Hi all,

I have a 90 XJS and I just started hearing what we call in the USA "spark knock". What is the cause and is there a cure?

Tony in NM USA

 

A few things can cause it, poor fuel with a low octane, timing is to advanced, you could even be running plugs that are two hot for the motor, even carbon build-up in the head can cause increase in compression and start knocking.
If the car was OK before and suddenly started knocking, I would say your using crap fuel, If not check your timing and your plugs, even try a bit of upper cylinder clean and see if it helps.
Good Luck

 

yes please fill us in on the fuel first. A lot of people think it is acceptable to use 87 or they use 91 and buy it at crappy stations to save money. Look up toptier gas stations and only use those. They have more/better detergents and usually their gas is kept well. I've gotten gas at a few cheap stations before that was WELL BELOW its octane rating on the pump.

 

I noticed the spark knocks seem to start when the engine temp is at the middle of "N". During a slow climb up a hill. (i put 185 thermostats last fall so it seems to run comfortably at the bottom of "N"). After climb, the temp goes back to normal, but knocks remain.


Tony

 

Tony
Is this a 6 or 12 cylinder car?
I still think it maybe a fuel issue.
From you have have said it starts knocking when under load and things get a little hot, overheating can cause knocking as well, but your temp gauge going up slightly is normal under load, but there's only one real way to test temp and that's with a proper thermometer, Have you access to a laser thermometer? I'm thinking take the car for a run, bring it up to temp and and go up that hill that makes it start getting hot and knocking, check the temp on both banks, or the front of the head depending if its a six or a twelve and see if it is getting a little to warm. If so start looking at the cooling system, if it is Ok I would change the fuel if that doesn't work start looking at the ignition system, I believe your car has the Marrelli ignition being a 90 model, I don't know much about it as mine is the Lucas but maybe someone can chime in on how to check and set the timing on them. Too advanced will make it knock but too retarded will make it run hot, can cause knocking as well
As for your thermostats I use 190 (90) part ebc3577 in mine that are recommended for the the motor, if you use the right ones that have that little jiggle valve on them you shouldn't have a problem, its also a myth that running lower temp thermostats makes the motor run cooler.
Good Luck Again

 

thank you! Putting colder thermostats only affects the opening temp. The cooling system itself (radiator and fan) affect the running temp.

you could have a 190 degree and a 100 degree with no radiator...would the 100 degree stat run cooler? No.

if it opened at 5 degrees instead of 180 would the engine only reach 5 degrees? no. The cooling system can only do a certain amount of cooling. given a certain airflow and certain radiator size a cooling setup can cool X gph of coolant by Y degrees. If the heat generated is greater than the X and Y of the cooling system it will only be cooled a certain amount and overheat. If the system is cooler than the capacity of the cooling system, it will over-cool and the thermostat will regulate the temp to keep it up.

T-stat temps essentially only affect minimum temp, which is important for warming up and keeping the system at operating temps when it is cold or the cooling system is capable of over-cooling

I also agree, check your timing

 

Hi all,

I think problem is in fact the fuel quality. I took it on an out of town trip a couple weeks ago. I filled up a couple of times at some "no-name" gas stations located in the middle of no where. I assume that may be where the last than optimal gas came from. After a couple of fill ups back home and some octane boost, the spark knocking seems to have gone away.

I also appreciate the info on cooling. When I got the car last May, it had some overheating issues. So last summer, I got the radiator serviced (cleaned out), replaced thermostats (to 180s), replaced all the hoses/clamps, the mechanical fan was replaced with an electric one and the overflow and expansion tanks were replaced. I am pleased to say that I haven't seen any drips under the car after parking and the system cools better than it likely has in years. I think the system is completely airtight.

I'd love to know how one can tell if all the air is out of the system. I have done the bleed several times and it seems the coolant is at the top of the radiator when last I checked.

Thanks,

Tony

 

G'Day Tony
Good to see your on top of your troubles, always nice to see nd here about a happy ending.
As far as bleeding the cooling system on an XJS you have to stand on your right leg while having your left hand touching your right ear and your tongue out pointed slightly to the north-east, if you can whistle Dixie it may help. LOL
There is a really good thread on here on how to bleed the system, which is very important air pockets make hot spots and give false temp readings so best to get rid of them. Ill try and dig it up, unless on of our other astute members beats me to it, saves a lot of typing.

 

Here you go, open the link to the post read post No8 By Doug one of our most knowledgeable members, follow his procedure you can't go wrong.
https://www.jaguarforums.com/forum/x...r-flush-37512/

 

Hi all,

I went out a purchased a timing light. Got home and logged on to my Alldatadyi.com to read up on how to check/adjust the timing. The alldata says that the timing is not adjustable. Is that true??

Tony in NM

 

Your car has Marelli ignition so, correct, the timing is not adjustable in the conventional sense. You have an ignition ECU that controls advance/ retard while driving.

I'm sure there's a procedure to position the distributor correctly as a baseline, though, as you don't want to rotor arm ahead or behind of the terminals in the distributor cap....but it's a visual adjustment; not done with a timing light. It's probably explained in Kirby's book....or others with direct experience will chime in to describe/explain.

Cheers
DD

 

It appears there is some sort of removable "Link" (wire/connector) that can be removed to set a low octane profile. I assume this also acts to retard the timing..

Thanks all. Love this car and have learned a lot. Looking forward to keep learning too.

Tony