jjjjwiz

Hello again,

I am still having trouble lowering my idle speed on my 84 vette. I tried the procedure to set the TBI settings, grounded the ALDL to set the IACs, set the TPS.....still runs about 900-1100 at idle and engine hot. When I remove the PCV valve and plug the vacuum port, the idle drops to 700-800. Thats where it should be - right? Any suggestions about what else to check or test. Feel free to be as technical as you want - I can understand it - THANK YOU

Sandpiper59

Sounds like you have a vacuum leak somewhere since plugging the PVC reduces the amount of air going in. A vacuum leak can allow enough air that the IAC can't close down enough to limit idle speed..

That said, what are you basing your idle RPM on? On my 87 slash 90, I was trying in vain to lower my idle.... until I finally got datamaster working right and found out my tach reads over 200rpm high.

jjjjwiz

i need to dig deeper to see if I have a vacuum leak. I am going by the dash tach. I know it is way too fast because if you shift into drive it slams. any "common" place for vacuum leaks?

Sandpiper59

On mine they seemed to be everywhere

Main ones were intake manifold to plenum was loose on passenger side, tubes to base was leaking at #5 and #7. PVC and line to brakes, hard and leaking around nipples. EGR grossly gummed up and not closing fully (this will usually cause rough idle more than high)..

Also if the IAC is defective, wrong or sticking it won't close all the way.

Sorry, just noticed your an 84, don't know much about the cross ram engines......

383vett

I'm sure you did, but just in case, did you close down the butterflies with the set screw?

jjjjwiz

i checked the IACs. I removed them and cleaned them and adjusted them to close the port. I know they are closed because I can spray carb cleaner into the IAC ports and it does not change RPMs. They are shut tight. I just am trying to locate a "common" place to check before pulling off the plenum and intake....

jjjjwiz

yes i did close the butterflys - anything else to check or try?

jjjjwiz

is is pretty easy to remove the plenm and intake - I know how to do it but just curious if I have to remove alot of junk to get it to come out easy

jjjjwiz

ALSO......what is the correct idle setting? It is an automatic. I cannot find a spec on the car.

Rohn

That may not work. Not sure how GM put the steps for IAC into tune, but lets say it is 10 steps retracted to maintain desired idle speed 650 rpms. Retracting idle stop screw to close fly's(lower RPM) will cause the ECU to move the IAC position to say 20 steps to maintain commanded idle speed. If there is a vac leak IAC may be at zero steps already attempting to maintain desired idle. Most likely vac leak. If there are a lot of wear on throttle shaft hole in TB unit it may be a source of vac leak and need rebushing.

hooked073

These throdle bodies are well know for coming loose and blown gaskets. Take a socket and try to tighten them down if they are loose. It is a 2 min check and you may be surprised at what you find.