Hi There, hoping someone might have some advice for a very frustrating issue that my local mechanic is having a hard time diagnosing!
2000 XJ8 4.0 V8 / 105k miles

*Took car into my local indy jag garage because of a rattling and noise around engine - was diagnosed as bad engine mounts which were changed.
*At the same time they also checked the chain tensioners and found they were the original plastic and very worn, so changed all of them and fitted a new chain.
*Also replaced plugs and ignition coils at the same time.
*Engine had been fine before taking it in, albeit with some shaking and rough idle which I put down to the tensioners and bad engine mounts.

After collecting the car the engine seemed perfect, better than it's ever been. After a few short journeys however, it went into restricted performance. Using my USB reader there were P0300 and P1316 errors, and then shortly after that P0301 through P308 errors, and the engine warning light came on.

That was 2 months ago and since then the garage has had it in 4 times, changed various parts, checked everything multiple times, and each time it seems to be fixed when I collect it (and during their own tests) but after a short while (sometimes a few miles, sometimes longer) the same errors always come back.

Most recently it also stuttered a couple of times when accelerating which was new.

One day I can drive it and it is perfect, and the next I can feel rough idle straight away and that seems to always lead to restricted performance and misfire errors.

The mechanic I use is genuinely excellent and I have full trust in him (he has proved he knows these cars very well many times in the past for me) so I don’t think they are missing anything obvious – they’ve spent hours (even days) at no cost trying to diagnose this issue for me which I am very grateful for. He has suggested changing the throttle body as a next step given everything else has failed so far, but apparently they are not easy to get replacements for.

Any suggestions for what could cause this type of intermittent engine issue? Hoping someone might have experienced something similar or have some ideas for what I could try that the garage may not have thought of.

And would the throttle body explain these issues and be a sensible next step?

Thanks so much!

 

I personally have very little faith in any ignition coils other than Denso in these cars. I’m going to assume that they put aftermarket coils in it. If they have your original factory (assuming they were the OEM’s before) coils, I’d have them reinstalled. Seeing as your in L.A., the longer your engine misfires, the more damage to your catalytic converters and the more likely you’ll fail your emissions tests. As long as you don’t see a P1121 or P1122 (codes pertaining to the Throttle Body), the TB should be ok. My advice to you is to reach out to them and see if they have your previously installed coils still. If not, I’d look into buying a used set of
Denso coils instead.

 

Thank you @Addicted2boost. They did offer me the old ones back but I said no, which I regret now! I'll ask if they have them still and also check which ones they fitted. Would you suggest it's worthwhile investing in a new set of Densos for a car this age, or are used generally adequate?

 

Would you think a used set like this on eBay is worth trying? Or does the condition (I can see rust on them in the photo) indicate they could have issues?

 

https://www.ebay.com/itm/31416458112...mis&media=COPY

This set is much more affordable, is in your state and has a 30 day return on them. I’d still reach out to the shop even though you declined the old coils. The goal here is to determine if the coils they installed are defective or not. The codes that come back are misfire related and that’s what I’d go for first. The other thing I just thought of is since the engine mounts were replaced, on the passenger side underneath the engine has a large ground strap. It goes from the bottom of the engine to the body of the car. You have to remove that ground strap at the body as it’s in the way when you go for that engine mount. It’s a 10mm bolt head that fastens the strap to the body. I myself have left it loose a time or two.

 

That set looks great, thank you very much. Visiting the shop today to ask if they still have the old ones else will order those. Will also check the ground strap as well (would love for that to be the issue!) Thanks again, I will report back!

 

Have the Technician also check the fuel pressure at idle and WOT. At idle the pressure should be around 300 kPa and around 380 kPa at WOT.

Has the fuel filter been replaced recently?

 

Thanks @NBCat, will do. The fuel filter was changed just under a year ago so not recently.

 

I have had mechanics tell me that coils should be replaced at 80k but my experience
is that they last much longer than that. I also noticed that when one starts giving you
issues, the others are not far behind.

 

As long as the timing is fine then it's not something serious
what spark plugs you're using? There might be small vacuum leak so check for leaks check the fuel pressure
I personally use set of ebay ignition coil which works great
​​​​​​

 

I had the same problem last summer and while on a trip. On the third day of the trip I started getting the dreaded CEL along with all the P0300 codes and random Restricted Performance. I had two scanners with me so I left one plugged in all the time to clear the codes and reset the CEL. Those codes would come back randomly, but it quit throwing those codes after that leg of the trip. Drove another day with no problem. The next day I got IAT #2 codes and Restricted Performance; four times during that leg. Did the same as when I got the P0300 codes, just reset and continue the trip. That was the only day I got the IAT code.

There were a few other interesting days of random codes but I made it home and the last day of the trip was a lot of fun with no codes. One of my neighbors is an electronic engineer who used to work for Mallory, so I got him to look at the car and the first thing he noticed was that the voltage was stuck at 12.6; it should be at 13.5 to 14.5 to run all the electronics. I replaced the alternator and have had no random codes since.

Just something very easy to check before going into all the other stuff that could be going on.

 

That does sound very similar @hispeed42. I tested the battery this morning and it was 11.96 and 13.43 with the engine running. Both seem on the low side so I wonder if that could be an issue. I'll check it again after charging as I left the USB reader in overnight which has drained the battery in the past... I know the battery was replaced just before I bought the car 3 years ago though so it's maybe worth changing anyway.

Thanks @mobile1 - perhaps unrelated, but it's always struggled with a cold start which no one has been able to diagnose and I've wondered if the fuel pump or something related was to blame - will definitely ask the shop to check the fuel pressure this week.

I spoke to the mechanic and suggested trying a set of OEM coils but he didn't think it was necessary - I presume that's because they already ruled out plugs, coils, and any vacuum leaks in their previous checks but I'm dropping it off tomorrow so will find out more then. Also, I wonder if it's because all 8 cylinders are misfiring together that they're ruling out the new coils and plugs?

Thanks so much for all the helpful responses so far - I was starting to lose hope but you've all definitely given me faith that there's a fix to be found soon!

 

The reason I posted about the coils is that sooo many times we’ve put aftermarket coils in and occasionally right out of the box they’re bad. And we’ve had quite a few customers come back within days, weeks and months with misfire codes all related to the aftermarket coils. Trust me, I want faith in all of the aftermarket parts as a suitable replacement to the expensive dealership parts. Here’s one thing to remember concerning aftermarket parts, Just because a part is new doesn’t mean it’s good.

 

Thanks @Addicted2boost - understood, and good to know. And given it has gone on so long with no diagnosis, trying some OEM coils so they can be ruled out as the culprit would definitely make sense to me. I'll see what the shop says when I chat with them in person tomorrow. As I said in my initial post, I do have complete faith in them so it's very possible they've already ruled out the coils being the problem one way or another.

 

Those numbers aren't too bad considering the overnight battery drain. You can also check for bad cables just to make sure the correct voltages are getting to the electronics. These cars are in their 20's now, everything is electric, and things do fail. Checking the electrical system to make sure you've got enough juice to run all that stuff before throwing $$$ at parts is easy and I'm trying to make that a habit when I get CEL's. Obviously it's not a habit yet since my buddy had to point out the low voltage when it was staring me in the face for about 4,000 miles during my trip.

I've attached an electrical guide that one of the Forum members put together. It shows where and what to check for the voltages. Once satisfied with the readings, press on with the rest of the diagnosis.

 

I hit "enter" twice for the above post and I can't delete it.

 

Just heard from the shop and the fuel pressure was indeed low (210) - really hoping a replacement pump clears up the misfire errors as well as the cold start issue but TBC...

That electrical troubleshooting guide is amazing @hispeed42, thank you!

 

Coils would be my first guess regarding misfires, especially newly replaced ones.

 

@moohead, prior to condemning the fuel pump, did the Technician check to see if the fuel filter is blocked?

How was the fuel pressure reading determined? With a physical gauge connected to the fuel rail on the engine, or through the OBD port?

 

Thanks @NBCat - fuel pressure was measured with a physical gauge on the fuel rail. I didn't ask but would expect/hope that they did check the filter first - they're fitting a new pump at the moment though so I'll know soon enough if it was causing the low pressure or not...!