Last week my wife and I decided it was a cool enough day to take a ride to a nearby little town about 20 miles away. The town is called the little town of Washington and it has a nice hotel and restaurant with a deck that overlooks the Yuba River. It is located down a long grade of about 7 or 8 miles with a narrow twisting paved road. So I wisely (?) dropped it into 3rd and used the engine compression to retard my speed down the hill. It worked great and I only occasionally had to touch my brakes. The parking lot at the hotel was a straight in shot so I basically coasted in.
When we got ready to leave we had a bunch of people admiring the car as often happens. I noticed as I backed out that the engine was running really rough. I managed to drive away without anyone noticing how poorly it was running, but I knew something was wrong. Now it was almost 100 degrees down in that mountain hole and we had around 7 miles of uphill to climb out of. Soon the restricted performance message came on, then the dreaded check engine light. I felt that I had no choice but to drive it as long as it would run. No cell service down there. To its credit it got us out, a little slower than I might have driven, but we made it.
So today I hooked the OBD scanner up and as I expected I got multiple codes saying random misses in 4 of the 8 cylinders. Not all on one side, but scattered. It also said something about damaged Catalytic convertors.
Now I suspect it has something to do with going downhill on compression for an extended distance. But I can't imagine why. My car is not an oil burner, never a whiff of smoke. But it seems as if the plugs got fouled. So today we pulled the plugs and replaced them with new. Some of the old plugs did have oil on them, but not enough to cause them to fowl (in my opinion) Well, the new plugs did not fix the problem. I still get codes that say random misfire. I guess the next thing to check might be the fuel injectors. But I can't imagine why they would be involved. Maybe the trip down the hill was not the cause. But I have a strong feeling that it was.
Does anyone have any ideas about what might have caused this. I have a working knowledge of engines, but I don't claim to be an expert. Thanks for any advice you might give me.

 

Which DTCs were stored in the system when you checked?

With the ignition OFF, what is the voltage across the battery terminals when measured with a voltmeter?

Use the Torque app with a smart phone to connect to the vehicle's PCM via the OBD port using a Bluetooth interface and read the fuel pressure when cranking and running. List the values here.

 

+1

For the best advice, always post ACTUAL codes and not just descriptions.

Graham

 

Try changing the plug wires. That happened to my '96 Mustang out of nowhere earlier this year. I just couldn't believe that was the problem, but it was! Checked plugs, compression, injectors and more. I had a misfire code on one cylinder and it ran rough. I also had it happen to my mini van back in the 1990's. Good luck!

 

The X100 doesn't have 'plug wires' as it has coil over spark plug units that can fail for a variety of reasons including oil fouling and cracking or the insulation.

 

The XK8 does not have plug wires. It uses individual coil packs on each plug. But thanks for the advice. It occurs to me that even with the new plugs there was no smoke at all when I started it up. It seems it should have burned off some residual oil.

 

I will list the actual codes the reader produced. Good advice there. But I am sort of stuck on the idea that coming down the long grade on compression had something to do with my problem. It just dosen't make sense to me that multiple coil packs would suddenly fail. The car was running fine before the downhill trip.

 

Remove the coil covers and coils and look for oil pooling around the plugs.

 

You may find the fuel pump is the culprit, but the only way to know is to read the fuel pressure during engine cranking and whilst running.

 

Yeah, I forgot about the coil on plug ignition, like my X300 used to have. Still electronic components often fail without any warnings, I've had fuel pumps suddenly go out. I'd check the coils as mentioned. I remember reading that running the engine against compression results in a high vacuum condition, though I can't think of what problems that could cause. At round of thorough diagnostics should reveal the culprit.

 

I did observe oil pooling around some of the plugs. Especially around #5 on the drivers side. But would this cause it to misfire? I am wondering how to check for spark?

 

I had experienced a very small leak in my part load breather circuit a few years ago. We had enjoyed a mountain trip to Big Top Apple Orchard in Flat Rock NC. all was well until as we had departed down that long, steep descent using lower gears (increased vacuum). Near the bottom, the P0171/P0174 codes set the "restrictive" notice along with subsequent (ramdom I would guess) misfire codes. I had "fixed" this leak previously, but engine heat took its toll (It took my stethoscope to hear my failed fix ...added for clarity...engine off, intake sealed off, low pressure <5psi compressed air thru full load breather circuit ), and I have since done a more robust "fix/hack" that is working to this day. Because mine has the supercharger, I will delay replacing the proper breather hose until the SC comes off.
Moral of my story:
Do not underestimate there being a very, very small intake leak to be a cause of such happening.

 

Thank you Dr. D. I am not really sure what you are telling me, but I will search for more information regarding this. It sounds like a possible answer.

 

here is a link with a pic of my hack that is working now.
https://www.jaguarforums.com/forum/x...9/#post2089364
I sealed off the intake ahead of the MAF sensor, that way, I could also check the accordian intake.
Note: I used < 0.5 psi of compressed air, instead of the <5psi that I typed earlier.

 

Thank you for that advice Graham. Here are the codes.
P1000
P0300
P0301
P0302
P0305
P0306
P0308
P1316

So basically a general misfire everywhere. I have detected a sound that seems to be coming from under the intake manifold that sounds like a small backfire. Could I have somehow blown a gasket and caused a air leak? It is very hard to locate . I did verify that the coil packs are firing the plugs. So probably not an ignition problem.

 

Look for splits and cracks all around the plastic pleated accordion section of the intake tube that runs between the airbox and the throttle body. If yours is factory original, that may be your issue. These intake tubes sit there and bake in the engine bay, eventually getting brittle and cracked.. Mina Gallery sells an all-aluminum intake tube that solves this problem permanently....

 

 

Well, I finally decided to dig into the engine to look for a cause to my misfire. The codes do not help when they simply say "major misfire" over most of the cylinders. I hate to go in blind, but as they say, "fools rush in".
What I found is that the intake manifold was awash in engine oil. I am pretty sure this should be dry. Remember that the problem started after I went down a long road, maybe 5 or 6 miles using the engine as a brake. (third gear) This would have subjected the intake to maximum vacuum for an extended period of time. I really don't see why that would cause a problem. Any idea why or how the oil could get sucked up into the manifold? My only guess is that it came through the PCV valve, but I am not exactly sure how that could work. The car never smoked, even though it was running on maybe 5 cylinders. It has never been an oil burner.
What other possible sources of the oil?

 

If you have a stock type intake pipe & Unless you have a K&N air filter that is passing oil, there is no other way for oil to ingress other than the full load breather.

I would also check the partial load breather for blockage (if you dare tempt the brittle plastic).

Z

 

I am not familiar with what a full load breather refers to. Can you elaborate please.