My XJ6 X300 3.2 N/A is lumpy at idle, splutters at part throttle until fully warm and recently (should have) failed an MOT for very high emissions. It stinks of fuel. Once hot it's perfectly driveable and quite powerful but the idle is still rough.

I've put used but uncracked exhaust manifolds on it and a new centre cat as both were blowing. I've also just changed the 02 sensors.
I've also just adjusted the pedal and cable to get the TPS moving from 12% to 95% whereas before it was 20% to 80%, which has brought the idle down from 800rpm to 650-700 where I believe it's supposed to be.

My ODB2 scanner shows the air inlet temp and coolant temp to be sensible numbers, and the TPS shows a good steady sweep as I put my foot down.
I've checked for air leaks around the inlet and found none, but that should only make it run lean or idle high, which it does neither.

What's odd is that even once I'd got the car hot (90C) my scanner was claiming the car was still in open loop due to insufficient temperature, and i couldn't get any fuel trim readings or 02 sensor voltages. The only value I could get was long term fuel trim and both banks were showing -100%.

The MAF shows 0.01 at idle and 0.02 g/s when revved, but I'm led to believe on an early 1995 car like mine those numbers could be junk.

Is my MAF dead? If so, where to get a good one from? Any other suggestions for the lumpy running and overfuelling?

 

It could be your MAF
There are secondhand ones out there on eBay, or generic ones that are about USD$100.
I went through all this in my XJR, and in the end it needed another engine ECU to sort it properly. Then it would go into open loop once warm.
But I would start with a new MAF

 

Ordered a second hand "guaranteed to be good" MAF from ebay, will update.

 

Hi. I had similar issues with mine but a used maf from ebay.co.uk improved it a lot. I first bought a new one for 100usd but it only lasted 3 weeks.
I'd also recommend to change spark plugs, air filter, and check your coils. I also ended up adding a cleaner called bg44k and it seemed to work well.
The whole story is here if you want to read it. https://www.jaguarforums.com/forum/xj-xj6-xjr6-x300-26/1995-4-0-sovereign-stalls-drive-169235/

 

I gave it a service not long ago, so plugs and filter are recent. The coils were recent when I bought the car but a brief check showed them all the firing fine.

 

New MAF fitted and no difference.

Things I've noticed are that it takes a very long time to warm up, so fitting a new thermostat and coolant sensor tomorrow to be sure. On a 30 minute drive last night it never got over 70C on the OBD logging, and the dash gauge agreed it wasnt getting to the N.

I took the plugs out and as expected they are black but cylinder 5 the plug was the normal colour. Sometimes that means a head gasket issue so I've compression tested the engine and it's equal across all cylinders (phew), plus I am loosing no coolant and it's not overheating. Maybe 1 injector is blocked? The coils were changed before I bought the car and I've lost the order so can't replace just 1 now.

I've got IDS coming soon so hopefully I will be able to get my ECU back into using fuel trims, and I'm going to test the injectors into jam jars for leaks and good flow.

Anyone else got any ideas?

 

Just wanted to update this, ended up buying another ECU and all the trimming started working properly and runs much better.
Now to do the pesky timing chains.