cold start, engine tapping/slapping, possible cause

OK, So if you search, chain slap, cold start, ticking, or a combination of these, you will read about an engine tapping/slapping some of us get when we start a cold engine. It lasts only seconds, maybe 5 or 10, and usually fades in, then fades out. I would best describe it as a dull muted slapping sound. Anyway, its well documented here in several threads. Most believe it to be chain slap due to worn chain tensioners and/or chain tensioner guides. And as the oil has all dripped from these components as the cars sits, it further attributes to it. So today I finally went to fix something I had noticed some months ago. There is an aluminum coolant pipe that runs in the front to back direction pretty much underneath the coolant tank reservoir. Probably 2 or 3 inch diameter. You can't miss it. Peering in from the engine compartment you can see it without removing anything. It runs forward until it disappears around a bend. It connects to a rubber coolant hose as it runs underneath the tank to the back of the car and turn towards the center of the engine, then the rubber hose goes downward. So that's the pipe I'm speaking of. Using a flash light, you can look at the aluminum section that runs towards the front of the car under the coolant reservoir. There should be a plastic brace that encircles this pipe about a foot forward of the tank. If you do not see it, it has broken off. Mine is broken off. It would be really difficult to replace it without removing the tank which is why I haven't fixed mine. The brace really prevents the pipe from moving side to side not so much up and down. I know this as I have grabbed it and wiggled it. Grab yours and try moving it from side to side, if movement, brace is broken. It has always bothered me that it was free to slide back and forth several inches without this brace. I don't know what it attaches to as it goes forward to the front since it disappears out of sight. But worst case I figured it may be slowly working its way out from where it attaches, and that can't be good. So today I used a velcro strap to temporarily secure this pipe until I remove the tank for some reason and replace the brace. Well, so what is all this leading to you ask? My car has always had the ticking/slapping at cold start. Been doing this for the year and a half I've owned it. It bugs the sh.t out of me and have done much research. Well to my surprise, having done nothing else to the car while I was in there fitting this velcro strap to secure the pipe, the ticking/slapping has stopped. As in, stopped the first cold start since securing this pipe and every cold start for the last week or 2. You can imagine my delight that it might have been something as simple as a pipe tapping as the engine started up cold. My working theories are as follows: 1) that perhaps this pipe is empty when the car is cold and the coolant has flowed out and it vibrates more when the car is first started since it is lighter due to having no coolant in it. Now this assumes there is no coolant in it at cold start. I am assuming it doesn't, but I could be wrong. I'm sure someone can confirm or deny this. If I'm right then when the car is warm and the pipe is full, it doesn't vibrate, therefore no ticking/slapping upon warm or hot start up. 2) that it is depressurized at cold start up and the rubber hoses that attach on each end of this aluminum section are loose and allows the aluminum section to vibrate. As the rubber hoses it attaches to on each end pressurize and become firmer and more rigid, it doesn't allow the pipe to vibrate. If it takes anywhere from 5 to 10 seconds for this coolant system to pressure, this would explain why it stops quite quickly. If I'm right then it takes some time for the system to depressurize and therefore no ticking/slapping upon warm or hot start up 3) it has absolutely nothing to do with me securing this pipe and just happened to correct itself on the first cold start and every cold start since. Stranger things have happened. So, I've taken the time to put this out there for those of you who have this cold start noise. It could be your issue. If your brace is missing, have someone hold the pipe as you start it cold. If ticking/slapping gone, your welcome. Then find a way to secure it permanently. I am very delighted that my car no longer ticks/slaps at cold start ups. It stressed me out every time I started it cold. I am going to leave it as is for 10 or so more cold starts, then I will remove the velcro strap and see if the ticking/slapping reappears on cold starts. I'll report back once I do this. If this is clear as mud to anyone having this issue, I can take pictures or you can PM me with questions.

Thanks for your posting. Mine has a slight slap on cold start up, but for no more than 2 seconds.

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That's about what mine did. Check your pipe for the brace. Have someone hold it while you start from cold. Let me know what happens. By the way, we have spoken on the phone several times. I had those coil packs available, which you decided against me shipping you. We chatted several time thereafter.

That's pretty interesting, will have to check it out on my car. Here's a picture of the coolant pipe that you are referring to when I replaced the tank. Are you referring to the clamp seen in this picture or one forward of this? This clamp is pretty much below the tank, where you say it's approximately 1 foot forward of the tank.

Great picture. That's the pipe and you can see the black plastic brace that holds the pipe. Without that brace that pipe is free to rattle around. When it is filled with coolant it is heavy and can really slosh around. My ticking/slapping at cold start has disappeared since I secured mine.

Sorry I am slow... but which color arrow are we talking about here? Waterloo, hope you dont mind I re-use your picture.

Green...

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I am honorrd! Rock star status.

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Since we have the same engine noise, I wonder when we might try this one, lol?

So last night I parked the car in my main garage so I could try this in the morning. Just got back from testing this theory. You know what happened? The cold start knock went away!! It does not look like my pipe clamp is broken, you can feel the metal part of the pipe and the clamp just below the coolant expansion tank (it's pretty much directly below it) if you put your hand to the outboard side of the air intake plastic piece that feeds the airbox and get it down below the tank. I felt around before starting the engine, nothing seemed loose. I double checked that the belt wouldn't get me before starting the engine, started it, it had the knock for a few seconds until I could get my hand down to the pipe again and then I exerted force on it in the outboard direction and the noise went away. I took my hand out after about 10 seconds and the knock never returned at idle, ran it for about 3-4 minutes and then shut it off. Normally it would keep knocking for 1-2 minutes (but now I just drive away and it doesn't happen after it warms up just a few seconds). Somebody else has to do this.....I will repeat again tomorrow morning. If it's just this pipe knocking until the forces on it are reduced due to coolant flowing or something (could be some slight water hammer, dunno).....that would make me sleep better at night!

And here is the other thread with the video: https://rennlist.com/forums/996-foru...old-start.html

The noise starts around 45 seconds in: It appears I can't figure out how to post the video.....it's in the link above.

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You Tube

I checked mine and did not try this because my clamp looked fine, but I will definitely be trying this today! This is insanely great news for some of us. It is too bad, there was a guy a few months ago who I believe sold his car after this sound occurred on morning starts. Could have just been a simple fix all along!

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And here is mine, too: https://m.youtube.com/watch?v=Z5IrhKTYEcc Wow, I cannot wait to try to see if it's the hose knocking instead of a rod or piston, lol

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You Tube yours sounds a bit more internal, constant, idk

Mike....I will be very interested to find out your results and I am looking to re-test tomorrow or later today considering I only ran it at idle for about 3 minutes. If you want me to come over and push on the hose so we can get some better data, let me know.

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Hey Paul, I'm the OP. I was hoping a few others who were concerned about this issue would try securing the pipe to see if the start-up noise went away. I am going to unhook the make-shift velcro strap I now have securing mine and see if the noise returns. But unfortunately I had to put down my dog last week, a 16 1/2 year old shih tzu. I just haven't been it good spirits to do much anything since. But I will get around to test mine soon and report. Let me know how yours goes. If this turns out to be a "thing" a lot of owners will be mighty relieved.

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True - this one did sound a little different than what my car usually does. When I heard Paul's car in person, it sounded exactly like mine, I thought. I'll have to check this out.

I just snapped 2 photos. Both are with the snorkel removed. The one with the arrows: the single tip arrow is pointing to the rubber hose that attaches to the aluminum pipe that then runs rearward. The double tipped arrow shows the make-shift velcro strap that I have holding the pipe secure. It wraps around the rubber portion then around the metal bracket of the SAI pump. It more less prevents the coolant pipe from moving side to side. If you look close I also have a plastic clip tie holding the straps down so it does not slip up and off the pipe. This was the easiest way to secure it temporarily without having to actually get back in there and replace the OEM clip which is actually on the aluminum portion. If your clip is completely missing, you can grab where I show the single tipped arrow and pull left and right, and the pipe will move several inches from left to right. As you do this look back at the aluminum portion and you will see it moving sie to side. FYI. The single tipped arrow is a good place to hold the pipe (the rubber part) when you cold start it. If you hold it here instead of actually reaching back and grabbing the aluminum portion, just make sure you push towards the left side of the car. It's not really an up and down motion that this pipe has when it rattles, it's side to side. The other photo simply traces the route of the velcro strap. Sorry in advance if the photos are rotated... Also, Paul, if I understand correctly, yours may be rattling even with the clip intact. Maybe the case that the clips look intact, but have become loose and not supporting the pipe as it should. Just a thought. Best test is to have a competent friend grab the pipe at start up as the slapping/tapping is occurring and see if it stops abruptly.

On the MkII engines, there's a clip that holds down that big house. not sure if you have that on the 3.4L and whether the clip will help. The big clip on the left head (bank 1) The clip holding the fat coolant hose.

Yes, that rubber portion but not the portion that goes downward. The upper portion that travels to the left and attaches to the aluminum portion that then turns and runs forward to the front of the car. See the photos in earlier posts. #8, green arrow.

Gotcha. I just couldn't tell from your photos in post #20 whether that big clip is missing and wondered if it would help.

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Cool. Even if that clip is missing, given the hose is rubber at that portion it wouldn't make much noise since it's rubber. The aluminum section will if it gets to dancing around.

Good finding btw. I'm sure some can benefit from it.

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maybe just a hunch, but when I did the 30k service on my car I paid special attention to reinstalling the heat shields (I had the bumper off as I was installing aftermarket muffys) when I think about the construction of those shields and the sounds they made when being handled yours could be just as simple as that, a loose heat shield why not start simple ?

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Thanks, but we haven't yet confirmed a correlation. As I mentioned earlier, my car has made this noise upon cold start since I've owned it (going on 2 years and 8K miles, many cold starts.) Once I secured this pipe, noise gone and yet to return as of about 10 cold starts. I am going to undo the velcro strap and see if the noise returns, but haven't yet done so. When I do, and if the noise returns, well then we have correlation.

Just did another test....had the knocking noise, put my hand on rubber coolant pipe (which I had put a tie wrap around the factory connector between it and the smaller pipe) and it made the noise for just a few seconds and I can't tell if anything I did made it go away or if it just wasn't "cold" enough from this morning. I only idled it for about 3 minutes this morning. You Tube

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Paul, next time you try this, remove the snorkel so it is out of your way and grab the pipe on the rubber portion (post #20, bottom photo, single tipped arrow). It connects to the aluminum pipe in question. Best way is to push it to the left which traps the aluminum pipe against the side of the car and holds it tight. As soon as you do this the noise should stop immediately if yours is like mine. Squeezing your fingers in like you do in the video is not as effective. If your clip is totally missing you should also be able to push it to the left and right. You will feel about 2 or 3 inches of travel. I believe it is the side to side motion that is of concern not so much it bouncing up and down. Given our engines are flat sixes the vibration is more side to side. Then once the pipe pressurizes and the rubber connector pipes stiffen the aluminum portion is stable enough and the slapping/tapping stops.

So I did another test this morning, I have to take back what I said yesterday.....the pipe seemed to have nothing to do with the noise. I think it was just so warm out that the noise went away so fast at idle, like less than 30 seconds. If it's cold outside, say less than 60 F, it continues to make the noise longer. That's a bummer. You Tube

Bummer. Did you check if the noise is coming from the cats? It will help if you jack up the rear of the car and have a mechanics stethoscope ready to poke around when starting it in the morning. Nice s8. I feel they're always under appreciated.

Yes....I will have to try the cats, you have written that before. With the warm temperatures outside, it only lasted something like 15 seconds if that. If it's 50F outside, it will last at least a minute if not two or three. But if you just drive it a block, it usually puts enough heat into it so it goes away on idle, and if you just rev it a little bit, it also goes away. It sounds like mechanical looseness to me. That is why on my last Blackstone report that copper went from 6 to 7 to 8 (last three reports) I am freaking out. But on each report, they had more and more miles, with the last one being the most miles. I was going to do an oil change at about 4,000 miles and see what I get. BTW, I bought a stock air box the other week and I will be installing it, replacing the K&N intake. Think this is a good move.

Definitely try a mechanics stethoscope on the cats first then. No need to jack up the car. Cat rattling sounds very similar to engine knock because the loose cat element is knocked around inside by the exhaust gas impulse, which correlates with engine rpm).

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Don't freak out. The likelihood that all of us 996 owners with these cold start noises have bad bearing or bore scoring, etc. is low. Besides, the real trouble is when it makes noise when hot.

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Oh well, there goes my Nobel Prize in the field of "cold start noises." Had my acceptance speech ready...

I pulled the serpentine belt off and verified that the cold start rattle on my car is still there. Kinda sounds like chains - maybe tensioners.