CorbinD

I swapped a used ls6 intake, ls6 valley cover and ported throttle body onto my 1999 firebird formula. This car has longtubes, full exaust and a moser nine inch. Also I swapped it from an automatic to a manual over a year ago and its been running wonderfully until now. I have the car tuned in speed density and I have HP tuners and a wideband through the EGR wire.

To swap the intake and throttle body I followed the guides on ls1howto and it went fairly smoothly except I broke the oil pressure sending unit, I did have to cut off the boss on the engine block but I feel like that went fine. Anyway I got everything all buttoned up but the car wouldn't idle at first, I did get it to idle pretty well by resetting the PCM and letting it relearn. Now it idles fine but if you lightly press the throttle the RPMs will dip really bad and the wideband will go off the charts lean. Then the engine either stalls or the rpms shoot up and the AFR goes normal for less than a second then goes lean again and the RPMs drop really low again.

If you move the throttle quickly it wont bog down and it will rev normally, but if you try to hold the throttle still at a higher rpm it will usually do the typical surging issue like at low throttle. The car is completely un-drivable because the low throttle surging just causes the car to buck really bad. There doesn't seem to be any vacuum leaks because it will idle around 800 rpm and I tried spraying throttle body cleaner all over the engine bay and nothing caused it to rev up. The only things I can this is that the swap changed the tune that much, There is a fueling issue like an injector. Or a cylinder is misfiring under light throttle but there's no check engine codes and I don't think it is at idle. Did I maybe get dirt in a cylinder? the ports were taped off and we vacuumed them out when we finished but they were really dirty and we could have accidentally pushed some of the dirt into a cylinder.

I have attached a short log of the car idling and me trying to give it various amounts of throttle. Hopefully this all makes since, im a little stressed and I could really use some help! Thanks

Edit: I should add the used intake came with fuel rails and injectors that I put on with the new intake. When we encountered issues I swapped to all of my old injectors with the fuel rail still connected to the car. It also came with a map sensor that is on the car currently but in my log that appears to be fine.

G Atsma

Check for a vacuum leak.

CorbinD

I appreciate the quick response and the help, however I stated that I did check for vacuum leaks and I cannot find any. I can check again, but wouldn't a vacuum leak cause a high idle?

G Atsma

If bad enough it will cause what you have. You mentioned it going very lean, another symptom.
800 is a pretty high idle. It's trying to get the AFR right in spite of enough air being admitted to idle that high.

CorbinD

Ok thanks, it seems like the intake sat right and sealed well when I put it on but I guess I could have something pinched under it causing it not to seal right. That sure would be a big vacuum leak. Whats the best way to check for a vacuum leak? I tried spraying stuff around to see if the rpm changed.

CorbinD

It was a vacuum leak. The MAP sensor that came with the intake must not of been sealing properly and it was a little loose. It's little gasket looked a lot better than my old one. I swapped the sensors and the car immediately ran perfectly. Thanks for the help, considering I had just been cutting part of the engine block off and had exposed I was kinda freaking out that I had somehow messed something up really bad.

G Atsma

Glad you got it going!
Not the first time I've heard of a poorly-seated MAP sensor. Easy to miss!
Probably a large enough leak NOT to hiss sharply, thereby missing it easily.