Chasing Misfires V8

[KTMsm]

I bought an V8 LS1 engined car, it has a history of misfiring as it's had three sets of plugs & leads in the last 3 years / 3,000 miles

I plugged it into a scanner and can see it was mostly on two cylinders

Cyl 7 5000+ misfires
Cyl 2 1000+ misfires

The others had between 0 and 50 which I'm going to assume is normal / sensor error

It has a separate coil pack for each cylinder

I moved the plug, lead and coil pack from 7 to 5

At idle 7 had no misfires and 5 did - so I thought it's either the plug, lead or coil pack that's faulty

However, when I then raised the revs to 2000, 5 stopped misfiring and 7 started again

I'm thinking perhaps it's the injector that's at fault and is has affected the coil or plug so they are now faulty too ?

There's clearly an issue with them at idle and an issue with something on number 7

2 - swapped the coil & lead - no difference so I'll swap the plug next

[KTMsm]

[GreenV8S]

Are the plugs gapped correctly?

[stevieturbo]

Are you chasing a code, or an actual fault with it running ?

[KTMsm]

Runs like an absolute POS never runs on all eight

This morning I couldn't buy the correct tapered plugs (TR6)

So I took a chance and put some flat washered ones in the two dodgy cylinders - they've 95% fixed it

Mostly running on all eight

I haven't touched cylinder 7 since I moved everything onto cylinder 5

Yet once I fitted a new plug in cylinder 5,Cylinder 7 isn't reporting a misfire - so presumably that's a sensor issue confusing the two

[KTMsm]

Originally 5 didn't have a misfire

I moved the plug, lead and coil from 7 and it misfired

I've now changed everything on 5 with 3 to try to work out if it was the lead or coil (as I fitted a new plug)

And it's still misfiring

[richhead]

try going back to basics, have you done a compression test?
Been a while since ive worked on a LS but used to run a race car with a LS7 and we used to have alot of valve and pushrod problems, might be worth pulling the rocker covers and having a look. Not saying thats the problem, but its easy to assume the problem is electrical.
Also can you test the injectors somehow?
Another thing, cam sensors and crank sensors can do strange things. anyway you can check these?

[stevieturbo]

KTMsm said:

Runs like an absolute POS never runs on all eight

This morning I couldn't buy the correct tapered plugs (TR6)

So I took a chance and put some flat washered ones in the two dodgy cylinders - they've 95% fixed it

Mostly running on all eight

I haven't touched cylinder 7 since I moved everything onto cylinder 5

Yet once I fitted a new plug in cylinder 5,Cylinder 7 isn't reporting a misfire - so presumably that's a sensor issue confusing the two

BCPR6ES will work just fine. That taper seat st is ust that, st.

[GreenV8S]

KTMsm said:

And it's still misfiring

Cylinders can interact with each other because they're connected via the intake and exhaust manifolds. It's reasonable to start by looking for an ignition/injection component failure in the cylinders with the symptoms, but don't assume that's the cause. For example you might have mixture problems in a different cylinder, or a mechanical problem in the cylinder or valve train.

Have you got good compression on all cylinders, no excessive valve/bore leaks, roughly equal pressure while cranking, all injectors holding pressure and flowing similar amounts? You can test all these without dismantling and without any expensive equipment.

[SystemOfAFrown]

Have you tried swapping injectors? Don't swap 5 and 7 since both are suspect, swap both with two known good cylinders.

[KTMsm]

stevieturbo said:

BCPR6ES will work just fine. That taper seat st is ust that, st.

Googling showed Craig (Tinker27) says the same

NGK state that you can't swap them but I suppose that's to cover themselves as no doubt there are engines where you can't

Why are you both suggesting the 6 when the std NGK is a 5 ? As it's had a history of misfiring and is a standard engine I'd have thought a colder 6 wasn't the best idea (although IME 6 is more common in other cars)

Thanks

[KTMsm]

I had to collect my wife 10 miles away so I took this car to see what it was like

There is the odd miss at idle but no check engine light - which is a first ! No fault codes either

I think I'll just change all 8 plugs and run it for a bit

I can't see anything wrong with the old plugs, they've done minimal mileage and it ran ok on the test drive so I've no idea why it all went tits up

[stevieturbo]

KTMsm said:

Googling showed Craig (Tinker27) says the same

NGK state that you can't swap them but I suppose that's to cover themselves as no doubt there are engines where you can't

Why are you both suggesting the 6 when the std NGK is a 5 ? As it's had a history of misfiring and is a standard engine I'd have thought a colder 6 wasn't the best idea (although IME 6 is more common in other cars)

Thanks

I see no reason to run as hot as a 5.

And presumably this car has ran correctly at some point, or have these issues always been present ? Are the leads routed correctly ? ie simply direct from plug to coil ? Not relocated coils with very long leads all bunched together or something stupid like that ?

[KTMsm]

stevieturbo said:

I see no reason to run as hot as a 5.

And presumably this car has ran correctly at some point, or have these issues always been present ?

Are the leads routed correctly ? ie simply direct from plug to coil ?

Not relocated coils with very long leads all bunched together or something stupid like that ?

From google it seemed an NGK TR5 or 55 is the standard plug in the USA, which I'm assuming is a 5 heat range

With the long gearing I guess it doesn't get very hot in normal use

That's the question, it's done less than 2,000 miles in the last 10 years since import. Is it running badly because it's rarely used or is it rarely used because it's running badly ?

Yes, standard short leads, in standard location

[stevieturbo]

KTMsm said:

From google it seemed an NGK TR5 or 55 is the standard plug in the USA, which I'm assuming is a 5 heat range

With the long gearing I guess it doesn't get very hot in normal use

That's the question, it's done less than 2,000 miles in the last 10 years since import. Is it running badly because it's rarely used or is it rarely used because it's running badly ?

Yes, standard short leads, in standard location

Any fault codes present other than these ? What are mixtures and fuel trims like ?

I would doubt lack of miles is an issue, mine hardly ever moves these days, but always starts and runs fine, unless the fuel is maybe 3-4 years old or something ? Fuel filter etc ok ?

[Rhino]

I’d recommend checking the small wiring loom for coils that runs from the white 7 pin connector to the individual coils. This loom is only wrapped in PCV tape and if loose could wear through on the rocker cover. I had this happen!

Admittedly it’s unlikely as you got the issue on both banks and there’s one of these looms per bank, but it could be worth the check and its not replacing parts.