louiethebats

91 305 tbi hi guys. i just finished my "vortec swap"305 vortec heads but now my car idles rough, when in park if i step on the gas pedal till about 10-15rpm it surges up and down, if i rev it real hard in park to close to red and lay off the pedal it comes down and feels like it wants to die. the intake i used required me to get rid of egr and move brake bootser vaccum line to the rear of intake. i know i should get my ecm tuned but i dont think that is the problem yet. ive also heard of people doing head swaps without having to go into that yet. just wondering if there is anything else that i should have swaped out along with the heads.

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Nub383

In my opinion, tuning your ECM is what you need. If you swapped to vortecs, your compression ration is higher, your engine has changed.

louiethebats

but honestly...would it change that much????
origional heads were 58cc. the vortec ones are 58cc too.
can this not be fixed with adding more fuel to make up for the increase in airflow. o2 sensor should pick this up anyways and adjust itself shouldnt it??

kevinc

Which intake gaskets did you use, and what did you torque the intake bolts to?

louiethebats

i used the gaskets suggested by SUMMIT Racing for the intake.
i tightened the bolts in a crisscross pattern but i did not torque them.
i did not know the torque spec. i got all of them pretty even though. and i did check for leaks by spraying starter fluid around the intake and tbi. i did find a leak at the tbi and fixed that but it did not change idle much.
it idles ok a cold start then gets rough as it warms up.

Stephen

What intake & TBI do you have on there now?

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louiethebats

im using the intake i bought from summit racing the Proffesional Products Part number 52028. basic operation is 1,500 to 6,500.

the tbi is the origional one that the car came with.

Stephen

Well....There goes my ideas of the possibility of mismatched Vortec head ports to an older style intake or of a 350 TBI on a 305.....

ls six

There should be little to no difference between the two heads at or near idle so I doubt the tune is in question .

Double and triple check everything you touched during the swap and be sure nothing is leaking or disconnected.

Check that you dont have a coolant leak due to the heads and block not sealing.

I had a similar issue when I replaced the TB and the gasket between the TB and IAC body leaked coolant into the IM. My solution was to bypass the coolant lines for the TB

kevinc

Aluminum intake manifold and no torque wrench...recipe for problems.

I'd pull the intake and verify the sealing surfaces are flat, using a steel straight-edge ruler. If they're warped, it's off to the machine shop.

louiethebats

thanks guys. please keep the suggestions coming. im glad that most of you are thinking outside the simple answer of "NEEDS TUNING".
i will pull of the intake, inspect for warppage, and torque to specs, though i highly doubt that this would be the problem since i sprayed the starter fluid all around the intake and didn't get a reaction.

RFmaster

Vortec heads were drilled for 8 bolts vs conventional 12 TBI (post '87 intake angle). Also, Vortec intake requires very little torque - IRC 12 to 15 ft-lbs which is easy to exceed with a 1/4" drive ratchet. Intake vacuum leak can be on underside - sucking oil-air mixture from lifter valley. Spraying carb cleaner on exterior surfaces will never reveal this vacuum leak. The leak can be very small to detect using conventional approaches, but enough to screw up idle quality. I like to use gasket shellac around intake ports to insure that gasket stays put during intake installation.

Regarding tuning - Vortec headed engine requires different spark advance vs. conventional TBI heads. Like it or not you'll have to alter your ECM SA tables to match your engine requirements. There are couple very good threads dealing with this issue.

//RF

kevinc

That test only detects intake leaks on the top side of the ports. It will not detect an intake leak at the base of the ports, exposing them to the lifter gallery.

When you pull the intake, look for traces of oil in the heads' intake runners, or poor indentation on the intake gasket surfaces.

ShiftyCapone

Did you swap cams? Don't overlook the tune. Little changes cause big tuning issues on these cars. A stock L03 with an LT1 cam will not idle without surging. If you have checked everything as mentioned above it is time to do some data logs and let it tell you what is wrong.

daatv

im not sure on your tbi but i had the same problem with my tpi the throttle linkage was sticking put a spring on it seems to have fixed the problem

truck5458

had the same problem 89 Lo3 also lost power assist to braking. check manifold tto tbi surface. .042 diffence between bolt holes. doubled gasket add spacer. and problem went away. almost bet it is vacuum related.