Hi There,

I am having trouble with a P0300 code, I get individual cylinder codes too, but no rhyme or reason as to what cylinder. I clear the codes and get different individual cylinders every time.

I have done a tremendous amount of the usual stuff. The random misfire is exactly the same despite fixing a massive amount of little problems. The car definitely is running better and putting out much more power than before I started, but the random misfire persists.

I have read every post I can find, they always seem to have a different scenario than mine. My car seems to run great at 1100+ rmp. Strong and does awesome burnouts the ultimate test of power ha ha. Seriously though, car runs great aside from the idle. I can get around it by two-footing it at a light (I only need to do this sometimes) to keep it just above idle.

- replaced octopus hose
- cleaned the entire intake system
- replaced all the vacuum hoses (no vacuum leaks, even fixed the oil fill cap leak)
- cleaned the throttle body and all related parts while doing the octopus hose.
- plugs are new and correct
- coils all test fine.
- New CPS
- New temp sending unit
- Injectors are cleaned
- brazed and machined exhaust manifolds, no leaks.
- replaced collector gasgets (now exhaust no leaks)
- confirmed all 4 o2 sensors are orientated properly (verified using the colours of the wires)
- added fuel line cleaner amd run it though
- "Italian-tune-up" (took her out on the hiway and was full throttle a bunch of times, for a long time. Definitely dusted out any cob-webbs)
- checked the ecu pins (weird, was a tiny bit of oil on the red plug... cleaned it. All pins and plugs in excellent condition. Zero corrosion
- checked the grounds on the firewall, (and everywhere) look great, tested with meter 0.0 ohm resistance, tightened them a little extra just to be sure.

Stuff I am suspicious of:
- fuel pressure regulator (I had both fuel pumps running and it still idled rough) I don't have a way to test it, other than I pulled the vacuum line off to see if it was leaking past the diaphragm.
- and the o2 sensors because everyone likes to blame them. Not giving me any codes other than the misfire codes though... amd no soot on tail pipes, does not smell rich at the tailpipe in any way.

if anyone can think of some other stuff for me to try I would be grateful, getting "editors blindness" with the car at this point.

I have an autelmax ap200 code reader, but it does not seem to be able to read the fuel trims...

I can upload a video of the stumbling idle if anybody wishes.

Thanks so much.

Aaron.

 

Cracks in the exhaust manifold and the donut gaskets leaking will make the engine run rich at idle

You can tighten the 4 downward pointing studs to see if that seals the donut gaskets up

!3 mm median to deep socket on a long extension before you dive , one nut has very little wall clearance against a pipe

 

Thanks Parker,

yes I fixed the cracks in the exhaust manifolds. I also had the gasket surface machined and put in new collector gaskets. No more exhaust leaks that's for sure

 

Last 2 coil wires swapped , Rearmost colored Light Green / Purple , second from rearmost Light Green / Orange , they will both have the same White / Pink wire as power ' sitting " on the coils

This would not explain warmed up occurrence but the O2 sensors are not used until ECT of 88 C ( closed loop engine regulation )

Fuse 10 , 12 , 14 , 16 right engine bay fuse box , one of those is the O2 heaters to ensure they read properly , fault may acure on throttle off cooled back down sequence

IAC valve rest ?

 

You seem to have covered most of the usual suspects. The fuel pressure regulator and the injectors themselves would seem the next logical step. The pressure regulator should be releasing the most pressure at idle, as consumption of the fuel rail will be low relative to delivery pressure. If it is faulty it might cause pressure fluctuations in the rail? It is easy to replace and I don’t think it is very expensive…….
Have you cleaned the injectors themselves? Easy to do, and they tend to be overlooked…..

 

How did you test the coils? The codes you're getting would indicate that they might be nearing the end of their life. You can purchase a cheap set on Amazon that would probably work for a few months, certainly enough to check this item off your list.

I'm assuming you have seen no oil on the boot plugs when checking the coils? Your problem is common when the gaskets at the plug wells begin to leak.

Another guess could be the harness wires could be brittle and shorting out somewhere along the way between the ECU and the coil.

Lastly, I want to make sure you checked the common ground for these coils. I know you say you've checked the grounds, but there are many, the one I want to make sure was checked was the one used for the coils.

If someone had run an O2 reorientation in the past, the wire colors would become meaningless, since it would have been changed with a reorientation. The car would run very roughly though, so I don't believe that's your problem.

 

I wish you luck. I had the same issue, which may or may not be there. I changed everything that you could think of to the point that anything that effects this code is new. It is currently on jack stands, waiting for me to finish up with the suspension. Ones I get all that fixed I will see if and when the codes come back. Mine is a little different in that when I clear the code, everything is fine for a little while. I am not sure when it will return, but I think it will. I also checked all the coil wires and did not see anything. The last three times that I saw mine, it was cool out. My house sits about 1200ft higher than the interstate( I need to get on it to go to the golf course twice a weekend), so there is a lot of coasting. When I get on the freeway and go 10'ish miles, it codes me. I pull over and clear the code till the next time I follow the same process. It is very weird, but it is electrical, and we all know there is always an issue with these cars when we purchase them.

 

Thanks to everyone for their thoughts

I am going to check a couple of things mentioned before I make my next step of buying the fuel pressure regulator.

the problem that I'm experiencing has developed relatively recently. I have put 70,000 km on the car with regular maintenance. It now has 200,000 km on the car. I live in Victoria British Columbia in Canada and our weather here is quite mild... so that helps a little bit as far as corrosion and other issues. For example my wiring harness is still flexible, and of course I do keep it moisturized with WD-40 just to make sure that it lasts as long as possible.

I am going to poke around under the hood for a little while and come back with some updates.

 

The FPR is a rubber diaphragm that doesn't last forever , around 90 dollars US

Common auto industry part and not specific to a Jaguar parts source

The FPR will regulate the pressure at 43 PSI at idle and 46 - 47 at throttle up

If you have a question on low fuel pump delivery you can directly jumper the fuel pump relay between socket 3 and 5 .

This jumping will drain the battery with the keys in your pocket , hot at all times power source at the fuse...........

 

Check your thermostat. I was chasing a stumbling/running rich/fouling plugs condition for months.
Scan tool told me I was in closed loop operation, but the ECT would not go over 150 degrees.
Pulled thermostat and sure enough it was stuck open. Replaced thermostat with a new one
and my fuel trims have gone back to normal and the car no longer stumbles, runs rich, etc.

 

A faulty Coolant Temp Sensor would also do that. You say you replaced the sender, which only tells the dashboard what the temp is, the sensor is what tells the ECU about the coolant temp.

 

Some more great info here :-) Parker yes I have tried having both fuel pumps running with no change... and I did take the vacuum line off and ran the car so that if there was any gasoline leaking through the diaphragm I would smell it. This doesn't mean that the regulator is perfect, however it does rule out a catastrophic failure of the diaphragm

the thermostat idea is a great one! I did replace the temperature sending unit on the thermostat housing but didn't bother with the thermostat. It looks good to me so I just put it back in. Nothing wrong with having a new thermostat, they're cheap and it's a good idea to have a fresh one. God only knows how old the one is in the car could be the original! Ha ha. Once I get a bit more time I'll get under the hood and come back with a report with what I've found. It's been a busy few weeks doing some other things around the house but I'll get back to it soon enough. On the bright side the car runs and it's insured I've driven it around and of course people just love to look at it ha ha!

 

If you change the thermostat it does not take a large o - ring , even if t comes with one , cracks housing as you clamp down

The 1 wire sender is for the instrument cluster

The 2 wire sensor is for the ECU engine regulation

 

I've not encountered this on a Jaguar (yet) but have with plenty of other cars. In case it helps, here is some of my general experience with a number of cars with the exact problem you are trying to solve.

For cars with cars with coil packs and conventional spark plug wires when I received a P0300, I always took the sledge hammer approach and replaced the coil pack, spark plug wires, and plugs and that resolved the problem. In most cases I think the culprit was the coil pack but I never proved it. Such an approach to replace all coils would be an expensive proposition for the X300. I have had multiple instances where I received a misfire on a specific cylinder (P0301 to P0308) with coil on cylinder and replacing that coil on the cylinder with the error always solved the problem. That's just been my experience and there are plenty of other possible root causes for this type of error. In both types of configurations, the coil was involved in resolving a P030x error code.

Regarding the specific misfire error designating the cylinder. You may want to try this. Next time you get a specific cylinder error code, Disconnect the coil on the reported cylinder and see if the idle changes. In most cases you will not see a difference in how the car idles, good or bad since that cylinder is likely not firing at that time. If there is no difference, you may want to replace that coil . Keep an eye on it and see if that cylinder is removed from future P030x error messages.

Another approach would be to use a thermal camera or infrared thermometer if you have one and compare the temperature of each cylinder. Any that are not firing should be much cooler. Since the issue is random, you could keep the camera or thermometer in the car and when you get a specific error code, check the temperature on each cylinder to see which one is cooler. If I found one not firing, my inclination would be to replace that coil and see if that cylinder is removed from future error codes. Doing it this way, you could pick them off one by one.

If the coil testing turns in to a dead end, my thought would be to focus on the fuel injection. You could start with using a noid light on each injector. This will tell you if the pulse signal is being sent to each injector. Noid lights are fairly inexpensive for most cars. Not sure whats available for the X300. Lisle makes a universal that works well on many cars if one is not available for the X300. You could also do this with a multi-meter. The noid light is easier in my opinion and given the cost of a noid light, I prefer to go that route. If all are pulsing correctly, next step would be to check the injectors themselves. My preferred method is using a mechanics stethoscope, press the probe on top of where the fuel injector and listen for clicking. No clicking will tell you it is not working. Given the intermittent nature of the problem it seems like you have something electrical or electronic, not mechanical. My experience with the fuel injector itself is that they typically always work or never work.

If you have not already done so, you may want to pull the plugs and compare them. This could help you get a line of sight to which ones are having a problem.