Broken Vacuum line casue rough idle/gas smell

[Hill185]

Hello all,

As mentioned in an earlier post, I recently changed the valve cover gaskets in my '91. While I had it all apart, I replaced the plenum gasket, throttle body gasket, EGR gasket, plugs, wires, cap, and rotor. Unfortunately, most of the vacuum lines are very brittle and in the process three broke (pics below) The green goes to the EGR valve, and I'm not sure what the red does, nor the white (smog pump?). The white line is on the passenger side.


The car idles high once warmed up, slightly rough, and the exhaust smells a little like unburnt gas/rich. At first I thought maybe I put the wires on the wrong spark plus, but they seem to check out. I'm beginning to think it has to do with the three broken vacuum lines (no check engine light yet). Any input is appreciated. Thanks for all the help!

[gadget73]

Vacuum leaks make the engine run rough, rich, and idle high. Only the red one is full time manifold vacuum. The green is controlled vac to the EGR valve, white is controlled vac to one of the thermactor valves. I don't remember which is which, but the one down by the pump is thermactor air bypass (TAB) and the one up by the AC is thermactor air diverter (TAD)

The most important one is center rear of the engine, going to the MAP sensor. The sensor is mounted flat, and is about the size of a deck of cards. It has one hose and one plug on it. If that hose is in bad shape, everything runs badly. The manifold needs to be removed to replace that one. Honestly at this point in time it might be worth pulling the intake and changing all of the hoses under it anyway.

[Hill185]

Awesome, thank you very much for the information. I think these cars have 4mm-5/32 vacuum lines. Ill have to pick some up. Thanks again.

[VicCrownVic]

For patching I use 3mm(1/8") ID silicone vacuum tube. The stock lines fit and stay put, so very well could be 4mm stock lines on these cars. I've bought various sizes and colors from the seller below for different applications. It doesn't work too well when the break is really close to a connector and doesn't leave much to put into the patch piece, so for that I'll head to the JY and get at least a new end with more length to splice to if not a full line. (We have plenty of boxes in the JYs here in the Detroit area).
I'm sure there may be better options out there, but that's what I do for my DD beaters.
https://www.ebay.com/itm/SILICONE-VA...G/400302581171

[Hill185]

Great, thank you for the parts Recommendations.

[gadget73]

When the plastic gets to the point of having more patches than line, just replace it with standard vac line and color code it. I've used small zip ties but colored heat shrink is a neater solution.

[VicCrownVic]

I like the heat shrink idea, colored to match even.

Agreed that there comes a point when replace is a better option than repair. So far the only replacements that I have came from the JY. I haven't looked at options for new lines, so don't know if there is anything that might be similar to the stock lines.

[Kodachrome Wolf]

I like the heat shrink idea, colored to match even.

Agreed that there comes a point when replace is a better option than repair. So far the only replacements that I have came from the JY. I haven't looked at options for new lines, so don't know if there is anything that might be similar to the stock lines.

Hard signal call line could work, not sure if heating it would allow you to make the factory style bends though.

[Hill185]

Thanks for the help guys. Replaced/repaired most of the lines. Turned out the MAP vacuum hose had some severe damage. Taking that plenum off is the worst part. Anyway, one last question, anyone have an idea where this large vacuum hose goes? starts at the back of the plenum, but I don't have any open lines to plug it in?

[87gtVIC]

The second photo looks like you have circled the PCV valve. You can kinda tell because some of it is dirty and some of it is not...the part that used to stock into the grommet of the lower intake. If it is in fact the PCV valve it plugs into dead center of the lower intake.

The first picture you have the source line circled for the vac tree mounted to the firewall.

[Hill185]

I popped it off, looks like two different lines but its not. And the vacuum tree on the fire wall is full with the exception of one spot that's capped.

[87gtVIC]

OKay. Thats definitely the PCV valve

The other end of that hose goes into underside of the upper intake but not on that metal tree.

You have some hoses mixed up.




this is from a mustang but its the basics.

[Hill185]

Ugh... Thanks for the pic. Must be from the former owner. Its strange cause the one supposed to go to the tree, is capped off at the tree, and it looks like it came from the factory that way. I took a bunch of pics when I had everything apart. Now I'm wondering what's in the PVC spot now?

[VicCrownVic]

Hard signal call line could work, not sure if heating it would allow you to make the factory style bends though.

Yeah, something that could be heated up with my heat gun and bent to the factory routing would be neat. (Or improved routing, say in a case where you are relocating solenoids on older boxes, by '91 it all seems pretty optimized.)

[gadget73]

If the line at the vac tree was capped off, you'd have only defrost, no power brakes, and no cruise control. There is one capped-off port on the stock tree though.

The pic posted looks like I'd expect it to. The PCV line loops from the front side of the upper, through the T in the corner, to the valve.