DOD Delete Theory

[RET423]

I have a theory that I think I am going to try, looking to bounce it off of our mechanic minded members.

2006 Envoy Denali 5.3
Low miles (98k), cylinder 1 intermittent ticking, throws misfire code when ticking.

I have disabled the DOD/AFM in the PCM and now need to address the hardware, all research says the cam must be replaced with a non DOD cam due to the profile of the DOD lobes being incompatible with non DOD lifters. The common result seems to be low compression and poor idle if you try to leave the DOD cam in place and just replace the lifters with the non DOD lifters.

My theory on this is that the altered profile of the DOD lobes results in too little lift for the DOD cylinders when the non DOD lifters are installed; reducing the lift & duration on those cylinders which reduces the volume of air that is available for the compression stroke.


So I thought about trying to determine the overall length from the DOD roller to the tip of the pushrod, then buying custom length pushrods to match that distance with the non DOD lifters, then using those custom length pushrods on just the DOD cylinders with the new non DOD lifters.

It seems like this should restore the stock valve train geometry & performance of the engine when it is in V8 mode and eliminate removing the valley cover for oiling modifications (the tune disabled that) and allow the DOD cam to remain in the engine.


I am in the pleasant position of not needing this car right away so I don't mind a 2nd tear down if this doesn't work to replace the cam but I was wondering if anyone sees any obvious flaws to this idea that would cause me doubt to it's viability?

Thank you for your consideration!

 

[Scottydoggs]

ditch the dod cam and lifters like the rest of the LS world. theres no combo of mix matched items that will let you reuse the stock dod cam, if its ticking loudly from a bad lifter chances are the cam is wiped out as well.

you want a bit of extra power, may as well as long as your in there. pick a small aftermarket cam that wont affect the torque converter. sadly if your doing the work your self, you need to pull the heads to get the lifters out of them engines, new cam, stronger springs are needed too. then a dod delete parts kit. most come with the timing cover and valley cover and all gaskets. ect ect.

 

[Marky Dissod]

You want to save effort / money by doing as little as possible / buying the minimum number of parts possible?
Maybe you figure that the only lifter(s) that need(s) replacing is / are the one(s) that clearly have failed?

Say you replace ALL eight of the (potentially) failed lifters, but you leave in the cam with the worn lobes.
Those brand new lifters are now riding on worn out cam lobes that are also weak / lazy.

Let's say you're one of those who worships GM OE, uber alles.
Why not replace that worn cam with the GM OE-equivalent cam WITHOUT the V4 humbuggery?
I bet that there's a GM OE cam upgrade, if your definition of GM OE is flexible.

Your engine got into this situation by being forced by GM to half-@$$ itself.
LITERALLY.

En Español, decimos: lo barato, sale caro.
In English: Cheap turns out to be expensive.

 

[RET423]

I will obviously replace the cam if there is any damage, but I seriously doubt it given the intermittent nature of the symptoms and how well it runs and sounds when the lifter seats correctly.

I am not trying to save money, I usually do this with a new aftermarket cam that improves torque and certainly will if it needs it this time; I have 2 good used stock cams out of non DOD engines I could use as well on the shelf from former engines I have upgraded.

This is about testing a theory that might make this common problem easier and more affordable for those who don't crawl into these engines all the time and are in a situation where they need the vehicle returned to service as soon as possible.

In other words, I just want to know if this theory is valid, I am quite aware that nobody has ever tried it (at least no internet evidence is available if they have) so I wanted to be the guinea pig and see how it works.

I know this site has several experienced mechanics so I wanted to present the theory for their critique to see if there are any glaring holes in it that I haven't considered.

I know how to delete a DOD system the conventional way, I want to know if this is a plausible alternative that would restore normal V8 functionality with less surgery on the engine.

 

[iamdub]

From what I've read, the difference is a smaller base circle to account for the longer AFM lifters but the different lobe profile is the real issue. Try it if you wanna test it and let the world know! It'd be easy enough to swap the cam later if need-be.

 

[RET423]

From what I've read, the difference is a smaller base circle to account for the longer AFM lifters but the different lobe profile is the real issue. Try it if you wanna test it and let the world know! It'd be easy enough to swap the cam later if need-be.

That's what I was thinking, the lift, duration and LSA pretty much has to be the same as the non DOD lobes for the engine to run correctly; so I think they just started with a reduced base circle so all the pushrods could be the same length. Like you say it's no big deal to swap the cam later if it doesn't work, all I am risking is the cost of 8 custom length pushrods that won't fit anything else; I think I will try it and see what happens

 

[RoadTrip]

... but the different lobe profile is the real issue.

That was my thought when I read the initial post. Lift and duration are separate things so the lobe design has to be accounted for.

...all I am risking is the cost of 8 custom length pushrods that won't fit anything else; I think I will try it and see what happens

Some of my vehicles have interference engines, meaning if the cam and crank are not correctly synchronized, the valves will contact the pistons, usually ruining both.

OP, you have the luxury of time and some $ to experiment, but if it were my project, I would contact a cam manufacturer and find out more details about the difference of the cam profiles between DOD and non-DOD models.

I'm rooting for your success, I would just hate to see this turn into an entire engine rebuild if it's attempted without full knowledge.

 

[Donal]

I have a theory that I think I am going to try, looking to bounce it off of our mechanic minded members.

2006 Envoy Denali 5.3
Low miles (98k), cylinder 1 intermittent ticking, throws misfire code when ticking.

I have disabled the DOD/AFM in the PCM and now need to address the hardware, all research says the cam must be replaced with a non DOD cam due to the profile of the DOD lobes being incompatible with non DOD lifters. The common result seems to be low compression and poor idle if you try to leave the DOD cam in place and just replace the lifters with the non DOD lifters.

My theory on this is that the altered profile of the DOD lobes results in too little lift for the DOD cylinders when the non DOD lifters are installed; reducing the lift & duration on those cylinders which reduces the volume of air that is available for the compression stroke.


So I thought about trying to determine the overall length from the DOD roller to the tip of the pushrod, then buying custom length pushrods to match that distance with the non DOD lifters, then using those custom length pushrods on just the DOD cylinders with the new non DOD lifters.

It seems like this should restore the stock valve train geometry & performance of the engine when it is in V8 mode and eliminate removing the valley cover for oiling modifications (the tune disabled that) and allow the DOD cam to remain in the engine.


I am in the pleasant position of not needing this car right away so I don't mind a 2nd tear down if this doesn't work to replace the cam but I was wondering if anyone sees any obvious flaws to this idea that would cause me doubt to it's viability?

Thank you for your consideration!

My theory on this is that the altered profile of the DOD lobes results in too little lift for the DOD cylinders when the non DOD lifters are installed; reducing the lift & duration on those cylinders which reduces the volume of air that is available for the compression stroke.
So, you have measured the lift of each lobe and can confirm the dimensions are different for dod cylinders verus non dod cylinders??

 

[Marky Dissod]

My theory on this is that the altered profile of the DOD lobes results in too little lift for the DOD cylinders when the non DOD lifters are installed; reducing the lift & duration on those cylinders which reduces the volume of air that is available for the compression stroke.
So, you have measured the lift of each lobe and can confirm the dimensions are different for dod cylinders verus non dod cylinders??

This has been repeatedly confirmed. The V4 mode lobes are ALWAYS slightly but consistently different from the permanent lobes.
Kinda like how every left lung is slightly smaller to make room for the beating of the heart.
This is why every THOROUGH Cylinder Confusion deletion includes a new cam with identical lobes.

 

[Donal]

This has been repeatedly confirmed. The V4 mode lobes are ALWAYS slightly but consistently different from the permanent lobes.
Kinda like how every left lung is slightly smaller to make room for the beating of the heart.
This is why every THOROUGH Cylinder Confusion deletion includes a new cam with identical lobes.

OK, so you have the dimensions?

 

[Marky Dissod]

GM has all their cams' specs, including the cams with Cylinder Confusion.

 

[RET423]

I won't know for certain until I do it but when a dod engine is in 8 cylinder mode I don't see how the lobe specs can differ without causing a cylinder imbalance; that is why I think the only difference is the smaller base circle to account for the extra lifter length.

If that length is corrected with a longer pushrod it should run exactly as when stock in 8 cylinder mode. I do know that the roller on the bottom of a dod lifter is identical to the rollers on the non dod lifters and the plunger travel is also listed as the same (but I haven't verified that yet)

 

[Donal]

Maybe you can measure the lengths of the two different length push rods and post the measurments here. Maybe note that the spring visable on the upper end of the dod lifter"s function is to keep the rocker arm, push rod and lifter internal plunger correctly positioned during the dod activation period.

 

[RET423]

Maybe you can measure the lengths of the two different length push rods and post the measurments here. Maybe note that the spring visable on the upper end of the dod lifter"s function is to keep the rocker arm, push rod and lifter internal plunger correctly positioned during the dod activation period.

I ordered everything but the pushrods from Summit today, when I figure out what length to order the 8 new pushrods I will post it as well as the results of this experiment

 

[91RS]

I don’t see this working. These are hydraulic lifters, not solid, and you’re trying to move length from the solid side to the “collapsible” side.

 

[91RS]

Maybe you can measure the lengths of the two different length push rods and post the measurments here. Maybe note that the spring visable on the upper end of the dod lifter"s function is to keep the rocker arm, push rod and lifter internal plunger correctly positioned during the dod activation period.

That spring is to allow the lifter body to move up and down with the cam lobe but the valves stay closed. The pushrod, rocker, and valve do not move in V4 mode.

 

[RET423]

I don’t see this working. These are hydraulic lifters, not solid, and you’re trying to move length from the solid side to the “collapsible” side.

This makes no sense, the length required is what is necessary to preload the hydraulic lifter to between 50 & 65 percent of its plunger travel to establish normal operation of the valves; which area in that straight line or which component has which individual length is irrelevant as long as the lifter plunger is within spec when the lifter roller is on the respective lobes base circle.

 

[91RS]

I don’t see it working. I would risk the amount of pain in the ass it is to get heads off of a V8 S/T truck. If it worked, this would be a million dollar idea and everyone would offer a kit to do it.

 

[Donal]

That spring is to allow the lifter body to move up and down with the cam lobe but the valves stay closed. The pushrod, rocker, and valve do not move in V4 mode.

That is exactly what I stated, thanks for agreeing.

 

[RET423]

I don’t see it working. I would risk the amount of pain in the ass it is to get heads off of a V8 S/T truck. If it worked, this would be a million dollar idea and everyone would offer a kit to do it.

They do offer a kit to do it, most DOD delete kits don't include a cam, just the option to add the cam you want; most kits also specify to verify the pushrod length needed after the heads are back on and order the appropriate lenth, don't just assume the stock pushrods will be right. People skip that step but many complain later about noisy lifters as a result.

There is no money to be made with this idea that is not already being made, just an opportunity to save some money and work for those whose cam is not damaged and are not looking for an improvement in performance.

If it works that is, the parts came today so I will see if I have time this weekend to get the heads off and eyes on the cam; if it looks good I will calculate the pushrod length and get them ordered next week.