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Discussion Starter #1
We have a 1998 beetle with about 130,000 miles. The engine is misfiring all cylinders once the car warms up after 10 minutes or after it rains.
We have replaced the spark plugs, wires and coil, mass flow sensor and a few of the vacuum lines. When watching live data it looks like we lose the feed back from upstream O2 sensor and fuel trim bank 1. Then the check engine light will flash until the 2 sensors appear to come back online. Car still won't drive great but it will stop shuttering. Looking for input as to where to start looking for the trouble spot.
 

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If you CEL comes on it will store a code so you can see what it is. But misfires can be cause by a weak fuel pump, clogged fuel filter, improper spark plug gap, bad battery, or low compression. Start with the simple stuff first. Once I had a misfire and when I looked at the wire for #4 it was loose so make sure you have a good connections to each plug.
 

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Discussion Starter #3 (Edited)
I have attached an image of the codes. I just cleared the codes and when it happened again it has misfires on all cylinders. The primary code P0300 with P0302, P0303, P0304 having misfires and a pending code P0411 and P0422. Let me know of you have any ideas.
SmartSelect_20200510-211030_RepairSol2.jpg
SmartSelect_20200510-210555_RepairSol2.jpg
SmartSelect_20200510-210924_RepairSol2.jpg
 

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Discussion Starter #4
If you CEL comes on it will store a code so you can see what it is. But misfires can be cause by a weak fuel pump, clogged fuel filter, improper spark plug gap, bad battery, or low compression. Start with the simple stuff first. Once I had a misfire and when I looked at the wire for #4 it was loose so make sure you have a good connections to each plug.
I posted an image of the codes.
 

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Was the MAF replacement OEM from VW or chinese knockoff? Chinese made ones will work but will make your car run like crap. Looking at the codes I would suggest pulling the spark plugs to check the gap. When you put them back put some dielectric grease on the end to make sure they have a good connection. I would replace the fuel filter since it is cheap and easy to do. Do you have to cycle the key to start the car to build pressure from the fuel pump? You will have to buy a gauge to check that the fuel pressure. Have the battery load tested since our cars are run by computers. It might be a good idea to put fuel injector cleaner in the gas tank to ensure injectors are spraying correctly. It looks like the O2 B1S1 is working just showing its running rich. Should be close to 0%. As for the secondary air injector make sure the lines going in and coming off are not broken, cracked, loose. Make sure it is turning on when you first turn the car on.
 

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Discussion Starter #6
The MAF was knockoff. But the issue was happening before we replaced it, anytime it rained or after it was driven for 10 minutes or so. The car starts good and the battery was replace a few months ago. We also did replace one of the secondary air injectors lines.In the last year we have replace: plugs, wires, coil, MAF, one of the secondary air injector line, 3 of the vacuum lines, timing belt, water pump and battery. The issue was happening before most of the stuff was replaced. We are also having a transmission issue. Should the fuel trim sensor bank 1 always be slightly changing. When the car is misfiring it goes to 0 and stays there until it stops misfiring. The car smells flooded during this time also. Let me know what you think. Thanks.
 

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Discussion Starter #7
Was the MAF replacement OEM from VW or chinese knockoff? Chinese made ones will work but will make your car run like crap. Looking at the codes I would suggest pulling the spark plugs to check the gap. When you put them back put some dielectric grease on the end to make sure they have a good connection. I would replace the fuel filter since it is cheap and easy to do. Do you have to cycle the key to start the car to build pressure from the fuel pump? You will have to buy a gauge to check that the fuel pressure. Have the battery load tested since our cars are run by computers. It might be a good idea to put fuel injector cleaner in the gas tank to ensure injectors are spraying correctly. It looks like the O2 B1S1 is working just showing its running rich. Should be close to 0%. As for the secondary air injector make sure the lines going in and coming off are not broken, cracked, loose. Make sure it is turning on when you first turn the car on.
The MAF was knockoff. But the issue was happening before we replaced it, anytime it rained or after it was driven for 10 minutes or so. The car starts good and the battery was replace a few months ago. We also did replace one of the secondary air injectors lines.In the last year we have replace: plugs, wires, coil, MAF, one of the secondary air injector line, 3 of the vacuum lines, timing belt, water pump and battery. The issue was happening before most of the stuff was replaced. We are also having a transmission issue. Should the fuel trim sensor bank 1 always be slightly changing. When the car is misfiring it goes to 0 and stays there until it stops misfiring. The car smells flooded during this time also. Let me know what you think. Thanks.
 

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Yes the O2 S1B1 does fluctuate to be as close to 0 as possible. Check your EGR valve. Make sure it holds a vacuum and isn't all carboned up clogging it. Recheck vacuum lines and make sure no air is coming in. Listen for hisses, spray soapy water look for bubbles, or use carb cleaner. If your car is smells like gas look to see if your injectors are leaking. Gas cleans well and should be easy to spot. Use a long screwdriver and listen to see if your injectors are firing consistently. You might want to put a new fuel pressure regulator on since you have a 99.
 

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Discussion Starter #9
Yes the O2 S1B1 does fluctuate to be as close to 0 as possible. Check your EGR valve. Make sure it holds a vacuum and isn't all carboned up clogging it. Recheck vacuum lines and make sure no air is coming in. Listen for hisses, spray soapy water look for bubbles, or use carb cleaner. If your car is smells like gas look to see if your injectors are leaking. Gas cleans well and should be easy to spot. Use a long screwdriver and listen to see if your injectors are firing consistently. You might want to put a new fuel pressure regulator on since you have a 99.
The EGR valve does hold a vacuum and doesn't look like it is blocked.
We did get a new error. P0134 oxygen sensor B1 S1 no activity. I need to order some new vacuum lines but if you have any other ideas to test in the meantime let me know. Thanks.
 

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I would check the wiring to the sensor for corrosion and see if it is getting power. If sensor is bad only replace it with what the car came with. I think it is Bosch. Only other thing would be a compression test to see if your engine is worn out. Bad compression would cause misfires. Minimum pressure should be 110psi.
 

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Oxygen sensor and wiring testing: (click arrow for the next pages):

 

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Compression testing:


Fuel Pressure testing:

 

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Discussion Starter #14
I would check the wiring to the sensor for corrosion and see if it is getting power. If sensor is bad only replace it with what the car came with. I think it is Bosch. Only other thing would be a compression test to see if your engine is worn out. Bad compression would cause misfires. Minimum pressure should be 110psi.
I did also notice that the one of the vacuum hoses sounded like it had a lot of air going through it. The one that goes to the connection with the crankcase hose. Does that help at all?
 

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No vacuum hoses should be sucking air. The only air should be coming from the filter. Unmetered air will confuse the computer since that air isn't being measured by the MAF sensor.
 

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Discussion Starter #16
No vacuum hoses should be sucking air. The only air should be coming from the filter. Unmetered air will confuse the computer since that air isn't being measured by the MAF sensor.
The one that is making a sucking sound but not leaking is. The orange one in the image below.
250260
 

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Is the car idling bad? If it doesn't leak should be okay. Connected together correctly? It just shouldn't be getting air from anywhere except it is connected to.
 

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You might check your fuel trims (+ or - 10% is considered a normal range); a vacuum leak, will affect this, watch the video, for more info:


 
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