GM TBI emission woes

[pamelarose]

When I first went for the emissions test HC @ idle were 2,400 I changed the timing to what I thought was TDC and my cruse numbers got a lot better, HC 98 --> 29 CO .33% --> 0.01% (they passed the first time anyway) but the idle HC's went up to 2900. I fixed a crimp in the fuel return line, ( we had done a tune up when the motor was out 3 yrs ago which is when the fuel return line got bent) and noticed the EGR was rusted out while I was doing that, so I replaced it. While I was at it I replaced the ignition wires which were OE and one was bad . The idle was high with an IAC of 0 and we thought fixing the return line would fix that which it didn't. Sooo we closed the throtle plate a smiggon and the idle went down to specs and the IAC started controlling it at around 6. Sooooo back to the emissions test and idle passed really low this time (HC 112 ppm CO O%) but HC's at cruise went over limits to 395 ppm. I could even hear the pop-pop from the waiting room when it was on the dyno @2500rpm so I guess I have an ignition miss. It really wasnt bad but you could hear it was a miss.

The final numbers were @ idle HC 112 ppm CO 0% and at 2500 rpm HC 395 CO 0.42%. I am going to try to find the vacuum leak which made me have to close the throtel plate to get the IAC to kick in, change the spark plugs, though all of them were pulled, regapped and looked good, and make sure #5 and #7 wires are not too close to each other and cross at 90º. Should I really have to worry about crossfiring through brand new 8mm high quality wires?

Anyone have any tricks for finding a vacuum leak on a computer controlled engine? Any ideas on why cruise numbers went up once we got the idle fixed?

FYI fuel pressure is 9psi (now) and the IAC goes high at cruise.

Thinking the vacuum leak might be leaning something out or leaking injectors ( $150 APIECE ! )?

Any suggestions would be appreciated

Pam

[fordin K]

vaccum check is same for computer vehicles. If any thing, I think I would begin checking sensors, as this would probably put you in the ballpark of which area is probably causing things to get out of specs. I don't know what year is your truck, but I take it with Tbi probably around '91 ? if you haven't updated the cheap stuff I would think now .... like Pcv and the like ...... good luck.

[BrianC974]

Spray carb cleaner at the base of the throttle body, the fuel sprayed into the TB soaks the paper gasket and causes it to deteriorate. If the idle changes when you spray at the base of TB, change the gasket.

Brian