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- 1992 XLT
Recently I was working through a problem on a car of mine, and it dawned on me I had never seen a basic thread on fuel pressure testing in a fuel injected engine on this board. There are a lot of "old hands" on this board, and we forget we have a lot of younger folks in their learning phase. I thought maybe a thread on fuel delivery and pressure testing might be a good idea. This will be a short, basic thread on fuel from the pump to the rail. maybe in another thread we can talk about injectors.
All the pictures were taken on a 1st Gen Explorer.
In a fuel injected engine, we have a fuel pump which pressurizes a line running to the injectors... on the engine it's referred to as a fuel rail. All fuel rails have a pressure test port... a schrader (bicycle type) valve... it has a cap on it, and (keep it in place except to test) when you remove it, here is what you see...
In the fuel injected engine, the electric fuel pump pressurizes the rail, which is often controlled by a fuel pressure regulator. (Engine driven pumps are all but gone from the scene). Any excess fuel not needed or any overpressure is sent back to the tank. So now we have dual lines to a fuel tank. Some fuel pressure regulators are vacum controlled, others are just spring controlled. A bad FPR can cause you fits. And, they DO go bad.
Like other things, FORD has to be different. Their schrader is smaller, so if you are going to use a fuel pressure gauge, you need an adapter. On the 92, the schrader is in a bad spot... you have to finagle the hose under a wiring bundle to hook it up, can you see the adapter?
Maybe this is a good place to talk about a fuel pressure gauge. On fuel injected engines it is an essential tool in your toolbox. They cost about $30. Money well spent. Any old mechanic will tell you that in order, when an engine has trouble, check fuel delivery, then spark. (Course we have alot of other things to worry about now too, but the old saw is still good advice... Especially in a non-start.)
The fuel rail will remain pressurized even after the engine is off. So, to help prevent fires etc, gauges today have pressure relief valves in them so you can sfaely unhook them... here's ours hooked up to the rail, notice the second clear line...
It is the overflow.... here's a closer look at the pressure relief valve on the gauge... a button to push when you are done.
and here's where I run the overflow line.... lawnmower gas I have even held down the button to get lawnmower gas once!
When I hooked up the gauge on the Explorer, which had sat for over a week, I get pressure of:
ZIP
I turned the ignition on until the electric fuel pump stopped (you can hear it run) then I did it again just to be fair to it.... now the pressure was
Good healthy pump. Pressures will vary and you should check spec, but 35-40 is pretty good. With the engine running I had 42.
Ok so we checked our pressure. We're done. [Note: not quite, read Mr. Shorty's post directly below!] All is well. Engine is shut down. We need to release the pressure, instead of spraying gas all over the engine compartment.... so we push the button, and the pressure....goes to....
ZERO!
And the lawnmower gets fed..... see the gas in the line? (frankly I still end up leaking gas all over the place once I remove the gauge set and pull it out of the engine bay, but at least I am not leaking it on a potentially hot engine...)
Unhooking the gauge is just the reverse of hooking it up. make sure you recap the valve... keep it clean.
I strongly recommend that you buy a fuel gauge. It is a beginning part of your serious toolbox on fuel injected engines. You need it rarely, but when you do, there's no substitute!
[I will add to this thread in terms of content about multi-port injection etc later, for now this is it]
All the pictures were taken on a 1st Gen Explorer.
In a fuel injected engine, we have a fuel pump which pressurizes a line running to the injectors... on the engine it's referred to as a fuel rail. All fuel rails have a pressure test port... a schrader (bicycle type) valve... it has a cap on it, and (keep it in place except to test) when you remove it, here is what you see...
In the fuel injected engine, the electric fuel pump pressurizes the rail, which is often controlled by a fuel pressure regulator. (Engine driven pumps are all but gone from the scene). Any excess fuel not needed or any overpressure is sent back to the tank. So now we have dual lines to a fuel tank. Some fuel pressure regulators are vacum controlled, others are just spring controlled. A bad FPR can cause you fits. And, they DO go bad.
Like other things, FORD has to be different. Their schrader is smaller, so if you are going to use a fuel pressure gauge, you need an adapter. On the 92, the schrader is in a bad spot... you have to finagle the hose under a wiring bundle to hook it up, can you see the adapter?
Maybe this is a good place to talk about a fuel pressure gauge. On fuel injected engines it is an essential tool in your toolbox. They cost about $30. Money well spent. Any old mechanic will tell you that in order, when an engine has trouble, check fuel delivery, then spark. (Course we have alot of other things to worry about now too, but the old saw is still good advice... Especially in a non-start.)
The fuel rail will remain pressurized even after the engine is off. So, to help prevent fires etc, gauges today have pressure relief valves in them so you can sfaely unhook them... here's ours hooked up to the rail, notice the second clear line...
It is the overflow.... here's a closer look at the pressure relief valve on the gauge... a button to push when you are done.
and here's where I run the overflow line.... lawnmower gas I have even held down the button to get lawnmower gas once!
When I hooked up the gauge on the Explorer, which had sat for over a week, I get pressure of:
ZIP
I turned the ignition on until the electric fuel pump stopped (you can hear it run) then I did it again just to be fair to it.... now the pressure was
Good healthy pump. Pressures will vary and you should check spec, but 35-40 is pretty good. With the engine running I had 42.
Ok so we checked our pressure. We're done. [Note: not quite, read Mr. Shorty's post directly below!] All is well. Engine is shut down. We need to release the pressure, instead of spraying gas all over the engine compartment.... so we push the button, and the pressure....goes to....
ZERO!
And the lawnmower gets fed..... see the gas in the line? (frankly I still end up leaking gas all over the place once I remove the gauge set and pull it out of the engine bay, but at least I am not leaking it on a potentially hot engine...)
Unhooking the gauge is just the reverse of hooking it up. make sure you recap the valve... keep it clean.
I strongly recommend that you buy a fuel gauge. It is a beginning part of your serious toolbox on fuel injected engines. You need it rarely, but when you do, there's no substitute!
[I will add to this thread in terms of content about multi-port injection etc later, for now this is it]
