1995 5.0l Intermittent Fuel Pressure

jrb

New Member
Dec 5, 2015
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I just bought a 1995 mustang GT with 100K miles. It is having an intermittent starting problem. I can always hear the fuel pump run when I start the car. I attached a pressure gauge to the fuel rail. When I turn the ignition on (but not to start the car), the fuel pressure usually goes up to 35 psi and stays there. However, some times the pressure goes up to 35 then immediately goes to 0. When the engine is running, the pressure is at a steady 30 psi. If I pull the vacuum hose off of the fuel regulator, the fuel pressure goes to 40 psi. I am not sure what to check next. The fuel regulator looks like it has been replaced (it is a Bosch). Also, I am not sure if this is related, but when the engine is under load (like going up a long hill), I can hear pinging (or an exhaust leak) coming from the passenger side of the engine. When this occurs, the CEL comes on temporarily. I checked and it is code 172 for a lean O2 sensor on the passenger side. Any help would be appreciated. Thanks!
 
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There are a few things that could cause the pressure to leak down... the regulator as you mention, a worn fuel pump bleeding down, and ( I 'think' the most likely thing is) possibly a sticking fuel injector, since you mention it's intermittent, and due to the age of our cars.

Symptoms of this would be the fast bleed down of fuel pressure, a lean condition in some cylinders (perhaps the whole bank), and a rich condition in the cylinder with the stuck-open injector. If you're getting a lean CEL (plus the pinging), but at the same time smell fuel or have white smoke coming out of the exhaust, it's very possible this is what's happening. If you're not, then the fuel pump or regulator are more suspect.

You could try running a cylinder balance test, checking your spark plug condition for rich / lean conditions, and replacing the fuel filter as quick & cheap diagnostics. If the O2 sensors are original, replacing those may be a good idea at 100k miles. Many will tell you to clean your MAF as ordinary maintenance too, it's probably not a bad idea to check the EGR valve condition too. Beyond that, I'd suspect the fuel pump since the regulator appears functional (if there's no fuel in the vacuum line, it's even more likely that it's working properly), and the pinging & lean condition seems to happen when fuel demand is high (up a hill).
 
Thanks for the response! I did run the cylinder balance test and it passed. I also cleaned the MAF. There is no fuel in the regulator vacuum line so I think I am going to replace the fuel pump and filter first.