I just did a short introduction in the "new members" area. I've been a reader on this forum for many years until now.

I own a 1997 XK8 convertible since 2011. It is not pristine but in good running condition. A lot of work has been done on the car. I now try to find the cause of a not really smooth idle of the engine. There are no (misfiring) codes. Drivability is fine, no problems during acceleration. The only thing I can further mention is that last year I had a code reader connected and the long term fuel trims for both banks where between -10% and -15%.

I'm trying to exclude parts and now am focussing on the coil on plugs. Being an AJ26 the COP's are the 2-pin version. I cannot find a diagram how these are connected internally. I can measure the primary, but not the secondary coil. One of the connections of the secondary coil is off course connected to the spark plug, I would expect that the other end has a common connection with one of the primary coil connections (both connected to the positive battery voltage for example).

The question is if the secondary coil can be measured with an multimeter (Ohm).

 

Coil on plug problems are more often than not related to insulation breakdown at high voltage. The only way to test or this is with a specialized high voltage tester.

If the COPs are the originals, at 20 years old, it is worth considering replacing them all.

 

I'm not sure how old the individual COP's actually are. They are all original (Denso). One or two where replaced a few years ago with used ones.

I'm now considering to replace all 8 with new ones.

I understand that a valid function test requires specific equipment.

Is there anyway to verify resistance of the secondary coil between two connections?

 

Sounds like the COP tester...

 

With high negative LTF's I would not suspect bad coils but instead, leaky injectors, marginal MAF or weak O2 sensors. Perhaps a BG44K treatment could improve injector performance.

 

The injectors have been treated in the car with a machine connected to the fuel rail (similar to TerraClean). It cannot be ruled out that one or more injectors are leaking, they will have to be removed to verify that. The MAF is new and original. The O2 sensors have been replaced also but not with original Denso's. They could also be responsible for false readings to the ECM. Emission tests however are always fine (need to be tested every year), as is fuel consumption (23 MPG US).

After ruling out the coil on plugs the next action was to have the injectors removed to be cleaned and flow tested. When I think of it that would be a more logical step to do first.

Thank you all for the input so far.

 

Update: I’ve just removed the injectors. Tomorrow I will bring them to a place nearby where they can clean and flow test them.

 

You need to do better here. The car's built-in diagnostic system is your friend. I'll say it again: get a cheap ELM327 wireless reader from eBay and the corresponding phone app, each for a few $. This will give you the error codes as well as a bunch of real time values, like air temperature, water temp, air flow, etc. Without being an expert, you might be able to pick up on something if it is obviously wrong.

Have you checked the O2 sensors? From memory, the upper sensors are wide-band, and the lower sensors are the older narrow-band type. The original Denso are not that expensive.

Best of luck, keep us posted.

 

Hi. Sorry for the problems you're experiencing. Can't really help much here but I thought I'd ask how did you find the task of removing the injectors? Is it a lot of work? Any special tools required? I realize my engine is different as it has the blower and associated bits in the way... I'm thinking of doing the same as I have a lean condition going on at the moment.

Cheers,
K

 

In the case of the engine in my car (AJ26 1997) it was quite easy. After removing the engine covers everything is accessible. Drain the fuel via the schräder valve. Then remove the electrical connectors and injector retaining brackets. After that the injectors can be pried out with a screw driver. Wiggle it until it pops out.

In your case I guess it is different because it is a AJ27 and a supercharged engine. I believe the everything above the injector rail needs to be remove to be able to access the injectors. It is a lot of work. I had this done on my XJR (AJ27 MY 2001) a few years ago and also had the cooling hoses under the supercharger replaced.