P38 gems 4.6 backfire/misfire sound

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Wesleyb69

Member
Posts
14
Location
Northampton
Hi newbie and in need of help . Sounds a bit like a backfire which is most audible from the plenum
New plugs
New leads
New coil pack
Any help please
 
They can suffer from worn cams, pull rocker covers of and see if all the rocker move the same amount that will confirm cams.
 
Just found this after plenum removal
IMG_0020.JPG
 
It looks awfully like coppaslip (or copperslip whoever manufactures it....) copper grease. But why would that be in the coolant?

The only other thing that may resemble it is Radweld.

However, a misfire since the plugs, leads & coils have been replaced could only mean one thing, one of the plugs (or more) isn't firing at the correct time!

Pull out the plugs and check them, the gap, the insulator and so on. Did anyone drop a plug and close the gap or such like?

As suggested, it could also be a mix up of the plug leads. Or worst case, a burned valve... But I would put money on plugs, leads or coils seeing as how they were just done and this just happened or a I reading it wrong and it happened leading to the replacing of all the above? In which case, burned valve if it's making a noise in the plenum.
 
Possibly cheapest & easiest thing to do 1st is a compression test, see if there is a vast difference between cylinders.
 
That really sounds as if it is exploding in the plenum chamber.

So, how would it be possible for fuel to be burning in the plenum?

Does it have knock sensor? Could even be continuing to run open loop, making it overly rich! Check O2 sensors, temp sensor and Throttle Position Sensor, which would all affect the fueling.

Afraid I am not used to working on V8's, but any petrol that is misfiring does so for a reason.

The timing is crucial on EFI engines, so everything needs ruled out.

Did it come on slowly or all of a sudden?
 
You really need to get it on a code reader, or you'll end up tearing your hair out :confused:

This may help build up a picture of how your system works in the meantime

http://blackbox-solutions.com/help/SM001.html

Check for splits on the pipework after the MAF sensor, as this will mess up the fuel/air mix if nothing, try unplugging the MAF and giving her a run. If the explosions stop, it possible that's the likely fault.

However, I can't recall if you mentioned MIL (Management Indicator Lamp) lit up or not?
 
Last edited:
Hi all bit of an update
Compression test done all 100 psi
Rocker cover off passenger side all seems to be rocking as it should
With all plugs out still getting back pressure form cylinder 8
Really confused now
 
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