Hi all,

Haven't really posted very much here since the introduction but I am on the forums very often and have learned an immense amount from you guys. My car has generally behaved itself well, whatever cropped up was quite easily fixed by referring here, but I knew the good luck had to end at some point!

I'm hoping you can help me with an issue I've recently started having with my car. As stated in the topic it is a misfire but the problem is that it triggers no codes whatsoever.

It's also intermittent, some days there's no sign of it at all but other days it's quite serious. Mostly noticeable at idle but then sometimes it pops up under acceleration also. What's concerning is the fact that it is clearly an issue but there are no codes triggered at all, as well as the random nature of the fault.

I have the car hooked up to an android tablet via a PLX kiwi and use Torque to real time monitor it. Only clue I picked up was fuel trims seriously out of sync when the issue happens.

At idle Long Term Fuel trim on bank 1 is registered at -15.6% while Bank 2 is at 14.8% (Positive not minus),

Short Term Fuel Trim on Bank 1 is -19.5% while Bank 2 is 19.5% (again positive)

I don't know very much about fuel trim but that doesn't seem right to me.

I do hope you could point me towards identifying and fixing the issue.

The car is a 1998 XK8 Convertible with 60k Kilometres on it and I live in Sri lanka

Thanks!

 

A failing ECU can do all that.


Dave

 

A little reading about fuel trims. Before you begin pulling things apart see in this information helps you to understand what is going on.

Link JagRepair.com - Jaguar Repair Information Resource

 

When was the last time you checked or replaced your plugs? On my 3.0 SType i had a poor idle issue and it turned out to be failing coilpacks with no codes.

 

+1 on failing or shorted coil packs as they often do not set DTCs nor trigger the MIL.

Check for oil getting into the spark plug wells as this eventually causes the coils to fail.

 

Thanks for the input guys, plugs were replaced in November as part of a major service. I didn't consider coilpacks as I thought they would always trigger a DTC code. Will investigate further.

Also, more worryingly, noticed a small amount of white smoke and a smell sorta like coolant from the left hand exhaust. Wondering if it might be a failing head gasket. No overheating noticeable though. Could that have a co-relation to the misfire issue?

 

Thanks Gus, that helped me understand Fuel Trims quite a bit. The concern I have on the Fuel trim front is twofold

1. The values are very high (19.5%)

2. The massive discrepancy between banks (-19.5% on bank 1 and +19.5% on bank 2) Also they seem to always settle at the 19.5 value when the misfire is happening.

Any ideas why?

 

It's so poorly it can't get consistent conditions thus can't set codes. But you may have pending codes (which some tools won't report).

You can still diagnose it but it'll be tougher. E.g. suppose you have an air leak. Watching STFTs and spraying burnable gas will still make them go crazy if you spray at the leak.

Things the car relies on: temp sensors, coils & plugs, battery, O2s, MAF, absence of air leaks, fuel pressure. (Also MAP if any, EGR, EVAP etc.) You can monitor a lot using OBD if you're thorough.

 

You mention you had major service performed, could you list what they did at least engine related? All good advice above and I'd suggest pulling all the intake filter / ducting etc and ensure maf/tb are cleaned and there are no cracks or air leaks. Smoke test would be good but.. Check all hoses, not sure your MY has a egr but it sounds like a open somewhere.

 

Jaguars do tend to put out a little white smoke during warm up in humid climates from condensation in the exhaust system. I live in a damp country so it's a regular sight.

Any evidence of coolant loss should be investigated immediately. The XK8 has a 'low volume - high flow rate' cooling system and a relatively small drop in the level can lead to overheating.

Don't rely on the temperature gauge to warn you of this. The temperature has to have risen seriously before it moves from normal.

As the V8's get older, there's a few more reports of failed head gaskets but it's still nothing like a regular issue. If it's a concern, have a compression test done.

A more frequent cause of coolant smell is tiny cracks in the header tank which allows drops of coolant to fall onto the hot exhaust manifold.

Graham

 

I'd look at the fuel for a moment. Pull a little fuel out of the tank. Check it with litmus paper for water content. You could pull the fuel filter like I did last and dissect it, check for water and or excessive dirt.
Theory, excessive water/ dirt could momentary clog injector or stick it open. Logic? Might not be enough to trigger fault code. Most times the computer is looking for electrical failures... Hope this is a simple check with great resolution. Rich

 

Thanks very much everyone for the input guys, much obliged! Decided to pull out the spark plugs and have a look at them today, found that the cylinder nearest to the firewall on the left bank (the one just under the coolant header tank) had some coolant in the spark plug well. so cleaned that out, let it dry, cleaned the plugs out and reinstalled which has sorted the misfire. Car now idles/runs fine and fuel trims are stable at close to 0.

What I need to figure out is how water got into that plug well. The coolant tank is less than a year old but I did recently change the clips for the small hoses connected to the top of it and some coolant did splash out. Wondering if it somehow wormed it's way down through substandard seals in the plug covers.

The other alternative is more concerning, that there is an internal leak in the cylinderhead.

On the subject of the temperature gauge, since I live in a very hot climate (lowest we see is around 22 C and the average is around 30-32 C) I'm very paranoid about overheating. I read here about the unreliability of the temperature gauge so I keep an eye on temps and other variables via a PLX Kiwi Bluetooth adapter and an android tab running the torque application, which is permanently set up inside the car.

 

The coil covers do not seal very well, so it's likely the coolant came from hose change.

 

Under normal conditions you will get codes in this situation the problem could be on the edge. I would first look at your plugs and see if they might need replacing I would then be looking at your fuel delivery. The first thing I would do is add BG44K to the fuel. This product is the best in the market! You could be getting bad readings from the O2’s. I would also try to find a shop that can read Jaguar codes or someone near you that has a IDS or SDD system that can read your codes.

I personally had failing coilpacke and a failing purge valve that did not throw codes but corrected my fuel trims.

 

Thanks Gus, since our fuel quality is not the greatest I use a can of BG44k every couple of months to be on the safe side. However in this situation it turned out to be coolant in a plug well, dried it out, cleaned the plugs and now it's running ok with the Fuel trims hovering around 0 with minor variation.

Will keep an eye on it and see if anything develops

 

When my coolant reservoir leaked slightly coolant managed to get into 2 plug wells closest to it even though the cover was in place. It gave me misfire and rough idle also but I got codes.



Dave

 

I would recommend that you do not over use the BG44K it is a strong and affective product. Use as directed on the label.

 

To start i would disconnect one injector electrical connector or coil connecto at a time while the engine is running to determine which cylinder is misfiring. You need to be very careful doing this since the engine is running and you could get injured easily. Once you determine the affected cylinder remove the coil and the spark plug. If there is a dark brown discoloration on the ceramic part of the spark plug below the point where the boot of the coil covers the plug then that indicates there is arcing from the coil to the metal groung base of the spark plug causing a misfire. Use some carburetor cleaner on a rag or some gasoline to clean the arcing off of the spark plug. Re-install the plug then use a liberal amount of dielectric grease on the inside of the boot that goes over the spark plug before installi ng the coil. Start the car to see if the miss is gone. If not, recheck to see if another cylinder is missing. If another cylinder is missing perform the same operation as before. If the next spark plug is showing the same problem I would replace all if the spark plugs and also apply dielectric grease on all boots before installing the coils. One other possible problem could be coolant getting into the rear spark plug holes causing the spark plugs to short out. This is caused by the intake manifold leaking coolant and getting into the soak plug holes. If coolant leaking into the spark plug holes is the problem the intake manifold will need to be replaced.
If your spark plugs do not have brown arcing marks then exchange coils from another cylinder to see if the miss changes to another cylinder. If it changes cylinders you need a new coil if it doesn't change cylinders then change a fuel injector with another cylinder if it changes to the swapped cylinder then replace the bad injector. If your miss is still in the same cylinder then I would recommend taking a compression reading on the bank of cylinders where the miss is and see if you might possibly have a burned valve. It is doubtful you have a bad valve. Most likely you will find a bad plug, coil or injector.