New engine - Idleing and accelleration issues

[ShadowFist]

I just had a new OEM engine installed and am trying to diagnose some issues with idling and missing/hesitating on acceleration. When I fire it off, it idles fine anywhere from 10 seconds to a minute and then starts to drop RPM's and start stuttering. Also, a loud hissing can be heard from the air filter when this happens. If I raise the RPM's by 50-100 the hissing immediately stops and idle returns to normal for 10-30 seconds and this repeats.

I spent a while trying to find a vacuum leak but I haven't been successful yet. The MAF has been replaced as has the IAC valve. An interesting observation is that when the engine is hissing/missing, if I unplug the IAC the hissing immediately goes away but the engine RPM's stay the same. If I plug it back in again, the hissing returns immediately.

Any suggestions?

Video of hissing/IAC disconnect:

[ame="http://www.youtube.com/watch?v=s1MgnuZlkUI"]YouTube- New Engine break in[/ame]

 

[justa03]

You running cams? Also what kind of hesitation are you having? I have stage 3 cams in mine after putting a motor in it and have a bad hesitation right when I come into boost then it comes out of it. Maybe we can help each other out here.

 

[rich5150]

I just had a new OEM engine installed and am trying to diagnose some issues with idling and missing/hesitating on acceleration. When I fire it off, it idles fine anywhere from 10 seconds to a minute and then starts to drop RPM's and start stuttering. Also, a loud hissing can be heard from the air filter when this happens. If I raise the RPM's by 50-100 the hissing immediately stops and idle returns to normal for 10-30 seconds and this repeats.

I spent a while trying to find a vacuum leak but I haven't been successful yet. The MAF has been replaced as has the IAC valve. An interesting observation is that when the engine is hissing/missing, if I unplug the IAC the hissing immediately goes away but the engine RPM's stay the same. If I plug it back in again, the hissing returns immediately.

Any suggestions?

Video of hissing/IAC disconnect:

YouTube- New Engine break in

Any chance you can datalog this? I can give you the PID's that you need to datalog if you need them. :??:

More than likely you have an air leak passed the MAF itself.... datalogging the IAC duty cycle and a few other PID's can isolate this pretty quick.

 

[ShadowFist]

You running cams? Also what kind of hesitation are you having? I have stage 3 cams in mine after putting a motor in it and have a bad hesitation right when I come into boost then it comes out of it. Maybe we can help each other out here.

Stock internals, just a KB strapped on top :beer:

Any chance you can datalog this? I can give you the PID's that you need to datalog if you need them. :??:

More than likely you have an air leak passed the MAF itself.... datalogging the IAC duty cycle and a few other PID's can isolate this pretty quick.

I have a SCT livewire but I won't profess that I'm an expert. If you can send me the variables to log, I can get to it in the morning.

Thanks for the responses!

 

[ShadowFist]

Bump. Anyone know what the hissing noise is? It was not occurring before the engine swap...

 

[justa03]

When you unplug the IAC does it still idle rough?

 

[ShadowFist]

Yes it does.

 

[ShadowFist]

Bump with new information: I probably should have mentioned that I've put 90 miles on the engine before having any serious issues. It did have some of the idling issues before, but not as bad as they are now. Before the car would stumble once and then run fine for several heat cycles. Now it barely stays running. The hissing would always start when the idle started having issues and would disappear when it was running normally.

 

[justa03]

Do you still have EGR on the car?

 

[ShadowFist]

Yes.

 

[ShadowFist]

I have datalogs now, and I hope I got the right ones. Let me know and I can send them over as it looks like I cant attach them here.

Thanks!

 

[Coderedsaleen]

Do you have the screen installed before the MAF? Sometimes that will cause turbulent air to flow through and give the rough or up and down idle. When you unhook the MAF connector it will even out. That is what happened to mine.

 

[ShadowFist]

I will need to check on this.

Thanks for the suggestion!

 

[ShadowFist]

Would a plugged/collapsed catalytic converter cause symptoms like this? I just found a good amount of cat material sitting behind my back bumper tonight after some datalogging...

 

[SVT GI]

Datalog your short term fuel trim. Does it smell really rich? Mine had the same issue and it ended up being a front O2 sensor that went out and caused it to act crazy. I did all the same steps as you with the vacuum leaks, MAF, IAC and still nothing.

 

[ShadowFist]

That is a possibility, but I also know that my cats are/were plugged and probably need to be replaced..

I've been in contact with my tuner and been working on a couple things. The short term trim was off, so he took out 10% as a test on the low end just to make sure it idled. It seemed to get better, but then started running like crap today. Thats when the cats decided to regurgitate.