Soo.. I have one of those OBD2 heads up displays which gives me RPM, Speed, and most importantly, coolant temperature.
After much experimentation I've come to learn (as other forum members may have learned as well):

The act of monitoring OBD2 data while driving adversely affects engine performance and fueling!!!
Car would run really poor off the line in 1st gear and act as if there was a large vacuum leak and fueling
would go lean > rich > lean. which in turn would cause coolant temps to rise above normal...

The act of observing coolant temps via OBD2 (at least on the 5.0 DENSO ECU) causes higher temperatures
and overall poor throttle response and poor performance.

I realise this could be device specific, but everyone should be aware of the potential.


Has anyone else encountered this?

 

I don't have those side effects. Rarely a P0171code.

 

Which ECU do you have? 5.0?
which obd2 are you using?

I didn't know I had a problem until I unplugged it.

 

PS you shouldn't be getting a P0171 unless you have a problem.

A P0171 code is a diagnostic trouble code that indicates your vehicle's air-to-fuel ratio is too lean, meaning the engine is getting too much air and not enough fuel. The powertrain control module (PCM) sets this code when it detects the ratio is outside of the factory-defined range.

A P0171 code can be caused by a number of issues, including: A leak or electrical problem, A dirty mass air flow (MAF) sensor, Intake gasket degradation, and PCV system issues.

When a P0171 code is triggered, the PCM will call for more fuel to be sent into the engine's combustion chamber. This is known as a positive fuel trim.

A vehicle with a P0171 code may experience the following issues: Loss of power, Stalling, and Rough idling

 

Maybe its the uncertainty principle at work?

 

Sounds like the cat is alive but barely.

 

I use the Car Smart Digital Meter model P8.

The code came after a lot of traffic on a hot day. Temperature touched 100 degrees C.

Since then I changed the original gas filter and the original PVC. As I said the code is rare.

 

I feel like I'm missing something: are you saying having an OBD reader plugged in is causing your car to run hot? Why would that happen, the device is only reading data from the PCM. If you have an emissions inspection requirement, the car would fail every time if that behavior occurs when reading data from the OBD connector. I have a Foxwell NT560 and have never observed that behavior, even when using Torque Pro and Track Recorder. I have a 2007 XK non-supercharged. Did I read this wrong; what am I missing?

 

I'm saying that having an OBD reader plugged into your car makes your ECU/fueling act strange, and for me run lean.
Lean leads to hotter engine temps, also you have a 4.2 and different ECU so it may not apply to you.

OBD2 ports are live and if left connected to your device will prevent your ECU from powering down
and the car going into full sleep mode, your battery will die within a few weeks as well.

I'm not the only one who has reported this, theres other similar posts on this forum.

I also thought it was improbable, being that an OBD2 device is usually only reading data,
but after I unplugged mine, car runs much better.

 

Something wonky with your reader. The one I’ve had the longest, and used most often, senses when the car is off, and shuts itself down. No battery drain. Only problem I had, was when I plugged it in while driving, big mistake, dash lit up like a Christmas tree and all kinds of warnings blew by on the console. Stopped and shut off the car, unplugged the reader, restarted and all was back to normal. Lesson learned, plug / unplug read while car is off.

 

I haven't experienced any glitches with mine.

 

Even if your reader shuts down, your obd2 port will remain live
and slowly drain your battery at about 500mah, (ive tested mine)
(In my case)

the cars body control module wont deep sleep with something
connected to your obd2

I suspect this messes with fuel trim.

 

Haven't heard of it myself. Which OBD II reader are you using?

 

Not an obd2 reader, it's a heads up display

 

Only if the thing wants to keep it alive. Plug unconnected wires into your obd2 port, it won’t keep the module active.
The manufacturer of your heads up display didn’t do right by you, and cheaped out on their obd2 interface design.

 

Going off topic a wee bit, show us a pic of the HUD if you have one. Curious as to the look is all.

 

Used Toque Pro for years and zero problems? Your wise to monitor coolant temperature too!
So I can't say as I have never seen it happen on my 4.2L SC or my 5.0L SC engines.

Really like the custom Jaguar PID pack that Cambo made years ago. If you don't have it take a look. It's free.
.
.
.

 

My OBD2 BlueTooth dongle is usually inserted on any drive over twenty minutes as to run the TSD Torque Pro gauges full-time. I've never had any issue ever.
It's a cheapo clear blue $8 one I got that included both Torque Pro and Dash Command on a small disk many, many years ago.

 

My OBD is basically never unplugged and feeds Cabrio Bob's ADU. I have had this setup installed for several years and have never had a problem. I only unplug it the OBD when the car goes in for service or for CA smog test.