Vacuum Gauges

[redmen62]

Hello everyone

I think I finally have caught up on all the show stopper problems with a new to me 83 Regal 277 with twin merc 228's and (new) counter rotating alpha one drives and can now start getting it to how I want it. I'm starting at the helm and checking all the gauges, lights, and switched (I know I have some electric gremlins hiding under the dash) and have NO clue what 2 of my gauges are measuring...

I have 2 vacuum gauges that have been added at some point in time. They look newer than everything else and are not a part of the standard cluster on the dash. Anyone have any idea what they might be measuring? My initial thought is fuel pressure; since I have oil pressure gauges on the dash, that's the only other thing I can think would make sense to measure

 

[GA_Boater]

I think they measure...er...um...Vacuum. :smile:

Do they work when the motors are running? There should be a hose from the gauge to the intake manifold or carb base. If the hose is connected to the carb, you should verify you have marine carbs. Marine motors don't use vacuum for any engine control like spark advance and carbs with vacuum ports are not marinized.

 

[redmen62]

Well I figured that much lol

Yes they work when the engines are running, as revs increase so does the vacuum. Carbs are for sure marine carbs, they have mercruiser tags on them. There has to be some vacuum on any engine... otherwise you wouldn't have able to hold pressure in fuel lines. No vacuum means no fuel to either the tanks to the pumps, or the pump to the carbs once you shut off the engines and the fuel pumps aren't pumping

 

[Bt Doctur]

There has to be some vacuum on any engine... otherwise you wouldn't have able to hold pressure in fuel lines. No vacuum means no fuel to either the tanks to the pumps, or the pump to the carbs once you shut off the engines and the fuel pumps aren't pumping

If they are vac gauges they should be around 14-21 at idle and go to 0 at full throttle

 

[Maclin]

Vacuum will not normally (or ever) go up in a linear fashion with the throttle. If those are measuring Vacumm (in HG) then it will be anywhere from 12-16 at idle and be pretty steady unless problems with the engine, and dip to zero as the carb is opened up, then go back up to way higher readings on the way back down to idle.

 

[GA_Boater]

Of course there is vacuum in an engine. Vacuum is the suction to draw air into the motor. I don't understand the fuel pressure, fuel pump and the rest in relation to vacuum. The fuel pump isn't dependent on vacuum to produce fuel pressure.

Anyway, my mention of marine carbs was purely that they have no vacuum ports. So the hoses in all likelihood are plumbed to the intake manifold. I don't believe vacuum gauges have much day to day purpose in a boat, but can be a good diagnostic tool in some cases.

 

[funk6294]

If the vacuum is 0ish at idle and only goes up when the the throttle advances it could be that it is hooked up to a timed vacuum port (above the throttle blades) and not manifold vacuum (below the throttle blade). It will go up if you are just slowly increasing RPM at idle, but under load it should start to drop off to 0 as the throttle approaches WOT. The proper way to connect one is via manifold vacuum, but most marine carbs don't have a lot of extra manifold vacuum ports. They may have 1 for a pcv valve but that's about it. You can either T it into the pcv port if you use a pcv valve, or add a barb fitting to the manifold. The vacuum gauge is a pretty handy tool to have permenantly connected. Great for monitoring engine condition, diagnosing problems, and from what I hear optimizing cruise efficiency.

 

[Silvertip]

Just check where the hoses are connected: as in 1) directly to the intake manifold, 2) at the base of the carburetor below the throttle plate, or 3) above the throttle plate. All three of those could produce differing vacuum readings. The "fuel economy" gauges in older cars were nothing more than a vacuum gauge calibrated in good, bad, ok bands. Hammer down and vacuum goes to zero. Idle it sits steady in the range of 14 - 15 give or take a bit depending on engine condition. Good fuel economy was at whatever throttle setting got you closest to the 14 - 15 range indicating the engine was not lugging excessively. The prior owner probably used the gauges for that exact reason as he/she understood how engine vacuum works.

 

[achris]

I suspect that a PO has put those vac gauges in as a cheap mans' synch gauge. To try and make sure that both engines are loaded the same. Being that intake manifold vacuum changes with load as well as engine speed. (and has absolutely nothing to do with tank or fuel pump pressure)....

Chris.........

 

[Maclin]

I had a car back during the late 70's gas crisis. It had been "born" during that crisis, and had an option called Fuel Pacer. Fundamentally it was a vacuum line that ran to a resistor/switch that would light up the left turn signal indicator in varying degrees of brightness as the vacuum went below a certain HG. When I told a hot-rod buddy about that he said he could not believe I had not burned out the bulb yet.