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calidude 02-17-2019 11:29 AM

Failed smog, high NO and HC
Hi guys,

New to the forum and just purchased an out of state 87 911 Carrera coupe. I went to go get the smog done and it failed :confused:

Any thought on what I should look for and recommendations of a shop in San Diego if anyone is local to the area.

eastbay 02-17-2019 11:35 AM

#1 is oxygen sensor disconnected or faulty

Trackrash 02-17-2019 12:51 PM

BLACK FOREST will take care of you.

calidude 02-17-2019 01:27 PM

Thanks guys, I was also searching the forum and came across this old post too in regards to connecting the brown wire under the seat. I did that as mine was disconnected, then went on a good warm up drive to go retest but the shop was closed already. Perhaps tomorrow if they are open during the holiday.

I'm also going to check the O2 sensor to see how old it looks. Black Forest looks good as near Kearny Mesa and the 163.

Superman 02-17-2019 01:57 PM

If you just reconnected a wire that is part of the fuel metering system, then it might make sense to just do another smog test.

calidude 02-17-2019 03:24 PM

I'm reading from the other thread that it's a bridge ground wire?

Nick Triesch 02-17-2019 05:58 PM

Do a complete tune up including valves, Put 40 pounds of air in the rear tires , change the oil, get it hot as hell before smog test!!!

spyerx 02-17-2019 07:18 PM

With that much variation something is off
See this thread when mine fails a few years ago

Things to check
O2 sensor
Valves adjusted
Plugs and ignition good
Stock tune
Connect the wire next to ecu (it is stock)

With mine it was the afm base idle running rich.

Based on how out yours is, suspect the cat or badly out of tune engine. Did it pass ok last time?

Black Forest is a good shop

pmax 02-17-2019 07:25 PM


Originally Posted by calidude (Post 10358812)
I did that as mine was disconnected

I believe that should do it.

Don't forget pics of the car.

Techno Duck 02-17-2019 07:47 PM

Whats the mileage on the cat? High NO is either lean mixture (vacuum leaks) or a bad cat. CA legal replacements are available.

Even your CO numbers are high. When was the last tune up? Plugs, cap, rotor and wires if needed. Weak ignition will raise your HC readings also.

calidude 02-17-2019 10:31 PM

Car has 128K on it and recently brought from Atlanta, GA where on 1/22 had the full service tune up including, oil change, plugs, cap and rotor, fuel filter, fuel lines and fuel pump replaced.

I don't think the O2 sensor was changed.

Going to get it retested tomorrow after hooking the wire up today. Appreciate everyone's help as glad to join a great community of owners.

autojack 02-18-2019 12:50 AM

I bought an 88 from Atlanta, and it didn't pass smog in CA either. Ended up needing a new cat. That was with the brown wire connected. So, just a data point that a 30 year old cat can be good enough for Georgia but not California. A replacement was about $850.

spyerx 02-18-2019 06:14 AM

Pretty sure that wire will not do it. It just runs timing a little more aggressive.
O2 sensor is easy
But given recent tune i suspect cat is shot

See of station will do a pretest

Techno Duck 02-18-2019 08:23 AM

The brown wire retards ignition. It will lower your cylinder temps which will lower NO. I dont think it will lower it enough to pass on NO with your numbers.

Another thing is check the fuel quality switch is set at zero on the DME. After 30+ years itís worth checking that it hasnít been adjusted.

Invoice shows plugs but no cap, rotor or wires. If they were inspected and reused thatís fine.

calidude 02-18-2019 08:45 AM

Yeah your right, looking at the cap I thought it was new but it's not on the invoice. The spark plug wires look original as the protective sheaths where they bolt over the fan are cracked with age.

I'll pop the cap off and look and run at a resistance check on the wires with an ohm meter but probably should replace them anyway. I was reading another article about how poor ignition or neglect over time will clog a cat :(

I'll post a photo of the cap and rotor later.

Sounds like getting an O2 sensor out is an act of God too -

Is the fuel quality switch inside or outside of the DME?

Synchro Joe 02-18-2019 09:32 AM

Here's some info on the fuel quality switch (internal DME): - Fuel Quality Switch

scarceller 02-18-2019 10:42 AM

Very likely the CAT is bad.

But the slightly high NOx could also be one or more faulty injectors. A lean running cyl can cause NOx to go up.

Then the O2 sensor sees the average O2 content from all 6 cyls, if one cyl is running very lean because of a clogged injector that will increase NOx and it will also cause the other 5 cyls to go slightly rich because the O2 sensor sees a slight lean condition because of the one lean cyl, the DME will then richen the other 5 cyls and then HC also goes up.

Usually if NOx is high HC is low, but the above explanation is something to think about. A faulty clogged injector could result in both NOx and HC going up.

proporsche 02-18-2019 11:02 AM

calidude..also on your 911 change your wipers the one on right should go on left and vice versa...

Also your hand brake need to be adjusted....

otherwise...congrats ,nice 911....

Techno Duck 02-18-2019 11:38 AM

O2 sensor is not hard to get off with the proper tools. Leverage is everything. Take the wheel off and you will have plenty of access. Cut the connector off and use a box end of a wrench, tap with a dead blow mallet and it will come off. If the wire connector is still in good shape, get a universal O2 sensor and splice it on. This is cheaper than the O2 sensor with the Porsche plug... but the original connector can crumble with age and is not always reusable.

If the insulation on your wires is cracking, its time to replace them.

calidude 02-18-2019 04:47 PM


Originally Posted by proporsche (Post 10359938)
calidude..also on your 911 change your wipers the one on right should go on left and vice versa...

Also your hand brake need to be adjusted....

otherwise...congrats ,nice 911....

Thanks Ivan, yeah I thought those were backwards as the paint guys who put them back on must be British or something, lol... :D

Agree on the brake adjust too as on the list of to do

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