Knocking Sound When Accelerate And Cold Start

[vissine]

I have been searching around but haven't found a right answer. It happens to my 96 LS400. Every morning, even in summer, when I start up the engine, drive and accelerate up to about 1700 rpm and I can hear the knocking sound from the engine (somewhere behind the air vent) the the power was not great. My AC and fans are off. The noise continues for about less than 10 minutes and gone after that, and the car power is back. Sometimes I let it warm up for 10 minutes, but when I drive, the noise is still there for a few minutes. I use Shell and Exxon premium gas. Sparks plugs and cables , pcv, and ECT sensor are new. Am I missing anything?

[landar]

Maybe a knock sensor is going bad? Have you checked for any engine codes?

[vissine]

I scanned the engine but no error found. I have 220k miles on it. Should I just replace the sensors anyway since they may have been out of specs?

[landar]

No, I would not just blindly replace them yet. You have to remove the intake manifold and its a pain. Instead, I would investigate a bit more. First, make sure that the sound you are hearing really is engine knocking. Maybe it is something else that just sounds like it. Or maybe it really is a knock or pinging (pre-detonation sound).

So, I know this might sound counter-intuitive, but what if you purposely put lower octane fuel in the tank to see if the symptoms get worse (more/louder knocking)? Or...you could buy an octane booster, add that to the tank and see if the knocking subsides somewhat. Neither would be used for long, just to get an idea if the knocking sound really is related to pre-detonation. Real pre-detonation can be harmful (in the long run) to your engine. It is very hard on the bearings, rods, pistons. How long have you owned the car and how long has it been making this sound?

[vissine]

I got this car about 2 years ago and it started to make noise about 7-8 months back. I have not tried using lower octane but I will try it next time to see what's going on. I recently added Chevron Techron but it didn't help much. The recommendation is every 1,000 so I will try it a few times next few thousand miles.

Do you think if the timing belt have anything to do with the sound? I am going to check the timing this weekend to see if there is any problem there.

BTW, I notice there is a leak on the rear of right CAT and plan to replace all the gaskets on both CATs this weekend.

[vissine]

Yesterday, I managed to replace all four seals before and after both CATs (passenger and driver sides). However, when I finished installing the gaskets, I notice that both flange gaskets (rear, the ones with 2 bolts,between the CATs and the y connectors) have open on the top side, meaning the tops are not closed and air is leaking out. Is there any suggestion what I should do to close them?

I found this kind of gasket that may work for my case, I will try and let you know.

BTW, the noise is still there.

[vissine]

Just to let you know that I've got the problem taken care of. I replaced all 8 spark plugs and the problem went away. It doesn't make sense to me because I replaced them last year.

[landar]

So what brand and model number were the old plugs and new plugs?

[vissine]

Iridium BKR6EIX-11. I was told not to gap them so I assumed the old plugs were gap correctly.

[landar]

Iridium BKR6EIX-11. I was told not to gap them so I assumed the old plugs were gap correctly.

Are these the old plugs?

[vissine]

Iridium BKR6EIX-11. I was told not to gap them so I assumed the old plugs were gap correctly.

Are these the old plugs?

No, these are new plugs. The old ones are BKR6EIX.

[landar]

So, both old and new plugs are NGK? Those are supposed to be good plugs. I have heard of some having problems with Bosch... but NGK Iridiums are fine.

You generally do not need to gap new plugs but you should do a quick sanity check to verify that the gap looks approximately correct. You never know if a plug got damaged somewhere along the way.

[Crazeecreed]

On 9/8/2014 at 6:04 PM, landar said:

Maybe a knock sensor is going bad? Have you checked for any engine codes?

 

Kmock Sensor will not cause a knocking noise. The sensor is for picking up knovk noise from engine so computer can adjust as needed.