89 K5 TBI PROBLEMS, OR IS IT?

HAYSEED

Active Member
Joined
Jun 8, 2009
Location
WAXHAW NC
the k5 sat for a few years before i bought it. the truck now has 137k on it. basically all new seals, and major tune up to get it right. reliable truck, but it idles rough after warm up. i replaced the IAC valve because the old one failed. the monitor shows lean code but it runs like it's rich. have also overhauled the tbi with a kit from autozone. it's getting proper fuel pressure. any ideas floating around out there?
 
Some of the early TBs had a problem with the spring on the throttle shaft producing a side-load on the shaft. This wore the bushings, creating a vacuum leak. The one on my Burb was replaced once or twice in it's life because of this, and it was the same symptoms. An overhaul kit won't fix it, it's in the body. You may be able to re-bush it, but I've never tried.
 
there is an IAC reset proceedure, I forget what it is off the top of my head but something with setting the ALDL jumper while having the IAC unplugged at a specific time. Basically parks the IAC so its in sync with the counts the ECM thinks it has..

Also check your MAP sensor and vac hose for that. Make sure its going to manifold vacuum and not a higher ported source.
 
thanks for the advice. i'll check all these tips. it's funny that something that runs so good at cruising speeds, will run like crap at and off idle. this truck gets 21 mpg, no lie!
 
My Suburban got 17mpg for the longest time. Motor's old and kinda weak now, not so good these days. Still runs like a champ, though. I've never had to deal with the IAC on mine, but the symptoms are exactly like what my Dad was going through with the TB issue. A little carb cleaner around the throttle shaft should help you decide whether or not there's a leak there.
 
yager said:
there is an IAC reset proceedure, I forget what it is off the top of my head but something with setting the ALDL jumper while having the IAC unplugged at a specific time. Basically parks the IAC so its in sync with the counts the ECM thinks it has..
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From Bill H.:
IAC reset/min idle speed- This sets the throttle plate on the throttle body to a specific rpm. I recommend about 500 rpm for most engines. Procedures- Go to sheet 7 of the pinout diagrams. At the top is the ALDL connector. You need to put a jumper between pins A and B of the connector. These are pins A9(wht/blk) and A12 (blk/wht) on the ECM. This puts the ECM into aldl mode. Turn the key on (not start) and the IAC should drive completely closed. You will normally hear a buzzing from the IAC. If you have the winaldl up and running on your laptop, you should see the IAC went from 145 to 0. Now unplug the IAC connector. turn the key off. Remove jumper. Start motor. If the idle is too low you may have to keep it running. On the front drivers side of the throttle body is the adjustment screw. It may be behind a silver plug. If so just use a nail or punch to poke it and remove it. Then use a #20 torx bit to adjust the idle speed to at least 100 rpm less then your warm idle speed. if 600 then set to 500 rpm. You are looking for the lowest consistant idle your motor will do. When set, turn off motor and reconnect the IAC. I try to keep the IAC count to at least 40 with the motor warm and idling.
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yager said:
Also check your MAP sensor and vac hose for that. Make sure its going to manifold vacuum and not a higher ported source.
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x2
MAP signal should be coming off the rear of the TB (between the supply/return lines) on an '89...
 
It's good to have some GM gurus in here. sounds like i'm not the only one to ever have these troubles. I'll get started checking all of these tommorrow. thanks for the help guys.
 
funny you should mention that. i found 2 vac. ports clogged. gonna take it to work tonight and strip it down and clean it good. guess i didn't clean it as good as i thought when i overhauled it.
 
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