Figured out why the 4t80e monte wouldn't idle, and I kinda feel stupid. I swapped the ECM from my daily to the 4t80e monte, in order to see if my tune would help the car run, so I could log what it was doing, and make changes if necessary. I don't know why I didn't think about the throttle body. The TB on the 4t80e monte was the one I had ported a long time ago, knocking out the hump at the MAF. I just recently decided to put that on the daily, cause I ordered a new TB gasket when I swapped my thermostat to a 180*. The engine wouldn't stay running at first, which is what the 4t80e monte was doing when that TB was on it. After a bit of fighting with it tuning wise, I got it to run. I recalibrated the MAF first, then the VE table. I've noticed the following things while running the ported TB the past two days.

Notes:
-Throttle response. Stabbing the throttle gets an amazing feeling, quick response of RPM. It feels like a much quicker rise in RPM than stock
-The RPM no longer hangs. With a stock TB, stabbing the throttle would result in the RPM to rise to whatever, lower to about 1200-1100, then slowly go down to idle. This would happen in other circumstances, I just listed this easy one to note. I tried fixing that with tuning, but I couldn't figure out how to. Apparently, this is caused by the hump in the TB. Now, when I stab the throttle, the RPM will rise to whatever, then immediately fall down to idle. I highly prefer this, as it's more consistent with how I think it should be. More "racecar", if you will. (I'm not saying this issue is universal to all 3800's and their throttle bodies, ths is just what I experienced.)
-There's no more bottleneck. I consider that hump to be a big restriction in airflow. The MAF sensor itself is still a restriction, but obviously a necessary one. Airflow is much smoother without that hump there, so boost will flow through the TB better.
-Low speed drivability is not affected as far as I can tell. Obviously I had to custom tune the vehicle in order to run this TB, but the car reacts about the same at low speeds as it did before.
-The car idles low. This could probably be solved easily by a professional tuner. I'm obviously not one, so my startup tune is now off, and the car idles anywhere from 500-700 depending on ambient air and engine coolant temp. Previously I had the engine idling at 700-800 easily, but with the ported TB, idling especially on a cold engine can be a bit tricky, much like with a carburetor.
-Peak horsepower is down. According to my way of logging horsepower with HPTuners, I dropped about 15-20HP from my peak. This should be taken with a grain of salt however, because the way I'm measuring HP isn't exactly accurate. I'd love to do a comparison between stock and ported on a dyno someday.
-The alternator is greatly affected. If I let the car idle on its own, the alternator doesnt charge properly. I'll have to look at exactly what the voltage drops to with HPTuners, but im guessing it drops two or three volts. I don't really notice any ill afects of this yet, but that can obviously change in the future. This could affect battery life especially on cars that idle for a long time, but could be delt with. Changing to a hairpin / 6 phase alternator, one with stock or higher output could solve this issue.
-The AC is greatly affected. Letting the car idle on its own means the AC clutch wont be engaged. I'm guessing this is due to how low RPM the engine is at. I've been working around this by pressing the brake with my left foot, and holding the engine at about 1000 RPM with my right foot. Again, if the low RPM is fixed in the tune, this wont be an issue. Also, on race only cars without AC, this isnt an issue, but I'm guessing there arent many race only 3800's anymore.
-The check engine code for idle air control is being set. I havent figured this out at all yet. When I first installed the ported TB and was trying to get the MAF calibrated so that the engine would idle, I saw the IAC working in HPTuners. But now it's only ever set at 255. I dont know much of anything about the IAC or the modes it can be in. The modes being the number I'm seeing in HPTuners, IE 73, 78, 255, etc.
-There's now a strange whistle like sound coming from the vehicle. I haven't figured out the cause of the whistle yet. I have several theories. Serpentine belt noise/squeak, alternator pully, AC clutch/pully, idler pully, or it could just be how the intake sounds with this porting. I don't know how many other people have done this, if any. I've included a video below, let me know what you guys think it is. (this noise was never present before installing this TB)

Whistle Video Link

 

Theres something wrong with the idle airflow; there's no reason a properly functioning system should be idling so low that AC and charging are disabled. The stock idle targets are well above that. The stock IAC setup should also be able to easily keep the engine over 1k rpm.

255 IAC means the idle air control valve is pegged wide open (and clearly still cant maintain desired idle as I'm assuming you didnt intentionally turn down the idle in the tune). You've got a problem there that needs to be fixed.

I do wonder if fixing the issue to get IAC back in control might also bring back the softened 'return to idle' after revving up (vs dropping straight back down directly to minimum).



The MAF doesn't have to be there if you're convinced its hurting power. You could always ditch it completely and go SD or you could stick a card style MAF in a giant 4" housing.

Quote:

Peak horsepower is down

Curious your speculation on why? If that's anywhere near real (and assuming it was tuned for the change) a 10% loss of power makes it sound like it's time for the dumpster, lol.

IMO using a scanner to infer hp isnt a great method though. The only useful direct reading is MAF (assuming WOT AFR is tuned for the same value before & after), but given you've screwed with the housing so much here, it wouldn't be useful for this specific change.

Without having access to a dyno, one other good option is to use mph. Pick a couple of points on a secluded street and do a few back to back before & after runs. Doesnt have to be crazy far apart, even 1/8 mile is fine. Certainly not as precise as a drag strip speed trap, but it can give you an indication as to if you're making forward progress or not.

 

I didn't turn down the idle desired RPM, in fact I maxed it out at 1200 RPM. So clearly the IAC isn't keeping up. It's really weird to me how that one change in the throttle body causes such a massive difference in airflow at idle. I also still want to do some tuning to see if I can get it to idle properly. I was thinking maybe advancing the timing at idle, but I'm a bit cautious of messing with the timing.

I'm not necessarily saying that the MAF is a restriction in airflow which should be removed. My point was that anything that protrudes into the intake track is a restriction, and in the case of that hump, a pretty big bottleneck.

I'm 100% possitive the reason I lost HP is cause the intake air velocity is down now, due to removing the restriction point. But I'd rather have no bottleneck and increase velocity with boost. The way I'm reading HP is taking the delivered trans torque PID in HPTuners, plugging that value into an equation, which spits out a HP number. Also, I didn't remember to look at engine coolant temp intake air temp, ambient air temp, etc. I just took a random peak HP number I had from a log file and compared it to one pull I made with the new TB

 

If all else fails and you cant get it fixed so it idles right, theres always the plan B option - just drilling a hole in the throttle plate. Certainly not ideal, but its proven to work.

 

Yeah, that would do it. I'd like to understand what is actually happening to make it need so much more air at idle though. Basically all I did was widen the intake track. I could see that affecting my AFR throughout the RPM band, but not kill the car at idle. I did a base tune you could call it so that the car idles without stalling, but I deffinetely need to go through it in detail. Does the MAF play any roll at all in idle airflow, or is it just the IAC? Additionally, where does the IAC draw in its air? I don't know much at all about that system.

 

The MAF sensor is used for measuring the airflow, the IAC is what adjusts how much air leaks around the throttle plate to keep the engine running.

Only way I could see the MAF sensor impacting idle is if the tune was so jacked up that the pcm couldn't trim it to get close to stoich. I wouldn't think that would be the case here if you tuned it as you would've seen it in your O2 readings / trims immediately and would've fixed it in your MAF tune.

I dont know exactly where the IAC draws its air from on the L36 TB, but it'll be before the throttle blade. Cant find a good picture of the front side either, only of the back / IAC outlet. TBH, I am a bit surprised you dont know having ported it though- you would've had to clean the passages out when you got done porting, right?

Sounds like the best step 1 is to pull the TB back off, pull the IAC and clean the passage out to make sure they aren't full of gunk and aluminum dust / shavings.

 

Fixed the IAC. Back when I ported this TB I filled some "unnecessary" holes. Turns out one of 'em was pretty necessary, lol.

This darker grey patch is where the IAC draws its filtered air from in front of the throttle plate.



Luckly, there is a freeze plug like thing on top of that passage that I could just knock out, instead of taking off the TB and reopening that hole.



Also, you were right Bumpin, now that the IAC is fixed, the throttle is no longer super responsive. Which is sad, cause I really liked that. I'm thinking about putting something like a resistor into the signal wire for the IAC so it halves the IAC position or something like that. Right now its over correcting by about 50-100 rpm. But I'll research that at a later date.

Another thing to note, is that whistling I mentioned earlier. I don't think its anything got to do with the serpentine belt or any pulleys, because it only happens when the engine is at operating temp. It is also still occurring even after fixing the IAC. When I go to shut the engine off, the whistle will slowly die out, making some super weird noises. Which makes me believe it's the engine. If it were a pully it would stop the moment the belt stops moving.

 

Did you replace the TB gasket when you had it off? Sometimes they whistle if you reuse the gasket.

 

I agree, sounds like a small vacuum leak.

I havent really been following these threads that closely, but what did you end up doing to seal the MAF since you cut off the sealing surface for that O ring?

I'd probably inspect around the things you've had off recently with either smoke / propane / etc to see if its sucking in anywhere.

Youd also hate to waste any time tuning if you've really got a leak somewhere.

 

So, I ended up returning to the stock TB, but I'm not giving up on the ported TB concept just yet. I failed to realise that knocking out that "freeze plug" would be unfiltered air as I mentioned, and therefore would screw with the MAF. Which is why I was perfectly fine running the ported TB on speed density, but the moment I plugged in the MAF, everything went wrong in the tune. As far as vacuum leaks and the whistling goes, I used blue RTV to seal the MAF. I did inspect all the vacuum stuff, and the whistle started the moment I put that TB on, so before I knocked out the freeze plug. When I put the stock TB back on, I reused the same gasket, which was new when I put the ported TB on, so it's about two weeks old. Now the whistling is gone completely. Surely other people have done this porting before me, and I don't believe any of them reported the same whistling. Meaning it shouldn't be just the sound of the engine taking in air. Anyway, I've decided to put this project off for now, while I tend to the cracked 4t80e, and finish up the headers for the turbo kit. But when I do return to porting the TB, I'm going to follow Maniac's writeup