Rafi1123

... TBI vacuum line

Hello all, new guy here and to the 3rd gen world. I tried searching the forums but could not find what I was looking for.

I have a 1990 RS with the 305 TBI. It has over 200k and obviously needs some work. It does run well, except I have an issue with engine stumbling and sometimes stalling around 1/4 throttle. It idles fine and runs fine if you give it more throttle. I've been scouring the internet looking for answers. I did notice something interesting under the hood of my camaro.

The previous owner had put an aftermarket air cleaner on it. I was tracing vacuum lines when I noticed according to the diagram under the hood, line "S" coming from the TBI is supposed to go to the "Temp Sensor" and then to the "VAC Motor" which (I assume) are both located in the stock air cleaner assembly.

Mine, however, comes out of the TBI and is plugged with a stick. No joke. If I have the engine running and up to temp, I pull the stick and I can hear the idle jump a few hundred RPMs. When I stick the stick back in the line the idle comes back down.

Could the stick and the missing temp sensor and vac motor be the cause of my stumbling woes?

Thanks in advance and here are some pics to give you an idea of what the hell I'm talking about. The red X in the diagram is where the stick is stuck in.

Thanks again!

Schurkey

You have a full-time warm-air air cleaner assembly. The original air cleaner was temperature controlled via an air door that blended heated air with outside air. The original air cleaner would reduce throttle-body icing in cold weather, speed engine warm-up, improve cold-engine fuel distribution; yet prevent overheating of the incoming air once the engine--and summer temperatures--increased.

Since you don't have the temperature sensor or the air door, the vacuum source has been plugged. The engine might (MIGHT) run without surging with the original air cleaner and vacuum system...but...if that car was in MY driveway, I'd be using a REAL scan tool to look at the computer data stream for clues to the poor part-throttle performance.

A failed TPS is my first thought, but there are other things such as improper EGR operation that could also cause the problem you're having with part-throttle stumble.

It would be worth your time 'n' trouble to verify that ALL the tune-up items are in reasonable condition--good spark plugs, usable plug wires, cap, rotor. Decent fuel filter and fuel pressure. No cylinders with weak compression, O2 sensor not ancient, proper ignition timing including functioning spark advance, etc.

Rafi1123

Thank you for the detailed response. I was told a tune-up was done recently, but I plan on doing another for piece of mind.

If problem persists, then I will look into the TPS first, that seems to me the most logical.