Vacuum testing from the dipstick
 

I have a 1998 2.5 5MT with a dipstick and what I suspect is a vacuum leak (codes 1124 and 1126, running fine at all temperatures), and I figured I'd shove a manometer into the dipstick tube.

Unfortunately, doing that gave me zero vacuum reading on the manometer.

Is there some sort of valve that prevents vacuum from reaching up the dipstick, or some sort of disconnect I'm just not thinking of? I would think that crankcase vacuum would be readable from the dipstick, seen as it reaches directly into the crankcase.

If my hunch is correct, that means I have a vacuum leak at the dipstick tube, which would explain why it's isolated from the rest of the system. Does this track?

More info:

Throttle response is good, no smoke at startup, oil level is 3/4 of the way to the top mark on the dash (exactly where I filled it to), I can remove the oil cap while idling with little effort, the car barely reacts when I do. I'd rather pay for a smoke test at a shop and find none than pay for a pair of O2 sensors and fix nothing, though of course I'd rather diagnose it myself for free if possible.

If you have no vacuum signal (which should only be 4-5 inches of water at fully warm idle, not Hg), the dipstick housing is open somewhere. You can confirm this by smoke testing the tube, which will show you exactly where it is leaking.

Thanks, that's what I thought but didn't want to dive headlong into a wild goose chase before confirming. I'll dig into the dipstick assembly tomorrow and report back with findings.

After digging into it again today, I noticed that my manometer is in inches of mercury, and the correct crankcase measurement of 5in-H2O barely registers in it.

I also finally found somewhere to shove it in the plenum and found very steady vacuum around 16 in-Hg. I tried searching, but can't find any info on what the reading should be. A general search showed me that most cars should be steady between 15-22, so things look positive but I'd like more certainty.

If anyone is wondering as I was before, I unscrewed the little blanking plate on the rear of the passenger's side and jammed the manometer adapter in there, it worked like a charm.

Next step, I'll order a fuel filter, as I should replace it regardless of what's causing this issue.

Intake vacuum and sump vacuum are two entirely different things. The AOS in these engines throttle the high intake vacuum down to about 4-5 inches of water in the sump when the AOS is working properly.

If you manometer is digital, you should be able to switch from inches of Hg to inches of water with a click, and then retest the dipstick tube. Again, you can also smoke test the tube as well.

You are too early in the process to claim victory......

Thanks for the input, I did make sure to record my plenum vacuum as a separate figure from crank vacuum, just not sure what to do with that information.

I know what a normal vacuum measurement should look like in a normal car, and mine looked perfect, I'm just wondering if normal rules apply to the Boxster or if there's some unique M96 thing I should be looking out for.

Sadly, I have an analog manometer, so I can't measure crankcase vacuum with my current precision, so all I've got to go by is plenum vacuum. I know I have some crank vacuum because I can hear/feel the sucking when I remove the oil cap/dipstick, and I know it's not too much because the car has a small reaction to it.

Do you think I have enough information to go on, or do I need to get a more precise manometer? I thought people used crank vacuum because it's easier to measure than plenum vacuum, which is why I was excited to get a smooth plenum reading.

You need a digital manometer or a smoke test to go any further. You can get a good manometer off Amazon for around $30 or so, and it is a useful tool in any toolbox as you can use it on any car as well as around the house. Smoke testing hardware is a bit more expensive and dedicated to one thing.

Without information from one or the other diagnostic tool, you cannot go any further.

Thanks, I'll get a fuel filter and a digital manometer in that case.