1987 Chevrolet 350 tbi high idle

[AuroraGirl]

So you’re saying I may have “PROM” issues and the front axle off center is normal? I wouldn’t doubt the axle off center being normal if my truck hadn’t been totaled forever ago

Yes you could have a PROM issue but I’m more wondering how your intake job went, you replaced gasket at some point didn’t you?

 

[iamtherealJayy]

Negative ghost rider, I haven’t done anything much to this engine other than changing sensors, spark plugs, tbi gasket etc. I haven’t taken anything apart apart you know? The intake manifold has been off before and I believe that’s part of my slow oil leak from the front of the engine. Why is the intake stirring your curiosity?

 

[iamtherealJayy]

Alright the funky idle is starting to bother me more and more as I drive the k10 more and more, the pressing clutch and rpms staying at 1500 so when you let off in the next gear it throws you around is getting old. It’s time to make some changes. I’m not going to spend a ton of money making patches to this engine if there’s a lot of problems I’ll just rebuild a 350 and carb swap this I guess? If it’s that simple. I’d like yo keep fuel injected for the colder starts but I guess that’s what a choke is for.

 

[AuroraGirl]

Alright the funky idle is starting to bother me more and more as I drive the k10 more and more, the pressing clutch and rpms staying at 1500 so when you let off in the next gear it throws you around is getting old. It’s time to make some changes. I’m not going to spend a ton of money making patches to this engine if there’s a lot of problems I’ll just rebuild a 350 and carb swap this I guess? If it’s that simple. I’d like yo keep fuel injected for the colder starts but I guess that’s what a choke is for.

you havent even tried to do the things suggested like 3x now. 1) PROM sounds like it may literally be it
2) set that idle lower you may have to change the minimum air screw if that isnt set right

 

[AuroraGirl]

Alright the funky idle is starting to bother me more and more as I drive the k10 more and more, the pressing clutch and rpms staying at 1500 so when you let off in the next gear it throws you around is getting old. It’s time to make some changes. I’m not going to spend a ton of money making patches to this engine if there’s a lot of problems I’ll just rebuild a 350 and carb swap this I guess? If it’s that simple. I’d like yo keep fuel injected for the colder starts but I guess that’s what a choke is for.

I looked more into it
There is a function in the ECM called Throttle Follower
and GM has released a TSB on newer vehicles on this exact subject. moral of story is its intended and youre not supposed to coast in neutral anyway so there is no fix. (from GM)

it also helps with emissions, downshifts if someone was a lazy shifter , and keeping driveline from clunking.

I would just stop rolling with the clutch pressed, put it in neutral and let out

 

[iamtherealJayy]

I don’t understand the first thing about prom and computer stuff, I can read about it if someone points me in correct direction but I couldn’t even figure out how to make the aldl cable work. Also I don’t coast with clutch in, I only use the pedal to shift and take off, I pull it out of gear coasting to stops and I even down shift from third to second a lot of the time, when I say rpms stay up when pressing clutch I mean when shifting, say I go up to 3k before shifting from first to second, the rpms only drop to 1500 so when you let out in second it kicks you around since it drops it to 1000 or so as you’re letting out

 

[AuroraGirl]

I don’t understand the first thing about prom and computer stuff, I can read about it if someone points me in correct direction but I couldn’t even figure out how to make the aldl cable work. Also I don’t coast with clutch in, I only use the pedal to shift and take off, I pull it out of gear coasting to stops and I even down shift from third to second a lot of the time, when I say rpms stay up when pressing clutch I mean when shifting, say I go up to 3k before shifting from first to second, the rpms only drop to 1500 so when you let out in second it kicks you around since it drops it to 1000 or so as you’re letting out

Oh,
do you know what size clutch you have? and I would think your idle is maybe too low and its thinking youre gonna stall.

 

[gmbellew]

I don't know if the ECM does something different with the manual transmission TBI during shifting vs how an automatic ECM adjusts parameters. Aside from TPS being at "idle" is there another sensor that detects when the clutch is engaged that let's the computer know to pull back RPM for a shift? or maybe your throttle cable is sticking? if you rev in neutral, does RPM drop quick or hang?

 

[iamtherealJayy]

The clutch is just a factory replacement, I think it’s 12”? But not positive. The idle is usually around 650 if the truck is warm and it doesn’t get much below 1000 for first few minutes of running. It doesn’t matter how warm the truck is it still stays high in rpms during shifting or pressing clutch, I can bring the rpms down in Low and press the clutch and the truck like shoots up in rpms. Revving in neutral the rpms take a few seconds to come back down even with just a blip of the throttle. I know the clutch safety thing has been bypassed in the truck since you don’t have to press it to start the truck. Other than that I don’t know on the sensors, I have a new in box TPS I put in but changed nothing so removed it for GM original parts.

 

[gmbellew]

the RPM shouldn't hang when you blip the throttle in neutral i don't think. I'd have somebody rev it while you watch under the hood and see why it isn't immediately idling down after the blip. throttle cable could be hanging up. maybe a spring is missing somewhere?

 

[gmbellew]

here are some pics I took of the area for another thread for information.

 

[iamtherealJayy]

Mine actually used to have extra springs, @AuroraGirl pointed out that there were carb return springs added on and the return spring js built into the tbi. I removed those springs with no change other than pedal is easier to press, still hard at first kinda so when you get it to move it revs higher than you want. I’ll get a video of a blip of the throttle later

 

[gmbellew]

maybe the return spring on the TBI is weak? could be why somebody added extra springs as a quick fix?

 

[AuroraGirl]

Mine actually used to have extra springs, @AuroraGirl pointed out that there were carb return springs added on and the return spring js built into the tbi. I removed those springs with no change other than pedal is easier to press, still hard at first kinda so when you get it to move it revs higher than you want. I’ll get a video of a blip of the throttle later

Well I forgot about that but good bringing it up. Your cable. either i thought it was stretched or the force over time needed to overcome springs + wear probably has yanked that cable out of spec. in the engine bay how much can you operate it with slack or does it feel/smooth/consistent

 

[AuroraGirl]

Also if your spring is weak or cable is not consistent, the TPS may be either "held" off 0% (.45v if I recall correctly ish?) or have a slow time returninmg to it, or just may inconsistently land at different values, not allowing it to properly learn. is your TPS new? Also how is the vac harness to the map sensor?