Seems to be running rich on bank two (left side): I guess I would start by cleaning: clean the air filter (or replace); clean the MAF and check each connector; clean the throttle plate and bore; run a can of Gumout's SeaFoam look-alike (or SeaFoam) through a can of gas.

No codes?

 

Not sure why my text didn’t enter into the original post.

MY2003 XJ8
Issue: Terrible misfire when first starting the car for the day. It clears up after a few minutes and drives fine and has a very minimal shudder at idle when driving for the rest of the day. Not a single pending code.

I’ve replaced coils and plugs.
Tried brand new BSE and Duralast MAF’s.
Cleaned throttle plate.
Cleaned part load breather
New air filter.
New battery and new positive battery lead to trunk main fuse distribution block.
Cleaned all grounds including engine ground strap and bulkhead connection near ECM.
I do not see any apparent vacuum leaks.
I get no MIL/ Check engine codes.
I’ve also hard reset the ECM several times after major changes.

Any help would be awesome.

 

It would be nice to know wich cylinders are misfiring. After that you can pin point the reason better when you know are you missing spark of fuel or both or something else. Try to pull out injector connector on each cylinder while engine running badly to pinpoint the cylinders that are acting one(s).

Oh I did not see the pictures. Rear O2 sensors aren't warmed up yet, so the mixture is not known yet. (Unless they are warmed up and stays bottom wich case engine is running very lean) 12.5% on long term trim is nothing severe on these cars. But on a high side that is for sure.

 

Downstream

Upstream
Getting weird O2 fluctuations.

 

Aftermarket MAF not that good for this car use the oem denso . Check if you have any oil around the spark plugs
​​​​​​what spark plugs you're running denso or other?
Use techron fuel injector cleaner

 

Just prior to the vehicle running rough the very first time, can you think of anything at all that may contribute to these issues popping up out of nowhere? Example only: the car was sitting unused for a number of weeks and the battery went flat, I ran the car out of fuel and got some fuel out of the shed I put in my lawnmower, I hit a large pothole and immediately started running rough. How many miles on the car? Any work done soonish before it started running poorly?

 

"Issue: Terrible misfire when first starting the car for the day. It clears up after a few minutes and drives fine and has a very minimal shudder at idle when driving for the rest of the day"

We have seen this problem frequently as weather changes: often a stuck Temperature sensor in the cross over pipe is not operating correctly. The temperature sensor acts like chokes in the bad, old days: if it is cold outside (water temperature is cold), the sensor sends info to the ECU (via MAF wiring) to tell it to enrich the mixture at start-up. Once warm, it leans out the mixture. If stuck open, it doesn't enrich (rough engine running, hard starting). If stuck shut, the opposite (which we see come summer). Bad thermostats can also show these kinds of issues. Since we don't know where you are . . . In OZ, in May; in northern Europe, November issue.

Just a guess.

 

Do you have catalytic converters installed ? When rear o'2 sensor voltage oscillates like that cat's does not work or are cold as can be.

 

Yes they're installed. That reading was while the car was warm and idling for quite a while. I was trying all day to try to figure out what the issue was so the car had been idling nearly all day.

To the people above. I am in the U.S in South Carolina.

There was nothing major changed or happened that caused it. It was random. So I've fired the parts cannon at it trying to resolve the issue since there has been zero MIL or pending codes to help diagnose it.

 

135k. No work done. I fired the parts cannon after the early morning misfiring started. No positive changes.

 

Perhaps silly question, but I didn't see it listed in the parts cannon list: how old is the fuel filter? Cheap and easy to replace...

 

Im not sure if it has or not. Definitely not by me.

I am seeing different data now that may be an indicator. My fuel trims look a little better. But the idle shuddering persists in gear or out of gear.

My short trim fuel bank 1 and 2 downstream don’t seem to be moving at all while revved, in gear or idling. They’re dead flat at -0.787. I’m wondering if my downstream sensors are dead?

 

Fuel trims on cold start when the bad idle happens

n




Notice in all screenshots that the downstream short trims never change in any condition.

would this indicate bad downstream O2’s?

 

You may have not so reliable tester readout. It takes like 30s to get closed loop to turn on. Before that everything goes as their own weight. If I remember right short term is showing choke at the startup.

 

How about water temp at start up?

 

The STFTs in the last picture are extremely high on the + side; the ECM is adding a lot of fuel, which would indicate an air leak.

I've attached some good reading on fuel trims, however, this is the first time that I've seen a "downstream fuel trim." Anywhere. I think that it's reasonable to assume that the reading is from raw fuel going through the cats as the downstream O2 sensor is telling the ECM that it's too rich of a mixture coming out. IMHO

 

With tester that shows same pid value with totally different value can not be trusted 100%. Unless verfied with a working car. I may have looked in to the short term trim behaviour in my car when engine is cold but I can not remember how it worked.