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Discussion Starter #1 (Edited)
Hello everyone,

Joshthenotsogreatmechanic here, and I am having issues with my people's car. My passat has 190k miles on it, and I recently had a coolant leak which required a coolant flange swap. After this I started getting p0302 and p0300 codes when first starting my car

Edit: the misfire goes away after running for a bit. Usually less than a minute.

What I've done so far is:
Coolant temp sensor
Ignition coil swap
Spark plug swap
Fixed cracked wire and ground on cylinder 2 in harness and went as far back as I could on top of the engine looking for other cracks.
Compression test all around the 140-150 area
Also replaced a broken vacuum check valve near the firewall.

I cant think of anything else. I dont have an icm or a cable throttle body if that makes sense.

Please help me. I'm losing my mind, and I don't know what else to check.

Any information from the volkswagen community is appreciated. Amateur mechanic, so please be gentle.

Thank you all! Glad to be here.
 

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Does the misfire go away when engine is warm?

Did it run hot?

What octane gas are you using?

Vacuum leaks can cause misfire. I would do a smoke test.


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Discussion Starter #3
Hey Axenstar! It does go away when it gets warm. I'll toss that in the original post if I can. I've never done a smoke test. Pretty simple procedure? Some one has probably done it on youtube I imagine?
 
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Discussion Starter #5
You could try swapping injector #2 with one of the others, and see if the code follows.
Hey Tomvw! That's a good idea, and you reminded me that I did that as well. I am at a total loss with this thing. That was one of the first places I decided to start since the plugs and coils didnt work.

This is such a weird issue, and I think about selling this thing to strangers regularly lol
 

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I would check the wiring for the coolant temp sensor. I expect it is cracked and causing a bad reading, or the sensor itself has failed when being removed.
 

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1998 Passat GLS Wagon AEB , 2003 Ford Ranger Edge
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I would check the wiring for the coolant temp sensor. I expect it is cracked and causing a bad reading, or the sensor itself has failed when being removed.
I second this, its common for the coolant temp on the rear flange to throw misfires when bad especially when not at operating temp.
 
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Discussion Starter #8
I would check the wiring for the coolant temp sensor. I expect it is cracked and causing a bad reading, or the sensor itself has failed when being removed.
T
I will give it a looksy then. I'll check it after work and let you all know what I find. Would it throw a code for the sensor though?
 

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A bad sensor won't throw an explicit code. If one of the wires was shorted to ground or something that might throw a code but I've never heard of it being reported. Misfires due to bad temp reading will be logged and I think that could eventually throw a code.

You can read the sensor output directly if you have VCDS and that is great way to see if it's working correctly.
 
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Discussion Starter #10
A bad sensor or bad sensor wiring won't throw an explicit code but if there misfires due to bad temp reading, they will be logged and could eventually throw a code. However, you can read the sensor output if you have VCDS and that is great way to see if it's working correctly.
That's a good idea. I didn't think about checking it that way. I do have vcds lite. That should work though. Am I just looking for a temp reading? Every once in awhile my car tells me to stop because of a coolant issue even though it looks fine. Do you think that might be related to the cts?
 

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There are 2 temp outputs from the CTS since there are 2 sensors. One output goes to the dash display and the other is the one that is sent to the ECM to control the fuel mixture. I'd check both but the one that matters is the one to the ECM. It will be a measurement block and you can find the details at the Ross-Tech website or elsewhere on the web.

As far as I know, the STOP message is from the sensor at the bottom of the coolant tank. I suppose it's possible that if the ECM thought the coolant temp was too high it might also trigger a stop message but I've never seen that and I had a 1.8t with a bad radiator that would peg the coolant temp on occasion if you weren't careful.
 
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Discussion Starter #12
There are 2 temp outputs from the CTS since there are 2 sensors. One output goes to the dash display and the other is the one that is sent to the ECM to control the fuel mixture. I'd check both but the one that matters is the one to the ECM. It will be a measurement block and you can find the details at the Ross-Tech website or elsewhere on the web.

As far as I know, the STOP message is from the sensor at the bottom of the coolant tank. I suppose it's possible that if the ECM thought the coolant temp was too high it might also trigger a stop message but I've never seen that and I had a 1.8t with a bad radiator that would peg the coolant temp on occasion if you weren't careful.
Oh boy. I figured that stop was from the coolant tank, but I feel like you can never be to careful with these things Haha anyways! I will check it out once the rain stops here.
 

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Not to put a worse case scenario on the table, but there is a possibility that coolant is getting pushed into cylinder #2 as the vehicle cools down, which would cause a single cylinder misfire at startup due to the coolant being burned off in the cylinder on the compression stroke and well the car is running the coolant evaporates at a faster rate therefore it doesn’t effect the drivability once the vehicle has been running for a short while. There are several ways to find out if this is the case, but I found that the easiest way to check is after driving the vehicle, slowly and carefully let the pressure out of the coolant reservoir and once the car has cooled down, (preferably over night) start the vehicle and if you don’t get a misfire you are probably getting coolant into cylinder #2. I truly hope this isn’t your issue and it turns out to be bad wiring or a leaking fuel injector that is flooding the cylinder or one of the hundreds of things that could cause a single cylinder misfire. When you mentioned the coolant system this is the first thing that came to mind and just a thought for anyone replacing a coolant flange go billet aluminum You’ll thank yourself later. Best of luck!
 
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Discussion Starter #14
Not to put a worse case scenario on the table, but there is a possibility that coolant is getting pushed into cylinder #2 as the vehicle cools down, which would cause a single cylinder misfire at startup due to the coolant being burned off in the cylinder on the compression stroke and well the car is running the coolant evaporates at a faster rate therefore it doesn’t effect the drivability once the vehicle has been running for a short while. There are several ways to find out if this is the case, but I found that the easiest way to check is after driving the vehicle, slowly and carefully let the pressure out of the coolant reservoir and once the car has cooled down, (preferably over night) start the vehicle and if you don’t get a misfire you are probably getting coolant into cylinder #2. I truly hope this isn’t your issue and it turns out to be bad wiring or a leaking fuel injector that is flooding the cylinder or one of the hundreds of things that could cause a single cylinder misfire. When you mentioned the coolant system this is the first thing that came to mind and just a thought for anyone replacing a coolant flange go billet aluminum You’ll thank yourself later. Best of luck!
You know the worst part? I've been thinking that's the case this whole time. I truly hope not, but I am so unsure. It is such a weird issue to have, and I genuinely worry that might be the case. I hope it isn't, but to be honest I've had the feeling that was the issue for awhile, and I dont really want to pull the head off again to swap out a head gasket. That would be a serious pain in the ass. I am going to do a smoke test on it though. See if there's some leaks and also pull the harness out and look for some cracks in that as well. I can only go so far back before the firewall becomes a pain.

I appreciate your input though. You've said what I've been dreading lol
 

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That definitely sounds like a good direction to go in, I will have my fingers crossed for you. I have had a broken pcv valve give me a Single cylinder misfire and helped a friend that was having a cylinder #2 misfire for months, which ended up being loose bolts on the combi valve that was letting unmetered air into the cylinder and both of these were at cold start. So chances are you could still get lucky and not have to remove the head. Good luck on the Diagnosis process.
 
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Discussion Starter #16
I appreciate it. I really hope it is something along those lines. I can handle nuts and bolts for the most part. I will build the smoke tester and let you guys know how this pans out. Don't expect much lol
 
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Discussion Starter #18
Hey VW Forest,

Thanks for reaching out. I still haven't been able to pinpoint my issue. I am going to toss the smoke machine on it tonight and see if I can find any smoke spots. I don't believe coolant is getting into the cylinder because it doesnt always misfire when starting. Still at a loss. I am tempted to replace the entire wiring harness, but I haven't seen any write ups or videos on doing so. I imagine it is a time consuming process. I will write on here this evening about my findings. Fingers crossed.
 
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Discussion Starter #19
Update:

Alright guys, I couldn't find a leak anywhere. Tossed it back on the reader and found out that I have some new codes as well. Most likely from my harness splice. So, we've got:
P0300 - random multiple misfire
P0301 - cylinder 1 misfire
P0302 - cylinder 2 misfire
P0303 - cylinder 3 misfire
P1358 - cylinder 2 ignition circuit/open circuit
P0456 - evap system: very small leak detected

I dont really know where to go from here. I guess I'm unsure on how to replace the wiring harness completely, but that is my next move. I swapped coil 1 and 2. We'll see if the issue follows.

Any advice? This is giving me mad anxiety right now. Dang Germans. They can take this thing back to the fatherland and launch it out of a GD trebuchet lol
 

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  • Faulty Ignition Coil
  • Ignition Coil harness is open or shorted
  • Ignition Coil circuit poor electrical connection With a P1358 code these are your usual suspects, I would start by checking all connectors and pins for corrosion and make sure there are no exposed wires that could be shorting out the ignition system, also check the ignition coil ground wire it can brake at the bend near the bolt on the valve cover. These car can definitely be a pain in the a$$ to fix, but It’s always worth it in the end. I have brought my vehicle back from impending doom several times and would do it again. You got this!
 
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