Hi folks. As per above I have quite a pronounced misfire on my 2003 XKR at WOT only from about 4000rpm onwards. It also occasionally occurs under very light acceleration. I suspect the coils as I know they're a mixed bunch some newish, some older but I'm not getting any error codes. Is there any way to rule in or out coils as being at fault and if I do decide it's coils how do I determine which one is failing? Any input much appreciated

 

Usually the coils will set a code error and check engine light on. The best way is connect a diagnostic tool and look for logged DTC and read live parameters. You can also check for oil leaks inside spark plugs holes removing the coils.

 

One simple little trick i did was to listen for the coils in the AM radio, and sure enough; crack snapple pop.. following the engine rpm. No faultcodes and the engine was still running without missfireing.. but il guessed it was just a matter of time before it would.

So.. the cracking noice on the radio has to come from somewhere.. right? Its a sparkgap somwhere.. sure enough, when i pulled out and checked the coils, they have a little spring at the end that pushes on to and connect to to the top of the sparkplugs.. they looked like a mix of burnt/corroded/tired metal.. and i realised that if the resistance in that little coil locally got to high, a spark will jump across from one turn to the next...

Cheap fix? yep. I got some copper solder braid and carefully treaded a short piece in to each little spring, from the top to the bottom of the spring and put everything back.

No more "snap crackle pop" anymore. Wallet happy.

/E

 

If there are no codes, I would use a multimeter and write down the resistance across the different LT and HT connections on every coil and see if any are out of line, either with the rest or with the expected resistance figures which you will probably find online.

Richard

 

Are you sure it's not a bit of fuel starvation, if it's at WOT over 4k? Have you changed your fuel filter recently/ever? Worth checking that both fuel pumps are working.

 

That was going to be my guess too. No matter how new a filter might be, it can get clogged.

( PS: the 4.2’s only have one fuel pump )

Z

 

I did not know that.- every day's a school day!

 

Also check your fuel trims at that load and rpm. The XKR plastic intake has been known to develop cracks at the resonators. These cracks can open under load and create massive air leaks. Air flow meter could also be faulty and report the wrong amount of air coming in.

Fuel trims are a standard OBDII thing, an ELM327 can report them on a mobile device.

Best of luck, keep us posted.

 

General question. How can one have repeatable misfires without the ECU logging them?

 

I had random misfires on my S Type 4.0 for many months with no codes ever triggered. Got fed up one day and replaced all the plug coils and plugs and never missed again. Sometimes codes are just not registered.

 

Thanks folks. I appreciate the feedback. Still no codes from car scanner with my elm327mini. Now thinking you might be right re the air leak. I've tried to attach a graph showing the STFT's showing what happened when I fed it the gas from about 3500rpm in 2nd. A few years ago I replaced the gaskets on the end of the supercharger elbow (hat) the bit with V8 stamped in it. One of the 3x bolts that holds the double barrel gasket in place was fairly stripped (not by me). Wondering now if that gasket is deforming/leaking under pressure of high revs and boost...? Anyone have any thoughts? Also can anyone read anything into the graph attached? I had the scanner on record so can supply other data if it helps. Thanks Jeff.

 

Such as:
1. not enough monitors have set (so, many codes cannot flag)
2. the misfires don't occur in nearly similar ways

 

Whith one of that gasket leaking you would see oil around the leaking one.

 

Check on that. Very small amounts in the low spots of the supercharger hat

 

It seems too little to justify an air leak. Maybe your problem could be fuel pressure related..

 

Change the fuel filter as a starter. Not expensive, and probably needs doing anyway.

 

Does your OBDII reader display fuel pressure ?

 

I'll have a look at the fuel pressure readings. I changed the fuel filter about 12000kms ago so should still be fine. Thanks will keep you posted. Any other ideas by all means fire em thru.

 

Fuel pressure is also available over OBDII for our 4.2l engines.Should be easy to display.

These fuel trims look super small. You might want to try and graph the long term trims as well, if possible. Trims are a function of load and rpm. See if you can capture the trims for different loads and rpms, and try to graph the trims as the elevation on a map of load and rpm.

Best of luck, keep us posted.