Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Need guru help on vacuum reading

Normally I can find the answer for just about anything car related on the internet but I can’t seem to get any positive direction for this vacuum reading anywhere…
you can see it at
Most mechanics I’ve taken it to refuse it.

It’s a newly rebuilt stock v6 in a 1974 FJ40 , running points, everything is new. No leaks anywhere on manifold. It runs great sometimes, but always starts spitting at some point.


Not a guru, but just Google pulsating vacuum reading. Possibilities: ignition, valve guides, compression. Might want to do a compression check.

Fresh rebuild

Has it been run long enough that the rings are seated?
Was the camshaft properly broke in?
Have you checked the valve adjustment lately?

Seeing it’s intermittent have you checked the vacuum / mechanical advance of the distributor ?

1 Like

hmm … well the intake manifold vacuum is created when a piston moves downward while the intake valve is open, and the exhaust valve is closed. It’s more or less constant b/c the next piston takes over for the one that moves from going down to going up on it’s own compression stroke. So something must be wrong with all that.

  • Exhaust valve not closing completely, check valve clearances
  • Intake valve not opening complete, check valve clearances
  • Camshaft timing incorrect w/respect to crankshaft, check valve timing
  • Piston rings or valves not sealing, check compression, both wet and dry
  • Lifter problem, checked for damaged or collapsed lifters
1 Like

I recall a few Toyota engines of that period with loose valve seats which would certainly cause the vacuum to bounce. Find the misfiring cylinder and run a leakdown test.

As for that old Toyota running with points, you might check for an igniter attached to the coil or near it. If you find an igniter the points rarely need replacement or service because they only carry enough current to trigger the transistorized igniter. and can outlast the engine.

edit; on second thought the 1974 seems to have the inline 6 engine with a cast iron cylinder head which is unlikely to use pressed in valve seats.The inline 6 was somewhat of a clone of an early GMC engine.


Thanks for the responses.

I learned elsewhere that the vacuum reading can actually be caused by the gauge itself not having a “dampner”…
My distributor has now quit making consistent sparks. Spark is intermittently lost on all the plugs. Basically the points are occasionally failing. No amount of adjusting, checking grounds, wires, etc. changes anything. It’s an aftermarket distributor so I am going to replace it with an OEM one. Hopefully that solves the problem!

I’m not saying it doesn’t happen but my gauge is a cheap one from Target about 35 years ago and has been pretty reliable and never pulsated. Sure might be a few percent off but they are pretty simple devices.

A failing condenser can cause that symptom too.

1 Like

I recall when replacing points, it was common practice to put in a new condenser too, as preventive maintenance.


Yep, the package of points contained the condenser and little pill of lube for the lobes. You had to buy the rotor separate though.

You say it’s an after market distributor with points. I’m amazed.

Is there an igniter on the coil?

No igniter. Engine had one before it was desmogged. But not anymore. I’m replacing with a OEM distributor that doesn’t need an igniter for sure. But I’m not sure if one was necessary for this or not. I’ve managed to drive it almost 200 miles on the rebuilt engine. It was working well initially. Manual says that if you pull apart the points with your finger and see blue spark, that indicates bad condenser. There’s definitely blue spark. Going to try a new condenser tomorrow just for kicks.

All cast iron. Engine weighs a meager 800 lbs.

Had this vacuum reading the other day…
Seemed stabilized…but it was short-lived.

Turns out condenser was bad/going bad. Replaced and have no more ignition issues.


Good for you for sticking with it. I’m happy to hear your LC is purring like a kitten again. Thanks for the feedback.