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Old 02-10-2009, 08:32 AM   #1 (permalink)
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What would cause a TBI 350 to idle poorly?

i'm going to have a look at a tbi injected 350 this weekend to swap into my CJ7. the guy that owns it converted it from carb to tbi himself using a factory gm computer and wiring harness. he is a mechanic by trade and the motor itself seems to be pretty healthy with about 15k on it and a mild cam.

the thing is, it won't idle until it's warmed up a bit. you need to keep your foot on the gas for a minute or 2 until it warms up enough so it doesn't stall. he just ordered and installed a new chip hoping to correct the problem, but it doesn't seem to have cured the idle issue. he said it has more power than it did before, quite a noticeable difference he says, but it still doesn't want to idle until it's warmed up. after it's warm, it idles fine. i've seen this motor once last fall and it had the same behavior with not idling when first started. but after a couple minutes it's fine. any ideas what would be causing this? the chip perhaps? the first one (that was just swapped out) as far as i know was the stock factory chip which wouldn't be tuned for the mild cam that's in there. but this new one was programmed roughly to specs to match the cam. he couldn't remember the exact specs on the cam he installed so he tried to get as close as possible when he ordered the new chip. i know next to nothing about chevy motors so any insight would be greatly appreciated.
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Old 02-10-2009, 08:38 AM   #2 (permalink)
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possibly the throttle position sensor or the idle air controls? Thats the only 2 things I can think of. Probably more likely the idle air control on the throttle body.
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Old 02-10-2009, 08:40 AM   #3 (permalink)
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thanks botie...
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Old 02-10-2009, 08:58 AM   #4 (permalink)
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The stock chip should work fine with a mild cam. My 93tbi has an edelbrock cam in it with the stock chip and it runs just fine. That shouldn't cause idle problems.

Along with what botie said I would check to make sure the injectors were clean.

Also, ask the guy what the TB came off of, 4.3, 5.0, 5.7? Same with the computer and the year of the computer. The different TB's all had different sized injectors, and there were different cpmputers used throughout the years. He would have had to know the year of the computer when he ordered the new chip.
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Old 02-10-2009, 09:11 AM   #5 (permalink)
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x2 with the throttle position and idle air sensors.

and i have a rv cam on my tbi wih stock chip and runs fine, PO installed it, but ive never had an issue with rough idle
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Old 02-10-2009, 09:12 AM   #6 (permalink)
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TPS -- my buddy had a very similar problem. It almost wouldn't start when cold.

We plugged in a scanner and it said the TPS was bad and reading something like 90% throttle at idle -- probably enough the set the 'clear flood' parameter when you try and start it. If it gets the clear flood signal it shuts off the injectors when you crank it and it will not start/barely start/not idle/etc.
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Old 02-10-2009, 09:24 AM   #7 (permalink)
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good info, many thanks fellas.

ugly, what was the fix?

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Old 02-10-2009, 10:37 AM   #8 (permalink)
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Best thing you could do is go here and look through the FAQ. Find the initial setup procedures and make sure they were done correctly, odds are they weren't. If that doesn't help then most likely the chip program. There again on that board you can ask for help with the chip..
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Old 02-10-2009, 10:57 AM   #9 (permalink)
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I've seen the gasket between the throttle body and intake give out on a couple vehicles and cause a lot of trouble. You might try spraying carb cleaner around the throttle body with the air cleaner still on and see if it changes the idle any. If you have some kind of vacuum leak it should idle up. good luck
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Old 02-10-2009, 10:59 AM   #10 (permalink)
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thanks binder, i'll check it out.
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Old 02-10-2009, 11:01 AM   #11 (permalink)
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ah.. vacuum leak.. hadn't thought of that. but would that still affect it even after the engine was warm? maybe just not as noticable?
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Old 02-10-2009, 04:12 PM   #12 (permalink)
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if you had a vacuum leak it would affect idle at all times, hot or cold, it would just go away when you gave it gas..
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Old 02-11-2009, 06:19 AM   #13 (permalink)
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that's what i was thinking, but didn't know if it would subside some once the motor was warm.
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Old 02-11-2009, 06:52 PM   #14 (permalink)
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what caused mine to run shitty was the rockers too far outta ajustment
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Old 02-11-2009, 07:09 PM   #15 (permalink)
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Quote:
Originally Posted by brooks294x4 View Post
if you had a vacuum leak it would affect idle at all times, hot or cold, it would just go away when you gave it gas..
Actually, on those gasket leaks it can go away once the engine is warm. The aluminum intake and throttle body expand more and faster than the steel bolts holding it down. The intake and TB expand and can seal up a marginal TB gasket. I know it sounds far fetched, but I've seen it with my own eyes. Of course our extreme temperature swings in this neck of the woods can do goofy stuff like that. That kind of stuff can be a real bastard to figure out.

*edit* Oh yeah...and have you plugged a scan tool in to see the datastream? I'd like to know what your CTS is reading. A faulty CTS will make the cold mixture lean and will not advance the timing enough for a cold engine. Opening the throttle some while warming it up would offset both of those.
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Old 02-11-2009, 07:16 PM   #16 (permalink)
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i agree the gasket between the throttle body and intake was my problem just the other day
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Old 02-12-2009, 08:15 AM   #17 (permalink)
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thanks guys, gives me some good questions to ask him. i appreciate the help.
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Old 02-12-2009, 10:07 AM   #18 (permalink)
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Mine was idling high, replaced the gasket and tried different chips. Turned out to be the Idle Air Control valve. Good luck!
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Old 02-12-2009, 10:33 AM   #19 (permalink)
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My Dads IAC valve would get stuck closed and not go into high idle when cold. I would just spray some carb cleaner in there (IAC port) while it was running so that it would burn off instead of sitting in the cyclinder. That worked pretty well the two times I had to do it. If I had taken the IAC off I might not have had to do it again though.

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Old 02-12-2009, 10:41 AM   #20 (permalink)
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do you know what cam???
does it go away when it is warm?
what is your fuel psi?
chipped ecu?


mine ran funny cause it is like 10:1 comp and a 272 cam i up graded to a holly thottle body and i tuned my ecu.
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Old 02-14-2009, 08:59 AM   #21 (permalink)
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Is the closed loop, open loop, circut hooked up/working? If the computer dont know the engines cold it wont richen the mixture for you.
Like a carb without a choke.
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Old 05-08-2011, 06:05 AM   #22 (permalink)
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hi, i've read all this and for me i have the same problem with my 350 TBI 1994.
it's in a gmc sierra 1994 5,7L
I've change:

cap + rotor
Spark plug and wire
Egr valve
engine coolant sensor

I've clean the IAC valve and the throttle body and i put a new gasket.

i don't know what to do next and my problem is when i'm in park ou in drive and my engine is hot, i have a rough idle but it never stall.

thanks
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Old 05-08-2011, 08:33 AM   #23 (permalink)
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If it was a vacuum it should be idling high all the time. Yes there is a chance that once it heats up the throttle body expands and seals the leak but not with in 2 mins... My guess is the coolant temp sensor is to blame.. If you don't have a scanner to read the computer then just toss one in, I think they are like 8 bucks! Also get a can of carb cleaner and spray the base of the throttle body to see if the idle changes when it's cold. That will tell you if there is a vacuum leak.. But my money is on the coolant temp sensor..

Keep us posted.
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Old 11-01-2011, 07:00 AM   #24 (permalink)
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350 tbi idle

I bought my jeep with the 350 already in it, ok with that said my IAC is bad i tested it and when i cover the hole my idle come back to where it needs to be. I bought a new one and installed and its not working. I read where you need to reset it via the underdash port so it can "find home" or something. The problem is i dont know if that port is still their cant find it any help would be great..
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