115,000 mile 2003 xj8

At idle my fuel trims hover right around 0%
However, on acceleration, the STFT on both banks can rise up to 10%
On deceleration the STFT on both banks can fall down to -24.2%
Long term fuel trims remain around zero.

No trouble lights, car runs fine most of the time, but gas mileage is questionable (about 15 city). Any ideas what could be causing this?

Another issue, possible unrelated, is poor idle on hot starts. The issues only happens sometimes, maybe once in 100 starts. No issues starting, no trouble codes, but when idling the car feel lumpy as though it's down a cylinder. If I rev the engine it smooths out. When i let off the gas, then the engine will drop to around 400 rpm and nearly die (occasionally it does die) before coming back up to idle. As soon as I start driving, the issue smooths out and doesn't come back.

 

Do you have (pending code) P1111? If instead P1000, which OBD monitors are not set?

Any other pending codes?

I'm asking because flagging codes needs most/all monitors set and reasonably similar conditions (*) to flag.

(*) or a definite bad condition, which you clearly don't have

A coil (or wiring etc) on its way out is a definite possibility.

I take it that required maintenance has been done (proper plugs etc)...

The trims don't sound worrying but you can feel something wrong so worth following up.

 

A rough idle when warm, may be caused by many different things such as low fuel pressure,
low voltage to the fuel injectors, dirty fuel injectors, a vacuum leak, a faulty Oxygen
sensor or a dirty or failing idle air control valve. I tend more toward the vacuum leak
simply due to the age of the car, particularly if it is a daily driver.

 

Yes, I have the the code p1111
Plugs were done 5,000 miles ago and 02 sensors 30,000 miles ago. Fuel pump was replaced quite recently too. I checked the MAF sensor and it's readings seemed accurate.

Is there any way to test coils without full replacement as they're quite pricey?

I'll try to look for vacuum leaks using ether.
If that doesn't work then I'll go for a fuel injector cleaning.
After that, I'll maybe try cleaning the throttle body.

My battery is also 8 years old. I had it load tested and it seemed perfect, but I'll get it replaced if nothing else works.

 

congratulations you’ve described a correctly operating fuel injection system

current ramp the coils make sure fuel supply is adequate you will never get bad trims from a dying fuel pump on these cars

 

24.2 % is the outside limit of what it will read which is a bad value

Negative 24.2 is a rich exhaust per the chart below

The go lean command is the direction the ECM should go to correct the engine regulation to a 0 % target value

Took me awhile to get the chart straight in my head

Seeing it on both banks makes you look away from an exhaust leak biasing the sensers seeing a high unburned O2 content

this points to a vacuum leak shared by both banks

On deceleration the " supply " of vacuum is not sufficient to overcome the leak

The supply is more related to engine low idle speed RPM then letting off the pedal and deceleration translon to idle range

My head's still not straight

On your V8 you have a specific fuel injector relay you can swap ( in case the power contacts inside are burnt ) that places power " sitting " on the injectors with the ECM providing the timed ground to control the injectors opening

 

Look downstream lambda signal voltage with the stft %, Engine needs to be warm or hot. What does the lamda signal voltage when trim is way off ? Don't make quick throttle blips. Stay on current position long enough that you can see what is happening. And if there is something that ecu can do. Where is lambda singal when trim is okay ?

 

Sounds like you do not have a problem! What do the LTFT readings show? STFTs will fluctuate, especially when cold, or when accelerating. The older the car, the more so (rings, valves, dirty injectors, here and there a vacuum leak, old plugs, errant temperature gauge on cold start), but yours seem pretty close to as good as it gets.

 

Over 15% trim is always telltale that there is somehing going on. The thing that short is high and long at the same time is low tells that that short is not yet been enough long "on" time that it has been moved to the long term side. It takes time or cycles to move +10% short to +10 long and when that happens at the current load spot short goes back to the 0% as long has the 10%. Recent code clear may reset both trims. Usually every atleast somewhat modern ecu will turn mil light on when long goes over around 10% trim. I dont know why x308 ecu does not. It is not uncommon to ecu display high short trims when engine is cold. Cold run enrichment is done via trim.

 

I believe that the ecu's throw a code when trims are over +/-25%. At 24.2%, I'm at threshold.
Long term fuel trims remain low, close to zero, but need to check them again to confirm.
These trims that I'm describing occur when the engine is completely warm.
I'll check lambda voltage when I get a chance, I believe it was close to the prescribed value, just a little bit lean, maybe 1-3%.
I've looked for a vacuum leak with ether but found nothing. Maybe I need to do a smoke test.

My current thinking is that it's vacuum leak before the throttle plate. Typically with a vacuum leak after the throttle plate, they're more pronounce at idle and their effect decreases at higher load because there's less vacuum. However, I believe that vacuum before the throttle plate increases at high load, thus causing a lean condition of acceleration. I could be mistaken so someone check my logic here.

I still have no idea why it would be rich on deceleration.

 

Vacuum leak before plate leads to misread of air mass to the engine. So ecu thinks that air goes in less than the 0% trim. Mixture goes lean and trim goes up. Leak here will affect at wot too. Amount may decrease.
Vacuum leak after plate causes lean mixture, increased trim, hissing noises and lumpy idle and or misfires. Leak affect dimishes here at wot or close to it because there is no vacuum in intake plenium at wot. Idle rpm may rise.
Leak amount makes big difference in both situations.

My car had +25% LTFT and +15% STFT For couple of years over 1/4 of throttle positions. No cel, no fault codes no nothing. Sometimes weird hiccups and ocassional weird throttle pedal response.

 

You now appear to be discussing the kind of leak that would result in LTFTs not near to zero, yet I read that they are.

What makes you think you have any leak?

 

Ah, found at least one missing piece of the puzzle lads.
Did a 200 mile road trip today, ran some diagnostics:
short term fuel trims were between +10 and -25
Long term fuel trims stayed at +1.6 and +2.3 most of the time, but during city driving (lots of acceleration and deceleration) they went up to +10 and +7. The ltft's never went below 0 so it seem like the engine's lean.

As for the other issue: the poor, lumpy idle on hot starts.
I happened to have by obd reader connected when the issue occurred this time. The lambda voltages were quite rich and the stft's were negative. Could this be leaky fuel injectors?
Also, when the rpm's drop to 400 from the poor idle, the lights start to dim as well. Is this a normal occurrence when the rpm's drop? Is this indicative of a bad battery?

 

Anything under + - 6% is completely ok. Sometimes under 8% is ok. In what situation that -25% showed up ?

400 is so low rpm that alternator does not spin fast enough.

Too much fuel at idle. Maybe leaky injectors. But if it does not feel that it runs on 7 cylinders or less then there something that affects to all of the cylinders.
If that -25% pegged during deceleration i'd check the fuel pressure regulator vacuum hose that it is tight and ok. Fuel pressure would be nice to measure during driving and when this idle happens.
During decel engine needs really little of fuel until overrun fuel cut turns on.

 

I think you're drowning in data, most of it of small value if not indeed misleading.

I'd do the more basic stuff first.

E.g. talk of fuel trims / air leaks - do the usual checks. Easier and lots of people have been there before you with posted results, fixes, etc.

Hooray. Gives you a chance to do the basic stuff.

Could be. Don't assume so, though.

400 sounds too low and the dimming worrying. I don't have your model so do others have the same things happen?

You can easily do the usual checks for battery (& alternator).

 

Checked for vacuum leaks again last night, couldn't find anything. Cleaned out the pcv system in case that is causing any issues.

I forgot to mention that the negative stft's during the lumpy hot start idle were about equal on both cylinders ~ -8 to -10. This suggests that it might be a problem that affects all cylinders.
I'm starting to think it may be an electrical problem because of the very binary nature of the issue. It starts up fine most of the time (no vibrations, normal fuel trims within 20 second) or it starts up all lumpy (vibrations and subtracting fuel). There's no in between.

I'm doing a battery (+alternator) load test and a fuel pressure test today.

 

After learning this the hard way, I'm a big proponent of checking the electrical system before speculating on any of the electro-mechanical parts. (I made up that term ... )

I use the attached process and it only takes about 10 minutes to make sure the proper voltages are there before going crazy trying to figure out what's going on.

My "learning the hard way" was a two-week 4500 mile driving vacation and on day two I started getting all kinds of BS codes. Figured it out when I got back home and it was the alternator.

 

A very thoughtful and easy to read trouble shooting guide. Thank you for posting that.
It should help quite a few diagnose the electrical problems that inevitably occur in these
20 plus year old vehicles.

 

Battery and alternator testing was mostly good: 800 cca's and proper charging. However, the battery is only outputting 12.24 volts when it should be about 12.6. I understand these cars are sensitive to low voltage, is this low enough to cause issues?
Fuel pressure testing came back with 33 psi at idle which seems low. It's 38 with the ignition on and engine off. No signs of pressure loss over time. What could be causing this fuel pressure?

 

With the engine running the battery is taken out of the equasion as the alternator should be putting out 13,5 to 14.5 ideally

A quick thing you can do with the fuel pressure is jumper the # 1 fuel pump relay between socket 3 to 5 in case the power contacts inside the relay are burnt

Blade type jumper wire ends recommended

Inspect the fuel pump motor connector for burnt sockets which is also a sign the fuel pump is failing

The fuel pressure at the pump alone ( negating the fuel pressure regulator and injector drainage ) should be higher or more at it's optimum with alternator 13.5 volts