**Cliff notes at bottom**

Yeah. This is weak sauce, I know. It's an intake manifold..... Install went fine. It's the after-portion that has me seething the more I think about it.

As stated, out with the LS6, and in with the 78. Everything went fine. Didn't break the OPSU (2nd sucessful time). Transferred all gaskets to the 78 since they only had 3k miles on them. I put them on all new in November when I did the LS6 valley. I also installed the required button head bolts in the valley.

I'm going to call myself out on this-

**I DID forget to attach the brake booster line.**

It was funny. My dumb *** actually listened to it for almost a full minute like, "Damn, this thing sounds like it's got a mild cam" while my wife had this screwey look on her face and telling me it doesn't sound right. Then I got to thinking, "Well, maybe she's right..." as I thought I was still in idle re-learn.... Sprayed a bit of MAF cleaner around and as soon as I shot it at the back, BOOM, idle soars. Instantly, I felt like an idiot, shut down the car, and attached the brake booster line.

After you dry your eyes from laughing so hard at me, read on-

My dilemma:

I went for a drive, a fairly hard spirited drive, for about 15 minutes and the car ran GREAT. Seat of the pants just felt smoother. Not necessarily more powerful, but just a tad bit fatter of a power band. I was pleased, went home, and cracked a bunch of Coors Lite in celebration.

Next day I went for a short 10 min drive to the store and the car ran fine. On the way back, it seems like its searching to come to idle. When I go to neutral, it hangs around 1400-1600 or so till I come to a complete stop, then goes back down to normal. When I switch gears, like 1-2 or 2-3, it wants to hang at the RPM I'm at when I switch.

Just wondering if this is due to the intake or what. Car has a tune for full bolt-ons from Frost, so I didn't think it would be a very significant change. I sprayed MAF cleaner all around searching for a vacuum leak and nothing. Idled great in the drive way. Touch the gas even a millimeter, and it hangs for about 4-5 seconds, then comes down.

Obviously I have no scan software. What do you guys think? Do I need a retune for the FAST?

Yesterday I pumped the brakes at idle checking yet again for a vacuum leak. When I push the pedal down hard, no change; It's when I let off the pedal the RPM's come up maybe 50-100, but immidiately settle back down.

**Cliff notes**

Had LS6 intake
Have full bolt on tune from Frost
Installed FAST 78
Car runs great first 15 minutes or so
Hot start-up and my idle likes to hang at 1400 or so for a few seconds before settling down
Quadruple checked for vacuum leaks; None
Stumped

 

It'll need a tune - it's a pretty big change even from a LS6 intake.

 

First off, do an idle relearn again after unhooking the battery for a while. You let the pcm learn with a vacuum leak.

Second, it likely still will need a tune to clean up the idle airflow values.

 

Almost sounds like you cracked that small plastic hardline that runs your hvac controls. Mine did the same thing and turned out that was the issue. It was just enough of a vacuum leak that it messed with idle but wouldnt show up anywhere else. I found it when I researched what woudl cause my HVAC to not switch from vent to defrost to floor.

 

Thanks for the replies so far guys; It is much appreciated. I'm back out on the rig now until the 23rd, so I get 2 weeks to pound sand and think about this.

Good point, and I even thought about that. It ran for almost 2 whole minutes like that before I shut-down and rehooked the booster hose. I thought and was hoping the driving would clean it up, but you very well may be right.


I did replace the 3 way check valve thing last year with a HELP! brand replacement. I had the default "floor/defrost" when I bought the car.

I absolutely DOUSED the HVAC lines with MAF cleaner checking for a leak, but found nothing. I was actually thinking since I deleted the AC, I could prob plug that nipple in the intake and see if it changes, but didn't get that far before I had to take off.

Anyhow, I've emailed Frost twice in the last 2 weeks checking about resending my PCM for the 78, but have yet to hear anything.

 

Any other ideas?

When I get home I'm def going to do the idle re-learn, and send the PCM back to Frost whenever he gets back with me.

I'm almost positive my HVAC lines are good.

 

Did u check if all is good with throttle cable?

 

I had the same problem when I swapped from ls6 to fast 102 and turned out it was my map sensor failing. Switched with another sensor and it ran fine, still hang at 1400-1600 when I drive and then shift to neutral but only for 1 second and then it goes back to 900

 

I did. I thought that was initially the problem. It was TIGHT, and not letting the TB bumpstops touch. It was the TCS/Crusie that I was hearing click instead. I adjusted accordingly and did a TPS "0" and thought all was well. Unfortunately the "test drive" was only 10 minutes or so, and it always happens after that. Normally on a hot start-up.


Funny you mention that. I cleaned my MAP w/ MAF cleaner before i installed it. I thought it couldn't hurt though, being the intake was a bit oily (Damn you Norris for being out of Catch Cans!! )

 

The solvent in the cleaner may have damaged the diaphragm strain gauge in the MAP sensor possibly.

It would be an easy diagnosis w/ a scanner ($150 or so via scantool.net plus a cheap laptop) to read the MAP sensor output.

 

Well, the youngest I could imagine that sensor being is 149k, so maybe it's just due for replacement anyway. I did happen to soak it pretty good. Hindsight being what it is, maybe I should have just left it alone. I think my bracket I made for it sucks anyway and sould have stuck with the damned RTV only.

Frost got back with me today and seems fairly convinced that it's a vacuum leak. Maybe it is. I guess it's a good excust to take it apart and smooth some casting flash at the very least.

Anyone know if the rope gasket they sell for the 102 will work for the older versions (78, 90, 92) as well?

Hell, maybe I got a gasket pinched or something during install that doesn't show up until the motor gets hot. Personally, tracing a vacuum leak is one of the most aggravating things to me. I just don't get it though. Only happens once the engine has been ran a good 10 minutes or so.

Wonder if it could be anything else, like having to do with closed loop / open loop, MAF going out. I just do NOT want to start throwing parts at the car blindly.

 

Are you using a torque wrench and tightening the intake bolts to spec, and in the 2-step bolt sequence?

 

2 step sequence, yes. (Flame suit zipped, retardent gel in hair, Nomex hood & helmet on)

Torque wrench.... no.

I use a stubby 1/4" drive socket for it though.

**Edit** I'm guessing this is where your avatar pic applies

 

try water instead of MAF/carb/brake cleaner for a vacuum leak. i use a regular water hose at work, and just let it run over the whole intake, just dont spray it with pressure and watch what gets wet, although the LS1 engine bay is completely waterproof so water ANYWHERE shouldnt hurt anything.

if your engine bogs with water you have a leak.

 

Do me a favor.... Go outside, right now, and haze yourself. Repeatedly.

 

I had the EXACT same syptoms that you are pointing out, althought not after an intake swap. (This happens to me all the time whenever i let my battery die. Sometimes the cars tries to idle at 400,... or if i'm driving the rpm will hang at 1800 in neutral or between shifts. ) The idle air control could be sticking, which cleaning it will fix the issue sometimes (othertimes you just have to replace it). Or, one fast and easy way to find out is to cycle the ignition 3 times for a few seconds. When you turn on the ignition, the idle air control valve "recalibrates" itself by closing and opening fully (or vise versa), and apparently cycling it 3 times lets it figure out exactly were it needs to be.

Just try it out, it can't hurt

 

Not a bad idea.

I need to do a new idle relearn anyways like Thunderstruck advised.

Frost seems to think somehow it's a vacuum issue though. I think I'm going to go ahead and take it apart to get that lip out of the runners anyways, and reseal it. Apparently it's never been apart and I have a feeling by now those gaskets are dry.

I replaced the TPS last year. I'd imagine a new IAC would be a good idea as well.

 

well before you buy, you could just check to see if its caked with oil/dirt/carbon/etc. You can also have someone cycle the ignition ONCE while you have it off, just to see if it's actuating properly (before you buy a new one). I'm sure you'll get it figured out either way, and i most certainly am not challenging Frost LOL. But i did want to share what worked in my case with those same issues you have.

 

Definately. I planned on giving it a good cleaning. I do appreciate the thoughts and ideas.

I'm just hoping there's no cracked HVAC lines from the firewall to under the dash

 

Cleaning the sensor with MAF cleaner should be fine. I would let your PCM re-learn your idle values. If that doesn't do it, recheck that your TPS has been set correctly and let your PCM re-learn your idle values again.

My vote goes for something related to throttle.... is the butterfly getting stuck?