ABS

So my car has been suffering from burning a quart of oil every 1000 miles or so for the last 300-4000 miles. I want to address this issue before it gets any worse so I have been researching causes. I will be performing compression tests within the next week to confirm if the valves or the rings are the problem and how many cylinders . . . I discovered this little article which I suspect might answer part of the problem:

http://www.automotiverebuilder.com/ar/ar79842.htm

Here is what I think may have happened to my car:

1. Performed tune up but didn't have the valve cover all the way down on the rear bank causing the car to run lean for a short period of time - few hundred miles at most.

2. Also discovered that I had a bad MAF which was causing a low beta value for fuel trim. This may have also contributed to the lean condition.

3. Lean condition caused high exhaust temps.

4. Slightly overheated valves caused damage to valve seals - possibility

5. Damaged valve seals causing excessive oil to leak into engine.

Now, if this is what is actually happening, it is both good and bad news at the same time. The good news is that IF this is what the case then only the valve seals would require replacement and the piston rings might still be okay. On the other hand the repair requires removal of the timing chain and the camshafts for repair and taking those stpes is not a small project. Additionally, the risk of damaging the catalytic converters resulting from excessive oil in the combustion chamber is a very real concern. This may actually explain, in part, why we are now seeing cat converter failures.

Does anyone know what material Nissan is using in their valve seals? Also, is there a shortcut to replacing the valve seals without having to take apart the front of the engine (i.e. timing chain, etc.). If there is no shortcut and the timing chain has to come off, what enhancements should I be looking at performing while the engine is open?

Thanks in advance.

Andrew

eng92

iTrader: (3)

Here is the inside of your VQ35 with the front timing cover and valve covers removed.

As you can see, our engines use a bucket style lifter with the cam lobe directly over the valve. To get the valves out, you have to remove the cams which also requires removal of the front and rear timing covers.

bigEL

What a vision of beauty.

danger300z

iTrader: (4)

wow im in love got more pics!!!

ABS

Eng92:

Thanks for the pic - that is a really good one - would you know if I have to pull the engine to do the job?

Thanks.

steven88

iTrader: (28)

Andrew, I heard precats could be the culprit for oil consumption....have you ever thought about checking your precats?

eng92

iTrader: (3)

FWIW the engine pic above is the after shot of the same engine shown in these pics
http://forums.maxima.org/showpost.ph...7&postcount=11
Both precats were in mint condition.

ABS

Eng92:

Would it be fair to say that your problems were caused by faulty or worn valve seals?

When you did the rebuild, did you go with the OEM Nissan seals or did you use a better aftermarket part? If you went aftermarket, could you share with me what product you used?

Regards,

Andrew

ABS

Steven88:

I am aware of the pre-cat issue. I am now thinking that the precats may be failing as a RESULT of the oil consumption and not the other way around . . . Oil and incomplete combustion in the chamber entering the cat could cause the cats to run hot and burn out . . . When that happens, the situation gets even worse because small ceramic cat particles get sucked back in through the exhaust valves and cause severe damage when the computer forces the intake and exhaust valves to overlap by changing the valve timing . . .

Thoughts?

Andrew

eng92

iTrader: (3)

I got this engine from a wrecker, so without knowing the history of it, I cannot really say what the root cause of the oil leakage problem was.

It would be very unusual to have oil leakage due to bad valve seals on all cylinders. I did not observe any damage to the original valve seal lips when I removed them.
I replaced them with OEM seals.

If the oil leakage was through the exhaust valve seals, I would expect there to be significant carbon deposits on the backside of the exhaust valves.

There were none.

00SEMAX19

iTrader: (4)

Nice pic!

I personally odn't believe the problem is the precats. when I installed my headers my precats looked in perfect shape. I couldn't see any cracking or braking of the honeycomb inside at all. Plus my oil consumtion problem has gotten worse since they have been removed.

I removed my IM and notice I was getting a ton of blowby down into the front 2 chambers worying me about burning a valve. I don't know if this is part of the problem or not but after seeing pics of oil seperators catching lots of oil that would be burnt away it makes me think that is where the oil is going (right into the intake and burning up)

just my 2 cents

BewstAdd1ct

im still curious... and would someone please explain to me how a partially clogged, or otherwise failed catalyst causes excessive oil consumption...



what is 'lots of oil'? as opposed to a normal amount of oil? what are you comparing to?

00SEMAX19

iTrader: (4)

I don't have a good answer to this.

Rumor is the cats start braking a part and small particles of the cats get sucked back into the engine causing cylinder wall damage.


http://forums.maxima.org/showthread.php?t=418752
Top post is on a 3.0 but I bet our cars get more and that was only a week of driving, but he doesn't say how many miles.
The question in my mind is why do we get so much blow by

It would be nice if two people with these oil seperators would moniter the oil caught by the cans. Perhaps one person who has a oil loss problem, and another who is not loosing oil. That would really help prove/disprove my theory.

JoelC

How is it possible that anything would get sucked into a combustion chamber through the exhaust? This makes no sense to me.

BlackBIRDVQ

iTrader: (15)

Bad cat- loose honeycomb particles rattling around inside the canister. When you let go of the gas pedal, fuel injectors are shut closed by the engine and they dont operate on deceleration. At that time you get whats called reversion of the exhaust gases- engine is sucking air in from the exhaust side of the engine- throttle body is closed so how does it get air into the chamber ? Exhaust way. So at the same time those loose particles in the cat get blown into the combustion chamber and make their way down between the piston and cylinder wall- where the piston rings are. Those loose particles might be large enough to cause scoring to the cylinder walls- and piston rings can't seal the cylinder wall good enough. Oil which travels under the piston and is controlled by the lower oil control ring gets by into the combustion chamber and boom- gets burned.

Not to say this is the ONLY reason these cars burn oil, cause it could be other things as well. I never experienced any oil burning on any of my Nissan engines. I just rebuild my VH45DE cylinder heads and put new valve seals in, performed a 5 angle valve job and cleaned them up quite a bit. That engine has had severely warped cylinder heads that I was able to get fixed by a special techinque, and at the same time I was able to rebuild them. I should have this sucker runing in a very short time.

JoelC

Your tb closes and opens with acceleration and contols air flow into the engine. Fuel doesn't shut off, it tapers down to a point to accomodate idle as long as the ignition is on. While the engine is running it is an "air pump"... one way. Regardless of fueling, the cams open and close intake and exhaust valves. The air flows out through the exhaust manifold.

For this theory to be right, one whole side of an engine would need to be NOT pushing any air out of the exhaust manifold at any given time and in fact have negative air flow in order to "suck" cat particles into the head. In reality this does not happen while the car is running.

I am not a engineer but know a bit about cars. Logically this just doesn't make sense to me.

ABS

Also, don't forget that for 2002 and 2003, the VQ3.5 engine doesn't have an EGR valve - the computer adjusts the valve timing under certain conditions to cause overlap between intake and exhaust valves on the "down" stroke of the piston. This effectively replicates the operation of the EGR valve meaning that exhaust gases are pulled back into the combustion chamber.

In adddition to impacting the rings as described above, I don't see why this couldn't also damage the exhaust valve seats? It also occurs to me that if the catalytic converter was overheating for any reason that the exhaust gas pulled back into the car could be even hotter than it was when it left the head on its way out. Could the super heated exhaust gases damage the valves/valve seals on the way back into the engine?

BlackBIRDVQ

iTrader: (15)

fuel injectors close and do not operate during deceleration- hence you see 0.00ms of injector opening on a scanner when doing so.

I am not telling you to go out and put your hand on the tail pipe to see if it will suck your hand in or not durring deceleration- but it does go into reversion. This function is also used as a superimposed EGR where exhaust gases cool down the combustion chamber to reduce NOx. Since 01-03 Maximas have no EGR (and EGRs function is to cool down the combustion chamber), reversion is one of the ways they get away with emissions. On my wideband 02 sensor I can watch reversion as it happens- let go of the gas pedal and my wideband goes to LEAN- it can't read over 22:1 AF, and 1 sec later it says AIR. Air is read due to reversion- where it gets pulled along with bad cat particles into the motor. Particles can score up piston walls as they don't burn- how does ceramic substrate burn ? It can cause damage to valves if the particles are big enough.

I'm a landscaper for a living, I just read that all from a book.

GBAUER

iTrader: (2)

I'm a mechanical engineer and I don't know WTF is going on! But, I'll tell you guys in about two months or so after I tear into my engine.

BewstAdd1ct

the injectors DO shut off on deceleration, and since you know quite a bit aobut cars, then you know that manifold vacuum is highest during high RPM and closed throttle, and that nature abhorrs a vacuum. one of the easier paths for the vacuum to be filled is a short distance from the exhaust ports. during overlap, the manifold vacuum extends itself into the exhaust and any ceramic dust in the area is prone to being sucked in, as well as platinum and/or palladium.

im still curious however, why these will cause an engine to burn large amounts of oil (due to 'compression loss'), but not run rough. and why doesnt this happen to the front bank. theres almost never a reported case of BK2 catalyst deterioration, yet the burned oil has reportedly been from ALL cylinders based on plug readings and even some teardowns. ive seen engines run like ****bags due to a low cylinder, yet not smoke from oil out the exhaust.

im voting for top end.


its MUCH more likely that burning oil is causing the catalyst failure, not the other way around.