Engine running rougher

[delco1946]

I just had my booster rebuilt and a new master cylinder installed to address a huge booster leak I’ve had since I bought the truck. However instead of running better the rpms have dropped so much that it practically wants to stall. In putting around the bay last night I noticed that when I reconnected my egr vacuum line ( disconnected as it was the straw breaking the camels back) it stalled my engine in park. I then noticed the line had light suction on it. It shouldn’t right? I thought it was ported but the only PV connection goes to the intake manifold whereas the egr valve is connected to “BV” on the vacuum diagram.

[Gary Lewis]

I got a bit lost on the EGR valve.

But first, your air/fuel ratio and engine idle RPM was probably adjusted to accommodate the extra air being admitted via the bad brake booster.  So now you need to screw the two idle mix screws in 1/2 turn at a time until you get the engine to idle best.  And then you need to adjust the idle stop screw to get the idle speed set correctly.  You can see those screws here: Documentation/Fuel Systems/Carb, Chokes, & EFI/Motorcraft 2150.

On the EGR valve, you are talking about vacuum line, right?  If so, it should not have vacuum at idle as that will cause the EGR valve to open.  Maybe you have it connected to the wrong port?

[Frank Wyatt]

You will likely need to adjust your carbs idle screw as it was previously set with a vacuum leak through the leaky booster. With too low of an idle speed, it would likely stall when put in gear whether the EGR was hooked up or not. If connecting up the EGR stalled your engine then you need to check for a vacuum port that doesn't have vacuum at idle with a vacuum gauge if you have one.

[delco1946]

Found my vacuum diagram!

[Gary Lewis]

So, is your EGR hooked up correctly?

[delco1946]

So my EGR valve is connected to a vacuum line coming straight out of the carb, perpendicular to the length of the truck. In other words straight out towards the right side/passenger side of the engine bay.....which appears to be correct. However, there never used to be a vacuum from this line when at idle and now there is. Even weirder is that the engine idle used to be much faster and now its much slower. If anything, I thought no idle should be present at idle and then at higher rpms, the vacuum would slow increase.

What would cause vacuum to be present as shown in the diagram?

And I will adjust the idle screws, per your recommendations.

[delco1946]

Will i think so! But the problem is that its now getting vacuum at idle. Even if my TVS and VCV (temp switch) in the radiator hose both failed open, i shouldn't get vacuum at idle to my egr.

[Frank Wyatt]

Sometimes, actually most if not all the time the vacuum hose routing actually inter-connects so the ported vacuum devices, the vacuum advance for an example, gets manifold vacuum through the VCV if it reaches it's preset temp. This is done so the timing being advanced will raise the engine RPM to speed up the engine fan to provide some additional cooling. The VCV on my truck is not being used and all devices needing ported vacuum is hooked up to ported vacuum directly. I'm sure you'll get it sorted out.

[grumpin]

When I got my truck most of the emissions were gone. And not done correctly IMHO.

I tried to get my EGR valve to work but without everything in the system, I could not.

If the vacuum line goes direct to the valve with nothing in the line you’ll have vacuum all the time.

[Frank Wyatt]

On the YFA that's on my truck, the ported vacuum has 0 vacuum at idle. If his is getting a small amount of vacuum at idle maybe a vacuum delay valve or restriction of some type might help. Not sure though as ported vacuum should not have any measurable vacuum. A vacuum gauge to check how much vacuum is present on each of the vacuum ports would help.

[delco1946]

I guess what im asking/wondering is have i connected to the correct connection under the carb? All the gizmos in line with the vacuum are restrictors, delays valves etc, so those only act to smooth and modulate vacuum fluctuations. The temp switches will open with a hot engine. But still with a hot engine at idle, i should not have vacuum at my egr, right? This make me question if the line is connected to the right port at the carb.

I'm curious if anyone with a properly working 5.8L 351m can share where their egr vacuum source comes from?

[Frank Wyatt]

Here's a vacuum port diagram for a Motorcraft 2150 carb I found on FTE. The "spark" port would be the ported vacuum source. https://www.ford-trucks.com/forums/1424779-for-anyone-who-needs-a-mc2150-vacuum-port-diagram.html I hope you find this helpfull.

[delco1946]

hmmmm that's a very interesting picture, thank you for that. I've been thinking about rebuilding my carb. i wonder if i might have some internal carb vacuum issues? It looks like my vacuum lines are all connected correctly. When i get home i play around with this more. Looks like the vacuum could easily be messing with my vacuum advance system.

[Frank Wyatt]

Yes, a bad/wrong base gasket could allow the ported vacuum port to get some vacuum all the time.