So my vac lines split down the seams and my engine would heat the manifold up extremely quickly and melted header wrap zip ties in under a min of idling so i figured it was running lean. I told a mechanic friend of mine who working on a 69gto resto (absolutely spotless!) And he told me since im running just headers that my o2 sensor doesnt have enough backpressure to run properly. This true?

O2 sensors do not need backpressure to work. as long as it is located within a foot or so of the exhaust manifold and there is a few feet of pipe after the o2 sensor so that oxygen cannot migrate upstream to the o2 sensor it should work fine. Obviously you need to fix all your vacuum leaks - check an replace all hoses as needed as this indeed can cause all sorts of problems.

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84 NB, 3800SC, E-30 Fuel, VS Cam, 3.0 Pulley, 4T65E-HD, HP Tuners, AEM Wideband, Regal GS Gauges, S-10 Brake Booster. 12.53 at 106.5 1/4 mile

Ok i put fresh rubber elbow vac fittings on the map sensor, fuel pressure reg and lower intake (only vac lines im running) and the order goes intake, then tee'd so one goes to fpr and one goes map. does the size of the vac hose matter because they are all on a very thin hose.

They just need to be rigid enough not to collapse. Soft rubber tubing from the hardware store won't work. Make sure the ports you are not using are blocked off well.

quote Originally posted by DimeMachine: O2 sensors do not need backpressure to work. as long as it is located within a foot or so of the exhaust manifold and there is a few feet of pipe after the o2 sensor so that oxygen cannot migrate upstream to the o2 sensor it should work fine.

Correct. O2 sensors don't care about back pressure. They need undiluted exhaust gasses (no exhaust leaks) and heat. They have to get to temperature before they work properly. So they need to be located within a foot or two of the engine. If your exhaust system has leaks, they will cause the engine to run rich. Vacuum leaks on the intake side often cause no problems, as the ECM will compensate, unless the leaks are more than the ECM can compensate for. A faulty ECT (engine coolant temp) sensor can affect the fuel ratio. I had one fail, resulting in the ECM thinking that the engine was at -40°F, so it set the mixture way rich, and it ran like crap. If the ECT were reporting a temp that was way high, it would run lean. Also, if your EGR system is not functioning, the combustion temps will go up. I'm not sure how much that would translate to exhaust gas temps, though.