Afm 4/8 Cylinder Economy Pcm

kaelrad · February 11, 2008

Is it possible to change the PCM to stay in the 4 cylinder mode longer to get better mileage?Is this wise and would it have any ill effects on my engine?

Who makes a PCM like this,my truck has plenty of power and I do a lot of highway driving.

2008 Silverado 5.3 Liter with AFM

mdub · February 11, 2008

Is it possible to change the PCM to stay in the 4 cylinder mode longer to get better mileage?Is this wise and would it have any ill effects on my engine?
Who makes a PCM like this,my truck has plenty of power and I do a lot of highway driving.

2008 Silverado 5.3 Liter with AFM

Presently, the 5.3L is programmed to operate in 4 cyl mode only under very lightly loaded conditions. This is to prevent excessive load on the four connecting rods and rod bearings that occasionally push the truck along fairly flat surfaces. It would be alot to ask only 4 cylinders to push a 5,000lb truck up grades or under acceleration, wouldn't you agree? Especially when those four cylinders aren't designed with any more strength than the other cylinders that share the load.

Regardless of what some on this forum occasionally state, the same four cylinders are constantly used for the 4-cyl mode. The engine controller does not alternate between cylinders to randomize the load.

I would anticipate 4-cyl mode will be used more often on these engines to help with the new CAFE mandates. If so, engine longevity will be sacrificed. Chrysler is already advertising increased use of 4-cyl mode on the '09 Hemi Ram. In my mind, this can only mean operating in 4 cyl mode under heavier loads.

98 Z71 SLT · February 11, 2008

Is it possible to change the PCM to stay in the 4 cylinder mode longer to get better mileage?Is this wise and would it have any ill effects on my engine?
Who makes a PCM like this,my truck has plenty of power and I do a lot of highway driving.

2008 Silverado 5.3 Liter with AFM

Presently, the 5.3L is programmed to operate in 4 cyl mode only under very lightly loaded conditions. This is to prevent excessive load on the four connecting rods and rod bearings that occasionally push the truck along fairly flat surfaces. It would be alot to ask only 4 cylinders to push a 5,000lb truck up grades or under acceleration, wouldn't you agree? Especially when those four cylinders aren't designed with any more strength than the other cylinders that share the load.

Regardless of what some on this forum occasionally state, the same four cylinders are constantly used for the 4-cyl mode. The engine controller does not alternate between cylinders to randomize the load.

I would anticipate 4-cyl mode will be used more often on these engines to help with the new CAFE mandates. If so, engine longevity will be sacrificed. Chrysler is already advertising increased use of 4-cyl mode on the '09 Hemi Ram. In my mind, this can only mean operating in 4 cyl mode under heavier loads.

Also, to add on,

V4 mode only will stay on for a certain amount of designated time; in essence it has a "timer"

The engine MUST return to V8 mode per GM.

V4 will stay on for a maximum of 10 minutes, then switches automatically to V8 mode for 1 minute, then can return to V4 again. That's the cycle GM uses.

mdub · February 11, 2008

Also, to add on,

V4 mode only will stay on for a certain amount of designated time; in essence it has a "timer"

The engine MUST return to V8 mode per GM.

V4 will stay on for a maximum of 10 minutes, then switches automatically to V8 mode for 1 minute, then can return to V4 again. That's the cycle GM uses.

That's interesting -- I wasn't aware of this cycling. Does anyone know the reasoning? Perhaps to ensure the cylinder temp stays up for emissions reasons. That's just a guess.

kaelrad · February 11, 2008

Also, to add on,

V4 mode only will stay on for a certain amount of designated time; in essence it has a "timer"

The engine MUST return to V8 mode per GM.

V4 will stay on for a maximum of 10 minutes, then switches automatically to V8 mode for 1 minute, then can return to V4 again. That's the cycle GM uses.

That's interesting -- I wasn't aware of this cycling. Does anyone know the reasoning? Perhaps to ensure the cylinder temp stays up for emissions reasons. That's just a guess.

I travel from centarl Ohio to Upper Michigan alot it rarley operates in the 4 cyl mode at 65-70 mph my milage has never been more than 15.3 I got better milage on an 03 Suburban

2000DV4.8 · February 11, 2008

Also, to add on,

V4 mode only will stay on for a certain amount of designated time; in essence it has a "timer"

The engine MUST return to V8 mode per GM.

V4 will stay on for a maximum of 10 minutes, then switches automatically to V8 mode for 1 minute, then can return to V4 again. That's the cycle GM uses.

That's interesting -- I wasn't aware of this cycling. Does anyone know the reasoning? Perhaps to ensure the cylinder temp stays up for emissions reasons. That's just a guess.

I travel from centarl Ohio to Upper Michigan alot it rarley operates in the 4 cyl mode at 65-70 mph my milage has never been more than 15.3 I got better milage on an 03 Suburban

How many miles are on the truck? The MPG will be a little low before the engine completely breaks in.

And if you're so concerned about gas mileage why'd you buy a fullsize truck with a V8?

strafingmoose · February 11, 2008

How many miles are on the truck? The MPG will be a little low before the engine completely breaks in.

And if you're so concerned about gas mileage why'd you buy a fullsize truck with a V8?

Sorry if I branch onto another topic (somewhat related) but what defines "breaked in" ? I tested out the MPG on my 4.8L 2003 today (filling it up, travel, then refilling it up to see how much it guzzled) and only got something like 13 MPG. The engine has 60k miles. Definitely breaked in eh?

Mark_in_Tulsa · February 11, 2008

Is it possible to change the PCM to stay in the 4 cylinder mode longer to get better mileage?Is this wise and would it have any ill effects on my engine?
Who makes a PCM like this,my truck has plenty of power and I do a lot of highway driving.

2008 Silverado 5.3 Liter with AFM

Presently, the 5.3L is programmed to operate in 4 cyl mode only under very lightly loaded conditions. This is to prevent excessive load on the four connecting rods and rod bearings that occasionally push the truck along fairly flat surfaces. It would be alot to ask only 4 cylinders to push a 5,000lb truck up grades or under acceleration, wouldn't you agree? Especially when those four cylinders aren't designed with any more strength than the other cylinders that share the load.

Regardless of what some on this forum occasionally state, the same four cylinders are constantly used for the 4-cyl mode. The engine controller does not alternate between cylinders to randomize the load.

I would anticipate 4-cyl mode will be used more often on these engines to help with the new CAFE mandates. If so, engine longevity will be sacrificed. Chrysler is already advertising increased use of 4-cyl mode on the '09 Hemi Ram. In my mind, this can only mean operating in 4 cyl mode under heavier loads.

If 4-cyl can't push 5000lbs safely, then is 8 safe to push 10,000lbs?

FREAKENHYE · February 11, 2008

Ok, well, On friday i did a riverside to northridge run, in California witch is about 80 miles, and averaged about 23.9 miles per gallon.

With about 500 pounds of extra weight in the truck.

07 Silverado 5.3L AFM

20 inch rims and 3.73 gears

now to do the pcm tune for a better MPG all your gonna get is about 2 mpg more, but that would do more damage to your engine then its worth at the end, now what you can do is slow it down a little on the fuel enrichment so that it doesnt enrich is fast enough you will lose some acceleration but not that much to matter but gain about 2% in my opinion,

I will try these new setting next week and i do some testing on how much more i can save. Will post results if i get it done.

mdub · February 11, 2008

Is it possible to change the PCM to stay in the 4 cylinder mode longer to get better mileage?Is this wise and would it have any ill effects on my engine?
Who makes a PCM like this,my truck has plenty of power and I do a lot of highway driving.

2008 Silverado 5.3 Liter with AFM

Presently, the 5.3L is programmed to operate in 4 cyl mode only under very lightly loaded conditions. This is to prevent excessive load on the four connecting rods and rod bearings that occasionally push the truck along fairly flat surfaces. It would be alot to ask only 4 cylinders to push a 5,000lb truck up grades or under acceleration, wouldn't you agree? Especially when those four cylinders aren't designed with any more strength than the other cylinders that share the load.

Regardless of what some on this forum occasionally state, the same four cylinders are constantly used for the 4-cyl mode. The engine controller does not alternate between cylinders to randomize the load.

I would anticipate 4-cyl mode will be used more often on these engines to help with the new CAFE mandates. If so, engine longevity will be sacrificed. Chrysler is already advertising increased use of 4-cyl mode on the '09 Hemi Ram. In my mind, this can only mean operating in 4 cyl mode under heavier loads.

If 4-cyl can't push 5000lbs safely, then is 8 safe to push 10,000lbs?

Of course 10,000lbs is safe with 8 cylinders, but not a wise idea in V-4 mode. The ECM doesn't have any input to know that you just added +5,000lbs more to your truck, which is exactly why the ECM should NOT be programmed to blindly use 4-cyl mode under acceleration. And Tow/Haul mode wouldn't be reliable enable/disable input, since you can't count on the owner using it.

Maybe it's just me, but it doesn't seem like a good idea to second guess GM on the safe ratings of the connecting rods and bearings while in V-4 mode. But I would take an educated guess that the rated load curve is non-linear as you reduce cylinders, since running in V-4 with 4 dead cylinders isn't exactly optimal arrangement for transferring torque. And without guessing, we know the V-4 cylinders will have accelerated component wear relative to the others when the load is significantly increased. Doesn't seem like a worthwhile tradeoff.

kaelrad · February 11, 2008

Also, to add on,

V4 mode only will stay on for a certain amount of designated time; in essence it has a "timer"

The engine MUST return to V8 mode per GM.

V4 will stay on for a maximum of 10 minutes, then switches automatically to V8 mode for 1 minute, then can return to V4 again. That's the cycle GM uses.

That's interesting -- I wasn't aware of this cycling. Does anyone know the reasoning? Perhaps to ensure the cylinder temp stays up for emissions reasons. That's just a guess.

I travel from centarl Ohio to Upper Michigan alot it rarley operates in the 4 cyl mode at 65-70 mph my milage has never been more than 15.3 I got better milage on an 03 Suburban

How many miles are on the truck? The MPG will be a little low before the engine completely breaks in.

And if you're so concerned about gas mileage why'd you buy a fullsize truck with a V8?

I still need to tow a boat and camper sometimes and if I could get 18mpg empty that would be great.I have about 3000 miles on my truck now and have not used it to tow anything yet.most of my travels are empty It would be nice if I could switch to an economy mode and back to a tow mode when needed.Other than that I love the truck!

CalixEros · February 12, 2008

AFM is load based. The ECM controls cylinder activation by enabling/disabling solenoids in the LOMA and enabling/disabling the corresponding fuel injectors. Increasing the load at which the cylinders switch would cause increased wear on those 4 cylinders. Over time, this could shorten the life of the engine. It may also be extremely difficult to change the setpoints, and therefore expensive. Your best bet is to leave it alone. Yes we all bought trucks with V8 engines. I need a truck and that V8 engine; I didn't buy it because it makes me cool. Anytime I can save money on gas when I don't need all that power, I'm going to take advantage of it.

FREAKENHYE · February 12, 2008

Also, to add on,

V4 mode only will stay on for a certain amount of designated time; in essence it has a "timer"

The engine MUST return to V8 mode per GM.

V4 will stay on for a maximum of 10 minutes, then switches automatically to V8 mode for 1 minute, then can return to V4 again. That's the cycle GM uses.

That's interesting -- I wasn't aware of this cycling. Does anyone know the reasoning? Perhaps to ensure the cylinder temp stays up for emissions reasons. That's just a guess.

I travel from centarl Ohio to Upper Michigan alot it rarley operates in the 4 cyl mode at 65-70 mph my milage has never been more than 15.3 I got better milage on an 03 Suburban

How many miles are on the truck? The MPG will be a little low before the engine completely breaks in.

And if you're so concerned about gas mileage why'd you buy a fullsize truck with a V8?

I still need to tow a boat and camper sometimes and if I could get 18mpg empty that would be great.I have about 3000 miles on my truck now and have not used it to tow anything yet.most of my travels are empty It would be nice if I could switch to an economy mode and back to a tow mode when needed.Other than that I love the truck!

But if you still would like to do it all you have to change in the ecm with hptuners is the vacume numbers.