mcox8051

I got the Grand Sport back together yesterday and fired it up today. This was the LT4 that had the rocker arm come off #6 exhaust valve. After weighing in everyones opinion I decided to trust the cylinder leak down test and the test I did that showed .011 runout of the valve head by pushing it down until it hit the piston. I figured .011 runout was not that bad and besides, the leak test showed exactly the same amount of leakage, 9% (cold engine), as #4 cylinder. I removed the intake and replaced that lifter with a new one in case that was the problem, and also replaced the spring, retainer, and keepers, along with the rocker arm. I flushed several quarts of oil into the head and valley and then changed it. Ran the car today for fifteen minutes and changed the oil again. Car runs awesome, just as before, EXCEPT, when I kicked the A/C on there was a very noticeable vibration and drag on the engine. I only ran the A/C a couple of times before this rocker arm incident, but I don't think it acted this way, at least I didn't notice it then. Maybe I am just very sensitive to anything now after the repair. Is it possible I may have a vacuum line crossed or something? Any ideas? Thanks in advance.

0Paul Ruggeri

I'd have the AC pressures checked. Leave the AC off until you get this done.

engle1147

Glad to hear the damage wasn't too extensive.

AC pressure problem? Check the pressures with a manifold gauge set to see if anything is off or if the system is just short cycling.

Make sure you reattached all the hardware on/around the AC area after you completed your head work.

Check the accessory belt to see if it is showing signs of excessive wear.

mcox8051

Well, I don't know if it is a pressure problem, but it seems odd that its doing this now since I had the intake off and it wasn't doing it before. The A/C was serviced about 2 months ago, a shrader was bad and refrig all leaked out. It blows good and cold, just seems to be pulling hard on the engine and there is a vibration. I may have something not hooked up correctly. What compensates when A/C comes on? Idle goes up? Timing? Doesn't the pc control all of this? Vacuum problem?

lt4obsesses

I remember your original post. Made me pull the valve covers just to check the rockers.

I can't recall though, didn't you say the p.o. put a hotcam in it? I've had experienced, read and heard that the hotcam is not very a/c friendly. But, a/c does cause a bit of a drain on all cars. It did on my LT4 prior to came change and even more so afterward.

jakers

I have the same issue on my 94 with the AC. It is a drag on the engine an a very obvious vibration at higher rpms. If I disable the AC clutch by disconnecting the AC clutch power line the vibration goes away!? If you figure this one out let me know!

mcox8051

Yes, he put a Hotcam in it. The A/C was fine, I didn't notice any power loss or vibration before I removed the intake. It feels like someone pulls off a plug wire when I turn the A/C on now. I think I don't have something back together right or its something with one of the numerous switches or sensors. I've seen talk of having to reset the IAC, how do you do this? Or maybe the MAP sensor.

STL94LT1

You need to get the PCM reprogrammed to increase the IAC counts with A/C on.

lt4obsesses

I ran my A/C today on my way to work. What mine does is pretty much just drag the idle down a bit causing the cam lope to get a little lopier. A couple of taps on the gas and it smoothes out. Driving is no issue. Not sure what your symptoms are. It could be possible that you didn't notice it previously because your mind was on something else. The thing is, there is nothing really you wrenched that has anything to do with the A/C. About the only thing I can think of is if when you re-attached it, the placement wasn't right or not torqued down causing the unit to vibrate. But I assure you that due to the larger overlap, 112 in the hotcam vs 110 in the stock, the efficiency loss when running the A/C will be exaggerated.