I have a 1991 5.0 TBI, just put in a Brand new motor apprx 10,000 miles ago. I drove it from arizona to virgina, and have had it here for 3 months now. Had no problems with it until the recently (lets say 20-30 day) when i notice a slight fluctuation at idle, it usually idles at 700 but then will drop to 500 then back up to 700. Didnt think anything of it but now I notice after driving it down the road, when i come to a stop the car hangs at about 1500+ rpms for 8-15 seconds before it VERY slowly comes back down to 700 rpms (and roams slightly). the motor is very strong still has lots of pull havent noticed any loss in power or response, BUT it now smells very heavy of running rich. I smell it at an idle and while driving slow (at highspeeds not cause the air is moving) and it never used to smell like that.

Everything on the motor sensor wise is brand new, only thing that has not been replaced/overhauled is the TBI itself. I was told the left injector leaked slighty and both injectors spray patterns where out of wack. I have done alot of reading and research, and have been told its one of these things

1) TBI Gaskets butting up to the intake manifold are leaking
2) vacuum leaks in lines
3) IAC is bad
4) Injector leak (drip of fuel causing fluctuation of rpm at idle)

I have a vacuum gauge hooked up to the motor itself and runs consistently at 22-23 inches at idle, when it drops to 500-700 it will go between 18-22 inches. So id assume i didnt have any vacuum leaks. Anyone have any ideas. I planned on replacing the IAC this weekend to see. If that didnt help i was going to overhaul the TBI in its entirety. Any advice or experience with this?

PS. If you werent looking for the problem you probably wouldnt notice it. but the car has an louder exhaust, and you can hear the engine roam at an idle, its very slight and not a quick up and down but you can hear the pitch change

 

Can you see any injector dripping? If so you need to inspect/replace the o-rings to determine if it's leaking from the o-rings or the injector itself. The new o-rings & gaskets can survive several dis- and re-assemblies. A light coat of oil on the o-rings eases reassembly.

A leaking injector, be it o-rings or the nozzle, could cause your symptoms.

I would hold off on replacing the IAC until after you inspect the injectors & their o-rings.

 

Any headway on this, mine does the same thing until it gets warmed up and am curious if you figured yours out. Check engine light comes on, so I need to get a code reader because it goes away about 5 minutes after driving it, then idles fine.

 

I you figure it out please let me know what it was my is doing pretty much the same thing

 

It appears the IAC is working but it wont hurt to clean it and the passage way as well.

 

Leaking Throttle shafts were the cause of my idle "hunt" problem...I put a new (ported) TB on, and right away idle quality was very improved, and steady.....Before it drove me crazy with it always hunting and taking a while to idle down....Cant say enough good things about my New TB....BTW, I had already replaced the IAC, and it did not change the problem....Rebuilt my old TB, and it did not cure it either.....

 

o60 is there a reason that you did not mention your supplier or cost?

Guys like to know who they can get good stuff from.

 

Get a volt meter and measure the output from your tps. This was my problem and no SES light was on.

 

No I had mentioned it on another thread....CFM Technology about $220..I have ordered several parts from them..great service....You can check your TPS as has been suggested, but generally unless there is a bad problem it affects off idle performance more than idle quality....At least in my case that was true..I tried everything I could think of to get my idle as near perfect as possible..It got even worse when I put the new Eddy intake on, because there is a divider on the new intake.I had replaced the Dizzy, the wires, the plugs, all sensors (ALL).fuel filter, coil, you name it....The TB cured all the idle issues for me..not sure if it will in your case, but if you have the cash its certainly wont hurt anything...good luck to you...Let us know what it turns out to be...Tom