TBI rough idle - Third Generation F-Body Message Boards

Ward · 10-01-2009, 03:38 PM

Not quite a thirdgen, I'm having this problem on my truck, it's a 95 silverado with the 350TBI - basically the exact same setup as a thirdgen. A couple months back, I re-gasketed the intake, because it was leaking oil at the front and rear seals. I also replaced the cap, rotor, plugs, and plug wires while I had it apart. Ever since then though, it's had a problem idling, it seems to stumble a bit, and stall every now and then. However, at any RPM above idle, it's perfectly fine -- it only stumbles when at idle. No SES light.

I've double and triple checked to make sure I plugged everything back in, and all vacuum lines are connected as they should be. I've also verified that both injectors are firing, and have a good spray pattern. I know on the TPI setup on my GTA, I had to reset and adjust the IAC/TPS after taking the intake apart, is this needed on the TBI setup as well? If so, what is the procedure for setting the IAC? Any other tips appreciated, I'm running out of ideas.

Edit: I forgot to mention, when I had the intake off, I did not take apart the TB. I simply removed it in one piece. Also, the truck has shorty headers and the muffler dumped under the bed, it's medium loud. Is there any chance the loud exhaust could set off the knock sensor?

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891-92camaro · 10-03-2009, 09:30 AM

I have a 92 camaro that i put a 350 truck motor in and i have the same problem and i have yet to figure out what it is. have you checked the map sensor?

8t2 z-chev · 10-03-2009, 11:48 AM

could be a fuel pressure problem,is the idle rough but
with normal idle rpm or is idle rpm down-if idle rpm is
lower than normal,low fuel pressure can cause that.

Ward · 10-04-2009, 02:34 AM

Yeah, I thought about fuel pressure, but it seems odd that it just started doing it after I had the intake off. I bought a new IAC, I'm gonna try replacing that, and resetting IAC. The idle RPM is fine, it just stumbles every few seconds, and drops down a little, then goes back to where it should be.

RFmaster · 10-04-2009, 12:29 PM

Take a look at your TPS. Over years in service TPS resistor may develop a worn spot right just above idle position.

Remove TPS, get a DVM to measure resistance and measure between pins A & B (or C and B). Slowly move TPS shaft from resting position (idle) toward WOT. Any sudden change in resistance indicates worn spot in TPS variable resistor. The tricky part about this test procedure is making a good contact between TPS connector pins and DVM test leads. IRC, 95 uses a compact 3 pin connector so a JY pig tail comes handy or tiny alligator clips.

Good luck, let us know.

//RF

Ward · 10-13-2009, 03:52 AM

Well, just to update, I replaced the IAC and it seems to have fixed it. The old one didn't really look all that dirty, I probably could have just cleaned it, but for $20 it was just quicker to replace it. I did notice that the little round IAC gasket was broken, that may have been the issue.