My 95 4.0 XJS 59,000 miles, has been out of commission for the past two years waiting for me to get around to some minor paint work (bumpers), which I recently finished. My tags expired in the mean time and the renewal required an emissions test. I rolled the dice and decided to have it tested with the old gas in it (it did have stabilizer in it). I drove it around for a couple of days on the expired tags and then I had a good highway run just before the test. The results were as follows:

At idle - rpm 701
HC ppm 55 (200 allowed)
CO% .03 (1.0 allowed)
Dilution 15.43 valid

Under load - 1376 rpm
HC ppm 54 (allowed 50)
CO% .03 (allowed .28)
NO ppm 1 (allowed 365)

As you can see I was just over on the HC under load 54 vs. 50 allowed, with everything else fine. The car is of course running smooth and fine with no codes. I elected to do some simple things and see if it made any difference. While I was doing the body work, I had changed the oil, air filter and did a through cleaning of the throttle body (removed but without touching the TPS). Also when I purchased the car 4 years ago the throttle return spring and EGR recall were performed by the dealer. Therefore, the only other tune-up item outstanding was new plugs which I did. I then removed the old gas filled it with fresh, added some cleaner and drove 275 miles, filled it again and went for a new test.

I heated the car up well, just as before, but unfortunately almost no change in the numbers. HC at idle went down by 4 ppm but HC under load actually went up one to 55.

So I’m looking for suggestions on my next steps. I will also say that when I purchased the car 4 years ago, the critical HC reading was 49, just barely a pass, so something is moving slowly out of range. My thought at this point is to replace all 4 O2 sensors since I don’t know their history. Any other suggestions?

Further, what is the story on O2 sensors? On RockAuto I see a number of alternatives. Firstly, some manufactures show only one number for both up and downstream while others show different models for each. Then there is the price difference, all the way from $80 for NTK (which they say is the OE Mfg.), to $503 for the Denso. Any comments on the O2 options would also be appreciated.

Thanks,
John

 

First off..........beautiful car.....could be my twin
My 95 has the OBDII diagnostic port, and here in Texas, they can test it using that and not the tail pipe sniffer. As long as all the monitors are set, and there are no codes present, they don't seem to do anything else.
Don't know if this helps...................

 

If you richen the mixture a bit it should bring HC down and the CO up. when and engine is lean the HC go up and CO goes near 0. Don't waste money on O2 sensors.

 

Hi John,

I'm a licensed smog inspector in California. I also diagnose and repair emissions failures. One of the tools I use is a lambda calculator. Go to this website and plug in your readings. This will help determine if your engine is running rich or lean.

http://www.smogsite.com/calculators.html Scroll down the page to find the Lambda Calculator.

You will need the O2 and CO2 readings as well. I hope that you have them on your vehicle inspection report. Report your readings if you have them. Judging by your "Dilution" reading, this may be your CO2 reading. If it is, your O2 reading should be near zero.

Please report your readings if you have them.

HCs are unburned fuel and anything that causes a misfire can raise that reading. Ignition, compression, mixture, and other problems can raise HC levels. Have you or can you perform a compression test? It's good to know what you're working with. A normal running engine will produce some HCs. The post combustion controls like the catalytic converter are supposed to clean them up. A good air fuel mixture helps it do it's job.

As far as O2 sensors, the factory sensors never give me trouble. The aftermarket sensors seem to be more problematic. It depends on the specific design of the sensor for that vehicle. Some cars are more sensitive to a variance from a factory O2 sensor. Others don't seem to be bothered at all.`

The Denso sensors have only let me down when the system was too sensitive to a variance from factory. A factory O2 made the difference. The NTK has let me down twice out of at least a hundred and it was due to a defective O2. A new NTK fixed the problem.

Sometimes they may be too fast or too slow or it may have a heater that is slightly out of specs from what the ECU wants to see.

Unless you have money to burn, I would wait on the O2s.

Honestly, I often see degraded catalytic converters that cause problems like yours. Let's see what your readings are and take it from there.

 

If I simply elect to try and richen the mixture, how do you do that on the AJ16?

One other thing I forgot to mention is that I cleaned the MAF at the same time I cleaned the throttle body.

With respect to jchavez76’s more technical approach, unfortunately the data I presented is all the report shows. The load test is stated as an ASM2525 test which is a dyno based part throttle run based on an EPA procedure. In both cases (701 and 1376 rpm) the Dilution is noted as 15.43 although I could find nothing that tells me what that means. So that is all the data I have.

I had assumed my code reader would not work on a 95 since I thought OBD compliance only began in 96 but I will give that a try in case there is any data there that could be useful.

Thanks, for input.

John

 

Well, that was somewhat stupid of me. Yes, my code reader works fine and low and behold there is one error code there, “P0420 Catalyst efficiency below threshold bank 1”. I am used to my Mercedes, you just look sideways at it and the CEL comes on so with no light on the Jag I assumed no codes.

So given the test scores and the history is that the likely answer or could poor o2 sensors be incorrectly reporting a poor cat while at the same time giving me a poor mixture?

If I have to replace the cats, Rockauto shows 2 different ones for the 95 with slightly different lengths. It shouldn’t be too difficult to determine the length on my car. The cost of $300 -$350 seems reasonable given the nature of the car.

 

There are many reasons for a cat efficiency code. Obviously a bad cat is one. An improper mixture, exhaust leak, failed O2 sensors are some of the common items.

I have a graph that shows the relationship of the 5 gases vs. mixture. Stoichiometric is 14.7:1. This is also refered to as Lamba of 1.

 

I can help, but first i need you to answer a few questions.