I know its a bit forum unspecific but i wanted to ask what you guys think about these plugs (they ran in the engine for just 1000km. But it was many short trips since i had to test the car a few times after fixing little things.)
Just today i renewed the plugs, plug wires, rotor and distributor. Just preventative maintenance but I hoped it would fix the small idle missfires i seem to have. It didnt.
It still idles alright at about 750rpm. But every 3-10 seconds you can hear a high pitched clonck sound sounding like a missfire. And the engine still shakes a bit here and there. The tailpipe also sounds blubbery when it missfires. It drives great tho. Good power and gas mileage is about 9-11 liters per 100km.
Number 2 looks a bit rich but the rest look ok. You might want to spray engine start fluid around the inlet manifold to see if you have leaks on the other 5 cylinders, (doubtful but you never know)
sprayed lots of starter fluid all around the intake manifold. (with a fire extinguisher nearby) So far no apparent leaks.
Engine still a bit rough in idle but holds 750rpm constantly. so rpm doesnt fluctuate at all.
Maybe it could be a leaky injector? I really wanna buy a injector test apparatus or build one somehow… ponders
I know I sound like a broken record to folks on this forum but my first question is ALWAYS … have you done a compression check on the engine (which is as easy as it gets on the XJ40) ??
Before you try to troubleshoot ANY engine problems you have to start at square one … which is doing a compression check that tells you the internal condition of the top half of the engine … piston rings, and the intake and exhaust valves. Only then can you move on to square two.
With a car (I’m assuming an original un-rebuilt engine) that’s over three decades old you HAVE to start at the beginning. Only then can you begin to track down the other 30 or 40 things that can go wrong.
If you suspect leaky injector swap injector to another cylinder and view results. Be careful releasing the fuel rail fasteners. Make sure to use two wrenches as the rail is aluminum and easily damaged.
compression tester–very inexpensive-available any on web auto parts…or in store. Easy. As to DOING a compresson check first–well ok, if easy…but most of the time misses and stumbles are in the systems being discussed here, do the easiest things first.
Nick
Here’s an example of a well equipped compression tester, cost about $25.
Since the XJ40’s plugs sit down in the valve cover this kit even includes a metal extender.
Remove all of the spark plugs.
Screw in the straight extension like you are screwing in your plug, then on top of that screw in the compression tester’s hose. I start at the front (#1 plug) and work my way back, just to make things simple.
Make sure you have a fully charged battery … very important !
Now jump in and while holding the accelerator all the way down crank the engine over about about 4 or 5 turns.
Now jump out and write down the PSI pressure on the gauge. The gauge will hold the highest pressure until you push in a small button on its side to release it.
Do all 6 cylinders, and record the numbers.
Total time about 110 minutes (less time then it would take your mechanic to go over your bill). WHAT COULD BE EASIER !!!
Don’t even think about doing any other troubleshooting on an old engine before you do this, you can end up chasing your tail and very frustrated. If the PSI numbers look good, congratulations you’ve now eliminated the most common internal problems and you can move on with your troubleshooting.
A good engine should have very close to the same PSI readings on all 6 cylinders. Actually if the highest and lowest numbers are all within 10 - 15% that’s still considered acceptable.
So for reference, what should the PSI be on a new-ish AJ6 4.0 engine? One with 100k miles? 200k miles? I realize there are a number factors involved and not all engines wii be the same but what might be typical?
I have a 3.6L engine with “low compression” pistons. Somewhere around 160 psi is what all my cylinders read right after I had my head rebuilt. Now this was on an engine that had 155K miles on it. I was really elated because all the readings were very close so … hooray for my cylinder linings and compression rings
Your 4.0 engine is exactly the same as my 3.6 except for a slightly increased stroke. So I would take the highest reading you get and use that as a baseline.
I was surprised when I learned years ago that the XJ40 had “standard” and “low” compression pistons.
Use a VIN decoder and it will tell you which you have (at least that’s what comes up on mine).
Obviously you can expect a little wear and loss of compression over the years but if the numbers stay close that’s the key.
if readings are very low it’s usually an exhaust valve or head gasket. kinda rare for the compression rings to fail unless you’re a boy racer.
I’ll definitely buy such a kit. Do you guys remove the injector power before testing compression?
btw…
Today i just took the throttlebody out and noticed some oil in the middle of the intake (not a lot, maybe a 1-2ml thick puddle) maybe that could also have lead to plug no.3 being a little dark since the air for cylinder 3 would rush over that little oil puddle…
However, I never had smoke coming from the pipes. Only some condensation when its cold
The oil puddle is pretty normal, it is caused by the venting os the cam cover into the inlet manifold. It could possibly be adding to the colour but I doubt it. And no because the throttle should be held at WOT (wide open throttle) the injectors are cut off from operating. Remember this if you ever think you have a flooded engine.
good to see that lil puddle is normal. after 230k km it definitely wasnt a lot (if noone ever cleaned it)
so… I have to push the gas pedal down fully for WOT while compression testing? And that makes the injectors cut off from operating? scratches head Wouldnt the injectors pump the most amount of fuel into the cylinders if you go full throttle/WOT?
Why hold the throttle wide open when doing a compression test ? … Simply statedl this allows the maximum amount of air to be sucked into the bottom of the cylinder on the down stroke. Increased air during the upstroke means a more accurate psi reading on the compression test.
Robin … “The fuel injectors are cut off from operating at wide open (full) throttle” ? That’s a new one on me. As Marmo very astutely queried “why”, and does this mean the engine stops running (fuel starvation) when you go to the “pedal to the metal” mode ?
So the ECU knows the car is not running and shuts down the injector pulses at full throttle. Interesting, this car is full of tricks. Thanks for that bit of knowledge.
But on second thought after a bit of searching … why is it common practice on this car to pull the fuel injector relay before doing a compression test ? The AA (Automobile Association) article that you reference seems to be generic to all cars. I can’t find anyplace that mentions this feature on our cars. Seriously not trying to doubt you but are you sure our Jaguar’s system does this ?
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