Hey everyone,

Just wondering your opinions on the reliabilty of engine Used Oil Analysis (UOA) samples?

Story: I was told a couple of months ago that I have worn big end bearings. I have a rattle sound that comes on around 2K-5K or so RPMs. Its the same rattle sound that occurs at start-up and goes away a couple seconds later. The only difference is that it comes back at 2K.

My car doesn't perform any differently. Still pulls hard, no issues there. No here's the kicker. I've been driving with this rattle sound the past 17K miles or so. I want to say its a little bit louder now that when it first started. 2 months ago (14K miles or so afer the rattle started) I wanted to get a tune when I got my new exhaust and the tuner opted against it since he says I had big end bearing wear. He told me to take it easy on the engine.

Now, in that 17K miles, I've driven Germany multiple times, pushing my G to 140-150mph for 10-15 minute increments. Figured if I had a bearing issue, my engine would be done by now.

The opinion from another shop is that my noise is actually timing chain/cam sprocket related. That seems more beliveable to me.

The same shop suggested I do a UOA. Well I did and the results came back with suprising news. My engine is slim to no chance of big bearing or any bearing at all. I'm showing great results with my 0w-40 Mobil 1 oil and my samples look good. I did this for the transmission as well and those look positive too. My engine has 167K miles on it and its still doing well.

My question is....could my UOA be wrong? Could I have bearing wear and be on the edge of engine failure?

Thanks for the help.

 

Well, it all depends on the lab really. I analyze jet engine oil on a daily basis. For us it depends on if the oil analysis unit is calibrated properly and the proper use of clean materials. Otherwise all should be well.

Also if you had recently changed the oil prior to taking the sample, you would have gotten rid of most of the particles.

 

Thanks for the info. I had around 1500 miles on the oil when taking the samples. I was told this would be a sufficient amount of use on the oil to determine whether I had bearing wear or not. My results came back without any signs of major wear to my bearings.

A different shop told me that the samples would not reflect wear on the bottom end. This seems strange and doesn't make sense to me. Doesn't all the oil sink to the bottom anyway? Maybe the shop is just trying to cover themselves in some way since they assumed I had bearing wear in the first place.

I don't expect shops to know everything from just listening to a car for a few minutes or so. Doing the samples would just help eliminate possibilities of the rattle I'm having. It's just a tool to use in doing a more thorough diagnosis. Feel free to chime in if you guys have any opinions.

 

Personally I would do it after 3000 miles or more.

Bottom end? Yes. Particles can collect at the bottom. I would suggest running the vehicle for a while, then pulling an oil sample. For example when I pull oil from the jet it has to be within 30 min so it doesn't all settle.

 

Sounds like a bad VTC sprocket(Intake), 90% sure. Commonly characterized by the 1-2 second rattle upon start. It would rattle around 2-5k since your VTC is changing around then, and I believe the VTC turns off after 5.5k. The UOA analysis will be pretty accurate. The shop is probably just trying to cover their ***.

 

Maybe they're trying to cover it up. But, they do have a point with the settling. What material are these sprockets made of? Aluminum?

 

I have no idea, it's a brass/copper outside with a spring and silver colored helical gear inside I believe, I couldn't find the picture. The noise is pretty common in older Maxima's/300zx's though, apparently they have an older design that fails pretty often. I believe I have a bad VTC pulley(exhaust), which seems different than the VTC sprocket(intake) and I was getting silver glitter in my oil. I believe they are made of different components though, and I don't think you get shavings with the sprocket.

BTW, what's the proper settling time before sending in a UOA? After you drain the oil, what's the proper procedure to get an accurate read?

 

As long as it drained after it's been driven. No more than 20-30 minutes. From there it doesn't matter. All they have to do is shake the sample real well prior to burning it.

But, if you recently changed your oil or let it sit to long before taking the sample you will not get an accurate reading.

 

Could a bad VTC sprocket cause a random misfire issue?

 

Yes, but it would have to be pretty bad at that point.

 

Wait a minute....this might be all coming together now.....my rattle sound started when I added a Z-tube, Rev-up airbox, and 5/16ths plenum spacer. It has gotten a little louder, but not performance loss. The strange thing is I have been having a (CEL) P304 cylinder misfire here a there. Nothing changes in the way the engine performs though. The CEL usually occurs at start-up. Then, it will go away unexpectedly. Tried switching the coil packs around and still get the random misfire. Replaced the spark plugs and still get the misfire. I don't see how the misfire could be connected to a worn VTC sprocket. Can anyone explain this? Is it dangerous for me to be using my car with a worn VTC sprocket?

Thanks.

 

Those mods will have zero effect on the reliability of the engine. And yes, the VTC sprocket has a gear inside, and if that freezes or skips a tooth or whatever, the VTC will be thrown off and over adjust , temporarily throwing off your timing for a second or two which causes the valves and cylinders to get out of time along with screwing up the A/F ratio and cause a misfire.

Get a mechanics stethoscope from the autostore and listen to the VTC cover, maybe have someone start the car and source the rattle. I'm about 90% sure it's your VTC sprocket. I don't know how dangerous it is to drive with a bad one, but I would replace it rather than take the chance. With my bad VTC pulley I did not have any performance loss either.

 

Wow! Cool...thanks for the explanation. That makes sense now.

I actually did a test recently. Instead of a stethoscope, the place I went to just did an un-official diagnosis with a long screwdriver. The sound is coming area behind the timing chain cover, exactly where the sprocket is located. Is this the VTC sprocket that you are talking about?

I was not charged for the quick diagnosis. They were the ones who were almost certain it wasn't my bottom end going out. They were fairly sure it had something to do with my timing chain area. These guys are not VQ specialists so they do not know they engine well, but they are known for building race engines for race cars. They are also the ones that recommended doing the oil samples as well. They did not charge me for this either.

In regards to the VTC sprocket...how easy is this to change? I thought it might just be better to do the entire timing assembly and replace the other sprockets as well. Aren't there 2 sprockets? Is there another sprocket down below as well?

Thanks

 

I was able to hear mine pretty clearly through the VTC solenoid, the brass cylinder on top of the timing chain case. I'm not sure what causes the rattle exactly, but it should be pretty apparent. If you're not 100%, I would get a second opinion just to make sure. It's not hard to change, but it would be somewhat expensive if you had someone do it. I've heard of people cracking the timing chain case open and replacing for around 6-700, but typically the price would be 2-3k. I'm waiting on the parts and going to DIY.

The sprocket is behind a cover, should be pretty obvious as to what it is. There's 4 on the engine, 2 intake and 2 exhaust. The problem one is the intake on Non-Rev ups. Here's the DIY for removing the timing chain and the sprocket:

http://my350z.com/forum/engine-drive...ing-chain.html

And a general idea of some stuff to remove to make the job easier. This is where a lot of the labor comes from, fairly easy stuff like pulling the bumper, support, radiator, etc. There's more on like post 9 or so.

http://my350z.com/forum/engine-drive...y-removal.html

The sprocket should be around $400 for the part, unless you can find a used one for around $100. Also, you can find the factory service manual by googling G35 FSM

 

Just to elaborate on the oil samples. I would only use a plastic or glass container. If you use a metal container, oil analysis units can actually pick up the material of the container.