93 regal - high idle

93 Buick Regal with 3800 engine. I recently had both the lower and upper intake manifolds replaced due to a coolant leak. The car runs fine except for the idle being high. At start up the engine idles at 1200 rpm and soon after increases to 1400 to 1500 rpms and stay at this number. Checked for vacuum leaks and could find none. Changed the iac and tps with no effect. Car idled fine before changing the manifolds. Any ideas would be appreciated.

This sounds like you have a vacuum leak somewhere. I would recheck the torque on the lower intake gaskets and also check all the vacuum lines again. You should also check the gasket at the throttle body as it may be leaking. Sometimes the hard plastic vacuum lines can be cracked and not really show an issue. You should also check the gasket around the PCV valve.

This sounds like you have a vacuum leak somewhere. I would recheck the torque on the lower intake gaskets and also check all the vacuum lines again. You should also check the gasket at the throttle body as it may be leaking. Sometimes the hard plastic vacuum lines can be cracked and not really show an issue. You should also check the gasket around the PCV valve.

+1^on the PCV suggestion. Starting in 1993 the Series I 3800 (VIN L, L27) used the hidden PCV design just like what was later used in the Series II 3800. It is located on the far passenger side of the upper intake and has a small plastic cap that you twist a little bit to remove. I'd strongly recommend doing this with the engine cold. Be careful because this cap has very sharp edges, and may be stuck on there pretty good on the new intake. It may also be a bad PCV valve that is sticking. I'd also check the air intake hose clamps and make sure they are tight.

I can think of four other things that could cause vacuum-related problems. The first is the big EGR port could be leaking. To remove this hose, unbolt it from the EGR pedistal then rotate the hose about 100 degrees counter-clockwise (or until it stops) and it should pull right out. Inspect the big O-ring and intake opening for carbon build-up.

The next suspect would be the vacuum tee between the intake ports and the throttlebody. It is actually removable and IIRC there should be an o-ring on the bottom side of it. If the shop used Dorman p/n 615-179 to replace the upper intake, then it would have came with a new vacuum tee and you wouldn't need to worry about this.

The last two suspects would be a bad purge valve or bad transmisson vacuum modulator. The latter item bolts to the front of the transmission and has a vacuum hose going to it. It is only held on with a fork with one retaining bolt. If you have a vacuum tester, the modulator should be able to hold a small vacuum that is applied to it, if it can't then it is bad.

If it did not do it before, then everything required to do the job needs to be rechecked. Pain in the butt I know, but do the simple visual checks first. Make sure every manifold vacuum line is attached and routed to the proper connection, don't forget the brake booster hose. Make sure the throttle and cruise control cable are allowing the throttle to return fully to its rest position. If you could monitor the IAC counts that would be helpful. Zero counts would indicate the IAC is all the way in and there is a vacuum leak.

Thanks to all who replied. Sorry for the delay in answering, just got the car back. As advised checked all possible areas for vacuum leaks. Replaced all hoses that seemed suspicious. Redid smoke test and still no answer. As a last resort took off plenum and noticed a very very slight distortion of the gasket. Repositioned gasket and installed. Idle back to normal. Thanks again to all who offered advise.