P38A 4.6 2000 X REG

Help needed please...its driving me and the garage mad. Excuse the lack of technical speak as I am basically trying to repeat what the garage team are saying. Vehicle had a replacement Turner engine, camshaft and heads about 25 k miles ago (along with rebuilt gearbox, transfer box diffs etc...a lot of money spent).

It developed a slight misfire when warm and the plug on cylinder number 3 looked a different colour. (I say slight misfire because only noticeable on idle/tick over which is around 800 rpm). Leads, coil packs and plugs replaced. A later compression test showed all cylinders above 180 except for no.3 which was 160. It was decided to remove the heads and check no.3. Pistons/rings/bores all good on all 8...however a valve leak test showed valves for no.3 leaked. Heads were re-faced (basically cleaned) and valves re-seated, new tappets, push rods.gaskets etc All compression's now good and from cold it runs smoothly.

However, as before, once warmed up it starts to miss again (presumably on no.3.... it will be checked to confirm on Monday). Camshaft sensor stud has been changed ..no improvement. Flywheel was checked visually when the sump was dropped during checks..nothing found untoward.

Any ideas where to go next? We were hoping it was electrical....was the valve leak misleading and unconnected?

Thanks in advance for any clues ....

Hi, what was the colour of the plug on #3?

O2 sensors perhaps @Henry_b?

Best thing is stick it on a Nanocom diagnostics, will stop the need for shotgunning parts on it and hoping it cures

That slight drop in compression wouldn't of caused a misfire.

Your issue is likely to be electrical.

Diagnostics are the way to go.

The CPS is known to start to "breakup" when hot and cause rough running.

s said above what colour was the plug? Any coolant loss?

Presumably they've tried noid lights?

My brother had an issue after modifying his Yamaha fzr.he tried everything and in the end I suggested he tried a colour tune tool and he found that, (in his case) the coil packs weren't strong enough. Not saying this is your case, but it could help identify any issues with combustion. Maybe you have a duff injector?

kermit_rr said:

Hi, what was the colour of the plug on #3?

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I ll have to check with them on Monday.....

RangeRoller dt said:

O2 sensors perhaps @Henry_b?

Best thing is stick it on a Nanocom diagnostics, will stop the need for shotgunning parts on it and hoping it cures

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Yes it was getting a bit off track in my book but I'm no expert. I believe they had it on diagnostics first and nothing was showing which is why it went "hands on"

Henry_b said:

That slight drop in compression wouldn't of caused a misfire.

Your issue is likely to be electrical.

Diagnostics are the way to go.

The CPS is known to start to "breakup" when hot and cause rough running.

s said above what colour was the plug? Any coolant loss?

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Nice to hear that the low compression shouldn't be connected...my thoughts from moment one...but I suppose it helped clear up a small defect. My concern is, was the electrical problem causing the valve problem after 25k miles on a nice Turner engine. Could a slight misfire actually do that?
No coolant loss...and plug colour needs to be clarified.
The previous owner spent a great deal on the car and there are many new and refurbished mechanical parts...but the car has done some 140k so other minor electrical parts may be giving up

Mark Piercy said:

My brother had an issue after modifying his Yamaha fzr.he tried everything and in the end I suggested he tried a colour tune tool and he found that, (in his case) the coil packs weren't strong enough. Not saying this is your case, but it could help identify any issues with combustion. Maybe you have a duff injector?
View attachment 194333

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Believe they swapped the injector over.....

Thanks for the replies...it confirms my thoughts that it should be electrical and I will hopefully learn more next week. Maybe the link with number 3 is a misleading one...the plug may have been a different colour because of the valve problem...and the electrical misfire is elsewhere?

To be pedantic, what do you mean by a misfire? Not firing at all on occasion or firing at the wrong time?

With mine it was a failed coil pack/lead that was causing a misfire when hot every time. Death by a thousand cuts as the garage replaced the leads one by one each time one failed.
Eventually all packs & leads were changed and no more problem.

kermit_rr said:

To be pedantic, what do you mean by a misfire? Not firing at all on occasion or firing at the wrong time?

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When idling the engine from cold it is smooth and quiet....as soon as it is warmed up, a run of 20 miles or so...it has a regular audible change in sound and visual movement of the engine in time with that sound. Difficult to imagine I know but the garage called it a misfire ....disappears when driven ....not noticeable at all. My mpg has dropped by a few of miles...usually 23-24 mpg and now in the 19-21 range.

DanClarke said:

With mine it was a failed coil pack/lead that was causing a misfire when hot every time. Death by a thousand cuts as the garage replaced the leads one by one each time one failed.
Eventually all packs & leads were changed and no more problem.

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Leads have been changed twice but coil packs only the once. I have my fingers crossed for something like this.

Grrrrrr said:

Presumably they've tried noid lights?

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I would have to ask....I ll read up so I know more.

Revititllclear said:

I would have to ask....I ll read up so I know more.

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Easier said than done on a thor engine

Revititllclear said:

I would have to ask....I ll read up so I know more.

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Sounds like your issue is temperature related so needs to be done hot. Sensors can start to fail when hot but why only 1 cyclinder? V8 is wasted spark so fires in pairs. Bit of a mystery.

Grrrrrr said:

Sounds like your issue is temperature related so needs to be done hot. Sensors can start to fail when hot but why only 1 cyclinder? V8 is wasted spark so fires in pairs. Bit of a mystery.

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How do you know that it's not miss firing on 2 cylinders?

Datatek said:

How do you know that it's not miss firing on 2 cylinders?

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He said just #3.