350 tbi high idle? Sticky choke? Constant throttle?

[swervin]

I have 2 gmc k2500 manual 350 tbi's, both have the same issue. When i start one up after sitting overnight i always let it warm up for atleast a minute befor driving (usually longer). When it first fires up its at a fairly high rev, after about a minute it calms down a bit. But it will stay like that (higher rev then i think it should be at, but not as high as it was when first started) for a while until i put it in gear... but inbetween gears or just releasing the throttle or engaging the clutch it stays at a higher rpm instead of dropping down to an idle or back firing a bit (need to work on the exhaust soon). If i pull over for about 5 min until its completely done warming up if not more then its usually fine after that but ocasionally still has higher rpm when it should be at an idle (while shifting gears). After a while of idleing at a decent rpm it will drop way down and people think its about to die. I dont want to have to warm it up for 10 minutes everday befor work, my neighbours hate it enough as is haha. Does anyone know what could be causing this? Sticky choke, dirty sensors, ecu, injectors, timing? Not a mechanic by any means, still learning lots, so anything helps.

 

[GarrettGmc]

What's the rpm range they're having

 

[swervin]

No tacometer.. i dont understand why they would ever decide against not having a tac from 88-94 or 93 on a standard vehichle... would the rpm values help alot to decide between a few different problems? Im on a low budget so i dont want to buy anything i dont have to whenever possible!

 

[Chewy1576]

Check the Idle Air Control (IAC) valve. It’s a stepper motor that regulates the amount of air that passes through the throttle body when the throttle plates are closed for idle. There is a function in the ECM called Throttle Follower that is responsible for the delay in RPM drop by keeping the IAC open when shifting between gears to better match RPM for the next gear than dropping all the way to idle then having to rev back up for the next gear.


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[swervin]

Ill check that out forsure! Deffinatly sounds like it could be whats causing the problem! Is there anyway to check the iac or clean it? Or is it fairly cheap and not worth the hastle? Also forgot to mention the check engine light comes on every now and then, but if i shut it off for a bit then turn it over later its off again. Seems like theres no rhyme or reason to it...

 

[DerekTheGreat]

Could be too that the TBI gasket has **** the bed. A leak there will cause a higher idle than the IAC can compensate for. Also, if the ECM sees a "Stall condition" it will increase the idle until you start it back up or for a set period of time. When I had the old 350 that was running on 7/8 it frequently went into stall condition mode and ran at 1000 rpm instead of 550 when idling. Could also be too that if the gasket is ok and your truck isn't going into stall condition mode that the base RPM needs to be adjusted. Has the truck always had this problem or is it new?

 

[Chewy1576]

Ill check that out forsure! Deffinatly sounds like it could be whats causing the problem! Is there anyway to check the iac or clean it? Or is it fairly cheap and not worth the hastle? Also forgot to mention the check engine light comes on every now and then, but if i shut it off for a bit then turn it over later its off again. Seems like theres no rhyme or reason to it...

I don’t think they are too pricey. The IAC passage could be dirty too and the valve isn’t sealing correctly. Probably wouldn’t hurt to pull the entire TB off the intake for cleaning and new gaskets.


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[swervin]

Could be too that the TBI gasket has **** the bed. A leak there will cause a higher idle than the IAC can compensate for. Also, if the ECM sees a "Stall condition" it will increase the idle until you start it back up or for a set period of time. When I had the old 350 that was running on 7/8 it frequently went into stall condition mode and ran at 1000 rpm instead of 550 when idling. Could also be too that if the gasket is ok and your truck isn't going into stall condition mode that the base RPM needs to be adjusted. Has the truck always had this problem or is it new?

Is the base rpm and timing related? I think it might be a bit out of time asweel, but thats just going off of what i can hear. Im going to check the timing this weekend aswell. What is stall condition? Is that an auto thing? Both of my trucks are standard.

 

[Chewy1576]

Stall Saver is when the ECM detects the RPM dropping too low, so it opens the IAC and gives and extra shot of fuel to try to keep the engine from stopping. It would almost seem like a surge, but unstable and barely running.


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[swervin]

New gaskets and some cleaning is probably past due. I just bought the 88 not to long ago, but i bought the 90 about 3 years ago. I think the 88 just started to have this problem and the 90 might have had it all along... either way it has slowly gotten worse over time. Thanks alot for the input should be a busy weekend ill let you know if any of that solves it!