JG928S4

I usuallly make sure all my Vacuum leakers are closed, ie: Heaters/AC off and vents on closed position. I also buy the Autoshop pass test in a bottle you add to gas. Then Italian Tune up. It is just such a hassle when you don't pass, I like to up the odds before test.

FredR

Bill,

Temp sensor working correctly? Enrichment occurs until computer sees correct temp- shot to nothing as it were- if others suggestions yield nowt.

Fred

dr bob

Bill-

The system is in closed-loop operation during both of the rolling tests. so the oxy sensor is 'correcting' for weak MAF and for minor vacuum leaks during the test. With that consideration, the sensor reads the combined CO mixtures from all the cylinders. So one or two weak/lean/poor combustion cylinders aren't enough to move the average CO the sensor sees enough to cure those cylinders.

Recommendation:
-- Find and fix any obvious vacuum leaks. If there are HVAC leaks, pinch the hose to the system from the splitter or pull it off and cap the splitter.
-- Put a new set of plugs in. New plugs have a better chance of igniting a slightly weak mixture.


After that, oil pooling in the intake belly can be carried into the chambers, but it's a long shot at the low speeds. The "Italian Tune-up" will clear the pooled oil, but at the same time is likely to add some back. So do a -good- one.



Similar Experience--

My car passed with flying colors when I bought it (22k miles or so) then NOx was getting gradually worse as the years went by. I'm a religious user of good gas, and I dump a couple bottle of Techron in the tank at each fill-up before and during any extended trips when fuel flow will be highest. Got to a point where, right on the edge of passing and with numbers similar to those you share, I talked the tester (former 928 owner, BTW) into doing the rolling tests in second gear. It was enough better to pass, but I couldn't depend on that tester being available forever. Others here have reported that their tester didn't want to do that for them.

---

A year ago, intake refresh included new injectors. Fords from BlueOval. Car seemed to run great before the change, but it is definitely smoother at idle with new nozzles, and an occasional miss/burble at idle is now gone. Bonus is that the NOx numbers are back to what they were when I bought the car almost 15 years ago now. Miracle? No. The intake refresh may have solved some unnoticed vacuum issues, so I can't lay it all on the new injectors.

Another data point: Luan in San Diego had similar NOX issues, and had a local shop do the three-part BG intake and injector cleaning routine to his car. His numbers improved a lot, and he was able to easily pass after the treatment. IIRC he spent in the $100-150 range for that process.

I've adapted a H-F injection cleaning canister and regulator with the pieces needed to use on the 928 fuel rails to force-clean the injectors. I used it for James M's '85 Euro project here and it smoothed that out a lot. I haven't messed with the intake spray bar piece enough to say that it's ready for prime-time yet. The BG chems are on ebay regularly, and I'd be happy to box the canister and connectors/hoses if you'd like to try a DIY injector cleaning yourself up there.

Don Carter

Bill,

I've been through this twice over the years in Houston (before my car turned 24) and both times I eventually passed. The only things I did were O2 replacement, get the car real hot before the test, and keep the RPMs up during the test (kept it in 2nd).

Wild Bill

It's getting tough here in Houston to find a shop that will still do the test and tougher yet to find one competent enough not to destroy the car putting it on the rollers!

I'm now in the same boat, having just failed my first test (high HC and NOx) that I had hoped would be my last. Previously, my S4 has passed with flying colors even without cats. But, the car has been sitting for months as I have been ill.

Seems to run just fine, but added some Sea Foam and will be checking vac leaks, spark plugs and o2 sensor. Will put the cat back on if necessary, but I'm enjoying the Italian tune ups in the mean time!

Bill Ball

Just to be clear, the air pump affects tailpipe lambda (not measured as part of the test here) in the HC section of the cat, but the NOX section of the converter is before the air input, at least after 1980, I believe. Vacuum leaks or other sources of air in front of the converter can raise NOX by either leaning the fuel mixture of reducing the efficiency of the NOX reduction section. So, I'm primarily examining things that can lean the mixture, such as an aged MAF, vacuum leaks and O2 sensor. if teh plugs are anything other than fairly fresh copper single electrodes, I will change those. The plugs should give me a clue
about fuel mixture as well.

Bill Ball

Dr. Bob: Thanks for the review and suggestion. If I get a clue from compression testing that carbon is an issue I may go for the BG, but time is of the essence - the buyer is anxious (the seller is anxious too), so i may just take it to a local shop in the few days I have to resolve this.

Bill Ball

Will check with diagnostic tester.

Bill Ball

I hope not. Thanks for giving that to me. I do have it handy.

Bill Ball

Jim: I appreciate that insight. I wasn't sure, but I thought the O2 was high. Moves vac leak higher up the list.

Bill Ball

Thank you, Wally. I had noticed the HC and CO were quite good, so that ruled out quite a few things right there. I understood that generally high NOX indicates a lean condition and I had planned to go that direction, just needed some thoughts on what to check/repair/replace first.

Jim Morton

Bill:

Do not rule out exhaust leaks in front of the cat(s) or part of the air injection system, check valve after the air pump ?

Soapy water can be useful before things get real hot (warmed up)... all assuming you can have the car up on your lift while the engine is running.

Best wishes on the sleuth.

Bill Ball

Andy: That's pretty close to the consensus. If I find any vacuum leaks and that the O2 sensor is original and I can check the MAF voltage, I may just stop there. That is an excellent price on the injectors. The 19 lb ones almost disappeared and got much more expensive for a while. If I get some indication the injectors are funky, that's a good solution, but I'm hoping I find something cheap and easy.

Bill Ball

Yes, the car will be coming down to my lift in the next day or so. I'll put that check on my list - THANKS!

Bill Ball

I'll put in a good word for you with the seller.