legacycentral bbs

So I have developed a slight knocking sound during cold start. Kicks in right around 1200-1300 rpms and sticks around for about 20-30 seconds then stops (or until rpms drop under 1000). Only happens after the car has sat overnight. Even when it sits while i'm at work (9-10 hours) it doesn't do it when i start it to go home. Only in the morning after it has sat over night.

Mods are in my sig, I use Mobile 1 synthetic 5w-30 and OEM oil filters.

Thoughts?

Does it sound like a tick or a knock? How many miles are on the engine since it was rebuilt?

Any idea what your oil pressure is when the knock/tick occurs?

definitely more of a knocking than anything. Motor has ~25,000 on it since the rebuild and oil pressure is 70-80psi (oil pressure on cold start is 90-95 psi and then drops off to warm idle at ~ 18psi. Cruising at 60mph pressure is usually 50-60 psi)

Hmm....oil pressure sounds good.

I know forged pistons sometimes are noisy on startup until they warm up and take up the clearance. I'm not sure why you're just hearing them now.


What part of the country do you live in? Does your car sit outside at home and work, or do you park in a garage? What's the daytime and overnight temperatures like?


A compression and leakdown test would likely give you some additional information on what the exact cause might be.

Have you always had the same brand and viscosity (spelling) on the oil?

5w-30 sounds a bit thin (for my taste) but i'm no oil expert.

I have a knocking on cold start, from my pistons, more noticeable under engine load. But you should have had that sound since day one from the engine build i think.

Live in Washington, car is outside during the day and night. Temps are in the mid 50's at night and 70-80 during the day right now.

Might try a compression test this weekend and see what it tells me.

Yeah, let us know what you find out with the compression test.

Haven't been able to do a compression test yet, i've been crazy busy the last few weeks.

Regardless, what do you guys recommend for oil, should I switch brands/weights? I live in seattle so our climate stays pretty temperate except for a few spouts of cold during the winter.

You could try a conventional 10w30. I use the dino Mobil oil 10w30. The clean 5000 stuff. Since you're using mobil 1 now, I'd suggest trying to stick with the same brand since they should have similar additive packages.

I too run 10w30. If you look in the factory service manual it recommends a 10w30/40 weight for our climate here in washington.

Josh,

You would recommend going from synthetic to dyno? Everything I have ever learned says that is a no-no.

The main reason not to switch between syn to dino is the seals. If your seals are relatively new, you should be ok. If you have some leaky seals, then they'll probably leak more.

You could try going to a 10w30 synthetic to see if it makes any difference.

UPDATE: Replaced the valve cover gaskets this past weekend (had a slight leak on the pass side and I hate oil leaks so decided to do both) and did an oil change to 10w30 mobile syn. As of 2 days of morning starts, no piston knock anymore so I think the simple fix of changing oil weight did the trick.

5 weight oil is for regions that see pretty cold temps such as below 0 I believe.

For the tolerances that are on these engines, yeah 10w30 is probably the preferred weight oil.