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Saxo Problems & Queries If you're having problems with your Saxo and you're after a bit of advice, check this forum out...

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Old 8th July 2020, 09:02   #1
ESD1711
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Default J4 vs JP4 and spark plug conundrum

One for all the 16v gurus out there. J4 vs JP4 and correct spark plugs....

So basically I bought my Saxo with what I was told was a C2 jp4 engine. Iíve no reason to doubt that was the case. However, when recently pulling the plugs on it, it seems to have the shorter plugs from a Saxo engine in it.

So either it isnít a C2 engine and itís a Saxo one running the correct plugs, or.... it is a C2 engine and itís running the wrong plugs? Unless thereís a reason for a jp4 to be running the shorter Saxo plug?

Anyways - this poses three questions for me:

1 - whatís the easiest way to tell a j4 from a jp4 without taking it to bits? Assumes thereís a code or markings somewhere which will tell me? Where should I be looking?

2 - assuming it IS a C2 / jp4 engine - what sort of effect will the shorter plugs be having on it? Should it run at all? Should it run badly? Should it pretty much run the same? In my head, the spark being higher up should surely have an effect on how it runs?

3 - assuming it ISNT a C2 engine and itís the j4 with the correct plugs - what damage do I risk doing by trying it with a set of longer plugs? Ie is there room in there for a C2 plug to fit without it hitting anything?

Cheers
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Old 9th July 2020, 16:31   #2
MartinObviously
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Quote:
Originally Posted by ESD1711 View Post
One for all the 16v gurus out there. J4 vs JP4 and correct spark plugs....

So basically I bought my Saxo with what I was told was a C2 jp4 engine. Iíve no reason to doubt that was the case. However, when recently pulling the plugs on it, it seems to have the shorter plugs from a Saxo engine in it.

So either it isnít a C2 engine and itís a Saxo one running the correct plugs, or.... it is a C2 engine and itís running the wrong plugs? Unless thereís a reason for a jp4 to be running the shorter Saxo plug?

Anyways - this poses three questions for me:

1 - whatís the easiest way to tell a j4 from a jp4 without taking it to bits? Assumes thereís a code or markings somewhere which will tell me? Where should I be looking?

2 - assuming it IS a C2 / jp4 engine - what sort of effect will the shorter plugs be having on it? Should it run at all? Should it run badly? Should it pretty much run the same? In my head, the spark being higher up should surely have an effect on how it runs?

3 - assuming it ISNT a C2 engine and itís the j4 with the correct plugs - what damage do I risk doing by trying it with a set of longer plugs? Ie is there room in there for a C2 plug to fit without it hitting anything?

Cheers
Wouldn't say it's particularly unsafe if it's the correct heat rating, although technically it will alter the ignition timing.

If it's had a JP4 swap, there's a chance they've been down the motor factors and given the reg and got J4 plugs by accident and "they fit" herp derp.

You can identify the differences in this thread, plenty of pictures

http://www.saxperience.com/forum/sho...d.php?t=323539
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Old 9th July 2020, 21:44   #3
welshpug
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if its a J4 you wont want the long plugs, the seat is the wrong shape being conical instead of a gasket, let alone possibly punching a hole in the piston.
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Old 13th July 2020, 22:41   #4
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Short plugs in the JP4 will cause very poor flame propagation because of the recessed spark and the exposed threads in the hole will soot up making it difficult to fir the right plugs later, but at least the threads could be cleaned out.

Long plugs in a J4, besides the possible damage, the plug's exposed threads will foul with soot and you'd be at great risk of damaging the head winding the sooty plugs out of the head.
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