So here's my take on how to check your fuel pressure on a 88-96(I'm not sure on the years) C/K1500 trucks that come with the TBI throttle body intakes on them.
There are two ways to do it. I have done it both ways, but I'll outline the pros to each way.
The actron fuel pressure tester way:
Order the actron kit for $38 from amazon, or call them up and order it. You'll need a 3/4" open end wrench, and a 3/8" (I can't 100% remember, but it could also be 7/16") line wrench. Take off your air cleaner, and stand on the driver side of the truck. Look at the back of the TB and put your open end wrench on the TB's fuel line nut closest to you (See below picture), and the other on the actual fuel line. In my case, my fuel line is a braided steel line. I have no idea if that line is factory or not, but it was like that when I got the truck.
Next you take the fuel line adapter form Actron, and screw it into the TB first. This makes it easier to get situated behind your throttle cable bracket. DO NOT OVER TIGHTEN! Your adapter is made of brass, and will easily strip inside those steel threads. The actual schrader valve on the adapter will swivel around, so don't worry about where it's pointed. Connect your fuel line. Same deal, DO NOT OVER TIGHTEN! Just get them good and snug. You can tighten the line and adapter, just don't go crazy tightening it down.
Connect your fuel gauge before you start the truck. This fitting is what was used on old R12 air conditioning systems, and leaks quite a bit while your connecting the gauge. Leaking gasoline on top of your engine can be dangerous ... you have been warned.
I suggest you tuck the gauge under the hood and hold it by your door while you stand outside the truck and crank it. That way you can watch the truck prime, and how the pressure looks after it's running.
Acceptable readings, as per what General Motors says, is between 9 and 13PSI. As far as I'm concerned, a little higher than 13 PSI wont hurt, but anything lower than 9 will definitely have an impact on performance.
Turn off the truck before disconnecting the fuel pressure gauge.
Second method, is the generic, OTC brand fuel filter adapter.
For this, you'll need to order the OTC 7665 fuel pressure adapter for $53.95. There are other brands who make this same adapter to do the same exact job! This adapter is just the first one that came to mind.
For this, you'll need a fuel pressure gauge, PB blaster or other penetrating oil/lubricant, the same line wrench, and possibly an assortment of other wrenches to hold the fuel filter. If you have not done it already, I would suggest you also buy a new fuel filter, because you'll have your old one off already.
Have buckets and lots of rags or blue paper towels handy, because your fuel lines will be gushing their fuel while you fiddle with the filter. The filter is located on your frame rail just underneath where the driver seat is. At least, that's where it is at on my standard cab, long bed 2x4 truck. It could be elsewhere for you.
You'll want to hold the fuel filter wile you try to loosen the line nuts with your line wrenches. DO NOT USE ANY OTHER KIND OF WRENCH ON THESE NUTS! The line wrenches grab all sides of the nuts, and pose a much smaller change of stripping them. Take your time, if your fuel filter has never been replaced, your lines will most likely be rusty.
Your goal is to remove the fuel filter, and install the adapter in the filter's place. So if you break your fuel line trying to get the filter out, it will need to be repaired before you can test your fuel pressure.
Personally, the fuel filter adapter that I used, had a really neat quick disconnect fitting that didn't leak any fuel when I connected it. So I did it with the truck running, but if yours leaks fuel easily, I would recommend you connect it with the truck off. There is hot exhaust right in the path of the fuel stream, if it squirts out of the schrader valve.
You can leave the Actron adapter there, but the filter style adapter needs to be removed when you are done.
Something I observed about the Actron tester. The adapter swivels quite nicely around it's fuel line, but the O-rings don't have the most perfect seal. Be careful when moving around the fuel gauge, as if you pull on the gauge, you can have fuel gushing out around the edges of the adapter. It needs to be pretty centered and with no stress on it for it to not leak.
I like the Actron tester the best, because it comes with a gauge that makes reading low pressure easy, it also makes cleaning your injectors easy with a professional cleaning machine, and you don't have to lay under the truck to mess with the fuel lines and filter.
You should check your fuel pressure at idle, under load (brake torquing(BE CAREFUL!)), and momentarily with your fuel return line blocked in some way. I wont begin to suggest how to do that, because mine was pretty easy to do.
If you're uneasy about doing this kind of work, take it to a professional! Gasoline is highly flammable, the fumes are dangerous, and you can risk damaging your fuel lines!
PS, here's what the actron tester looks like (I bought it and really like it):
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