs to be. Also the reason you feel more power.

When the diverter sends fresh air into the exhaust, it helps any unburned mixture continue to burn. A simple analogy is blowing air over hot coals.

There is a check valve that keeps the exhaust from coming up the air tube and into the diverter valve. If that check valve is bad, that could've caused the old diverter valve to fail. Disconnect the hose from the diverter valve, start the car and if you feel any exhaust you know the check valve is bad.

Not sure about the lopey idle though....

Danglerb · 01-28-2009, 11:39 PM

Lopey idle = vacuum leak.

Idle adjustment screwed all the way in = vacuum leak.

Could be a vacuum leak.

MPDano · MPDano's Garage

Hi Guys,

Diverter Valve is Brand Spankin New. The exhaust smell is a lot better since I am not crying lke a baby everytime I step back there. Looks like I am on a vacuum leak hunt. Or maybe that Fuel Distributor plunger needs cleaning like Rjm65 told me to do. Hmmm.

Danglerb · 01-29-2009, 11:45 AM

You might also want to check that vacuum limiter and see if its working.

MPDano · MPDano's Garage

Vacuum Limiter? Is this the Decel Valve? Where is this located?

Danglerb · 01-29-2009, 11:53 AM

I'm thinking of whatever device you have that bypasses the throttle with two fat breather hoses on the side opposite to the linkage.

MPDano · MPDano's Garage

I believe that is called a Decel Valve, i also read that you can just eliminate that by looping those 2 fat hoses together and it it doesn't affect smog levels. I do have a spare that came on the other engine, but replacing it will be a b*&^%$ since it's in a very hard to get area.

Jim Bailey 928 · 01-29-2009, 02:16 PM

Quote:

Originally Posted by MPDano

I believe that is called a Decel Valve, i also read that you can just eliminate that by looping those 2 fat hoses together and it it doesn't affect smog levels. I do have a spare that came on the other engine, but replacing it will be a b*&^%$ since it's in a very hard to get area.

If you eliminate the decel valve (vacuum limiter) you must BLOCK both big hoses or you will have a very, very fast idle !

Danglerb · 01-29-2009, 08:27 PM

Quote:

Originally Posted by jim bailey 928

if you eliminate the decel valve (vacuum limiter) you must block both big hoses or you will have a very, very fast idle !

b i n g o

MPDano · MPDano's Garage

Just ordered a used one from 928 Intl and new connecting hoses. When I removed it, the smaller hose literally cracked off, this could have been one of many vacuum leaks.