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I am just wondering what are the nominal fuel trims for the 4.2 SC? And the timing advance at idle. Purely because I am curious about this stuff. Long Term trims...
I have the SDD software, can this interrogate this data? I usually use my scanner for this stuff but wonder can SDD fish this out.
I'll pull data from my car later, no issues with this vehicle just wondering.
Short term and long term fuel trims are additive so the short term plus the long term is the actual trim at that instant. Over time, and once the engine is operating in Closed Loop mode, the short term should settle close to zero and all the trim will be set by the long term and stored for when the engine is next run.
Ideally, an engine straight off the production line should have a zero trim as that is how the engine was designed. However, in reality, engines are not ideal so there will always be a +ve (engine running slightly lean) or -ve (engine running slightly rich) correction. I believe that zero +10% or - 10% is supposed to be achieved and +/- 5% is very good.
You might well be able to read live data fuel trims and ignition timing with your OBD scanner.
Richard
Last edited by RichardS; Jan 17, 2025 at 04:16 PM.
Much easier to use a simple OBD tool with live data than SDD.
Get engine properly warmed up then park and look at idle trims. Don't clear codes!! You'd reset the OBD monitors - oh, check them, or just check you have pending P1111 (means they're all set happily).
So I plugged in my OBD reader and used Torque to see what is going on and my long term fuel trims are around 7 percent positive, Bank 1 is higher than bank 2 slightly - not massive but it points to a vacuum leak somewhere around the MAF.. any ideas? The trims go back to 0 at around 17000rpm so that confirms its a leak - guess I will be having to take off the wheel arch liners, again..
The timing advance graph is here, this is from a brief spin to the shop - engine was warmed up
Update - looking into this I can see I fitted new air filters last october - expensive BMC - I also had a torque log from last summer - fuel trims were 1.5 percent long term...
I think we have our answer here?! I know the AJV8 is really ficky with air filters and here we can see it clearly --
** actually scratch that, I think this might be normal due to the cold weather-- not entirely convinced its the air filters as I looked at more logs after they were fitted and the long term trim at idle was around plus 3 which is fine, I will inventigate my piping...
7% isn't bad but yes worth seeing if it will go lower.
17000rpm is amazing
I think this is a case of "much ado about nothing" - I have looked into this and fortunately have torque logs from when I got this car a year ago and looked at them - the fuel trims were always hovering around 6 or 7 percent positive at idle, which makes me think it is what it is - the car runs fine and has just passed the MOT - 50k miles, o2 sensors are responding well and its not the Catylic converters.
I have a MAF sensor <somewhere> from my Subaru 3.0 SpecB which is the same Denso as used in the Jag, what I will do before I decide to get new MAF sensors as they could improve the fuel trims, is put in this MAF and see how they react - its just that like anything on this car doing jobs is a bit of a pig.!
None of this interests me, other than trims should be close to zero and if they change a lot maybe something should be checked. Timing means nothing since it's completely computer controlled.
None of this interests me, other than trims should be close to zero and if they change a lot maybe something should be checked. Timing means nothing since it's completely computer controlled.
The MAF sensors are crucial for trims and when I studied previous data I can see both banks reacting the same, an easy check is to swap out the MAF and see what happens, if it fixes it I can get new MAFs but not before I get some confirmation.
You aren't going to get zero trims on Jaguar XKs that are over a decade old, plus or minus 2/3 percent would be pretty good - fortunately I have another Denso MAF somewhere to try out, I can fit that and run for a few days and see what happens
If your fuel trims are trending positive, it means that your ECU is adding time to your injectors to increase the fuel in the mix to achieve stoichiometry. As these cars age, various O-rings start to harden and shrink. If you're diy, best thing to do is a smoke test of your own. If not, start by replacing the O-ring at the top of the dipstick, if you have one. Then, replace the old ring in your oil filler cap. Third, replace the O-rings that are at the end of various plug-in vacuum hoses. These are all very cheap, and might take a half an hour to do. After these obvious ones, a smoke test is the best way to find additional vacuum leaks as they may be subtle and small but add up in overall impact. If you're less than +10% ltft, you don't have any major ones, but some minor ones and perhaps some cumulative degradation. I don't know your mileage, but take care of these simple and cheap issues before stepping up to a MAF sensor.
Fuel pressure sensor perhaps ?? What do you think Panthera ??
I remember having issues with fuel trims being way out of whack last year, replaced the sensor and they're close to identical from left to right banks now
Fuel pressure sensor perhaps ?? What do you think Panthera ??
I remember having issues with fuel trims being way out of whack last year, replaced the sensor and they're close to identical from left to right banks now
One thing I found on my 4.2 NA was that the fuel pressure sensor had a vacuum line attached to it. This vacuum line was extraordinarily fragile, and eventually I had to replace it, as it failed and created a significant vacuum leak. This vacuum line connected several points, and was a truly crappy piece of bad design. That said, the op has a SC, and I don't have enough experience to comment on that model.
Last edited by panthera999; Jan 22, 2025 at 09:17 PM.
I don't think there's anything wrong with the fuel system, the fuel filter has been replaced and the car drives fine. The mileage is around 50 thousand miles
Going back to the lean fuel trims, last night I was looking around the engine bay making sure all the hoses were good and I saw a small hose going into the supercharger with a Y split - this fell apart after moving it (all of it crumbled) and apparently it's a vacuum hose - it's going to a solenoid and also connected to the front of the engine with a secondary pump. ...
I have ordered all the bits needed, I'm not a mechanic but I think this could explain why the trims were a bit lean, many people on other forums said a positive trim of 5/6 percent is fine and you are looking at a ghost.
Now I need to deal with this slight inconvenience, no idea when the parts will arrive, I think I'll bodge up that with some windscreen wash piping or something
Yes, those little vacuum lines are ridiculous. I cut off the connectors with about a half inch of line on them, and used "real" vacuum hose by pushing the connector stub into the end. You can replace most of the brittle line this way. No problems thereafter, for me. Good luck
Yes, those little vacuum lines are ridiculous. I cut off the connectors with about a half inch of line on them, and used "real" vacuum hose by pushing the connector stub into the end. You can replace most of the brittle line this way. No problems thereafter, for me. Good luck
I ordered various tubing, 3mm 4mm and 5mm hoses - a cheap 3mm hose arrived today and it fits good over the existing stubby ends - i went ahead and ordered some silicon 3mm hose of better quality which will arrive soon but in the mean time I will use the cheap 3mm hose I dont think it will last very long due to the heat
I also had to order a 4mm Y split connector, I have a 5mm one but its too large. I will also seal up the ends of each stubby end with rubber sealant.
4mm Y pice arrives tomorrow so keen to start this up! and yes, that Jaguar vacuum line is ridiculous, surprised its not a sticky on the Jaguar XK
The Vaccum line at the front is conected to the secondary pump and terminates at what looks like a one wave valve of sorts - then its joined to the other line leading to the SC - I have photos of my Engine from last year and was looking at this and could see that part of the connection at the valve end was split - one is going to an outlet which was broken and the other is going to the valve - I suspect this was the cause of the lean fuel trims, but truth be told the entire vacuum lines were terrible, surprised the trims were not worse
Fuel Trims and Timing Advance Q
