TOPIC: '87 to '95 GM TBI Rerouting Idle Air Pathway

Currently I am in Las Vegas Nv, But I just landed a job in Redding Ca. I will be relocating soon.

fellow Gadgetmen,
I just made a discovery I felt needed to be shared and it also created questions.

I went to the wrecking yard and picked up 2 TBI bases. The TBI's were so picked over that I was lucky to find what I did. I noticed that the throttle position sensor was different than the one on my truck, but the wiring colors were the same. I cut the wiring harness connector off so I could adapt it to my truck. BUT.... before I decided to mate it to my truck, I decided to do some resistance checks.

I was surprised to discover that they are not the same. The one on my truck uses the round plug, whereas the one I was experimenting with was a more oblong style plug. here is the differences

The round style plug unit measures 1.853K ohms at rest to 10.10K at wide open throttle
The oblong style plug unit measures 1.039K ohms at rest to 5.61K at wide open throttle
Both track linearly.

I picked the unit up to experiment with so it isn't a real big loss to me, but I wanted to share these findings with you guys to prevent someone from trying to adapt a different style TPS and wonder why they have issues.

Hope this helps.

Reason for this: Most every cable-operated TB has a provision for getting airflow around a closed throttle. TB's often also have idle stop screw like a carb. The Idle Air Control(IAC) is a computer-controlled reversible DC motor that extends/retracts a shaft w/ pintle on the end, this varies the opening for bypass air. This bypass air is used to maintain/control idle speed. IAC opens up more for cold start (like a choke on a carb.) or to increase RPM to support increased loads like from a/c at idle.

Most TB's have throttle blade(s) closed or very nearly so at curb idle or engine off. On a carb. we can turn idle stop screw to vary the idle RPM. And there's a choke for cold idle enrichment and a fast idle cam w/ it's own speed adjust. screw.

W/ fuel injection, there's no choke/fast idle cam or throttle solonoid for A/C. MAF/MAP sensors, Throttle position sensor (TPS), temp. and O2 sensors tell Computer what's going on. Basically that means that if we just allow more air in the ECM will add fuel to maintain operation.

But, factory IAC and IAC air ducts are sized to allow a HUGE amount of air in, to cover every conceivable situation even a really grungy TB and IAC system--
AND often the IAC never completely closes. IAC's dump air into intake manifold below throttle blade(s). That means this air is entering the area of Manifold Vacuum.

Put simply the IAC is a Variable Vacuum Leak!! The Groove increases manifold vac. intensity. any vacuum leak acts to defeat what we are doing.

So we need to restrict how much air can go thru the IAC syatem. Enough to give suitable fast idle speed as needed. And if needed we can adjust the idle stop screw a bit to give needed idle RPM, many/most TB's have one.

BTW- always clean the IAC pintle, shaft and ducts. A REALLY grungy one can stick open, there are documented cases of dirty IAC's sticking open un-expectedly causing sudden high RPM's you can guess the rest!

computer controlled drive by wire TB's usually don't have IAC.

hope this helps, now ya see what this mod to your type of TB is about?

And again, counterman, Who Dat and Where ya At? If you're a real parts counter guy is that auto Parts? If so you likely know stuff that folks here need ta know!

Tracy G

Well in that case you will have to meet GM Tim after you're up in Redding, he's a cool guy!

Tracy G

can you tell me how deep the grooves are?

Counterman wrote: can you tell me how deep the grooves are?

I'm not sure frankly, I'd say about 2 millimeters maybe? I don't know for sure though, just try to gauge it off the pictures, I know it's hard but I can't take the TBI off and measure it for ya!

Wayne, SC
Gadgetman Carolinas
352.507.5309 cellular