My 1989 XJS just started running rich in the middle of a road trip. I say it's running rich because the exhaust fumes smell rich and my fuel economy is about about 20% below what I expect. There is no smoke from the exhaust.

I did some research and have the finding results to share:

Air Temp Sensor @ about 70 degrees ambient temp - 2.07 Kohms
Coolant Temp Sensor after about 45 seconds run time - 1.89 Kohms
TPS sensor voltage @ idle - .32 volts
TPS sensor voltage @ full throttle - 4.3 volts
Full Load Vacuum switch - closed @ atmospheric pressure; open around 1.1 mm/Hg
Full Load Throttle switch is flaky but generally open circuit until the capstan hits the switch, then it is a closed circuit.

Any ideas on next steps? Is there a better way to check to see if it's running rich? I'm thinking about tapping into the o2 sensors to see what they're reading but wanted to see what the group thinks.

 

Pull the vacuum lines to the fuel pressure regulators, and see if there are signs of gas in either one. If so, you need a new regulator.
To see if the O2 sensors are working, put a 47 uf capacitor across the leads of a voltmeter, and measure voltage between wires on test plug at the ECU. Full procedure is in the Book. Even better is to look at the waveform on a scope. Do it cold, and you should see a jump when the O2 sensors start working. You have to test each sensor separately, but I don't think it goes to closed loop until both are working.
Also, make sure your enrichment throttle switch or the vacuum backup switch aren't sending signals all the time.
I think someone posted a list of CTS resistance values vs temperature a few weeks ago. A search might turn it up.

 

Thanks for the ideas. I pulled the o2 sensors and tested them using the propane torch trick. The right one was showing about .4 volts but the left one was dead - it showed 0 volts.. Those were Bosch 13032's that I bought less than a year ago and have less than 3,000 miles on it.

I ordered a set of Denso 234-3048's that will be here this week. We'll try those out and see if we can get more than a year out of these.

 

Thorsen
This thread gives the Mityvac reading required from a working vacuum switch:
https://www.jaguarforums.com/forum/x...p-both-243178/

specifically post no. 3:
"The vac switch is, in my view, the one to keep. UK spec cars had this switch only, no back-up microswitch. The two do the same job, they are wired so that whichever closes first switches to full enrichment on the same ECU circuit. The vac switch is passing current, thus enriching the mixture, at a vacuum LESS THAN 3.5, and it is easy to test it with a MityVac and a test light and a 12v source. These are very reliable switches. I got worried about mine a few months ago and managed to find a NoS one. I tested the new one and it closed at exactly the correct reading. Then I tested the old one, and that did too, in spite of 35 years in the engine bay and 150,000 miles.
So i do not think the microswitch is needed, and it is certainly a less robust switch than the vac one.
"

 

When fully hot I'd expect the coolant sender to be around 200 ohms. As a test, try unplugging the sensor and unbend a paperclip or similar and stick it in the terminals of the plug end to bridge them. That will signal fully hot and will pull out any cold enrichment.

 

There is a site on YouTube ( Living With A Classic ) Good site regarding the Jaguar V12 and maintenance / repairs. Has a video on how to properly tune the V12 - adjust fuel mixture etc, etc. Walks you through the whole procedure. Worth watching.

 

Thanks for the recommendation. It turns out I had an o2 sensor that stopped working. I replaced it and all was well again.