Dennis Burford

Hi enthusiasts,

Going to try to smog (California) my 86 928S friday morning again. Original attempt mechanic said O2 and Cat were bad. Have replaced cat and O2 now and am looking for other advice. Have browsed and since tightened all the intake hoses and looked for vacuum leaks. I do notice I do not have an "off" for climate control as air is always passing through, at a higher rate at higher speeds. Perhaps a vacuum leak in that system (AC still not working). I had to rewire my O2 sensor heating circuuit as wiring was cooked, and it seems to be working fine now - 13volts after starting up. M

My last numbers were:

At 15mph/1330rpm
HC measured 253, max allowed 128
CO measured .87, max allowed .80
NO measured 1155, max allowed 1106

At 25mph/2085rpm
HC measured 169, max allowed 103
CO measured 1.41, max allowed .60
NO measured 636, max allowed 936

Any advice what else to look for greatly appreciated. This car runs awful cool, anything like thermostat and/or coolant temp sensor causing rich running?

Thanks for the help!

Dennis

GregBBRD

The next step would be to try a rebuilt MAF, but I doubt it will fix it. The HC numbers indicate a missfire. Check the spark plugs. If they are Bosch Plantins, throw them away and put in the exact plug listed on the decal on the hood. If both those fail, screw in a new set of injectors.

Dennis Burford

Gad I just replaced with bosch plats dual prongs, had single prong plugs in their before and failed with, all looked like a little "cold".

GregBBRD

We love those plugs. Helps the retirement account. Use the correct plug.

The CO at cruise bothers me. If the O2 sensor is working correctly, it should control the CO better than that. Make sure that the little wiring loom that goes to the throttle switch is actually hooked up. They deteriorate badly on the '85/'86 models and can short out. The engine might think you are at WOT. That little loom is no longer available, last time I checked. Someone making one?

If that stuff all works, I'd try the MAF 1st.

Mike Frye

Dennis,

Were these numbers from before or after the cats and O2 were replaced? I had some interesting 'issues' with inspection. Check this out:


Emissions 'issues'


My vote is for caps/rotors/wires and plugs. Can't hurt and it will eliminate a lot of wondering.

Also, did you get the car good and warmed up? Like really hot? I think the cats work better that way.

Dennis Burford

These numbers are before cat and O2 sensor were replaced. THe car was left with the shop so I'm not sure how warmit actually was. I'll inspect wires and caps/rotors tonight and clean up, won't have time to get a set in before tomorrow morning. Guess I'll be changing out plugs again too....

leperboy

If those numbers were from before new O2 and cats, I think you're going to be fine. I would test as it is before I did anything more. A properly working O2 and new cats can make a significant difference and you were close already.

Matt

Mike Frye

+1 to what Matt said. I would just go for it.

(But hey, I was ok with going 3 or 4 times, just to see what effect each change had, so what do I know?)
Some of us are OK with the big read 'FAILED' sticker on the window.

Good luck.

dr bob

Here, the test stations will allow you to just test with no reporting. Or, the tech can abort the test when he sees it isn't going to pass so the "fail" never makes it to Ahnold's computer in Sacharin Tomato.

I'm with Greg on fixing all possble ignition issues right away. The HC going up as load and speed increase is a sign of weak ignition. The CO increasing under load means that the oxy sensor probably isn't working, and possibly you have a vacuum leak someplace. Sounds like you are addressing the oxy sensor issue already. After all that, a new set of cats will allow you to pass the test by masking the failing symptoms, but doesn't really address the underlying issues.

+1 On the recommendation to use the Bosch copper plugs instead of the platinums. On my car the HC and CO didn't change noticeably, but the NOx did when I tossed the new Plat +4's in favor of the copper plugs. I pull the plugs at least once a year to inspect them, so the claimed 100k life expectancy on the +4's means nothing to me. Great for a M-B with a required 80k emissions life warranty, no benefit except to Bo$ch when I buy them for the 928. Just MHO, of course, your own milegae and rsults will vary.

Dennis Burford

Howdy all and thanks for all the advice. I now am driving the cleanest emission car I have ever owned. Must be told, the new aftermarket cat (smaller, cheaper) and O2 sensor, along with plugs and tightening up the clamps on the intake rubber, allowed this beast to sail through. Here's the new numbers:

At 15mph/1330rpm
HC measured 5, max allowed 128
CO measured 0.00, max allowed .80
NO measured 192, max allowed 1106

At 25mph/2085rpm
HC measured 1, max allowed 103
CO measured 0.00, max allowed .60
NO measured 98, max allowed 936

As you can see a considerable difference from the previous numbers! So now I have my new in-state plates and she is already gobbling up curved pavement!
Thanks again for all your help and advice. The learning curve continues

Dennis
1986 "Mountain Shark" 5-speed
Champagne on Chocolate all leather

Mike Frye

Congratulations! Now enjoy!

GregBBRD

The downside:

Those little cheap cats will be junk in two years and you will need to have a couple of others installed. They are cheap because they have very little precious metal in them.

I have a really good friend that owns a muffler shop. The guys that buy the "junk" cats for the metal. pay him more for the used Porsche cats than he charges for the new, little junk cats. Go figure this out!

Dennis Burford

Yeah Greg, I figure you are right. This gets me two years on the road. Next set will be same as OEM if budget allows. Lots more to do to this car, needed the funds for those shocks, motor mounts, AC problem outscource, etc. etc.

GregBBRD

It never seems to stop on a 20 year old car....still one of the best cars ever made!