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73-87 Chevy _ GMC Trucks => Fuel Systems and Drivability => Topic started by: p7387 on October 02, 2012, 07:01:22 pm
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87 305 TBI
is it advisable to ck the pressure at the filter or at the TBI?
i went to get a pressure gauge but the only one the local parts store had seemed too wide a range (20-100 psi) to accurately register 12 psi, any recommendations? also my research uncovered the idea of some small permanent adapter which can be installed by the TBI to easily ck pressure any time, is this advisable? once the gauge is hooked up is the pressure measured at the time of 2 sec prime when the key is switched to pos1 or must the motor be running?
concerning the fuel pressure-
is it possible that a lower than ideal fuel pressure could create a sudden crank but no start issue in spite of the fact i clearly see flow from the TB fuel supply and spray from the injectors? it's hard for me to understand that.
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* Use a 0-15 or 0-20 PSI gauge for TBI, which will normally yield a ~1/2 to 2/3 of full scale reading (~11 PSI). If you use a higher pressure gauge, you will lose resolution.
* You can take an accurate pressure measurement anywhere between the fuel filter and TBI inlet.
* Installing a pressure tap anywhere in the supply line that is accessible is convenient, safe and speeds diagnosis. If the service fitting you mentioned is designed for TBI, it should be fine. Otherwise, a brass or steel 1/4" SAE flare (i.e., an R-12 A/C service fitting) with an internal Schrader valve and service cap is a reasonable choice, but be sure to install it in the fuel line using double flare fittings - DO NOT use compression fittings!
* Pressure is measured any time the fuel pump runs - both at start-up and with the engine running.
* Low fuel pressure in a TBI system (or any fuel injection system) can cause all sorts of havoc, because fuel pressure impinges on both spray pattern and volume/rate of fuel delivery. The OEM fuel and ignition maps are calibrated in part to a given injector flow and have a limited range of adaptability. Aftermarket systems are more flexible, but they are also more expensive.