Originally Posted by jeffg
hi! i have a 1987 gmc pick up with a 305 tbi motor. i pulled out the o2 sensor, egr, and put on headers, and a new exhaust. i have a terrible backfire when i let off the gas, and i dont know what to do... any input would be great! thank you!
This sometimes happens with MAP/TBI, it doesn't have an understanding of what happened. With the headers and duals the engine is able to move more mixture through the cylinders without a change to the relationships from the sensors. The removal of the EGR did the a similar thing in that with EGR gone the volume of exhaust gas is made up with air but the TBI has no way of knowing that so the mixture goes lean. By removing the O2 it's not reading the mixture so it can't make corrections.
The way MAP/TBI systems work is that Manifold Absolute Pressure (MAP) (manifold vacuum if you wish) is mapped on a dyno for its relationship to throttle position, RPM, and load on the crankshaft. A map of these relationships are built and programmed into a Programable Read Only Memory chip (PROM). Since this program is somewhat general as every situation, combination and permutation just can't be anticipated an oxygen sensor is put into the system to provide information on mixture ratio that can be used to fine tune the mixture to the need in the moment.
What you did with headers and elimination of the EGR is to introduce more air into the engine, but the injection system has no way of knowing that because of the loss of the O sensor.
You can fix this in one of two ways:
1) Currently this probably runs OK when cold as the TBI has a "choke" function that adds more fuel under 180 degrees of operating temp. So if you put in a 160 thermostat the computer will never read the engine as warmed and will stay in open loop/cold start mode. This isn't good for mileage nor upper cylinder life. What you're getting now is open loop cold start and once warmed and not finding an O2 reading to enter closed loop function, it's probably dropping into Limp Home mode which is enough function to keep the engine running, just not well.
2) Reinstall the O2 sensor and the EGR which will let the computer rebalance the mixture. Keep in mind that this isn't an unlimited function, the O2 sensor can swing the mixture only within some limits. EGR only functions at cruise, it drops out at idle and WOT. So it's effect is mostly seen at cruise power settings and has no impact on maximum power. So besides upsetting the cruise mixture, its removal has no effect on the search for more power. With the EGR missing the O2 sensor may not have enough total authority to correct the mixture for the addition of headers/duals and the loss of EGR function.
With the customs I build on these older systems I put the O2 sensor into the H or X pipe. This gives it exposure to both cylinder banks. You can also choose one or the other collector or head pipe and just read one side. Since TBI uses a 180 degree, dual plane intake either bank has 2 cylinders feeding from the high side plenum and 2 off the low side, so it will see an average of what the TBI is injecting. The loss of using just one side is that a miss-firing plug or local vacuum leak on a cylinder of the unmonitored side would go unnoticed by the O2 sensor.
For changes that are greater than headers a new PROM with a program is required. Camshafts are limited to about 200-210 degrees at .050 with a wide LSA (110 or greater) to reduce/eliminate overlap. This is necessary with MAP sensed systems because of the need for high manifold vacuum to provide an easily discriminated signal by the sensors. See these guys for info and chips http://www.tbichips.com