7 replies to this topic

[philip663]

Hey all, after some research iv decided my vacuum pressure is to low and not right!!

 

It ticks over at 16-17 on a smith vacuum gauge, well for that matter sticks at 16-17 costing down a hill or when you lay off the throttle.

 

As far as im aware it should go higher when you lay off the throttle going down hill??

 

The engines pretty new, rebuilt SPI 1310 with going on 1800 miles running smoothly as far as im aware getting 45ish mpg doing motorway style driving, Im sure i saw the gauge above 20 when i first got the engine running but cant be sure and its been at 16-17 ever since then.

 

I have changed the manifold and exhaust gaskets as i presumed it would be them, but no such luck. all breathers are new or what not, no blockages etc

 

Am i right in thinking its wrong?

 

Phil. 

[sledgehammer]

firstly - the gauge could be faulty - and is the needle steady or flicking ?

 

It can make a difference where it is piped in as well

 

but with that mpg - the engine can't be in too bad shape

 

I would check valve lash , do a compression check , plug gap , timing

 

see if it goes up

 

best of luck

 

ps anyone know where to plumb in a vac gauge on a SPI / MPI ? - I don't have a clue - near the throttle body ?

Edited by sledgehammer, 15 December 2013 - 11:07 PM.

[philip663]

firstly - the gauge could be faulty - and is the needle steady or flicking ?

 

It can make a difference where it is piped in as well

 

but with that mpg - the engine can't be in too bad shape

 

I would check valve lash , do a compression check , plug gap , timing

 

see if it goes up

 

best of luck

 

ps anyone know where to plumb in a vac gauge on a SPI / MPI ? - I don't have a clue - near the throttle body ?

 

Thanks, its in for mot atm so shall do comp check on its return so long as it passes its emissions lol. But as far as im aware its working fine, a steady 16-17

 

by the way i plumbed mine in to the back of the manifold one one of the take off's, the one used on the air filter as mine just wasnt working so put it to use on the vac gauge. 

Edited by philip663, 15 December 2013 - 11:26 PM.

[dklawson]

Vacuum gauges need to be connected directly to the intake manifold in most instances.  For SU carbs there are also insulator/spacers that have vacuum takeoffs tapped into them.  Be careful if connecting directly to a carburetor or to things like PCV valves as the vacuum there may not be what you expect.  Many intake manifolds have taps for brake servos.  As long as a check valve is NOT screwed directly into the intake manifold, you can "T" off of that line for your vacuum gauge.  Alternatively you can tap the intake and add a connection for the gauge.  I have no idea what is available or suitable on an injected engine.

 

When decelerating in gear, a vacuum gauge will typically shoot WAY higher than 20 inches.  Since you say your gauge does not ever read higher than 16-17 inches, it suggests that the gauge is out of calibration.  I suggest buying a tune-up kit that includes a vacuum/pressure gauge.  They are often supplied in kits to both measure low fuel pressure and engine vacuum.  If that tune up gauge matches your Smiths gauge, look for intake manifold leaks.  If the gauges read different from each other either replace your Smiths gauge or pay to have it calibrated.

[carbon]

There's not too much to go wrong with these vacuum gauges, they work in a very similar way to the Smiths oil pressure gauge / mechanical water temp.

 

There is a gear wheel inside driven by a 'rack' attached to the pressure tube. I have had these get sticky in the past, a judicious squirt of WD40 may help.

[philip663]

There's not too much to go wrong with these vacuum gauges, they work in a very similar way to the Smiths oil pressure gauge / mechanical water temp.

 

There is a gear wheel inside driven by a 'rack' attached to the pressure tube. I have had these get sticky in the past, a judicious squirt of WD40 may help.

 

Hmm this has had me thinking now, the gauge its self is coming on 40 years old, my dad used it in the 70's on his capri and is been sat in a box for 30 years or so before i came to use it. I used the same oil pressure gauge he used back then too and after about a month's use that 'broke' n started dripping oil down on to my radio. Presumably these things perish? 

[carbon]

It's just possible that the pressure tube inside the vacuum gauge has developed a leak, this could explain why it is not reading right.

[philip663]

It's just possible that the pressure tube inside the vacuum gauge has developed a leak, this could explain why it is not reading right.

 

Hopefully thats the problem (I feel bad saying that!). But to check it out iv just bought a vacuum testing thingy bob so will test it later in zee week