I've lurked here for a while and this board has been very useful to say the least. I have a 91 that I 've owned for about 6 months. The car has had the inlet tract extrude honed, Watson's, a chip, and borlas, all done by Exotic Muscle in Phoenix. The car has 70k miles on it and has run strong since I bought it, and is my daily driver. I recently had it dynoed: http://www.67fso.com/dyno.jpg
I was pleased with these results....car showed 132mph in 4th gear on the dyno.

The car never idled right when I bought it and was hunting all over the place, varying the rpm from 700 to 2000 at idle. After some reading here, I figured a big vacuum leak was the problem, so I pulled the inlet plenum the week after my dyno run. Sure enough, the hose for the fuel vapor canister system was completely open and not connected to the bung on the underside front of the plenum. Problem solved I thought. Just to be sure, I used a hand held vacuum tester and pulled a good vacuum on every line in the secondary system, and no leaks were found, and the electric secondary pump worked fine. The fuel vapor recovery system on the other hand leaked like a sieve, especially at the control solenoid under the plenum. Rather than screw with this, I plugged the bung on the inlet plenum for the fuel vapor line, so I would be sure there were no vacuum leaks. I figured the car ran hard without this system before and the 1990's didn't even have it....I'll deal with the tank pressure, valve, and canister out back if I have problems.

My problem is the secondaries have not worked since I reinstalled the plenum and buttoned everything back up. I was very careful to make sure all the lines and electrical connectors were put back in place. I even did a port matching on my inlet plenum gaskets and insulator plates to pick up a few more hp. The car idles smoothly now and runs nicely, but with no secondary operation (yes, the valet key is permanently on). No codes are being thrown from the ECM, and I have disconnected the battery a number of times on various occasions to clear any codes, and see if any new ones would set. I still have no vacuum leaks on the secondary system, when tested from the electrical pump side or the drives side inlet hose. The secondary electric vacuum pump is functioning normally on turning the key, and pumps a hair about every 60 seconds after initial pump up.

Does anyone know what the problem might be? Could I have hurt any of the
secondary canisters or reservoir or check valves by pulling too strong a vacuum on them with my hand held pump? I really would hate to take the plenum off again, but may have to if there is not some easy trick you guys may know...
What do I need to test if I do pull it off? Should I use the hand held pump to
open the secondary butterfly's? The factory service manual flow chart is about useless....

I do also get a 10 to 15 second stumbling around when the car seems to be going from open loop to closed loop operation right now, that I didn't get before pulling the plenum. Power just goes way down and the car coughs some for about 10 seconds, then snaps to and runs perfectly the rest of the time. It will happen if it is warm or cold, but appears to happen right on the transition of open to closed loop, judging from the coolant and oil temp readings. Any clues for me here why this is happening??
Thanks much!

 

First of all - welcome - and second, that is one hell of a long post!

Bottom line: You have got to pull the plenum. Don't worry - it'll be much faster the second time. I now have it down to about 20 minutes off, and 20 min on again.

As for your stumbling - my question to you is : Is the SES light working? It sounds distinctly like a big vacuum leak that eventually throws a SES light, and then runs open loop. I could be wrong, but check for SES light operation, and get that plenum off for inspection!! :seeya

 

Before your car reach temperature of 177 degrees it run with 2 fuell pump and goes from open to close loop, after reaching 177 degrees then shut off one the pump. Most important check all your connections under the plenum , wouls help to look at a sketch of the original connections.

Joseph

 

Does car run poor with power key off. I had injector that went bad and only was noticeable with power key in the on position. I always leave the key on, but when I ahd problem, I checked the simple things first. It wasn't in the closed open transition as you mention, but I would check a few simple things like that before you remove plenum again.
Mike

 

The car runs the same whether the valet key is on or off. I thought of the duel fuel pump problem too, but just the fact things happened from taking the plenum off leads me away from the fuel pumps.

I did notice the inlet plenum has a fair amount of oil on the inside and that the pcv valves and lines had oil in them. While I had the plenum off, I used a rag with acetone to clean the secondary inlet ports above the butterfly's. I was told the pcv valve system is easily overwhelmed on these cars by high rpm operation. I even cleaned oil out of my map sensor line, and then tested the sensor for proper output...all good.

I'm still at a loss....plenum removal time I guess!

 

All the LT5 engines accumulate oil on the intake system. I have a remote tank to reduce the majority of oil build-up. Time for plenum pull, but as mentioned, it gets real easy and simple after the first time.
Mike