What the title says. Car starts and runs beautifully when cold. After reaching operating temperature, if I slow down (i.e. idle into a parking spot, reverse out of somewhere etc.) the idle dips, and it will occasionally die.

I found some vacuum diagrams and was going to try and take a look at the hoses. What else should I check?

Thanks!

 

what year model ?

 

Sorry- 1987 XJ6 Series III

 

Make sure the throttle body is clean and the throttle blade is set to .002" gap

Cheers
DD

 

Thanks! I found an old thread about water coolant and O2 sensors, so maybe I'll check them out as well.

 

if it was a 1985 or earlier Series 3 XJ-6, I would tell you to install a Ground Strap from the oxygen sensor to the chassis, but 1987 models already have a grounded oxygen sensor.

 

Ok, further testing: after driving about 15minutes, idling in a parking lot, the the engine speed bounces up down between 200 and 800. Otherwise seems to start, run,
and drive great.

vacuum hoses seem good (haven’t checked exhaustively but so far seem solid)

fuel pressure regulator holds vacuum

throttle body looks clean, and is gapped at .002”

Coolant temp sensor is showing proper resistances

it will start right back up so I doubt overheating electronics, but of course never say never

What else to check?

EDIT: After running the tests detailed here: O2 Sensor Test Information & Resetting the Light (jag-lovers.org) I believe the O2 sensor is bad. When I initially hooked up the multimeter, I registered voltage as expected. When I started the car and revved it for a while, the voltage was stuck high. I created a vacuum leak (used what I believe is the cruise control manifold port). The voltage did not respond immediately, then slowly, over a couple minutes, dropped to a low number. When I replugged the vacuum port, the voltage did not increase, but stayed "stuck" below 0.2. The engine ran rough for a bit then died. I'm interpreting that to mean that for whatever reason the O2 sensor is unresponsive or slow to respond to air/fuel mix changes, and should be replaced. Let me know if you have any different thoughts!

 

Closing out this thread. Replaced the O2 sensor today, and the issue appears to be resolved. Possibly could have cleaned the old one, but new was $40 so I went ahead and replaced.

 

yes, it's usually the O2 Sensor. In earlier engines, adding a ground strap to the sensor stabilizes the idling. I am not a scientist and I don't know why, but I did it to my engine and the behavior change was instant.