So here's my first newbie question. My 2001 NA XJ8 had misfire codes on 4 cylinders, so I put in new plugs and coils, thinking this would be the quickest way to solve the problem. Previous owner had tried moving the coils around but couldn't tell for sure which ones were bad, and I wasn't having any better luck. The new parts didn't help, I'm still getting P0301, P0304, P0305 and P0306 codes. I checked the power to each coil and they are all getting 12v. Next step is to do the Pinpoint Test A3 Ignition Coil Ground Circuit but I cannot find the Ignition Modules. One diagram shows them mounted next to the brake booster but they are not there, and I have spent the last 30 minutes looking till I'm almost blind. Are they mounted someplace else on the 2001 model? Thanks for any suggestions.

 

Ive attached a wiring diagram of the ignition coils for a 2001 XJ 4.0L. Hope this helps

 

Since you have a 2001, the coils are 4 pin. The earlier models were 2 pin and had the Ignition Modules that were remotely mounted. On yours, the ignition coil is the actual module. If the diagram you have shows 2 separate modules between the ECM and the coils, then you have the wrong diagram.

 

Thanks, that explains it, mine are 4-pin. Now I'm really stumped because I'm getting the misfire codes with new coils. I was hoping to find a simple wiring issue causing the problem.

 

Fuel pressure? Bad injector?

 

indeed the injectors can contribute or cause these codes.

 

Didn't realize the injectors could cause those codes, I assumed injectors had their own codes. I will test injectors next. Thanks.

 

Hi Pasquo Wrencher,

Whenever I have misfires in multiple cylinders, I look for issues that can affect all of the cylinders that have flagged codes, since it is unlikely that four coils would fail simultaneously. It's also unlikely that four fuel injectors would fail simultaneously unless they are obstructed due to contaminated fuel. Which makes me think that it would be a good idea to replace the fuel filter just to rule it out.

I don't know if you have the correct Diagnostic Trouble Code guide that gives possible causes of these misfire codes, but if not you can download it here:

2001 Jaguar X308 DTC Summaries Guide

As Jeff mentioned, of the possible causes, low fuel pressure seems like a good suspect. If you have a fuel pressure gauge or can borrow or rent one from an auto parts store, it's easy to check the pressure at the Schrader valve on the left bank fuel rail. Check at idle and while revving the engine a bit. Then while the engine is idling, pull the vacuum hose off of the fuel pressure regulator to see if the fuel pressure rises at least a little. Also check the vacuum fitting on the FPR for any signs of wet fuel that indicates diaphragm failure. Then shut down the engine and watch the gauge for any significant drop in pressure within the first 30 minutes or so that could indicate a problem with the FPR, leaking fuel injectors, or a faulty check valve. If the pressure leaks down significantly, further investigation would be required.

Problems with high resistance in the ground circuits are also very common. It would be worth cleaning all grounds associated with the Engine Management System (EMS). The location of each ground is shown on the page before the wiring schematic in the Electrical Guide, which you can download here:

2001 Jaguar X308 Electrical Guide

Does your engine behave as though it has low compression?

Are any other codes present?

Cheers,

Don

 

Thanks Don. Those are the codes, except for one related to an air intake leak but I fixed that one by tightening the hose clamp at the MAF, which was hanging loose. I plan to start the car again tomorrow and run a fresh set of codes to zero-base the issues. This evening, I've checked the resistance between the two prongs on the fuel injectors (all ok) and checked the power to all injectors, which were all good. Tomorrow I'll borrow a noid light and check injectors while the engine is running. Your links to the DTC codes and electrics, and other suggestions, will be helpful. Thanks.

David

 

Have you checked the compression?

 

No compression check yet, I will add that to the growing list.

 

Do it first. On a car that is new to me, I always check the basics first, compression being the most basic of basics.

 

I drove the car about two miles yesterday with new plugs and coils, then pulled it into the garage for more diagnostics. First I pulled a fresh set of DTCs as another baseline. The CEL was not yet on but I could tell there would be codes because it was still running rough. Fuel pressure was 44psi at the schraeder valve. Next I checked compression (fuel pump relay removed, battery charged, throttle plate wide open). The first photo shows compression results. For the cylinder showing 135/140, the 140 is after squirting a little oil into the cylinder. The photo of the plugs shows the front two plugs on the left side of the engine. The dirty one is from the 142 psi cylinder and the clean one from the 165 psi cylinder.

 

Timing? Check the tensioners.

 

I looked at the right bank tensioners a few days ago and the "looked" okay, no obvious deterioration of the plastic, but i know that isn't any guarantee. There is a little valve rattle when it cranks or revs, but I've heard much worse. I was hoping to determine first if the engine is healthy enough to justify the effort.

 

Any fuel trim info? Vacuum leaks?

 

Have you checked the air intake pipe? The section that looks like an accordion? Sometimes they crack on the bottom, but not enough to hear the leak, this could contribute to the P0171 and P0174. As for the misfire, yes injectors can cause this to happen and not always trigger a code. You can try swapping the injectors for the cylinders that are misfiring to other cylinders and see if the codes move to those cylinders. The compression readings look ok, could be a little more even.


Timing could also be an issue as was mentioned earlier. You need to check the secondary tensioners where the chain rides, there should a " plastic " pad that the chain rides on, if that pad isn't there, its time to change them, and the slack could be causing the rattle.


Also worth mentioning, I've come across a few instances where the intake itself was warped, since it is plastic itself, but those times I think were quite extreme. You can take some brake clean and spray down various areas of the engine while running to see if the idle changes/starts to run smoothly.

 

Thanks for the suggestions. Intake pipe checked thoroughly off the car and looks good. When spraying anything that looked like a vacuum hose with starting fluid to check for leaks, I also sprayed the air intake between the MAF and the throttle body but no reaction. As for timing, I've checked the tensioners on both sides, even going down into the engine with a scope but saw nothing obvious. The left side tensioner looked newer than the right side, being much lighter in color. I even lined up the flat spots on the camshafts to see if they were out of line, and they appear to be okay. Is that a valid way to tell if the timing chain has slipped a tooth?

Next step, I will start swapping around injectors because it feels like a miss that could be fuel related.

Any guess as to why the two new plugs look so different after two miles?

 

Sounds like you know your way around a car; you have carried out a lot of tests recommended by other members and reported the results rather quickly. It's unfortunate that this misfire issue has stumped you/us.

Side note on the vacuum leak testing: I usually use a little smoke machine I have because no other method seems to reliably work for me. Your leak would presumably have to be large in order to cause misfires so you could probably have found it using the starting fluid method. When I have used starter fluid to look for leaks on a running engine I HAVE to be watching fuel trims to find a leak. I have seen huge spikes in fuel trims that indicated I had found a leak while the engine itself made no discernible change in RPM. The computer adjusts so quickly I guess that it would have to be a substantial leak for it to be apparent based on engine sound alone.

My only "idea" other than what's already been mentioned would be the fuel pressure regulator. It would seem that a failing FPR would cause random misfire spread over all cylinders, but I guess if some injectors are more restricted than others then those would be the injectors to regularly cause a misfire because of inadequate fuel pressure at the fuel rail. Just throwing that out there though.

 

Thanks Jeff. I'm no mechanic but I've had my share of cars that should have been in the junk yard, I just enjoy the tinkering. I'll read up fuel trims, I'm not familiar with those but looks like I need to be.

Here's a shot of the left tensioner. To me it looks pretty good but does not appear to have the metal body of the newer models I've seen on-line.

David