Hi all,

Firstly can I say this forum has been fantastic source of information and I hope to post a lot about the my first Jag and project as it goes on! I feel I'm nearly at the source of my issue but wanted some advice or confirmation that I'm on the right track before buying a bunch of parts.

OK, so I've had lumpy idle on cold start, and then once warm hesitations under throttle. No eml light but code p1646 and p1647 always reoccurring.

Disconnected the MAF and I got codes p0102 and p0112 as expected, but it ran like a dream... all the way up to 3k revs where it wouldn't go past in limp mode.

Tested both pre cat lamdas with a multimeter, the engine side of the harness has good earth's and about 11v to the heater circuit on both banks. All good there so hopefully no ecu issues or anything like that.

Metering both lamdas was a pain with the engine running. Burning my hand on a coolant pipe to reach it 😂. Both sensors had no continuity or resistance on their heater circuits suggesting Open Circuit. The rear most had a steady voltage on the sensor side, reaching about 0.55v which is in the right range between 0.2 and 0.8v, the front bank sensor however had barely any voltage... not the easiest to measure in situ but I'm fairly sure the max I read was 0.11v.

So the exam question is do I need two new lamdas... my answer was yes right up until I disconnected the Maf to rule it out. I suppose with the Maf disconnected the engine isn't using the lamdas to control fuel mix anyway? It must be something in the chain that's controlling the air fuel mix.

Bonus points, can I bypass the maf in the interim such that the car runs properly but doesn't go into restricted performance at 3k? I suppose that by running rich i would risk damage to the cats?

Thanks all! Again love the forum and the car.
I'll start a project log, if there is a section and interest for those.

Chris.

Just a quick aside, so far I have changed spark plugs, spark Plug gaskets (oil found in plug well) and rocker gaskets, imt gaskets, resealing everything and testing for vacuum leaks with carb cleaner - none found.

I also have a fuel filter to fit as I though it may be blocked, but after it running perfectly with the Maf disconnected I'm happy there shouldn't be any fuel pressure issues. I'll still change it when I get a chance to be safe.

Chris

Sorry to spam the thread, updated my signature to include my car details. I can see how to edit a post but I'll come back to that in the morning.
Chris.

Hi Chris,
Firstly...never apologize for adding details as they come to hand....all goes towards getting a clearer picture.

Can I clarify that you disconnected the MAF sensor (in the intake duct with 3 pin plug) not the MAP sensor which is located in the rear section of the intake manifold (with 5 pin plug)?

How does the engine behave at first start up?
Does it exhibit the same rough idle and rev limiting behaviour?
I believe the ECM will be running in an open loop state until sufficiently warm before starting to draw upon reliable feedback from some of these sensors such as the Lambda sensors.
So what it does cold may also help eliminate some sensor inputs from having any effect on engine management.

As to your question regarding O2 sensor operational voltages, this link might help you with understanding the voltages you are seeing, what is normal and when.
Also how to diagnose bad sensors and how you can sometimes trick yourself while testing.
https://www.linkedin.com/pulse/how-t...ndy-concepcion

Thanks for your response H2o2, the exploded diagram calls it a mass air flow, however its definitely a 5 pin plug. No 6 below:

on start up it idles initially at 1200ish revs, before dropping right to 800 but seeming as though about to stall. Haven't looked at fuel trims as I don't have a laptop logger but I'd say very lean. At this stage it's undriveable... kangaroos and almost a delayed response to throttle. But given 5 minutes to warm up the idle settles and then I can set off. Once there there is still hesitation through the rev range... perceived flat spots on the acceleration curve where the power drops off and then comes back by the time I'm changing up. Not sure if that helps at all.

Thank you for the link, I'll have a read in the morning.

Chris.

Hi Cmayes91,

My bad....MAF is 5 pin (air intake duct), MAP is 3 pin (rear of intake manifold).

If you have cleaned the throttle body disk and the MAF sensor, then the ECM should be getting reasonable input for a cold start engine.
O2 sensors will not be factoring into mixtures when engine is cold.
Disconnecting the MAF sensor will only be forcing the ECM to revert to baseline fuel mapping table settings, but bad MAF input could certainly trigger the ECM to be referencing the wrong data set.

1200 start idle is about right for initial start and mine drops away quite quickly into the sub 900 range within a few seconds.
Sounds like you probably have an air leak, but I get that you have done extensive hunting for one already...but they can be tricky to locate.
You might also want to see if you can get a OBD11 reading of your fuel pressure as something sounds quite hinky.
Don't know your mileage on your 2001 vehicle, but fuel pumps can become weak or intermittent with brush wear, generally though you would see no fuel or intermittent fuel, not low pressure.
My thought would be to check the fuel pressure sensor on fuel manifold, as it tells the ECM how hard to run the fuel pump to deliver the correct pressure as there is no return system on these cars.
Low rail pressure will stuff up your injector delivery, spray patterns etc and make it run like a bag of a-holes.
Fuel filter could be restricting flow, but pressure reading will give you initial indications anyway.

Hi, Mark,

Bit of an update, I've now got a fuel pressure tester and the new fuel filter so I'll get some pressures before and after to see if the filters blocked or makes any difference. Do you know what good should look like?
I also have managed to borrow a known good MAF sensor from my father in laws 2.1 V6 so I'm hoping that's an easy way to diagnose or eliminate it as a suspect.

Will keep you posted once I've had a free day to work on the Jag! Weekend goals, haha

Chris.

Hi Chris,

You should be seeing something around low to mid 50 PSI at the fuel rail when running.
FYI...Here is a recent thread discussing fuel pressures.
https://www.jaguarforums.com/forum/x...-p0193-252275/