Hi all,

I've been working on a intermittent miss fire issue and I think I've removed the plugs and the injectors as an issue. Maybe it is a fuel injector that is causing it but with the mechanics stethoscope it seems to be clicking evenly.

So I've found that each time I've changed the spark plugs that number 5 (counting from the front of the engine) is always carbon fouled and has some oil on the thread. All other plugs look healthy.

The tip of the plug is not wet with oil, just the thread, so maybe this is nothing to worry about. However the tip is certainly Carbon fouled, as per the photos.

Also to know: the engine has always had a loud clacking sound that comes from under the cam cover on the exhaust side, with the stethoscope I have found that it's coming from cylinder 6 (again, counted from the front). Maybe this could be a valve issue?

My compression tests found: C1: 165psi C2: 165psi C3: 170psi C4: 170psi C5: 160psi C6: 165psi

Again counted from the front on the engine.

Service manual suggests: 130 - 150 psi

She's done: 98000 miles or 157000 km.

Valve clearances are all reset to be 0.014

What do you guys think? I'm a bit stumped.


Thanks in advance,
Harry.

 

I had similar and it was a dirty injector squirting but not atomising the fuel, swap the injector to a different cylinder and see if the issue moves with it

 

Just read a diagnostic in "Hot Rod". Mish/mash SBC fouling all holes. Conclusion, poor part match at intake valves. Valve stem seal failure.


So, I add that to the one on your car. It is taking in oil, but burning most of it off.


A not uncommon malady on older I6 Chevy's. Old tech fix, a steel umbrella for the intakes.


I've no idea as to how Jaguar chose to deal with sealing the intakes?


Another guess. Is there a drain back in the cam chamber near the oily cylinder. Is it open? Or is it allowing excess oil to pool there. I did that on an old Dodge V8. Smoked like a train....


Carl

 

A 'cylinder leak down test' would identify some valve problems, not all. Such as a burned/leaky valve.

A puff of blue smoke occurring only after sitting overnight is one sign of a leaky valve seal....although the plug in question looks very dry which typically would negate the idea of oil fouling

Has the misfire ever occurred immediately after installing fresh plugs?

How long does it take for a fresh plug to become discolored? Does it become much worse that what we see in your picture, or is this as bad as it gets??

Personally I would regard what you have as a slight fouling; I'm not even sure if it's enough to cause a misfire. If this is as bad as it gets it might defy concrete diagnosis, as the cause might be a subtle fault. In other words, some grasping at straws might be in your future.

I agree with the swap-the-injector idea. You could do the same with the spark plug wire, perhaps, if the wire lengths will accommodate doing so. Any abnormality in inside the distributor cap at the post pertaining to that cylinder? You be looking for a slight fault.

The loud clacking merits a visual inspection to determine of the tappet bucket has climbed out of its bore and is contacting the camshaft. It happens. Or perhaps a broken valve spring? Although, I'd think a broken valve spring would present itself with constant rough running


Cheers
DD

 

Ok so here are some results after some further testing:

Hooter: Ill swap them over and see if it moves, if it moves the issue with the injector then replacing that injector could solve my misfire issue!

JagCad: Do you mean this drain hole? Thats a pretty big hole to get plugged! haha it was clear when I last had the cam shafts out.




Doug: I've checked for the puff of blue smoke, no dice, couldn't see much I did my best to make it easy to see too, used a light to highlight it.

The misfire happens regardless of new plugs, and such I agree with you in saying that the plug is not a fault but a symptom.

So how long does it take? Well this plug is less than 1000km to get like this, and interestingly enough this is worse than the plug that was installed when I brought the car.
I've only ever done 1000km in the car since I've owned it, so I can't guess how long it was in the car before I removed it.
I have swapped the plug to another one and took a couple of 20 minute drives over the weekend, I know this is a sort test for this sort of thing but I checked it and it looked pretty healthy to me. At least we know that its not fouling super quickly.

Yea good idea on the leads, I have been meaning to replace them (not keen on the colour) so I think Ill just replace them to eliminate that.

And now, on to the grasping at straws:

I did the old dollar bill trick on the exhaust pipes to see if it would get sucked back into the exhaust pipe.
Well it does, only on occasion though and I think I felt the car misfiring at the same time it got sucked towards the pipe opening.
So with this information I had another look the pipes in the system and found a small patched hole that had come unstuck, Ill need to fix this before I believe the dollar bill trick. however I think that this points towards a valve that's not sealing correctly.

Looks like I'm getting closer and closer to that leak down test.

 

Harry:


Indeed, that sure seems to be a more than adequate drain down. Note, messing with this engine not in my experience. My comments are generic. But, yet is that passage open all the way down to the sump!!! My Dodge issue was an oddity of the head gasket!!!


Rough and ready dodge/bodge: A hotter heat range plug in that hole.


Carl

 

Stick with it, basic fixes in sight. Some call it stubborn, I term it persistence.


Carl

 

The plug in the pictures is light years away from being fouled and causing a misfire.

I'd say it's indicative of that cylinder running slightly rich or the plug just not firing.

Doesn't look like oil burning to me.

 

Thanks again for the replies people!

JagCad : not a bad idea actually, I do feel that the runs slightly hotter than it should currently.

Mikey: I agree, that could point to a bad lead or injector, i assume that the injector could be over fueling and causing it to misfire. Man I can't wait to sorry this misfire haha, it will put my mind at ease when I'm driving about haha

 

I think one point needs to be made. This rather ancient EFI system, (L-Jetronic), fires all the injectors together twice per engine revolution, it is not sequential per injector like modern systems.

We have been told the engine has a mis-fire, so any plug getting a bit fouled could be the ignition system, and this points to the ignition leads or the distributor not allowing the full volts to that lead giving an ocasional mis-fire. The distributor cap could be allowing volts to leak at No 5 lead. You wouldn't need much to cause a misfire when the conditions are right.

Only other thing, as said before, is that with fuel rail pressure at each injector, maybe Number 5 has a very slight leak so supplies more petrol than needed, so that cylinder runs slightly rich.

 

Right, the ECU batch fires All the injectors are given the same command from the ECU, so to speak. But any individual injector can misbehave

Also, I don't think we've discussed the possibly of a fault in the injector harness.

And.....

The discolored plug, however suspicious, might be a red herring ! Whatever fault is causing that anomaly might not be the cause of the intermittent misfire.

Cheers
DD

 

OK, so I've replaced the leads, which was on my list to do anyway so that eliminates them. Ghetto distributor and cap looks fine to me and the Rotor is new.

The new leads made a slight Difference, maybe lessened the misfire so that's a start.

I've also called up my favorite jag parts guy (Rodney Jaguar and Rover for anyone in New Zealand) and he will lend a me tested second hand injector to swap in to see how that goes. Great business by the way. I'll go pick that up this morning and test that today and report back.

I've also leak tested the exhaust with a smoke machine incase of a leak, Mostly for the sake of it.
Found a very small pinhole in the bottom of one muffler, a small dent from hitting something that's rusted through, and a fairly significant leak where the resonator slides onto the exhaust, easy fix.
I'm not concerned about these leaks as they are all past the muffler which I assume is still going to provide the necessary back pressure.

And lastly the wiring looms, I've given them a pretty good wriggle while the engine was running and it didn't seem to cause any more misfires, although the wiring looms will be on the replacement list when and if I repaint this car.

The plug could well be a red herring, I guess I'll need to swap the injector between all cylinders so as to cover all bases.

I'll report back once I have more info. Thanks again for the help guys! It's great to have some people to bounce ideas off, it also helps me keep moving forward with my project car :P

 

Have you tried a fuel injection cleaner thru the fuel harness. Not additives but a system you plug into the injectors, unpower the pump and run the pressurized cleaner thru the injectors to see if it cleans up? Did this a lot on the XJ6-12 and XJS when we had miss problems. Sometimes took the injectors out and bench tested before replacing the costly little things. We used paint thinner to reduce the fire hazard at the bench.

 

Hmm I haven't done that, but it's not something I can do at home currently so I'll resort to that if need be.

OK so I've swapped out all the injectors and nothing really got better haha, there might have been a combination that was slightly better so I'll retest that tomorrow and report back.

I have been inspecting the loom today as well and it's looks OK, no cracked wires that I can see, and I replaced all the connectors a couple months ago after one came loose and I lost all power.

I did find a little ground strap that wasn't connected to anything in the process though. It's a strap that comes from the ground point on the coolant rail and goes who knows where, I assume the engine block, next to a very similar one that comes from the bunch of grounding wires from through the firewall with the rest of the loom.

I'll ground it to the block anyway and see what happens, I don't think it will have much to do with the fuel injection though, other than messing with the general grounding of things.

I also redid the compression test because I was doubting how methodically I did it last time, any how here are the new results, in order from 1 to 6.

160, 164, 165, 155, 145, 153.

similar to the last ones, and it does show that the low one was low again.

So I dropped a table spoon of oil in there and tested again, now it got 155, I guess I've got a bad piston ring, that sort of rules out that the valve is bad.

My thoughts on that are as follows, tested all cylinders and waited 5 minutes each time to see if the reading on the gauge went down, showing a leak. However all held pressure. Unless the tester doesn't go down as pressure drops? And with the jump in pressure with the added oil improving the low cylinders results that sort of proves that's it's not a valve issue.


Anyone have any thoughts on that?

Cheers,
Harry.

Here's the ground strap, it will only reach the block.
Wider view for perspective.
.

 

Hi Harry,

Sorry about delay, had to take some shots in the dark, middle one a bit fuzzy.

On my one the following;

That earth to earth strap which you are holding connects top left bolt on inlet manifold to next bolt forward of current one connected on your on water rail.

Mine has an earth strap from the wiring loom which connects to the bolt on the water rail which your other one is currently connected to.

I hope said photo's help.

Cheers,

Nigel

 

My shots came out upside down, sorry. Probably because I'm from down under/ :-)

That earth wire that you are holding could do with a thorough clean. CRC Brake Clean works a treat on those.

 

Don't let the exhaust leak/s issue interfere with the diagnostics. That engine like many others should run just fine, sans any pipes. Noisy, oh yeah.


But, #5 is a bit worn. Adding oil for the compression test temporarily improves ring seal.


Now, I might have mislead you a bit as to a "hotter" spark plug. I referred to spark plug heat ranges. The tip design primarily. A plug of a "higher" heat range will resist fouling better than one in a "cooler" range.


You might "Google" up "Spark plug heat ranges for some better tutorials.


A few decades ago, our 71 Hornet developed a propensity of dropping one or two of it's 6. A fresh set of spark plugs, one range "hotter" resolved that. Durn slick six, the precussor of the 6 in my 94 Jeep.


Carl

 

Is it your driving habits that tend to plug fouling? A freeway run at times is good.


Carl

 

The plug in the picture is not fouled so discussion of heat range is not relevant.

 

Mebbe so, mebbe not. Can't see up into the space between the shell and the insulting ceramic. The cylinder misfires, compression is down, the plug is at the very least, dirty, hence my "diagnosis" from afar.


Carl