ok lets try this again, 2000 Jag S type 4L


Code PO307, replaced all plugs and boots, coils all fire, only 94000Km or 52000miles, car has sat a lot and not driven, I can clear the code and it appears again on acceleration, when gas applied, slowly, trans kicks down at appx 80KPH and car misses and that code comes up, no code if I hard accelerate???? any ideas??? Thanks Peter


peter36@xplornet.com

 

The coils do not throw codes unless totally dead. We had coils constantly go out without codes on my old 01. Even if it seems like it is working they can fool you especially under load like you mention.

Don't use aftermarket coils, they are known to be crap

 

Yes, fully agree. Additionally, the only way to diagnose an intermittent misfiring coil is with a COP tester.

 

Indulge me for a moment please, as I climb up on my MisfireSoapbox(tm). The term 'misfire' is easily misunderstood in OBD II speak. The word makes me think solely of an ignition failure, that is, a spark plug that didn't fire for whatever reason. In my not-even-remotely-humble opinion, a better label would have 'low cylinder output' or something similar, to cover a broader range of faults, not just ignition-related. I do not care to discuss how I learned this...

The P0301-P0308 codes merely indicate one (or more) cylinders is producing less power than the others. (The PCM determines relative power by the miniscule RPM increase as each cylinder fires.) It could be an ignition problem. It could be a fuel problem. It could be a compression problem. Ignition is the most common fault, but don't overlook the other two. I'll get down from my soapbox now. A thousand pardons, please, if this is old news to you, but I didn't want you to end up lost in the weeds if you don't find an ignition fault.

Have you replaced the coils? You could always swap with another from a known-good cylinder and see if the fault follows.

After that, I'd consider swapping injectors with a different known good cylinder. Plug in a noid light to see if the injector is getting a signal to open.

After that, it would be time for a compression test.

 

Let me add the following. Check each electrical connection to each injector. Make sure they are fully seated on the injector.

Again, these coils can work at less than 100% capacity and not set off a misfire code. Trust me, been there.

Do not buy cheap coils. From my experience (my 2000 4.0 is closing in on 250K miles) only go with Jaguar, Ford Motorcraft or NAPA gold brand coils. Anything less, your asking for trouble.

 

Sort of. One thing the car looks for is the little jerks (yes, the RPM increase) from each fire of a coil and burn of the fuel. When those jerks are not as expected it may throw a misfire code. Another thing is the wrong O2 pulse in the exhaust showing the air/fuel mixture did not burn.

The later cars (at least) can detect certain coil issues (such as shorts) or certain injector faults (such as shorts) and throw other codes (e.g. P035x) so if you don't get those but do get P030x you are looking at misfires (which might be and almost always are coils) but not likely at those P035x circuit faults.

I recommend not using noid lights at all with any circuit on modern cars unless you KNOW it can stand the current draw. I wonder if (doubt) a noid is any good (or safe) with an injector and anyway you can expect a code for an injector.

P030x codes are nearly always coils unless there's an obvious other cause e.g. you suddenly get P030x where x is 1,3,5,7 i.e. a whole bank - well, 4 coils do not go bad at once...

BTW on injectors, normally you can read the pulse width for each injector and if one is screwy go hunting why.

 

Thank you ALL!!!!Problem Fixed, new plugs and boots and Gasket set, Sea Foam in Tank with Fuel Injection Cleaner, and brought it up in the RPM range for a good ride of about 300 KM, Runs like a new vehicle!! Thank you all very much, PEter Olson