TBI Drivability problems Please Help

[Jason915]

I have an 89 TBI 350 in my 70 K5 and am having a couple of drivability problems. 1st off it surges and hunts for idle when cold and will even die sometimes and when it dies I can hear a weird noise from the IAC. When it's at normal operating temp it runs just fine except for randomly it'll die for no reason when I come to a stop it starts right back up just fine though. At first I thought that it was my coolant temp sensor but that checked out fine. I am now thinking that I have a bad IAC but would hate to spend $40.00 and still have the same problem.

Any suggestions?

[Russell]

Try turning your idle up a touch. There is a possibility that it may be just too low for the IAC to be able to save the engine from stalling when it is cold.

If you've never adjusted your idle screw, you'll need to pop out the little plug with a small screwdriver, and use a torx bit to turn it. Half a turn to a turn should be all you need, and despite being an idle screw, it should not affect your idle speed as the IAC should compensate for the difference.

[Jason915]

Ok thanks Russell, I'll give that a try.

[Jason915]

I adjusted it last night but then pulled code 21 and before I adjusted it I cleared a code 15 but I think that was becasue I was unplugging the CTS and testing it while running. It's still not right but I am still learning how to get it tuned. Check engine light was on this morning and it was real hard to start so I'll check for more hard codes tonight.

[aggiedave98]

I didn't relealize that TBI had an idle screw. Isn't it controlled by the computer?? I think that's what you're saying by saying the IAC controls it, but I'm confused...

thanks.

[cableguy0]

YOU SHOULDNT EVER touch that screw on a stock setup. If the idle needs adjusted something else is wrong.go after the codes its giving you. take the iac off and give it a good lock if its all built up with carbon chances are it isnt working

[Jason915]

It's not a stock set up, and the idle screw was messed with prior to me getting the set up. I didn't realize but the plug was already pulled off. I jumped A&B with the key on to run in field service mode I waited for the IAC to completly close then unplugged it then tried to start it up (instructions from my GM fuel injection tuning and trouble shooting book) Great book BTW. Minimum air wasn't even enough for the engine to run someone really didn't know what they were doing when they attempted to adjust the idle. Once I got it running I set the idle and then pluged the IAC back in. That has fixed my Idle hunt/stalling problem now it runs like it should and no more codes.

Thanks everyone

[aggiedave98]

Can you tell me where to find out what field service mode is? I only knew how to jump a&b for finding the code without it running. I didn't realize you could do that and then start it to find out more info...

[ChevyTech]

Originally Posted by cableguy0

YOU SHOULDNT EVER touch that screw on a stock setup. If the idle needs adjusted something else is wrong.go after the codes its giving you. take the iac off and give it a good lock if its all built up with carbon chances are it isnt working

A little stronger statment they I would have made, but Cableguy is right, that the screw should VERY rarely ever be touched.

Another place that should be checked for a build of deposits is the area in the TBI bores just below the throttle plates. Most of the time, if the minimum air rate (Idle speed screw) is to low, all that is needed is some cleaning. If it is to high there is usually a vacuum leak someplace.

Originally Posted by aggiedave98

Can you tell me where to find out what field service mode is? I only knew how to jump a&b for finding the code without it running. I didn't realize you could do that and then start it to find out more info...

Here is info on the feild service mode:
If you leave the diagnostic terminal grounded ( A&B connected) with the engine running the system will enter the “Field Service Mode” of operation. In the Field Service mode you can tell if the system is in open loop or closed loop by watching the “service engine soon” lamp blink.
In open loop it flashes two and one half times per second.
In closed loop it flashes once per second.

Also when in closed loop, in the field service mode, if the service engine soon lamp stays OFF most of the time the system is running lean, if it stays ON most of the time it is running rich.
No codes can be stored and the closed loop timer is bypassed in the field service mode.

[aggiedave98]

Chevy,

thanks for the info!!

[Jason915]

I'll bet whomever set the min air on the set up I got from the J/Y had a vacuum leak and tried to compensate it by lowering the min air. Before I installed the FI on my K5 I cleaned the TB IAC and everything else so I know the min air and IAC wasn't plugged with grime.

Yeah like chevytech said with A&B together while engine running, it also locks the timing at initial and shows 02 sensor operation; a correctly operating 02 sensor will blink the check engine light at a rate of every other second that lets you know if it’s constantly monitoring A/F and reporting status to the ECM.

[Russell]

Originally Posted by cableguy0

YOU SHOULDNT EVER touch that screw on a stock setup. If the idle needs adjusted something else is wrong.go after the codes its giving you. take the iac off and give it a good lock if its all built up with carbon chances are it isnt working

I noticed that he was running a setup in a 70 pickup, with TPI heads, and figured that it probally wasn't stock

I had a similar problem with my 89 when I put the TBI onto a 73 350, so I figured I'd get him to try it rather than waste money buying sensors.

I do agree with you otherwise though. If the throttle body is on the same engine as it was originally, and suddenly it picks up an idle issue, there is something else that is a problem.

Glad to hear you got her running properly Jason! Enjoy the EFI