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Discussion Starter #1
Hello,

2000 GLS Passat Wagon 1.8t 5 speed manual

On cold start up for the first minute or so the idle is extremely rough- bouncing from 600-1400 rpm. Engine sounds like it is struggling to run and could die any minute. Also will stall unless you rev up to 2000 rpm before trying to release clutch. After about a minute or so the idle steadies itself and everything is normal. Symptom has been there for last 3-4 weeks (have driven around 5000 miles since I first noticed it).

Replaced all coils and plugs in June.

Work done 2 weeks ago: I had a cracked SAIP outlet hose which was replaced. Intake manifold and throttle body cleaned. PCV hoses replaced with 034 Motorsports kit. PCV valves replaced. Vacuum lines redone in silicone.

No engine codes (no VAGCOM but regular code reader), no flashing CEL.

How should I start diagnosing this?

Thanks in advance
 

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Discussion Starter #2
To clarify, cold start meaning the engine has been sitting overnight, not necesarrily below freezing ambient air temp.
 

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One suspect would be the CTS (coolant temperature sensor). There are 2 sensors in the part; one output is used for the gauge in the dash the other is used by the ECU to control the air/fuel mixture. It is not uncommon for them to fail. Can you read the temperature supplied to the ECU with your reader? VAGCOM/VCDS can do this so it may be available on your reader if you can find the right menu, etc.
 

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Pretty sure I have seen this on my reader. Is the only reading out of the OBD II port the ECU reading? Or do I need to pick between the dash reading and the ECU reading?

In terms of the reading itself, what exactly should I be looking for?
 

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If the sensor isn't reporting ambient temp range on your scanner when the engine is cold, then the sensor is probably bad.
 

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Pretty sure I have seen this on my reader. Is the only reading out of the OBD II port the ECU reading? Or do I need to pick between the dash reading and the ECU reading?

In terms of the reading itself, what exactly should I be looking for?
With VCDS I think you can read either one based on which output is choosen. Check on that though since my VCDS has been down for awhile and I'm working from memory. Also, the output in most likely in degrees C so keep that in mind.
 

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What should I be looking for? Couldn't find anything after a quick search
A failed combi valve will either throw a SAIP code or cause an exhaust or vacuum leak.

If it fails stuck open, the exhaust gases will flow back into the SAIP when the pump isn't running. This can cause moisture to accumulate there and ruin the pump motor.

If it fails stuck shut, then you'll get a SAIP CEL code since the O2 sensors won't see the extra air the pump supplies to the exhaust stream.

If it has a vacuum leak all the normal vacuum leak symptoms would show up.
 

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Did you adapt the throttle body after cleaning it? How was the intake manifold cleaned? If removed, did you use new gaskets? While I also suspect the CTS, it's best to check everything.
 

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Discussion Starter #12
Did you adapt the throttle body after cleaning it? How was the intake manifold cleaned? If removed, did you use new gaskets? While I also suspect the CTS, it's best to check everything.
Wow it's been a while.

Throttle body was adapted after cleaning. I left key in ignition for ~1 minute, heard the throttle body whine and stop whining. Then started.

Intake manifold cleaned with engine degreaser, brake cleaner, carb/engine cleaner, purple power, and a lot of water to rinse it out. I had to scrub for ~2 hours with a toothbrush to get it clean.

New gaskets: intake manifold to block and throttle body to intake manifold.

I will try to diagnose the CTS this weekend.

Thank you again for all your help.
 

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I would connect the VCDS and see if this only happens when the air pump is running and what the CTS is reporting. Since you only have a code reader, you would need to find someone with one or buy the cheap version. The SAIP runs when the combi valve is open, so a blown combi can cause a vacuum leak. You can test it with a vacuum pump/gauge combo to see if it holds vacuum. The CTS is more likely and would cause hard starting now that temps are cooler. Again, VCDS will show the exact temp the ECU is getting from the CTS. I've had 2 fail recently (out of 3 Passats) and they both read about 248F when cold.
 

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Discussion Starter #14
I would connect the VCDS and see if this only happens when the air pump is running and what the CTS is reporting. Since you only have a code reader, you would need to find someone with one or buy the cheap version. The SAIP runs when the combi valve is open, so a blown combi can cause a vacuum leak. You can test it with a vacuum pump/gauge combo to see if it holds vacuum. The CTS is more likely and would cause hard starting now that temps are cooler. Again, VCDS will show the exact temp the ECU is getting from the CTS. I've had 2 fail recently (out of 3 Passats) and they both read about 248F when cold.
Thank you. I will start looking around for VCDS or maybe I will just purchase it.
 
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