1997 5.7L Suburban, ~160k miles too rich both banks

Three tests--all using the schrader valve just behind where the two steel lines enter the intake manifold.

1. Engine running
Result: gage holds steady at 55 psi. NOTE: Specification for this vehicle is 60-66 psi

2. Engine running, then turned off.
Result: gage begins descending as follows:
After 8 seconds from turnoff, pressure 20 psi
After 18 seconds from turnoff, pressure 10 psi
After 28 seconds from turnoff, pressure 6 psi

3. Engine not running, then turn key to start--BUT NOT RUN position.
Result: gage immediately jumped to 64 psi, but then within a second or two started to decline. Did not measure the rate, but seemed to be the same as in #2 above.

Questions:
1. The Helm manual flow chart points to the fuel pulse dampener. What is it, and where is it? I probably can find it if I keep looking, but.
2. I wonder why Helm did not suggest leaking injectors as the reason for the pressure drop?
3. This is strange, I would think low pressure would never result in rich codes. ?

In order to better diagnose the fuel pressure issues, the Helm manual recommends a pinch test†the fuel return line is pinched while running the fuel pump. The Pinch Test Results:

When fully pinched, the pressure did go above 70†once it was 70, and the other time 79. Because of the warning about not going over 75, the key was immediately turned off, and the pressure fell†even if the line was still fully pinched.

Questions:
1. I am in total agreement with the responder who said it is lean. I mean that is what the gauge is saying, but that is opposite of what the on board computer is saying. It is reporting rich on both banks. Can the computer be that wrong?
2. After reading the description by the responders about the pulse dampener, I am thinking maybe I don’t have one. The Helm manual covers several engines. Some have injectors for each cylinder, but some have Central SFI. I have the latter. I guess I am saying that since the responder says it goes on the fuel rail, then that would leave me out?
3. My understanding of the fuel regulator (now) is that it does not regulate by restricting flow. Rather it regulates down by sending extra fuel back to the tank. That being the case, shouldn’t the pressure have held steady with me pinching it? Where did the fuel (the pressure) go?

Sure I could go ahead and replace the regulator (but not today), but that is a lot of work, so I don’t want to be just guessing. From the description in the manual, it seems about as hard as a timing belt, or removing/replacing a carburetor (both of which I have done).

Thanks.

Jim,

Thanks for responding. I can keep looking for the dampener, but I agree with your diagnosis.

Besides, on this vehicle if I want to look at the injectors, I have to open up the upper intake manifold and the pressure rugulator is there, too.

What you say about the codes makes sense, too (although Helm needs to get that in their shop manuals). A rich code could mean things are trying to be rich.

The only thing that points to the injectors are the bleed off when the return line is pinched, and the code for #5. I don't have the Kent-Moore scan tool to let me control each injector.

I will let you know what happens.

RIGHT.

The pinch test seemed to show the fuel pump has some life left in it. (Happy about that)

However, to answer the quesiton about the fuel pump: I don't know. We bought it used 10k miles ago.

Work so far: starter, battery, and vacuum booster (that was fun).

Then wires, and plugs for PM.

We also still have our 85 Suburban, but that was purchased new from the dealer.

Thanks again. I will let you know when I get into the intake manifold to change it.

Finally got into it tonight (been driving the 85 Suburban).

Did the step-by-step in the Chervrolet factory Helm manual up until removing the fuel lines from the bracket that is bolted to the block. [Anybody but me think the GM manuals are generally less helpful than they were 20 years ago? Haynes has simpler instructions--but much less complete]

Everything seems straight forward and generally easy exceept for getting that bracket off, and especially getting the metal fuel lines disconnected from the bracket. I don't even see how I am going to get the line wrench back there (it is pretty crammed up against the firewall with interference from the plug wires), much less the backup wrench on the bracket.

Oh, well it is dark and cold and late and I am hungry--maybe it will all be better in the morning.

Got the regulator replaced and all back together.

Ended up taking the lower off, too and replacing the gasket as we remembered it had leaked months ago. I got help for that because I was afraid of messing up the distributor replacement.

Seems to run fine. Need to cycle it a few times anyway to see the check engine go out. Then I will check the code history.

Although, the pressure level is 55 psi which is 5 psi less than the spec.

I wonder when I will be replacing the fuel pump.