Hyundai Repair: Sonata possible fuel issue, fuel pressure gauge, air flow sensor

Question
Morning. I have a '93 sonata, 2.0, 5 spd and it's not running properly. It seems to be starving for fuel. Starts no proplem but idles rough. You apply the pedal it coughs and spits but will catch and rev properly. Taking off from a stop it does the same(spittin and coughin) until I can let off pedal enough, it will catch. Could be a multiple amount of problems but seems like fuel. Fuel filter on firewall was recently changed but car has been around for long time and low kms makes me think could be dirty tank clogging the works? Where do I find the pressure regulator? Can you tell if it's bad? Any ideas I guess. Thanks  

Answer
Hi, Mark.  If you have a tip-in hesitation but after that the car revs up normally, it's unlikely you have a fuel delivery issue.  Under acceleration, constant fuel delivery pressure is required.  Once fuel pressure drops, it doesn't tend to come back until the load on the fuel pump has been somewhat releived.

I'd recommend checking the air intake bellows between the air cleaner and the throttle body.  I've seen cases, where there is a tear in the bellows (usually in the bottom or back where it's hard to see).  When you step on the gas, it pulls the tear open, in turn causing a large influx of air that isn't measured by the air flow sensor, causing a large delay in reaction of the engine control module (ECM) to the throttle application.  

If you'd like to check your fuel pressure, you'll need a fuel pressure gauge and an adaptor to attach to the fuel line where it attaches to the fuel filter.  When the engine is running you should have about 35-40 PSI.  If you remove the vacuum line from the regulator (at the end of the fuel rail), the pressure should increase about five PSI.  If the fuel pressure is operating normally, it should hold this pressure as the car is running.  When you turn the car off, it should go no lower than 20 PSI.  In addition, you can also test the relative health of the fuel pump.  With the engine running, take a pair of needlenose pliers and momentarily pinch shut the fuel return line.  The pressure should jump to 70 to 90 PSI while the line is pinched shut.  If this pressure is at 50 PSI or below, the fuel pump is in poor health.