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Low fuel pressure often has this symptom: Engine idles fine, but when you "give it the foot" it runs poorly, backfires or runs slows down. High fuel pressure can cause poor fuel economy or flooding (raw fuel running down throttles). Perform the fuel pump pressure test to be sure the fuel pump is working correctly.
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Overview
Testing the fuel pump is easy and doesn't require exotic tools.
Caution: don't let the gasoline spill!
If you find the pressure and the volume are within spec, you can be confident the pump is OK, and start looking elsewhere for cause of poor running, such as tune-up, ignition or carburetion problems.
Pressure test
Use a tee line to tap your pressure gauge into the fuel line near the carb -- after the fuel filter.
Pressure with engine running should be 2.6 psi for the A12 engine. For other Datsuns between 3.0 and 3.8 is normal at all engine speeds.
- Below 2.6 is bad
- Above 3.8 is bad
If the pressure is significantly higher with the stock fuel pump, replace the pump. If the pressure is low, replace the pump
Pressure Gauge
All that is needed is a simple vacuum/pressure gauge. Borrow one or buy one (about $25 at Sears).
Most troubleshooting vaccum gauges can check fuel pressure too
Volume test
- Disconnect line from carb, attach a hose going into a quart/liter container. I've used soda bottles and tin cans
- Start engine and run at 1,000 rpm for one minute (use a stopwatch)
450 cc or more should be pumped
If it pumps less than 450 ccs, try the same test but without the fuel filter
- If it improves, replace the fuel filter
- If it doesn't the pump is likely bad. It could also be a clogged line, but this isn't too common unless the car has been sitting for years without running
Empty Fuel Bowl
If the carburetor is empty, as viewed through the carb site glass, then it will take considerable cranking to fill the bowl. The engine won't start until the bowl is about 1/2 full.
Causes
Low
Low pressure or low volume can be caused by:
- Clogged filter
- Bad pump
- Clogged fuel line - especially if car has been sitting for years
- Aftermarket fuel shut off valve (fuel cock)
- Engine starts, but won't idle: Anti-diesel valve
High
NOTE: High volume is no problem. It will not cause carburetor problems.
High pressure is bad. Causes include:
- Incorrect or out of spec Fuel Pump spring. The spring controls the pressure. Replace the pump.
- Electric fuel pump. Replace with stock type mechanical pump
- many electric pumps are too high pressure
- fuel pressure regulator -- many go bad after some time
NOTE: A blockage in the fuel line or carburetor cannot cause high pressure. The pressure is controlled internally by the stock fuel pump.
Ditto for lack of return line -- it won't cause high pressure output from the pump. However if the tank is pressurized it can push fuel through the pump. To rule this out, repeat fuel pressure test with the fuel tank filler cap off.