vanster

Today I went to my local smog test station in Berkeley ... same one I take all my cars to and my 928 twice before and never a problem

Today the results came out as a pass at 25mph but a fail at 15mph
Too rich. Just a bit of tweaking but which tweak?

I have a few known issue, one is a very small vacuum leak in the HVAC behind the radio. Timing is with in 2 degrees. ( I timed it by ear when I did the big belt) Do I lean it out a bit by twisting the dizzy?

Yes I have a cat but they never look under the car anyway

fraggle

I'd cap the vacuum line going through the firewall to eliminate that part of the system, take it out for an Italian tuneup on a new tank of fuel, and try again. Not sure about the timing on your car, hopefully someone will chime in.

vanster

The irony of it is that the car runs great and I'm getting around 17mpg
Looking at all the basics, one item that I have changed since my last smog was I went back to a stock air filter from a K&N but I doubt if this makes much difference.

Speedtoys

Ya..see if the vacuum system holds, if not..plug off the leaky section..that'll hurt some.

michaelathome

You need to take it in HOT!!!

All 928's run rich to warm up. You failed at 25mph? That's less than 2000rpm +/- probably less in 3rd or 4th.

Try to see if they can run it in 2nd or 3rd after taking it in when the engine is nice and hot.

Michael

polecat702

Add a pint of Acetone to the tank, and you'll pass.

vanster

A) are you kidding about the acetone?
B) it failed at 15mph and passed at 25

What about race gas and what does it do to the cat?

MainePorsche

Mechanism ?

michaelathome

Ah,

With an '80 you are one of the earlier or CIS guys. I am not familiar with that injection/ignition system at all. Sorry I shouldn't have piped up. All S4's and later should be able to pass stock if hot.

Maybe get your injectors re-flowed?

Michael

danglerb

80 US is Ljet 4.5L, barndoor flap maf, so no K&N oil on the wire issues.

Maybe post all the numbers, not just failed rich at 15, passed at 25. Could be good to see everything that changed. Even better post both this time and previous results.

I suspect some vacuum leak, they make more trouble at low throttle settings, and less at higher. Rich should be easy clean up for the cats though.

FiveElements

Don't do that.. you can ruin your fuel pump.

Better check all your vacuum lines to the fuel pressure regulators and dampers. Low or incorrect vacuum will cause slight richer mixture. You may have a loose line or incorrect connection somewhere in those areas. Check your ignition systems like fresh spark plugs, cap/rotor, good ignition wires and good 91 or 92 fuel like Chevron or Shell. Drive the car hot and strong to the smog station and don't turn off the engine. Test right away while the Cats are still hot.

FiveElements

If you think there is oil on the MAF wire, (from the K&N filter) simply run the engine and lightly spay carburetor cleaner gently on the wire.

polecat702

3 ozs of acetone to 10 gals of premium pure gasoline, will help with the vaporization of the gas. Acetone can dissolve certain rubber tank liners, and fuel lines.

A lot of hot rodders have used it for years, to get older cars to pass smog.

dprantl

Be careful with the amount of acetone. If you put in too much you will get better HC numbers but the O2 number will get out of whack and you will still fail.

Dan
'91 928GT S/C 475hp/460lb.ft

RedPanda

You guys need to mention that hot rodders actually maintain, and replace parts on their cars to a point where tank liners are literally brand new.
With our 928s..... you got people driving with 30 year old fuel lines and tbelts.