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Old 08-22-2019, 07:57 PM   #1 (permalink)
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Default Constant misfiring after a half throttle pull

I've been having a blast driving my TT since I got it back together after a head gasket repair / 60k service until today. I did a quick half throttle pull to show off the new BOV to my girlfriend and ever since the car has had what feels like a constant misfire. It genuinely feels like the car is running on 5 cylinders. I got to about 10psi of boost when this happened. My first suspicion was that I blew off a vacuum line, but I can't find where any are loose. She was running absolutely perfectly before this happened and I just got done with hours upon hours of labor putting the car back together properly so I'm a bit bummed out.... Can anyone point me in the right direction to start looking for the cause tomorrow? A boost leak test is the first thing I plan to do....

Edit: I have fully equipped my car with everything I should need to data log the car with Blackstealth's converter and the Torque app, but I don't know what I'm looking at well enough to sniff out a problem using it. No CEL are showing either.
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Old 08-24-2019, 11:02 PM   #2 (permalink)
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Default Re: Constant misfiring after a half throttle pull

Did a boost leak check today. Had a tiny leak from one of the vacuum hoses on top of the throttle body. I fixed that but the problem still persists. The car runs and idles fine, but it feels like it's doing it with one less cylinder. The whole car shakes when you accelerate. New spark plugs, wires, and coils. Spark plugs gapped at .38. This one has me pretty stumped. I can try to capture some logs if one of you could be so kind as to help me know what I'm looking at.
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Old 08-25-2019, 11:45 AM   #3 (permalink)
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Default Re: Constant misfiring after a half throttle pull

Check out my thread here for getting a good setup for datalogging and some key parameters to look at:

https://www.3si.org/forum/f35/datalo...sion-q-811626/

Are you logging using bluetooth OBD2 converter? If so, you wont get much speed/resolution.

In terms of your misfire, is the RPM gauge off? If its PTU related, typically your RPM gauge will read wrong, but i believe that would also affect 2 cylinders (1 coil) at once. Any pops or other odd noises when the issue first started?

If you think it is down a cylinder, pull off the plug wires one at a time at idle and see if one of them does not make a difference. Slim possibility, but its possible one of the plug wires came off on the plug itself? If you can isolate it to a specific cyliner, check that wire, and the plug if its in the front. If its in the rear you may need to result to pulling the plenum again, and while you are in there you should probably just do a compression check.

Basically, isolate it to a specific cylinder if you can, then make sure that cylinder is getting fuel and spark. You can also check injector harnesses. If you just put everything back together its worth going over all electrical connections as well.

If you have a wideband, do your AFRs look way off now that the misfire is there?
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Old 08-25-2019, 03:08 PM   #4 (permalink)
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Default Re: Constant misfiring after a half throttle pull

Quote:
Originally Posted by jrhorne View Post
Check out my thread here for getting a good setup for datalogging and some key parameters to look at:

https://www.3si.org/forum/f35/datalo...sion-q-811626/

Are you logging using bluetooth OBD2 converter? If so, you wont get much speed/resolution.

In terms of your misfire, is the RPM gauge off? If its PTU related, typically your RPM gauge will read wrong, but i believe that would also affect 2 cylinders (1 coil) at once. Any pops or other odd noises when the issue first started?

If you think it is down a cylinder, pull off the plug wires one at a time at idle and see if one of them does not make a difference. Slim possibility, but its possible one of the plug wires came off on the plug itself? If you can isolate it to a specific cyliner, check that wire, and the plug if its in the front. If its in the rear you may need to result to pulling the plenum again, and while you are in there you should probably just do a compression check.

Basically, isolate it to a specific cylinder if you can, then make sure that cylinder is getting fuel and spark. You can also check injector harnesses. If you just put everything back together its worth going over all electrical connections as well.

If you have a wideband, do your AFRs look way off now that the misfire is there?
I am using a USB OBD2 converter run directly in to the back of my android head unit. The responsiveness on this adapter is nearly instant and updates very quickly.

My RPM gauge is working properly so I'll assume that isn't the problem for now. It was replaced by the previous owner in 2015. As far as any other noises when the car started acting up, there were none. I let off the throttle after a half throttle pull and upon the next time I gave it gas the car started acting like it was missing.

I individually disconnected the plug wires from the coils one at a time and the idle bogged down equally for every one of them, so I guess all cylinders are in fact firing.

I do not have a wideband installed yet. It was a consideration for the future though.

I'll go ahead and check out your datalogging thread and see what I can come up with. Thank you!
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Old 08-25-2019, 05:05 PM   #5 (permalink)
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Default

Ah if you are using wired USB your speed will be great. You should be able to look at numerous PIDs in torque and get real time info. Its the bluetooth that is super laggy.

Seems a little odd that it's even across all cylinders... I was suspecting one plug or one injector type problem. Getting torque setup to monitor and log some key PIDs might provide some clues. Then drive around and watch things in real time and do some logs.

Any other symptoms you can describe? Does the car smell like it's running more rich now? Same behavior on cold start and hot start? I wouldn't try driving it too hard but if you roll into boost does it clear up or behave the same?

The more I think about it one thing that can affect all cylinders evenly is timing. I would definitely check timing belt, make sure it's not loose or didnt skip a tooth, and perhaps check base timing as well. This would definitely make it seem like it was misfiring.
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Old 08-25-2019, 11:44 PM   #6 (permalink)
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Default Re: Constant misfiring after a half throttle pull

Quote:
Originally Posted by jrhorne View Post
Ah if you are using wired USB your speed will be great. You should be able to look at numerous PIDs in torque and get real time info. Its the bluetooth that is super laggy.

Seems a little odd that it's even across all cylinders... I was suspecting one plug or one injector type problem. Getting torque setup to monitor and log some key PIDs might provide some clues. Then drive around and watch things in real time and do some logs.

Any other symptoms you can describe? Does the car smell like it's running more rich now? Same behavior on cold start and hot start? I wouldn't try driving it too hard but if you roll into boost does it clear up or behave the same?

The more I think about it one thing that can affect all cylinders evenly is timing. I would definitely check timing belt, make sure it's not loose or didnt skip a tooth, and perhaps check base timing as well. This would definitely make it seem like it was misfiring.
I just set the timing less than 100 miles ago during a 60k service. It was still dead on when I checked today. However, I have not set my ignition timing after having to remove the CAS to have my heads resurfaced. I tried to put it back exactly where it were before I removed it. Do you think incorrect ignition timing could make the car run like garbage constantly? It was completely fine for the 100 miles or so I put on it after reassembly. If so I'll need to buy a timing light.
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Old 08-26-2019, 12:03 AM   #7 (permalink)
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Default Re: Constant misfiring after a half throttle pull

What happens if you go back to the old BOV? if that was the only modification and it started doing that..
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Old 08-26-2019, 12:40 PM   #8 (permalink)
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Default Re: Constant misfiring after a half throttle pull

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What happens if you go back to the old BOV? if that was the only modification and it started doing that..
No change when switching back to the old BOV.
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Old 08-26-2019, 03:34 PM   #9 (permalink)
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Default Re: Constant misfiring after a half throttle pull

Quote:
Originally Posted by Sparkko View Post
However, I have not set my ignition timing after having to remove the CAS to have my heads resurfaced. I tried to put it back exactly where it were before I removed it. Do you think incorrect ignition timing could make the car run like garbage constantly?
Short answer is yes, absolutely timing being off will make the car run terrible.

Now as to why it ran fine until you did a pull, I am not sure unless the CAS was not fully tight or something and moved when you did that pull? Did you make marks during your previous reassembly so you can tell by inspection if it moved since then? Also check the electrical connection on top of it; i had mine come loose and it car would all the sudden hit a wall like it was fuel cutting because that plug had backed itself off. Just pushed it back down and that went away. Not sure a loose plug would cause your issue though unless it was somehow causing timing to be misread.

In any case, it is probably safest so set your base timing, and it would be another thing to check off the list.
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Old 08-27-2019, 03:28 PM   #10 (permalink)
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Default Re: Constant misfiring after a half throttle pull

Quote:
Originally Posted by jrhorne View Post
Short answer is yes, absolutely timing being off will make the car run terrible.

Now as to why it ran fine until you did a pull, I am not sure unless the CAS was not fully tight or something and moved when you did that pull? Did you make marks during your previous reassembly so you can tell by inspection if it moved since then? Also check the electrical connection on top of it; i had mine come loose and it car would all the sudden hit a wall like it was fuel cutting because that plug had backed itself off. Just pushed it back down and that went away. Not sure a loose plug would cause your issue though unless it was somehow causing timing to be misread.

In any case, it is probably safest so set your base timing, and it would be another thing to check off the list.
The connector is secured on there really well. I don't think that is the issue.
Here is a little video I took of the way it sounds. You can barely tell through the video, but it runs perfectly, just with a weird miss.


See what you think of that. Hopefully I can get a timing light soon to get that out of the question.
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