Engine rough running - at wits end

[Captain Obvious]

It does respond that fast.

You should see the pressure blip up when you goose the throttle (because there is a quick blip reduction of manifold vacuum).

[Captain Obvious]

But with no load on the engine, it really doesn't (shouldn't) take a lot of throttle to get the RPM's way up sitting in neutral.

Point is... Sitting in neutral there won't be a lot of change of manifold vacuum at any steady state RPM level. And because of that, steady state fuel pressure should be about the same. You should see a small increase in pressure as you raise the engine speed, but not much.

You'll see a blip nearly to 36 psi when you goose it, but steady state should be about the same regardless of the engine speed assuming that speed is constant.

Does that make sense?

[Patcon]

Maybe, so when I rev it, blip it, fuel pressure should spike up, but reving ot up and holding it probably wont spike the fuel pressure like that...

Also what is supposed to hold pressure in the system?  Ie prevent bleed off after shut down

[Yarb]

Fuel pressure regulator. What year fuel rail are you using or should I ask is it the early two piece rail or the later one piece?

[Patcon]

The car is a 77. I am pretty sure the rail is original

[Yarb]

Is the rail solid except where the regulator is plumbed in. Late 77 on had the solid rail.

[the_tool_man]

The rail is two piece. I installed a new FPR when I got the car running years ago. After encountering drivability issues I let it sit. So while it's not new, it probably has less than 2 hours run time on it.

[Captain Obvious]

9 hours ago, Patcon said:

Maybe, so when I rev it, blip it, fuel pressure should spike up, but reving ot up and holding it probably wont spike the fuel pressure like that...

Also what is supposed to hold pressure in the system?  Ie prevent bleed off after shut down

Right. When you blip it, fuel pressure should spike up. But revving and holding probably won't raise the pressure much. Maybe a little, but not much.

And as for holding pressure on the system.... There are two things. First, on the supply side, there should be a check valve screwed into the fuel pump the hold pressure towards the rail. And second, on the return side, the fuel pressure regulator should hold pressure in the rail. The exact pressure number on shut-down is a little fuzzy because of the timing of events, but it should be "non-zero" and probably "between 15 psi and 25 psi". 

We could get into the reason why it's hard to predict on engine shut-down, but it's academic and probably not necessary for what you're working on.

[siteunseen]

You could clamp off the return line coming out the bottom of the FPR to raise fuel pressure. That's how I found out mine was lean and then added the potentiometer to add fuel through the coolant sensor.

[Patcon]

So here is where we're at.

I rigged up a temporary fuel tank with a 5 gallon gas can in the hatch. It's only got a gallon or two in it. The car starts and runs ok but here is the fuel pressure while warming up and the the vacuum gauge that is plumbed in before the brake booster check valve

It was my understanding that the vacuum to fuel pressure reduction should be 1:1?? we are seeing more like 1:2. Should this fuel pressure be 21-22Psi with 15...

Never mind, I think I figured it out. His vacuum gauge is not reading Psi!!! It's reading Hg. 0.5 bar is about 7psi. We had a static fuel pressure of about 35 psi in the start position with the solenoid disconnected. So that looks pretty good from that standpoint.

Which is good and bad. It seems our fuel pressure is regulating properly but our vacuum seems really low. Best places to start? I know he has block off the EGR. How do I verify that it is actually properly blocked?

Also @SteveJ

Battery is August 2022

At idle

Just after shut off. Where to start??

 

 

[the_tool_man]

The EGR looks like it's open because I had to reinstall the EGR spacer to get it sealed. The mounting studs aren't fully threaded and at the time I couldn't get them out. But there is a block-off plate and gasket under that spacer.

[Patcon]

I understand, but I know some of these devices aren't easily blocked off because of how they're designed. I want to be sure that that method is perfectly executed and does not present an opportunity to internally bypass. I was under the impression that vacuum should be higher than that but that could be a units of measure issue again?