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Short and long term fuel trims - do I need to worry?
3 Attachment(s)
I have read an old thread on fuel trims where there were some very strange LTFTs.
My XJR has had a fixed, unchanging LTFT of 10.9% for the past few thousand km. The STFT fluctuates by a few percent as per the graphs from Autodoctor that I have attached. Under harder throttle the STFT goes to zero as per the graph that also shows %throttle opening, presumably because it goes into open loop when the pedal is down. Rest of the time both banks fluctuate up and down in a fairly matched way. The car is going like a rocket and I don’t want to mess with it. I just did a fairly fast 500 mile road trip and it didn’t miss a beat, used no oil, and returned 22 mpg (Imp) despite me driving at 70mph most of the time, lots of overtaking up hills at upto 95mph. However I have mucked around with various air flow mass meters past few months. I also had a replacement ECU about a year ago, but swapped Andy’s reprogrammed EPROMs into the new one. It hasn’t had any sort of recalibration on the proper Jag equipment in that time, but my mechanic has the right gear - I’ve seen it, looks like part of Dr Who’s tardis. So maybe that’s needed.. Any opinions? |
Vee suggested to me that a rich fuel trim at idle going toward zero on throttle up is a sign of a vacuum leak . Found mine to be a missing hose on the EVAP valve as it hooks into the center of the intake manifold on the under side .
After a while the O2 sensors will cloud over with soot and your engine trim regulation will get lost . Mine were nasty . |
FWIW....
My 1995 XJR suffered fixed trim readings (both LT and ST) for all the years I drove it. After a couple years of trying all the usual fixes I finally threw in the towel. I can't even remember all the things I tried and checked I made. Same for the (apparently phantom) P0430 trouble code which popped up several times a year. The car ran like a watch, though....except the various times I had failed coils, of course :) Cheers DD |
I can’t tell what MY you have. Perhaps you have it in a signature that’s only visible when I view it from a desktop. Anyways, I believe if you are 95 or older, trims are not as useful as the car seems to store some crazy LTFT numbers. It should be useful from 96 on. 10+ would concern me, however I’m not sure how to resolve it. I was at 10.8 for awhile and have fought to get to 7.8 and now I am at 7.0. I believe I am due for a camcover gasket change since I allowed my oil catch can to get full of water and it has blown out areas when the engine overpressurized. If your LTFT is at 10, you probably have some unmetered air coming in. Not sure you want to bother with checking around for a leak. Camcover gasket (including spark plug rings), IACV gasket are two potential suspects? Lazy O2 sensors perhaps? it might not be worth the effort. I’ve been chasing mine for quite some time, however I will say, at one time, it was at zero. This spring I am planning on removing the riveted on pieces that pass for a camcover exhaust filter. With the catchcan in place, I don’t need that unserviceable problem in my system. Ill let the forum know if there’s anything to report on when i do it. (Just been waiting on warmer temps) |
I discovered that the root of my poor fuel mileage was the TPS.
https://www.jaguarforums.com/forum/x...g-rixh-191174/ It could also be your problem. Its a rather expensive part. |
A new fuel filter and pressure regulator has cured my long term fuel trim - within 50 miles of driving, it had dropped from +10.3% to +2.3%, and now, a few hundred miles later, is down to -2.3% and seems to be sitting steady. however due to throttle spring issues which will hopefully get resolved next week, I only have 50% max throttle opening on the XJR, which may have some impact on fuel trims. |
...and today my LTFTs have reached the holy grail of zero edit - 10 miles later the LTFTs jumped back up to 2.3, but the new fuel pressure regulator has fixed the LTFT which was fixed on +10.3% for 1000s of miles |
I had 10.3% LTFT on both banks for too long as well. Over time I have replaced many many sensors and o-rings looking for something that would affect it. The TPS is Sensor that is known to do unpredictable things when failing, simply because bench testing it does not reveal any problems with it. Good luck keeping that trim down. I am back to 0.0% for a week now! |
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