97grand4.0

So I see on my little Actron scanner the waveform for my TPS is showing at idle it sits about 16.1% which is what it always sits at, however, lately I notice some idle issues so I checked it and there is a periodic drop out down to 15.8%. The waveform looks like a quick spike down then right back up.
In checking, the throttle plate shaft in the throttle body has movement in it from the 200k miles, and when I jiggle it it will go down to 15.8 then recover. Seems to idle to low. Cleaned the iac.
Engiine off it sits at 15.8.
Does this sound like a tps issue, or has anyone a tip on how to restore the tightness of the throttle shaft short of replacing the whole throttle body?

It looks very much like this but it is not dropping way out like this, only from 16.1% to `15.7 or 15.8

97grand4.0

boo

dave1123

Sorry, no.

97grand4.0

Alrightie. Ill live with it for now. A little extra unmetered air...whats the diff.

dave1123

Worn throttle shaft bores used to be a problem with carbs and would give you 2 idle speeds, one when snapped closed and one when eased closed. Holley carbs had little nylon bushings on the shafts and weren't quite as bad.

Throttle bodies have no fuel going thru them to lubricate the shafts, so I can see where this could be a real problem. I thought the IAC would compensate. That doesn't solve the TPS angle though. My only other thought was the shaft blade that connects to the TPS may be a different size causing a delay. The throttle plate itself may have worn the throttle bore enough to not fit tightly at idle causing air leakage around it. Again, in carbs this will show as lateral motion in the shaft when open.

97grand4.0

Thanks. It does have 2 throttle angles depending. Hard to know if the butterfly is worn as I think you suggest or the bore is worn, or the tps itself is flaky. Regardless, it seems to settle at a couple difft spots depending how you move the throttle.
My buick 300 had a 2bbl rochester that was so worn you could just move it all over. Then I did the valves lifters and holly 500. that small block would hit 60 in first gear no prob at all.

dave1123

Yeah, my 67 SS Camaro had the old Rochester Quadro-jet 4bbl with the 2- and-an-eighth secondary bores that was like dumping a trashcan of fuel into the engine coming off the line, but it didn't idle for beans. (better word starts with an "s")

Actually, I made a lot of money on bets with that carb. I told everybody I had a Carter Quadro-jet and they bet me I didn't. I lifted the hood and showed them. Molded into the side of the float bowl was the inscription, "Manufactured by Carter Carburetor for GM." It seems with all the muscle cars GM built in 67, Rochester couldn't keep up with the demand, so they jobbed them out to Carter. All the parts were interchangeable. The Quadro-jet made it's debut in the 65 396 Police Pursuit.

97grand4.0

Interesting. My buick was a '67 also. Buddys had a 67 malibu 283, and other buddy had '67 LeMans with a junkyard (1965) 389. After he cleaned it up ( chain, lifters) it was turning 112 in the quarter, with the stock rochester and 2 spd turbo hydromatic.. Only later did he find out it was the police and taxi 389 with 11.75:1 compression vs the standard 11.5:1. At that point he went all in and really built it, did the tranny himself and everything in the block but the bearings. He had a theory about his loose bearings and his 30w synthetic oil. That was in '74-79.