long crank

[SUBURBAN5]

@Schurkey makes a good point about checking the actual guage itself...I think something happened to mine and I had buy another guage from the local parts store, but I don't recall being any difference in readings in my case.

I actually dislike those pressure guages. The hose is really short and when you tighten up the fitting it seems like the guage dial always wants to face back. I've thought of looking for like a 3' extension or rigging my own. I've seen YT videos and the guys with the MAC and similar pro guages and those come with long hoses. Would be nice to stick to your windshield and go driving to see what pressure is under actual driving conditions for diagnosing.

Yeah in my case I've used 2 different types of fuel guages. Same readings lol. As for the short ones. I hate it to. Hf (my newest guage) is short. I had to rig it to make it longer. Also doesnt have a bleeder for the fuel.... but other then that works for what I needed it for

 

[kennythewelder]

I understand what you mean. Yes it's a mpfi. Everything practically new just periodically check fuel pressure. Runs good for the most part lol. I've just wondered why everybody says 60-66psi and I've never achieved that . Highest I've seen is 60 but that like on the first prime, then it kicks down to 55 and stays there for the most part.

I have replaced the fuel pump in my 97- 5.7 L about 6 times in the 19 years I have owned the truck. You can check this info for your self. Before you start the truck, the ECM, cycles the fuel pump for 2 seconds. This is where you will have 60-62ish PSI. Once the truck starts, the fuel pressure will fall into the mid 50s. If you rev the engine, the fuel pressure will come back into the low 60s, but once the RPMs fall, the fuel pressure will fall back into the mid 50s. All 6 fuel pumps I have, installed in my truck, did this, ALL OF THEM. There is no adjustment on the OE system for this. The pressure is regulated by the fuel pressure regulator, and that is inside of the intake manifold, on the fuel Spider itself. There are a ton of videos on youtube that will also back up this info. NO, this is NOT something I saw on YouTube. This is something I have seen with my own eyes, first hand on my own truck. The fuel pressure regulator works off of vacuum. So more vacuum = more fuel pressure untill it is a max on that regulator, and the less vacuum = less fuel pressure, to a point.

 

[SUBURBAN5]

I have replaced the fuel pump in my 97- 5.7 L about 6 times in the 19 years I have owned the truck. You can check this info for your self. Before you start the truck, the ECM, cycles the fuel pump for 2 seconds. This is where you will have 60-62ish PSI. Once the truck starts, the fuel pressure will fall into the mid 50s. If you rev the engine, the fuel pressure will come back into the low 60s, but once the RPMs fall, the fuel pressure will fall back into the mid 50s. All 6 fuel pumps I have, installed in my truck, did this, ALL OF THEM. There is no adjustment on the OE system for this. The pressure is regulated by the fuel pressure regulator, and that is inside of the intake manifold, on the fuel Spider itself. There are a ton of videos on youtube that will also back up this info. NO, this is NOT something I saw on YouTube. This is something I have seen with my own eyes, first hand on my own truck. The fuel pressure regulator works off of vacuum. So more vacuum = more fuel pressure untill it is a max on that regulator, and the less vacuum = less fuel pressure, to a point.

Awesome I appreciate the debunk lol. Glad I dont have to go down a rabbit hole

 

[kennythewelder]

Awesome I appreciate the debunk lol. Glad I dont have to go down a rabbit hole

Especially with a Suburban. I can change just the pump. On a Burn, you have to change the complete unit. IDK why, but you do.its cost about 3 times as much for the complete unit VS just the pump.

 

[Schurkey]

Before you start the truck, the ECM, cycles the fuel pump for 2 seconds. This is where you will have 60-62ish PSI. Once the truck starts, the fuel pressure will fall into the mid 50s. If you rev the engine, the fuel pressure will come back into the low 60s, but once the RPMs fall, the fuel pressure will fall back into the mid 50s.

Sort of.

RPM has nothing to do with it. It's dependent on intake manifold vacuum. You rev the engine, vacuum drops, fuel pressure increases. RPM falls, vacuum increases, fuel pressure decreases.

The fuel pressure regulator works off of vacuum. So more vacuum = more fuel pressure untill it is a max on that regulator, and the less vacuum = less fuel pressure, to a point.

Other way around. More vacuum = less fuel pressure. Stab the gas pedal, vacuum drops to nearly nothing, fuel pressure is at max.

Point being, the injectors are firing into whatever vacuum the engine is developing. When the vacuum is high, it takes little fuel pressure to create a significant pressure drop into the deep manifold vacuum. OTOH, with heavy throttle, manifold vacuum goes away, there's atmospheric pressure or close to it, so it takes more fuel pressure to squirt into the atmospheric pressure in the manifold.

Couple that with the fact that high manifold pressure typically means you're looking for horsepower--fuel delivery needs to increase.

 

[kennythewelder]

Sort of.

RPM has nothing to do with it. It's dependent on intake manifold vacuum. You rev the engine, vacuum drops, fuel pressure increases. RPM falls, vacuum increases, fuel pressure decreases.



Other way around. More vacuum = less fuel pressure. Stab the gas pedal, vacuum drops to nearly nothing, fuel pressure is at max.

Point being, the injectors are firing into whatever vacuum the engine is developing. When the vacuum is high, it takes little fuel pressure to create a significant pressure drop into the deep manifold vacuum. OTOH, with heavy throttle, manifold vacuum goes away, there's atmospheric pressure or close to it, so it takes more fuel pressure to squirt into the atmospheric pressure in the manifold.

Couple that with the fact that high manifold pressure typically means you're looking for horsepower--fuel delivery needs to increase.

Yes, that was my whole point. Vacuum is what regulates pressure. And yes, you are correct, I did have it backwards. Less RPM = more vacuum.

 

[stutaeng]

I've used this video several times for my use and have posted it before as well.

It's an excellent source of the theory and mechanics of the fuel system.

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Unfortunately, the spider setup makes it somewhat of a challenge on these checks...at least they gave us a Schrader valve, LOL.

 

[honkon]

This truck has a computer and sensors which you can use to diagnose a fuel delivery problem without ever popping the hood...