This car is certainly sensitive to vacuum leaks. Last time I just replaced everything in the intake manifold, the rubber gaskets between the plastic plenum and the aluminum runners and the gaskets themselves between the runners and the engine.
That helped a lot but not everything, next I replaced the o-ring around the Oil Filler Cap, again, improvement but not quite. Then I replaced the o-ring around the dipstick and that even helped.
This is all after changing every coil, injector, MAF and o2 sensor.
Honestly it is still not right. It is still just a bit rough in the morning but the trim values are decent (much better than the +25 before). It also has a bit of a hesitation, not bad but there.
Next I am going to replace the cam cover gaskets and solenoid gaskets along with a new set of plugs (I think I am wasting my money here but for completion). It has some seepage around the gaskets and there is one the wets the plug cap. I figure if oil is leaking out then air is leaking in.
Hope that helps.

 

Hi @mbwillgrubs
I hope you and your Jag are well.
I read this thread a while back and am scrutinizing it again now. I and another member, @Mac Allan , are experiencing similar symptoms, lean-when cold, and I am very curious if you ever completely resolved them by resealing everything, or if you found a more significant culprit. I have gone through all of the normal vacuum leak detection tests and thought about all potential causes that come to mind, but have yet to find a satisfying answer. Lately I have been preoccupied with other things in life, but Mac's troubles, which are a little worse than mine, has got me focused back on this issue. Thanks!

-j

 

I have not. In fact I was trying again this weekend as it has started setting codes again. I am thinking intake manifold gaskets as I can spray carb cleaner in that area and it will effect short term trim. I am puzzled as I have replaced the gaskets before.
I think the next step is to pull the manifold (again) and take a close look at the sealing surfaces to check for warpage or cracking. It will not be soon but will feed back if I find anything. Best of luck.

 

Thanks for your quick reply. I am sorry to hear you are encountering these problems again. Have you had cold temperatures lately? I have found that mine will only go into restricted performance on particularly cold days, because LTFTs briefly exceed 20% before the engine gets to operating temperature.

We have fuel trims that exceed 40% when the engine is cold, but are nearly normal when at operating temperatures, which I do not think can be explained by typical vacuum leaks. I am struggling to accept that the explanation lies in the expansion of seals, parts, and gaskets. I doubt that these can correct that much leakage of unmetered air, however I do concede that it is possible, but it just seems unlikely to me. The fact that there are no problems at other RPM rates, suggests that it is not a fueling issue and it is almost certainly a leak somewhere, but the binary nature of all our leaks is the most confusing part. From 40% to nearly nothing at exactly the operating temperature. It really seems like the ECM is enriching the fuel to match the oxygen density of the cold air. I suppose this could be an indication of a failure of the intake temperature sensor, but my readings are slightly warm but reasonable (~75 degrees f this morning when ambient was around mid 60s) and enrichment, I think, should be consistent across the RPM range as temperature, and thus AFR, would stay constant.

The one important difference between our cars is my O2 sensor currents are around -.2 mA and yours are around 5.8 mA when cold. Another difference is that I don't seem to experience any loss of power when at operating temperature, although I have not extensively tested it.

-j

 

Hi guys. I'm getting the exact opposite. LTFTs are nearly 20% when cold, but STFTs come down by nearly 15% to cancel them out.

But after about 10 minutes the STFTs don't go down and sit at about +3%.

Bizarre. My HO2 sensors constantly read negative. I believe they may be faulty as previous owner has fitted cheap ebay ones.

 

Hmm, interesting, @XJR_hunter . So I am clear, when cold your STFTs are around -15% and then rise to a constant +3% when warm?

 

Well they fluctuate obviously, but that's roughly how big the change is from cold to hot.

In other news, I went through a ton of receipts from the work the previous owner had done, and turns out he has fitted O2 sensors off ebay costing 23 quid each. So I've ordered 4 genuine Densos at great expense to see if that helps. Will update accordingly.

 

At least you know you'll have quality sensors for a very long time! And you have backup cheap ones that can be used for future diagnostics, as you now know exactly how they behave.

I can't remember, but have you checked to see how your LTFTs and STFTs behave throughout the RPM range when cold and at operating temperature?

-J