AFM (Active Fuel Management) Cylinder Deactivation in GM V8 motors....is it a dud ?

Greaser · Canadian Poncho Superstar!

AFM (Active Fuel Management) Cylinder Deactivation in GM V8 motors....is it a dud ?

ashmore · Guru

I have a bit of experience with these engines.The lifters pump up with oil to keep the valve open when the cylinders cut out. These lifters collapse as a result of faulty oil control manifold. Cylinder head has to be removed to replace lifters. End result it is an expensive repair.

4SPEED427 · A Poncho Legend!

I have heard lots of negatives about these engines. Our experience at work has been very good. I think I've only seen two lifter failures on one of these over all the years. However, reading online is scary. I read the G8 forum daily and the guys there tell me our G8 6.0 litre will experience lifter failure any time (200,000km)... Too bad, what a great engine.

hawkeye5766 · Canadian Poncho Superstar!

Pretty sure there is a delete kit. Lifters,program etc. Just had a customer that deleted his after a lifter blew. Not sure on cost though....

Prefectca · Poncho Master!

The AFM actually keeps the valves closed. From an engineering standpoint the theory is that the air trapped in the cylinder creates a cushioning effect to smooth the transition from 8 to 4 cylinders. In the real world, because air cant get into the cylinder the motion of the piston going down in the cylinder creates a vacuum pulling crankcase fumes into the cylinder. This causes the oil burning and piston ring troubles.

Paul

Canadian Poncho · A Poncho Legend!

A bit OT but it reminds me of the issues GM has with the 3.0 and 3.6 engines wiping out timing chains. Apparently the PCV system creates too much vacuum causing oil consumption issues. Low oil causes the chains to eventually fail. The "fix" is a revised rear valve cover gasket that pretty much blocks most of the PCV passage other than 3 small pinholes.

4SPEED427 · A Poncho Legend!

Prefectca wrote:
The AFM actually keeps the valves closed. From an engineering standpoint the theory is that the air trapped in the cylinder creates a cushioning effect to smooth the transition from 8 to 4 cylinders. In the real world, because air cant get into the cylinder the motion of the piston going down in the cylinder creates a vacuum pulling crankcase fumes into the cylinder. This causes the oil burning and piston ring troubles.

Paul

It's interesting that I read online about the oil consumption you mention but our G8 6.0 litre with AFM goes 5000 km between oil changes and doesn't use a drop of oil. Some other guys say the same. It seems to be a totally random occurrence.

DANO65 · Poncho Master!

Carl if your foot is "in" it , AFM won't activate, I'm sure your G8 very rarely goes into AFM........

4SPEED427 · A Poncho Legend!

DANO65 wrote:
Carl if your foot is "in" it , AFM won't activate, I'm sure your G8 very rarely goes into AFM........

Oh no, when I drive it to the lake it's in AFM a lot, getting me 30-31 mpg on the flat land at 110 km/h. I'm real impressed by the mileage.

Prefectca · Poncho Master!

In my experience, the people that have problems seem to be people that only drive in the city. I havent worked on any, but the people that I worked with that own these trucks and drove into town doing 80 to 100 kph and probably 20 or more km to work dont have problems, just the city dwellers idling around the city.

Paul

DonSSDD · Canadian Poncho Superstar!

I wonder what the fuel mileage benefit is for AFM? Does it add to emission control issues?

Carl, my dads 305s or 307s in his early 80s rear wheels drive Olds 88s would get 30 mpg, driving it like you would be to see a benefit from AFM? About 65 mph? Yes emissions would be bad compared to the modern engines but it was also a huge brick of a car with a 4 speed auto and room for 6 people plus a lot of luggage.

Is AFM worth it if it maybe adds 1 mpg? Or is it just something all the V8s need for pr purposes?

4SPEED427 · A Poncho Legend!

Those full size cars with 307's were wonderful. What a great combination of a great riding car with lots of room, some very good looking cars, reliability and as you say really good mileage. It was tough to compete with GM in the full size category in those years, they had it all.

The G8's have a fuel mileage display in the Driver Information Centre (DIC). Sometimes when I'm driving I will scroll to that screen in the DIC and leave it on just to see what the car will do. From what I can tell the AFM is worth about 3 mpg which to me is a worthwhile improvement. I do notice that conditions need to be ideal, very little side wind or cross wind, pretty flat terrain and steady on the accelerator pedal. It takes very little to deactivate it. One day we were following a semi on the interstate. If I stayed about 150'-200' behind him the AFM stayed engaged but if I dropped back another 100' or so it would kick out. There was a sweet spot there that was nice, didn't have to follow too close and yet allowed him to draft!

DonSSDD · Canadian Poncho Superstar!

But hard to believe a 35 year old V8 barge with lots of add on emissions stuff is competitive on fuel consumption with a modern fuel injected V8 with AFM?

4SPEED427 · A Poncho Legend!

Yes, but 1/2 the horsepower, not anywhere near as much fun!

DonSSDD · Canadian Poncho Superstar!

True. But not a BB Carl? These LS motors will blow the doors off a BB, at least the stock specs of both, and fuel economy is amazing for the power they have. But the fuel economy of those old tech v8s most people would not believe. And not many modern vehicles half their size get that much better fuel economy on the highway? Almost 30 years later. How many of the suvs are better?

4SPEED427 · A Poncho Legend!

True, it's not a BB! I don't see myself ever "playing" with LS engines. I like the power of the G8, I like the fuel mileage but as far as getting into modding them etc? Doubtful. My favourite powertrain has long been and will likely always be a nice stock 427 and a Muncie. Power? Decent, but not up to LS levels certainly. Fuel economy? Nope! Why are they so much fun then? For a guy with very little hair, and gray hair at that, I guess it's a sentiment thing. They are certainly easier to fix and likely more durable than an LS!