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The Saga Continues


lexxus

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This is the car with the morning "knock". I changed the oil, and the knock decreased, both in intensity and length. It doesn't knock as long or as loud since the oil change. The dealer says, "Noise when first starting due to carbon buildup. Top engine cleaning needed. If noise continues, needs head job, $2800."

How could carbon make this knocking sound? And, why would an oil change have any effect on carbon deposits? And if it is carbon, why would it only cause a knock in the morning, and not later in the day?

The engine runs good, has good power. In fact, I've mentioned before, this 300 has more guts than my 400. If it had really bad carbon, wouldn't that cause a decrease in power?

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Following the "saga" -- imagine your girlfriend truly appreciates your efforts on her behalf. Hopefully will turn out all this worry for nothing!

I personally believe with symptons so far, the main issue (excuse the pun) is still main bearings. But is possible that reducing compression might help, but still need more background substance from mechanic to believe carbon build up is actual cause or solution relief. Definitely do not want to invest in head job for $2800 and still have rattle/knock problem of some sort!

MOST RECENT DEVELOPEMENT:

Latest round of issues and your immediate questions -- I am speculating what the most recent dealer/mechanic might interpret according to their findings. I know you already understand most of this, but to keep perspective here are my (non- qualified mechanic, new to Lexus engine) responses to your recent questions. We really need their detailed qualified explanations.

Your questions:

"How could carbon make this knocking sound?"

If mechanic tested and measured cylindar compression and found compression much higher than normal specifications, their educated view might be that additional compression contributes to the loose mains knocking as the higher compression puts abnormal additional force on the lower end. That additional force might make normally worn main bearings "rattle"/knock until flowing oil gets to the bearing surfaces between the bushing surface and the crank surface. Once the oil flows between the surfaces there is no metal to metal "slapping" or noise.

"Why would oil change have any effect on carbon deposits?"

Because the core issue is the excessive gap between crank and bearing surface -- therefore quicker or better flowing oil will stop the rattle no matter the contributing factors. But the point you are really making is true.

"And if carbon, why would it only cause a knock in the morning, and not later in the day?"

Same as above -- the metal surfaces are coated sufficiently until finally drains down overnight. And your point is correct that proves it seems to be more than just a carbon issue in my view as well.

"If it had really bad carbon, wouldn't that cause a decrease in power?" (In reference to how strong the engine seems.)

This may be truly a case where carbon build up over time increases compression without degrading valves to the point it does in fact increase performance of an engine as long as there is not detonation/pre-ignition. If you use 93 octane that helps the engine cope with the increased compression caused by carbon build-up. You have mentioned several times how "strong" the SC 300 engine feels and the excellent performance compared to your SC400. Remember that I commented early on we use to gap main bearings loose to lessen engine friction and that contributed to better performance for drag racing. Well in this case, it could be the engine has loose lower end combined with higher compression resulting in a stronger running car performance wise. Higher compression (whether turbo driven or carbon buildup to lessor degree) increases performance, but is harder on all components -- but as I understand the SC300/I6 engine, it has been historically designed with strong lower end to accept Supra/Soarer turbos.

In fact, you said it yourself, "... the car runs like a rocket.!"

If the mechanic feels top end needs carbon cleaned -- what are the recommendations and associated cost? Were you able to have an in-depth discussion with dealer/mechanic to understand their view and experience with this issue. They are suppossed to be the experts -- but do understand you have had several different opinions on same issue. But a sit down give and take with mechanic may be helpful to each of you to come up with definite conclusion.

BACKGROUND ISSUES:

How strong and for what duration is the "rattle/knocking" on start-up?

You mentioned changing the oil improved the main bearing rattle. What viscosity weight oil and to what extent did it make a difference/change (one/half second, one second, one and a half second etc) from engine start to rattle quit?

What specific oil filter did you use?

What is current measured oil pressure (at cold and hot) compared to what is considered expected oil pressure? If the oil pressure runs abnormally low when warm or cold, that may hint at another cause or confirm worn engine (especially main bearings).

What is experience level of the mechanic who actually diagnosed your situation this time?

What is your next step? One option is to drive and enjoy car with improved oil/filter and see if gets worse in near future. Apparently has been going on for a long time according to the prior owner's records.

Good Luck...

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Get yourself a long screwdriver. Start the car when it's cold then hold the screwdriver with the working end on the valve cover and the other end to your ear. You may have to move it around to find the loudest noise. We used these "listening" sticks at the phone company to hear when a relay operated. Make sure nothing is hanging loose that could be caught by the fan, belts, or pulleys.

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My God what a weird explanation, but it does make sense. I see the logic.

We can't get 93 octane here in CA, only 91. No, I didn't have an in depth discussion, because my girl picked the car up, and when I saw the diagnosis and the cost, I thought it was crazy. I have no way to gauge the oil pressure, but no lights have come on at any time. The oil change probably brought the seven/eight second loud as hell knock down to a three or four second somewhat muted rattle that is still unpleasant.

My mechanic says it's a lifter, and he'll replace all of the lifters and the timing belt for $950, which seems like a good deal, especially compared to the dealer. But after what you've told me, I better go to the dealer and talk to them. I'd hate to pay to fix the wrong thing.

Thanks to everyone who has responded. It's very kind of you folks to help.

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Summary:

Seven to eight seconds of "loud knocking" on start up is extreme. Glad that suggested oil/oil filter change resulted in reducing to several seconds of "rattle" instead. But your situation needs to be resolved -- if main bearings, then they are excessively worn.

Background response:

If timing belt due for replacement then at least not wasted effort with your mechanic. But, if problem is sticking hydraulic lifter, then the noise you would hear most likely would be more of distinct singular loud tapping -- not knock/rattle deep within engine. Using the excellent screwdriver "listening" technique suggested by jdowen2 you can tell if localized at top end. Seems hard to believe it is just a lifter to the extent you had "loud knocking" before changing the oil. But no harm in replacing the lifters except for the dollars.

Have you asked either dealer or your mechanic the cost to drop the crank and replace the main bearings and the oil pump? You could have the timing belt replaced at same time displacing some of the cost since timing belt due replacing anyway, and the harmonic balancer has to be removed to drop the crankshaft. If you are going to spend money at risk, I would suggest this is best place to spend it.

Considerable cost is saved if they can do it with engine still in the car -- I do not know if possible on SC.

Oil pressure -- if thorough analysis at dealer, they should have measured oil pressure given your symptons.

What viscosity oil and which specific oil filter was used to reduce the time and extent of knock?

I hope the problem turns out to be simplier than main bearings. However you proceed, will mechanic or dealer guarantee that their prognosis and your spending your money with them will guarantee fixing the problem?

Good Lexus Luck....

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  • 5 weeks later...

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