rattle on startup

March10k

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Volkswagens-for-life

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March10k

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Only when the engine is cold, and only when the AC is on. I hadn’t considered the chain, I was thinking the belt or something the belt touches, but you’re right, that is a possibility. Why would the fact that it goes away when the engine warms up lead you to the chain, though? I would have thought that more likely if the noise was present at idle, rather than under load (and only with the AC on).

I’m going to try some belt dressing as a diagnostic step. If that temporarily fixes the problem, then a new belt should be the permanent solution, right?

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March10k

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Qmulus

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How long does the rattling last? How cold does it need to be? It is easy to troubleshoot if it is the AC compressor as you just remove the belt and start it up. If it makes the noise, it is the timing chain and in my experience you don’t have long before it fails and takes out the head. If it is quiet, then re-install the belt and start again. If it rattles then, your compressor is failing. These compressors don’t really have a clutch, but the compressors are known to fail. Not nearly as often as the timing chains though.

Why does the chain rattle when cold? Because the tensioner is hydraulic, and pressurized with engine oil. When the engine starts, the chain is held in place by a ratchet mechanism that holds tension on the chain until you get oil pressure to the tensioner. These chains need a lot of tension on them to keep from jumping. If the chain is rattling, either the ratchet mechanism is at the end of its travel due to chain wear, or it is failing/has failed. You don’t mention the year or mileage. Late (’13+) tensioners do not fail, but the chains are still susceptible to wear. The early tensioners can fail or the chain can wear enough to jump. If you have 85k+ on the engine and do 10k+ oil changes, I would be guessing your chain is about done. Most of the failures that I have seen on these happen right around 90k miles.

March10k

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How long does the rattling last? How cold does it need to be? It is easy to troubleshoot if it is the AC compressor as you just remove the belt and start it up. If it makes the noise, it is the timing chain and in my experience you don’t have long before it fails and takes out the head. If it is quiet, then re-install the belt and start again. If it rattles then, your compressor is failing. These compressors don’t really have a clutch, but the compressors are known to fail. Not nearly as often as the timing chains though.

Why does the chain rattle when cold? Because the tensioner is hydraulic, and pressurized with engine oil. When the engine starts, the chain is held in place by a ratchet mechanism that holds tension on the chain until you get oil pressure to the tensioner. These chains need a lot of tension on them to keep from jumping. If the chain is rattling, either the ratchet mechanism is at the end of its travel due to chain wear, or it is failing/has failed. You don’t mention the year or mileage. Late (’13+) tensioners do not fail, but the chains are still susceptible to wear. The early tensioners can fail or the chain can wear enough to jump. If you have 85k+ on the engine and do 10k+ oil changes, I would be guessing your chain is about done. Most of the failures that I have seen on these happen right around 90k miles.

It’s a 2014 with about 60k miles. I live in Honolulu, where it’s never under 70 degrees. The rattling may come and go several times, but I can’t recall it ever being present more than 2-3 minutes after startup.

Correction: I was so focused on the rattle at startup, I forgot it also happens long after the car is warmed up. It’s louder and more sustained when the engine is cold, but I also get the rattle for like half a second or so with a warm engine. Basically any time the car is under 10mph and 10% throttle or less. So pulling away from a stop sign, I get that half second burst before the throttle or the speed cuts it off. I have managed to make it last longer by barely touching the gas pedal so it takes a few seconds to get up to the

10mph where it stops no matter what the throttle is doing. But under normal driving conditions, you just get that half second burst of a rattle as you quickly throttle up past the 10% or so where it cuts off.

I really appreciate you engaging with me on this.

Qmulus

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So, you are in a pretty much perfect climate where the temperature never goes much above 80 or less than 70. AC runs all time when you drive. Are you looking to repair this yourself or just trying to figure things out? I you had it checked for fault codes? Often codes can be set without turning on the check engine light. Unfortunately, your description is not terribly clear to me. It sounds like low RPM right when you tip into the throttle. A «rattle» under those conditions can be a lot of things. Other than what has already been mentioned, I can’t think of any other common issues so «internet troubleshooting» is not really the way to go here. It needs to be inspected and troubleshot. You can try taking videos and posting, etc., but honestly that is probably wasted time. If you do get it to the component, do you have the tools to repair it? Working on the HVAC system takes specialized tools, and compressor failures can cost a lot of money and they often blow metal through the system, requiring everything in the system to be thoroughly clean or replaced. That said, until it is properly troubleshot, I wouldn’t make any more speculation.

I would check it with a mechanic’s stethoscope or long screwdriver up to your ear. It is amazing what you can hear in the engine. Be very careful around the belt, pulleys and fans.

My recommendation would probably be to take it to a shop that works on VWs.(maybe Tai’s VW & Audi from the reviews? — no connection with those guys, it just came up in a search) and have them take a look. Chances are the issue would be quite obvious. I prefer a good independent over a dealer when you are out of warranty.as they tend to be just as knowledgeable, and less likely to upsell you.

March10k

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So, you are in a pretty much perfect climate where the temperature never goes much above 80 or less than 70. AC runs all time when you drive. Are you looking to repair this yourself or just trying to figure things out? I you had it checked for fault codes? Often codes can be set without turning on the check engine light. Unfortunately, your description is not terribly clear to me. It sounds like low RPM right when you tip into the throttle. A «rattle» under those conditions can be a lot of things. Other than what has already been mentioned, I can’t think of any other common issues so «internet troubleshooting» is not really the way to go here. It needs to be inspected and troubleshot. You can try taking videos and posting, etc., but honestly that is probably wasted time. If you do get it to the component, do you have the tools to repair it? Working on the HVAC system takes specialized tools, and compressor failures can cost a lot of money and they often blow metal through the system, requiring everything in the system to be thoroughly clean or replaced. That said, until it is properly troubleshot, I wouldn’t make any more speculation.

I would check it with a mechanic’s stethoscope or long screwdriver up to your ear. It is amazing what you can hear in the engine. Be very careful around the belt, pulleys and fans.

My recommendation would probably be to take it to a shop that works on VWs.(maybe Tai’s VW & Audi from the reviews? — no connection with those guys, it just came up in a search) and have them take a look. Chances are the issue would be quite obvious. I prefer a good independent over a dealer when you are out of warranty.as they tend to be just as knowledgeable, and less likely to upsell you.

Thanks for the advice. Tai’s is a good shop, and I do prefer an experienced independent to a stealership. The latter will just replace parts at random until the noise stops. Actual diagnostic and troubleshooting steps (checking the alignment of the pulleys, frinstance) would never occur to them.

I did figure it out, though. I put on some belt dressing last night. This morning, no rattle. So it turns out to be the cheapest fix ever: a new serpentine belt.

I remember another cheap one. Google «pink thingy replacement.» Chrysler cheaped out on the shift interlock in my SRT8, making it out of pink plastic. Guaranteed to break in a few years. The billet aluminum replacement cost me $25 and 45 minutes. The Stealership would have replaced the whole shifter for $750, of course.

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