I have a 91 XJ6 with 156k miles that I've been having problems with for most of the last year. It will start up and idle perfectly and it will run at WOT perfectly as well but if I go to drive down the road pressing the gas will usually make it bog before it actually decides to "go". If i leave it to idle for a while ~15mins or so it will stutter really badly, spit out some black smoke, smell like gas and give me a check engine light with FF44 although once its done stuttering it will continue to drive exactly as it did before.
Heres what I've replaced, and in order:
O2 Sensor
Air Filter
Carbon Canister(Unrelated, it was physically broken like something hit it)
Battery(it was time)
Spark Plugs
Spark Plug Wires
Cap and Rotor
Mass Airflow Sensor(albiet with a used on off of ebay)
ECU(also used)
Crankshaft Position Sensor
Both Coolant Temp Sensors(Theres one for the gauge and one for the ECU)
Coil

I've cleaned the throttle body and every electrical connection I can find and I'm stumped because the car still runs the same as before.

Things I Plan on Doing:
Compression test
Replace Injectors
Hollow out downpipe catalyst
Find a 93 or 94 in good shape with low miles

Anyways, am I on the right track here guys? Any ideas on where to go next? I really love this car but I'm pulling my hair out at this point. It also shifts rather hard and is getting progressively worse. Any help is appreciated

 

I had ' 88 and that was nothing but pure trouble . Of course the car was thrashed long before I bought it . So, I went for '93 which is my fave XJ40 . It's not been perfect either . However, later models seem better and you can also have fancy gadgets that you could not have in '91

 

Fuelling Failure 44 indicates poor mixture control rich or weak (or misfire). Could be caused by a clogged air filter (which you've replaced), O2 sensor (which you've replaced), then theres a small pipe on the throttle body that sometimes blocks up, or you could be using the wrong fuel.

 

Maybe have a look at the fuel filter, and do a fuel pressure test.

 

thank you for reminding me, I actually have replaced the fuel filter but I haven't done a pressure test yet

 

Hi, seems as though you've replaced nearly every fuel/ignition related item that would cause some of your problems. It's the part where you state that it runs fine at WOT, you can check the cats but I believe if they were clogged you would not be able to get the engine to run so great at WOT.
I don't think you mentioned cleaning the TB and while you're in that area check your TPS, I know you are not getting a code for it but a bad one will not always throw a code. Sounds as though you have a dead range (could be dirty) that causes the problems you are having. The TPS is a type of rheostat and all rheostats wear out eventually especially when they see as much use as in a TPS. You can easily find new ones on Ebay or many online parts suppliers. They are not cheap, 200-280 seems to be the going price these days. For that reason you can test and possibly clean the one you have, also you should try to clear all passages that breathe through there to remove oil residue.

Good luck!

Charlie

 

thank you, I'm in the process of looking for a good used throttle body/TPS. Any members on here have a spare to sell?

 

You need to check the fuel pressure. High pressure can cause black smoke (rich) and fuel pressure problems can trigger FF44.

bob gauff