My 1988 Jaguar XJS V-12 has started to misfire. I took the mufflers off and could hear this misfire (I’m installing electronic cutoffs). So my question is: Does the XJS mufflers give back pressure to the engine? Is straight piping it a cause of misfire?
P.S. Misfire happens often and is very small. Almost not noticeable
Ironically, i’m chasing a misfire now on my 88 V12, soon after putting on new straight pipes. I now only have rear mufflers at tailpipe.
For me, I believe the misfire has been there for a while, it’s just more pronounced now, easier to hear in the exhaust?
And/or perhaps the straight pipes have increased engine performance (I can tell my RPMs go up much faster) which has progressed a pre-existing issue by putting more stress on it?
Colin,
I see that you are new to the list. Welcome!
Member @Gregma posted about exactly this matter recently. He modified his exhaust system and then had more pronounced misfires. Search the archives for his recent posts, or maybe he will respond.
Paul
I was thinking the same thing. I think now that the mufflers are gone I can hear the misfire
Yeah, i’m realizing with a V12, if just one cylinder is misfiring, especially intermittently, everything still runs quite smooth. And with the original mufflers/cats, you can’t really hear it out the exhaust. Even the $1 bill test, it didn’t seem very apparent.
What found the misfire for me, was the $1 bill test. After replacing exhaust, car still seemed about the same, a slight vibration at idle, but everything still seemed rather smooth, and plenty of power, even more so. I decided to do the $1 bill test one day, and lo and behold, a big steady misfire on A bank! I thought it was new, but now I believe it’s just more magnified.
What is the 1$ bill test?
it’s a great simple test to find a misfire. You take a $1 bill (or any strong piece of paper) and put it tight against exhaust pipe at idle. Not ‘too’ tight, but tight enough to vibrate.
If there is no misfire, the paper will vibrate nicely like a dragonflies wings. If there’s a misfire, you’ll get a ‘pop’ of air on the paper.
Obviously, it will also help isolate misfire to A bank or B bank.
And help differ between constant misfire and intermittent.
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And, as has been noted before- if you take the car to a dealer they will require a $100.00 bill, which , after the test, is not refundable.
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Correction: Fold the dollar bill in half, and hold it over the pipe, not tightly. A miss will be apparent in the flapping of the bill.
I have an intermittent misfire if the idle Is too low. If she just warms up sitting in the shed this will happen. After a run it’s fine. It’s the difference between 550 and 650 rpm.
Wow,
Do you think it will work with British £££ Steling as well? or at least €uros?
Not sure about V12 however if you can hear it - that means you can test it via disconnecting of the ht leads on idle? No difference = you got the affected cylinder…
Euros will not work post-Brexit.
I heard the test was to hold the bill by one end, near the exhaust … The bill should “blow back”, like a flag flying, equally on both sides (pipes). If they don’t, the side on which the blow back is not as great has some kind of exhaust restriction going on (e.g. semi-clogged or clogged muffler, cat or pipe). 