New issues after motor swap

[bassist169]

Ok guys, so here is what is going on. I was forced to swap my motor due to a bad piston and due to age of the car doing a motor swap was more logical than just changing the bad piston. The car is a stage 3 2000 S4.

First issue: misfires-
Prior to the motor swap I was getting bad misfires only in the cylinder with the bad piston. Now with the new motor I am getting random misfires across the board. Not bad, but noticeable, especially at idle. It's not a boost leak in the vacuum hoses since I replaced the hoses and rechecked after. It could be as simple as the o-rings on the APR bi pipe to throttle body, one looked a little rough when I reinstalled but like I said, it was fine prior to the new motor going in. Regardless, I've ordered a new set of o-rings and will install later this week. Anybody think that would be the cause? Or any other causes that I haven't thought of? I'm kinda stumped since it was fine on all the cylinders with the old motors aside from the bad one. I might throw new plugs in but I wouldn't think that would be the issue. Any input would be awesome!

Second issue: rough start when cold-
So, prior to the motor swap my car didn't want to start when it was cold. It would turn over and die. I had to give it a little gas the second time and it would run, very rough mind you due to the piston. I figured this was all caused by the bad piston, however even with the new motor it is being very finicky about starting. I turn it over, it runs for a second then dies. Always starts on the second try but is still having cold start issues. I thought the problem was with the old motor but now I'm kinda stumped. I was thinking that it might be the temp sensor but the newer motor has a new temperature sensor, not brand new but what came with it. What are the odds that both sensors were/are bad and causing this problem. I STFA and it seems to always be the sensor or a fuel issue. I could swap the fuel filter but I was hesitant to do so unless I narrowed it down to that in particular. Anyone have any other potential solutions to this? I am kinda stumped since it all started when the piston went bad.

Any help is much appreciated!

Thanks,
Justin

[reverse.rake]

any codes? do you have fsi coil conversion? get some bkr7e plugs and try a .028 gap

[Royal769sr]

Agreed.
Smaller gap helped me a bunch but my misfire was higher in rpms

[christianb5s4]

Spark plug gaps could definitely cause misfires across the board. I had to experiment with mine and eventually found that on stock coils and ICMs, the optimal gap with BKR7Es was 0.024.

[bassist169]

Yes, I have done the FSI conversion. I'm running the NGK side fires but I'm not sure of the exact gap right off hand. Any reason the plugs would be more temperamental with this new motor since they seemed to be running just fine with my old one?

There is just the multiple misfire code. One of the front O2 sensors (tears are coded out) is throwing a code so I will swap that out tonight but in my experience in the past a faulty O2 has never thrown misfires across both banks, only across the one side. I could be wrong though

[Royal769sr]

Are your cam tensioner plugs in both sides?

[bassist169]

Are your cam tensioner plugs in both sides?

I haven't checked but I would assume so... Don't know why they wouldn't be, honestly I haven't personally looked though.

After I got the motor swapped and fired up it was running really rough so I had the the car towed into my shop. Issue there was a vacuum line I missed connecting..oops. But, My family has used for these guys for years and I wanted them to do the timing belt job since I don't know the history of the new motor... That pretty much the only thing I won't mess with on my car due to there being no margin of error. (they only work on German cars and are Audi certified before anybody goes there lol) But something like that I think they would've caught. We talked about doing the cam tensioner gaskets but since I'm assuming they looked ok to them they didn't swap them.

I'll take a look though just to be on the safe side.

[bassist169]

Update:
So I got some new o rings for the bi pipe from McMaster Carr and some new plugs- BKR7Es. Went ahead and swapped the O2 sensor, although on my mutism eternity wasn't reading high enough resistance to be considered "bad", I did it anyway. So I got everything in and out back together and took the car out for a drive. Seemed a little better but not great. Realized I didn't gap the plugs before I put them in, so I pulled them back out and gapped them to around .28. Took it out again and it hesitated pretty bad until it got warmed up a bit then drove just fine. However, it still seems to be missing a little bit at idle. Not bad, not enough to shake the car or throw a code but you can kind of hear it. Curious why it stuttered with the new plugs? Does the ECU take a few minutes to adapt or something?

Anyway, any other ideas about the slight misfiring at idle? Or is it just something I'm going to have to live with?

Thanks

[bassist169]

Got everything fixed yesterday...turns out it was all due to a missing check valve in the vacuum system. New motor was a post facelift and didn't have the valve in the same place as my 2000 so once I relocated the valve everything seems to be running great