being new to the world of electronic fuel management, I'm trying to catch up on 40 years of engineering advances in a few weeks.

What are the normal numbers for:

1) short term fuel trim

2) long term fuel trim

3) downstream fuel trim

?????

I'm not getting any fault codes, but the numbers are confusing nevertheless.

For short term fuel trim it's consistently under 1% Usually something like -0.79%

for long term fuel trim the numbers are higher, like in the range of -4.50% to -7.0%

But the downstream fuel trim numbers are really high, consistently over -40.0% Do these number have anything to do with the catalytic converter health?

I've looked in the 1999 shop manual I downloaded from this forum (is there a 2002 manual ? I couldn't find it.) And did a few searches here as well. But I think my questions are too basic, and I've come up empty.

thanks,

Z.

 

here is the link to Gus's website which has some good info on Fuel trims. This should also be in the stickies at the top of the XK8/R forum

JagRepair.com - Jaguar Repair Information Resource

 

A good place to get started on fuel trims is this thread. I've never heard of 'downstream' fuel trim.
https://www.jaguarforums.com/forum/g...ed-quiz-49317/

 

 

Thank you for reposting this link! Wow now I understand the importance of fuel trims and why experienced posters ask for them. I am now going to get mine read in an effort to find out why my cat monitor wont reset.

 

Remember, fuel trims are like a 3D elevation map, they are a function of Load and rpm. It is only a single number for a given Load and rpm, e.g. at idle. Off idle, it will likely be a different number. Also likely a different number between idle with transmission in "N" or "D" as the load varies. So posting fuel trims numbers only makes sense if Load and rpm are posted as well (or a shortcut like "idle"). Fortunately, Load and rpm are available over OBDII as well.

 

www.onboarddiagnostics.com/page02.htmConsider them with a hot engine (thus also hot cat convs) when parked. At idle ideally they'd all be 0 but that won't happen so somewhere near would be good.

If any LTFT is quite high, rev to about 2500 and see what happens.

With an air leak the LTFT would drop because there's so much more air going in (thus the effect of a leak is far less).

To get a feel for roughly what the PCM (aka ECM) is doing see OnBoardDiagnostics.com - Engine Basics

 

 

Most of todays fuels have sufficient detergents, so I wouldn't expect to see a change unless the car had been sitting for months or had poor quality fuel. If your highway mileage is below 22 at 75-80 mph it may help.

 

 

Zray,
I just got back from a 700 mile trip at 75-80 mph most of the way and on board dash shows 26 average MPG in my 97 XK8 conv.
Mike in TN

 

Nothing of interest. (Can't see why anyone would care, either, but no matter.)

Don't bother with the stuff but if you are keen on such try BG44K (think I've got the runes right).

 

is that with regular driving or with the cruise control ?

any XKR owners willing to share mpg numbers ?

Z

 

Zray,
My drive was without cruise control... and using 87 octane fuel ( the only thing this car has ever used) through the mountains of TN, and now has 113k miles.
Mike in Tn,

 

I'm just a tad over 125,000 miles, and have been using 91 octane pure gasoline when I could get it. Does the XK-8 not require the 91 fuel as the XKR does ?

In any case, the only way I'll get above 22 mpg, albeit briefly, is by driving off a cliff.


Z

PS will be driving thru your state on the way to Marietta GA in Sept. Any notorious radar traps to avoid ?

 

at higher altitudes regular octane gasoline can be used in a premium only engine. in the high elevations of Colorado, the sell lower octane than even regular for this reason. it is a premium required car (91 US system) and not using such at sea-level conditions is not the best idea unless one drives very flat roads and does not push the vehicle in the mid-range. speed also makes a big difference in the highway fuel economy. using regular octane, the fuel economy will be slightly lower under all conditions for both energy content (there is a very slightly higher energy content to the octane boosting chemicals, in premium) and engine tune. this all differs from racing fuel in that these fuels are not just higher octane but formulated for higher energy content per unit measure of fuel.

 

regarding Techron Fuel Treatment, very good results if you have the problem of dripping or somewhat clogged fuel injectors. two treatments consecutively made a big difference in performance, fuel economy and idle smoothness. use the larger bottle, add before filing the tank. run down to 1/8 tank and repeat. most additives are a waste of money; this one is definitely worth it.

at 55 with the cruise control, both calculated and dashboard indicated 30 MPG, 28.6 at 70.

 

Another vote for Techron here. All of our vehicles get a 20-ounce bottle down the hatch at every oil & filter change interval. It is the only fuel injector cleaner I will purchase. I watch the sales at Advance / Pep Boys / Autozone and grab several bottles when they run their BOGO deals....

 

around here there are 2 formulations of Techron, the fuel injector cleaner, and one labeled "total fuel system cleaner", which I believe is a little stronger, and it costs $ 0.40 cents more than their fuel injector cleaner product. I buy it at Wal-Mart Unfortunately W-Mart does not carry the larger 20 oz. size. But two bottles of the smaller size is still cheaper than the sale price of one 20 oz. bottle priced at the usual Auto Parts stores, So I just dump in 2 bottles every 3rd or 4th tank of (pure) ethanol-free gas. Probably more often than I have to, but I'm trying to clean up the system that may have been neglected by the previous owners.

Z