Charley B

Took the '88 auto for testing yesterday and failed on the NO scale for 15 and 25 miles per hour. Allowable is 772 and 711, I scored 937 and 775. I asked the tech what that meant and he said probably too much gas and running a little hot. Of course I took it out for an Italian tuneup before pulling in for the test. Any ideas?

Herman K

If you still have your air pump remove the vacuum line from the air pump diverter valve.

When I did mine and installed new plugs it passed without any problem

Bill Ball

Hi Charley:

Hi NOX usually indicates too lean, not rich, and high combustion temps. Vacuum leak, bad fuel filter, bad injector(s), high compression from carbon buildup, incorrect timing.

How were the other numbers (HC, CO)?

Charley B

HC = 105 and 70. A little high.

CO = .29 and .26

Charley B

I think I'll change plugs and run a compression test; if somewhat high, maybe I should do a little water treatment on the heads.

What's best plug for the S4?

Leon Speed

Bosch WR7DC

Mongo

How old is your fuel filter and how many miles are on the car? I was wondering this due to the age of the cats. I have never had an original fail but have heard of them failing still.

IcemanG17

since its failing on NOx but the HC isn't exactly great either....it could easily be the cat getting old.....Sharky would barely pass on HC when it was cold....but the italian tune up always fixed that....

Mongo

I remember my recent Italian tuneup I drove mine for about 10 miles straight in 3rd gear at 70 mph. When I got to the smog shop, the car failed 3x the amount of everything from O2, to CO, NOx and HC. Now those are failed cats!

Ducman82

i had the same issue, due to low fuel pressure.... fixed that, passed

Charley B

The car has about 103,000 miles. I've had it for five years and don't recall replacing the fuel filter.

It passed fine a year ago and has only had about 1,000 miles since.

Speedtoys

There's yer problem...gotta drive the thing.

Charley B

Point taken.

dr bob

The engine is at low load and low RPM's for the NOx test at 15 and 25. Fuel filter isn't the problem, unless the car won't go past 40 in normal driving. So don't waste your time/$ on a fuel filter. But, seeif the tester will retest with the engine in second gear. That will get the air and fuel masses up some and make it easier to pass.



How old is the oxy sensor? Still have the original non-rebuilt LH brain? Running cheap gas?

Failing/worn/whatever sensor has a tough time regulating mixture. 50k is end-of-useful life for a sensor, and there are reasons why it should be replaced sooner. One is fuel costs.

Tired/failing brain gives less-than-perfect fuel mixture. My NOx numbers got a whole lot better with te LH rebuild, and mileage improved a lot too. Borrow the brain from the GTS.

Cheap gas doesn't burn evenly.


Worn plugs, tired MAF, wires/cap/rotor, are regular maintenence items that too often get ignored. Bring all that stuff current before you test again. On the plugs, stick with the WR7DC's, avoiding any temptation to use platinums or multi-electrode super wiz-bang stuff. If the MAF is 100k old, you probably need to have it refurbed to get proper engine performance anyway. Tired, it makes mixture control tougher during th etest, especially with a less-than-new oxy sensor.

Speedtoys

When it all comes from the same refinery in an area..splain this?