Alloytec V6 misfire, dealer says rebuild...

[tHe_sTiG]

I've got a VZ Commodore Executive Alloytec V6. So a couple of days ago I started the car and was greeted with the engine misfiring. At idle, the engine splutters and coughs, struggles when revving it, but everything's fine once up to 2000 rpm and above.

Dad's WM Caprice had a similar problem not too long ago, solved by switching out the ignition coil... so I suspected I had a similar problem.

Took it to our local GM dealer. They got back to us saying one of the cylinders failed the compression test, and the engine needs a rebuild. Quoted $7200

Something doesn't sound right... the car is clocked at 228,000 km; full service history, never skipped an oil change and never abused... I told them to check the ignition coil, they said they'll get back to me tomorrow.

Could you guys please chip in your thoughts? I'd be surprised an engine fail at fairly average km's and with good care. Would there be anything else causing the engine to misfire? Or am I due for a rebuild?

Thanks

 

[diysv6]

Had a similar problem over the last two weeks.

Misfire, then OK, then misfire again, engine was very unstable during this period.
Above 2000rpm the miss was much reduced and hardly noticeable.

Finally located it to be Cyl #2 ignition coil under the intake manifold.
Replaced it and spark plug, then all good. Looks like my coil was performing badly over a period of time.
The new coil and plug really made a HP difference.

Can you swap the coils over and see if the engine miss follows a faulty coil?

I guess the local GM dealer would have done all of the Tech2 tests to arrive at his diagnosis?
Have any DTCs been cleared and don't come back when the engine is running?

EDIT
Spark Plugs have done about 15,000kms
Engine has done about 106,000kms

 

[blackve76]

I've got a VZ Commodore Executive Alloytec V6. So a couple of days ago I started the car and was greeted with the engine misfiring. At idle, the engine splutters and coughs, struggles when revving it, but everything's fine once up to 2000 rpm and above.

Dad's WM Caprice had a similar problem not too long ago, solved by switching out the ignition coil... so I suspected I had a similar problem.

Took it to our local GM dealer. They got back to us saying one of the cylinders failed the compression test, and the engine needs a rebuild. Quoted $7200

Something doesn't sound right... the car is clocked at 228,000 km; full service history, never skipped an oil change and never abused... I told them to check the ignition coil, they said they'll get back to me tomorrow.

Could you guys please chip in your thoughts? I'd be surprised an engine fail at fairly average km's and with good care. Would there be anything else causing the engine to misfire? Or am I due for a rebuild?

Thanks

If they did compression test they would have pulled plugs so should have seen a bad plug or signs

I smell bull, i'd change all the coils and plugs

 

[tHe_sTiG]

Can you swap the coils over and see if the engine miss follows a faulty coil?

I guess the local GM dealer would have done all of the Tech2 tests to arrive at his diagnosis?
Have any DTCs been cleared and don't come back when the engine is running?

They went straight to the compression test before trying anything else... they said they would carry out the rest of their tests tomorrow, which will include coils and spark plugs.

If they did compression test they would have pulled plugs so should have seen a bad plug or signs

I smell bull, i'd change all the coils and plugs

Yes, I'm a bit skeptical about all this... if they don't get the coils and plugs done and insist on a rebuild, I think I'll get a second opinion elsewhere...

 

[Smashfist]

Get compression figures. That will be pretty telling.

If it's OK when over a certain RPM, I very much doubt an electrical issue. I'd say a valve is sticking open and at higher RPM it slams shut more which explains poor compression on 1 cyl. Maybe a top end clean might help if it's a carbon buildup around the valve but best way to tell is to remove the inlet manifold.

Coils do fail on these, but so do valve springs. Carbon buildup isn't uncommon either.

They went straight to the compression test before trying anything else... they said they would carry out the rest of their tests tomorrow, which will include coils and spark plugs.

I can't blame them. The ECM is smart enough to tell you if it's a failed coil pack or injector. If it's not logging misfire codes then you can assume a mechanical cause.

 

[Benboy]

If you are a member of an Auto Club (RACV, NRMA, RACQ, RAA etc) you can avail yourself of their services and an independent mechanical assessment of your vehicle. If not a member it might pay to join - it could save you heaps in the long run.

 

[tHe_sTiG]

If it's OK when over a certain RPM, I very much doubt an electrical issue. I'd say a valve is sticking open and at higher RPM it slams shut more which explains poor compression on 1 cyl. Maybe a top end clean might help if it's a carbon buildup around the valve but best way to tell is to remove the inlet manifold.

Coils do fail on these, but so do valve springs. Carbon buildup isn't uncommon either.

The ECM is smart enough to tell you if it's a failed coil pack or injector. If it's not logging misfire codes then you can assume a mechanical cause.

Right, the GM dealer got back to us. They checked the coils and spark plugs... all fine. Yes, the ECM did not log any misfire codes. Its definitely a mechanical issue... they're suspecting a blown head-gasket.

The spark plugs and ignition coils have been changed. As the car is still drivable, the dealer told us to keep driving it until it becomes a real problem, then bring it back in for an engine rebuild... how long for a "real" problem to manifest itself they say could be anywhere from next month to next year.

At any rate, Dad and I did the sums and decided an engine rebuild is the way to go if it has to come to that.

 

[Commo Baba]

A blown head gasket does not need the whole engine rebuilt. (yet).

 

[Benboy]

Get an INDEPENDENT assessment BEFORE making any decisions.

 

[Smashfist]

I've never seen an alloytec do a head gasket - very unlikely as they're all alloy (and therefore the head expands at the same rate as the block when heated). Do a top end clean and see if it changes, or whip the manifold off, it's easy to do using the spark plug guide on here. Poke a torch down the inlet runners and see how much carbon is on the back of the valves.