For about a month I've been getting a p0174 code (sometimes it will go restricted performance for a short time as well). Comes on when coasting or light on the gas. Today it did it while I pulled over to take a phone call and engine slightly stumbled like it had a miss for a couple of revolutions. I've reset the codes and it is good for a few hundred miles before coming back. It is our 03 XJR. Any thoughts on what to look for first. I would think a bad maf or vac leak would likey throw codes for both sides right? Oh and which bank is the right when looking from the front of the car. My dislexia gets me confused on that one all the time. Thanks!

Don't reset it. You're slowly (possibly quickly) causing damage.

Check for pending codes then check fuel trims. More data is what can help you rule in/out what may be the fault and thus items to change.

That DTC lists the following possible causes,
Engine misfire
air intake leak between MAF and throttle (THIS IS MOST LIKELY)
fuel filter/system blockage
fuel injector blockage
fuel pressure regulator failure (low fuel pressure)
HO2S harness condition fault
Exhaust leak before CAT
ECM receiving incorrect signal from ECTS, MAF, IATS, TPS.

My money is on an induction leak, it effects both banks. Check the induction tube for cracks around the corrugated section, the seals on top and underneath the throttle body, the MAF connector onto the induction tube all can and do leak leading to DTC. Give the MAF a blast with electrical contact cleaner (in a can) and start from there....

Then wouldn't I also be getting left bank lean code as well?

sometimes, depends were it's leaking, if big enough.

That's why you need to check the fuel trims. If it is just the right bank (seen from the back) it could be a manifold gasket or crack.

Couldn't figure out how to screen shot the fuel trims but I also have a code now for p01533 (o2 sensor slow to respond) so I am wondering if that might be the issue and it just didn't throw a code til now. The snorkle was on good and tight no cracks etc. I didt pull it off and reinstalled just to be safe. Don't have a place to work on it at the moment. TVR is in the process of new rear brake calipers and rotors which will probably take another week. Imagine the Jag inboard IRS in a tube frame chassis and you get the picture. Not much fun at all!

DTC P0153

Check for exhaust leak(s) around catalyst.

No leaks that I can feel or hear. That o2 sensor was replaced last January with a bosch aftermarket one though assuming bank 2 is the pass side.

See attachment below for cylinder numbering and location. Bank 2 is the left side when viewed from the driving position.

Check for leaks around the welds on the catalyst and connecting pipe union. Check exhaust manifold for cracks.

Okay so that o2 is original then. I replaced the pass side last January. I did check it and no leaks or broken welds. Put some injector cleaner in. Thinking back, fuel filter is probably 2 years and 15k old so I doubt that would be an issue.

If the electrical connections to the oxygen sensor are good, the next thing is to check the fuel pressure. The P0171 and P0174 DTCs can also be caused by low fuel pressure, which in turn may also cause the 'slow response' of the upstream oxygen sensor(s).

Check the Workshop Manual for correct fuel pressure at idle and under load.

Below is the procedure for checking the fuel pressure on the X308.

Thanks NB, it will probably be a few weeks til I can get at it as I don't got the garage space with my TVR being on jackstands at the moment. Plus I need to scronge up a gauge as I don't think my bike air pump would work real well. Both pumps and the pigtail were replaced within 20k or so but of course new parts do fail as well. Put some Techron in to clean the injectors for the time being. Got lots of driving to find snow coming in the next couple weeks.

You could use a gauge like this one:

https://www.ebay.com/itm/Professiona...FYPUyv&vxp=mtr

If the fuel pressure is within specification, the upstream oxygen sensor (before the catalyst) is probably the cause of slow response.

I think I still have one for my bosch k jet system on the Tasmin but I don't know if the kit has any fittings that would work on the XJR. Hopefully I can adapt it to work. There isn't a valve on both injection ports is there? Can't remember.

If memory serves, there may be a schraeder valve on the fuel rail on the Bank 1 side of the engine covered with a plastic cap.

Commonly you can't see or hear them.

That's why to use OBD to check fuel trims.

Knowing the common issues with the X308. I would be looking to replace that O2 sensor and the fuel pumps. The exact O2 sensor (direct fit same Bosch) is available outside of the dealer network .. so get the exact replacement.

I had some strange operation prior to me understanding I was getting an intermittent fuel pump failure. I give my independent a lot of work -- they install my sourced parts. I did two OE pumps and attached filters and replaced the main fuel filter. The main filter is a GM filter .. get a good one (the last a long time).

The restricted performance could be another thing all together ..... if the engine stumbles and the output from the alternator drops below the threshold ... even for an instant -- you will get the RP. Especially if you are idling coming to a stop with the brake lights on.

I was getting RP as a coasted to a light .. and also while sitting at a light a while back. I almost sent the Tbody away. The battery showed no signs of weakness ... it was a middle aged battery (4 years) -- I replaced it anyway and it been two years now. The battery must be perfect on an aging X308 .. any strange flux in the voltage .. from wipers/AC compressors .. throws them off.

I'm not sure that we're getting through to him. All the suggestions to throw parts at it should be put on hold until there is more data.

CHECK THE FUEL TRIMS.