High deceleration vacuum, and how to reduce it

[Bigsyke]

I unknowingly purchased an oil burner when I purchased my 350z Rev-up. Faulty rings and design on the Rev-ups simply means you have to deal with it. Some owners have had up to 4 engines replaced by the dealer, and still burn oil. I burn 1 qt every 250-500 miles.

It seems the burning stems from faulty piston rings, and the unnaturally high vacuum of the engine on deceleration, will suck oil past the rings. A clogged PCV valve exaggerates the oil burning problem because of even more increased vacuum within the plenum (no controlled vac leak).

A solution I have pondered, was a reversed boost-controller, to provide a mild vacuum relief at a certain vacuum level. The relief valve is closed at -21hg/in, or what ever idle vacuum is, and the relief valve opens at something like -25in/hg allowing. It sounds like this would alleviate vacuum inside the combustion chambers on heavy deceleration. It doesn't look like the ECM adjusts fuel/timing on deceleration above 1500rpms, so I cannot see a negative affect of a large vacuum leak on deceleration only.

This would solve my consumption problem. I have tested 500 miles of driving by doing absolutely no engine braking or coasting in gear. No oil consumption.

I have searched, and came up empty handed with any examples. Does anybody have knowledge of a vacuum leak on deceleration causing engine damage?

Thanks!

 

[SnowDrifter]

So long as you are in fuel cut and not a lean condition with the extra air I can't think much harm could result. Certainly no worse than having oil continually sucked into the cylinder and blows through the cats

 

[Bigsyke]

The cats are gone. Passenger side was completely destroyed (the bank with the highest consumption visible via the plugs).

Yes, the consumption is pinpointed to deceleration. After a hard pull, lifting off the throttle creates what sounds like rocks hitting the firewall. Its pre-ignition from the oil igniting. Only does this when the oil is nice and hot.

On paper, a vacuum relief sounds logical, but the fact I have never heard of it before makes me hesitant to try it.

 

[slowdime]

High vacuum on deceleration is normal. I had a vac gauge hooked up for a while and it would always jump to about -25" or so on deceleration in gear. If your pcv is clogged replace that and see where it leaves you
Is it a standard or auto trans?

 

[Merkava_4]

Has any aftermarket companies come out with a fix for the poorly designed piston rings?

You need some rings with a lot of tension to bite hard into the cylinder wall.

 

[berniedd]

Why not make your own? It could be a simple metal tube with a spring and ball of appropriate diameter inside. You can experiment with different spring tensions that you can make on your own. Each end of the tube will be tapped to accept an ordinary metal cap for valve stems you use for your tires. Drill a hole at each cap. One end of the tube is attached to a simple air filter. Heck, it could be just a fine cloth mesh initially. Connect the other end to a Tee fitting plumbed to any hose from your engine's intake manifold (like a brake vacuum hose, for example).

Let us know how it pans out. I'm interested to know the results.

 

[Shannow]

SU carburettors used to come with an "overrun poppet" (google it) to prevent excessive vacuum on over-run...it made engine braking lacklustre also.

Wondering if one could be retrofitted.

Otherwise, find your manifold vacuum at idle, fast idle, and cut in a pressure relief valve a little higher.

 

[MolaKule]

How does an engine with loosy-goosey rings create a higher vacuum than an engine with good rings?

 

[Kira]

What's a 350z Rev-up? Is a 350z a Nissan? Kira

 

[brandini]

What you really need is an oil pan baffle/crank scraper/catch can combo.

 

[Bigsyke]

Originally Posted By: MolaKule
How does an engine with loosy-goosey rings create a higher vacuum than an engine with good rings?


Not exactly sure to be honest. I've heard the piston bore is warped towards the bottom on certain cylinders along with faulty piston rings. Nissan never came up with a solid fix on the VQ35DER, so some suspect dealerships swapped in an older model [VQ35DE] short block. All of Bank1's spark plugs are completely ashed up, so maybe that bank holds vacuum. New PCV valve, catch can, etc.

Originally Posted By: brandini
What you really need is an oil pan baffle/crank scraper/catch can combo.


Already installed those, mainly for the extra oil capacity. Little oil vapors going into the can. I have also installed KrankVents with no change.

Originally Posted By: Kira
What's a 350z Rev-up? Is a 350z a Nissan? Kira

The Rev-up is the engine type, which came in some 2005's and all 2006's.

Originally Posted By: slowdime

Is it a standard or auto trans?

Its a 6-speed trans. Supposedly we have excessive valve-overlap, which has sucked catalyst pieces right back into the combustion chamber on very rare events. That's a rumor, but I have heard it more than once.

Thanks for the reply everyone. I will attempt to create a device similar to a reverse boost controller and run it into the cabin. I will use my tuning software and UltraGauge to monitor readings.

 

[CorvairGeek]

Sounds like you need a NOS GM deceleration valve or a Ford 'gulp' valve from the 70s or early 80s, if you would like something fairly simple to play with. They work off differential pressure between ported (if it is available, may kill the idea if not) and not ported vacuum, which opens the big port to lower the vacuum on deceleration. This kept fuel from being pulled out of the carburetor, a real emission problem on vehicles with manual transmissions.

 

[CorvairGeek]

"Its a 6-speed trans. Supposedly we have excessive valve-overlap, which has sucked catalyst pieces right back into the combustion chamber on very rare events. That's a rumor, but I have heard it more than once"

I thought there was even a TSB for engine damage/replacement over the catalyst failure issue on 3.5L engine, and I have very little knowledge of these ???

 

[Rand]

gee where has this problem occurred before (sucking in exhaust catalyst pieces)(sarcasm!)

Nissan!

 

[brandini]

Originally Posted By: Bigsyke
Originally Posted By: brandini
What you really need is an oil pan baffle/crank scraper/catch can combo.


Already installed those, mainly for the extra oil capacity. Little oil vapors going into the can. I have also installed KrankVents with no change.

Those don't have anything to do with oil capacity, unless you got a different oil pan with a larger capacity. A pan baffle can reduce aeration so you don't foam it up and shove it out the breather when you lift and cause massive high rpm vacuum.

Most cars have both PCV system and a separate breather (my 4-popper does) and you must put a catch can on both systems separately to catch both sources.

Also possibly look into tuning the throttle to not close quite as much. Some cars opt to leave it open during overrun.