TheSamba.com :: View topic - Vacuum gauge readings

Tue Apr 09, 2019 1:29 pm

Hey,

1978 FI 2.0 here. I just hooked up a vacuum gauge to try and track down potential vacuum leaks due to the unsteady idle. The jumping around is kind of freaking me out though - does this indicate the need for a rebuild? My cylinders are all between 130 and 140 PSI. There is a sufficient amount of oil as well. Could a bled-down lifter be causing the erratic readings?

edit: video here - https://www.youtube.com/watch?v=QBNuVb5Ip30

Tue Apr 09, 2019 1:42 pm

If the needle is jumping around you have a dead cylinder, generally speaking.

Considering your compression numbers, could be ignition, fuel supply or vacuum leak at cylinder head.

Tue Apr 09, 2019 1:47 pm

Generally when you set up a vacuum gauge you should have an in-line regulator. Basically this makes your vacuum gauge read steady rather than pulsing each time a cylinder pulls vacuum.

This video shows a good example of what happens when you don't have a regulator.

https://youtu.be/TAhhMMnfN7Q?t=146

I think I got mine from a pet supply store in the aquarium section.

Tue Apr 09, 2019 1:58 pm

advCo wrote: Generally when you set up a vacuum gauge you should have an in-line regulator. Basically this makes your vacuum gauge read steady rather than pulsing each time a cylinder pulls vacuum.

This video shows a good example of what happens when you don't have a regulator.

https://youtu.be/TAhhMMnfN7Q?t=146

I think I got mine from a pet supply store in the aquarium section.

The guy in the video is working on a bike with separate carbs for each cylinder, with a 4 cylinder FI system running through a plenum you are not going to see this. If you did the AFM would go nuts.

I would agree that there is a cylinder that doesn't seem to be firing. Sounds like there could be a leak in the exhaust.

Tue Apr 09, 2019 2:04 pm

I hear some ticking in the video, does your '78 bus still have hydraulic lifters, and are they adjusted correctly and fully pumped up?

Robbie

Tue Apr 09, 2019 2:06 pm

advCo wrote: Generally when you set up a vacuum gauge you should have an in-line regulator. Basically this makes your vacuum gauge read steady rather than pulsing each time a cylinder pulls vacuum.

I 1000% disagree with that. setting up multiple carbs is way, way different than pulling vacuum from a central location.

a little (and I mean almost non existent) bounce is normal. if you have a bounce, it could be due to a weak valve spring, bad valves etc.

Tue Apr 09, 2019 2:41 pm

I was always taught that was most commonly a bad valve spring. However it could be a lifter too. Where is the vacuum being pulled from? The plenum? The S-Boot?

Tue Apr 09, 2019 2:55 pm

The thing about a vacuum gauge, it’s analog. OBD can tell you where and what, we don’t have that with the ACVW. A deal like this is difficult to figure out not having the rig at arms reach. My advice was just a guess considering the info posted above. Personally I love a challenge like this when it’s in my shop.

This very type of diagnostic is what separates men from boys, y’all.

Tue Apr 09, 2019 3:10 pm

Reading is being taken at the plenum where the brake booster hose normally goes. Valves are in adjustment, but it seems like one tends to bleed down quite easily, so that could definitely be the case. I’ll check 3&4 and report back.

When driving it drives really well. No missing or anything, smooth acceleration.

Tue Apr 09, 2019 3:12 pm

To clarify some above posts, a jumping needle at idle can mean a few different things, but retarded idle timing specs (DVDA & SVA distributors) usually bounce the needle at idle from exhaust/intake air reversion even if the engine architecture is sound. (This isn't necessarily applicable to this engine, but still useful to remember.)

Do you trust your plastic gauge enough to bet an engine teardown on? 8)

Does your AFM wiper/flap twitch like the needle does?

Robbie

Tue Apr 09, 2019 3:47 pm

Now that’s what I’m talking about! Hell no I don’t trust the gauge (it’s an original). That said, I’ve twisted the dizzy back and forth to see if I could get it to calm down to no avail.

I’ll try some of this stuff and get back.

Tue Apr 09, 2019 4:05 pm

Err, that wasn’t a suggestion, but I’m glad we have more diagnostic information :)

Robbie

Tue Apr 09, 2019 4:22 pm

reversion at idle? pretty much non existent. this wouldn't really ever show up in a closed intake, because what little (hardly none) is there would be smoothed out by another cylinder taking in air as the valve opens. you should have about 20inhg vac at idle, throttle plate closed. if not...you have intake leaks

if the vac gauge isn't dam near rock solid at idle....there is more going on

If weak valve springs are causing a misfire problem, the vacuum readings on the gauge will oscillate as engine speed increases.

However, a rapid vibration or variation (up to 4 in Hg) in the vacuum gauge reading at increased engine speed can also be caused by a leaking intake manifold gasket or head gasket, burned valves or ignition misfire. You need to rule out these other possibilities before you start wildly twisting a distributor all over the place

if your compression is what you say, that's an 8% difference...not bad, but not great either

buy or rent a smoke machine for EVAP systems. doing the carb/brake clean spray is inconclusive at best...unless you have a massive leak.

Tue Apr 09, 2019 4:49 pm

Seen this link and chart on vacuum readings....

https://therangerstation.com/Magazine/Summer2003/VacuumLeaks.htm

https://www.onallcylinders.com/2016/03/31/vac-visual-quick-guide-vacuum-gauge-readings/

Tue Apr 09, 2019 5:39 pm

Can't comment for the other guys here with lots of experience whom are to be respected but most of the mechanics I've known KNOW HOW to use a vacuum gauge but they rely on other pieces of equipment including their ears to diagnosis a car engine. Most mechanics I've known don't rely on a vacuum gauge because it may show an issue exists, but not THE issue. A weak cylinder when running usually shows up in a balance test, or in the engine sounds. I suspect that while the post subject is a good question, most folks here would make their analysis of your engine using other tools including their experience.

In my eyes, a vacuum gauge test is more a 40's - 50's kinda tool from when everything ran off vacuum - fuel pump, windshield wiper, distributor advance only, (and timing by ear) etc..

Tue Apr 09, 2019 6:21 pm

SGKent wrote:

In my eyes, a vacuum gauge test is more a 40's - 50's kinda tool from when everything ran off vacuum - fuel pump, windshield wiper, distributor advance only, (and timing by ear) etc..

and you'd be right. a cylinder contribution test would isolate a bad cylinder, or a weak one anyway. it's a fast way to know the overall health of an engine

I think the o/p is chasing a red herring to be honest with you, and I give him/her credit for trying, their diagnostic strategy is lacking. could be as simple as a vac leak....the main complaint is an idle issue...so jumping into other aspects of mechanical failure is just putting crazy thoughts in their head.

#1 thing to diagnosing all these old pigs is to make sure it's mechanically in spec...adjust valves (check hydros) set dwell, timing etc...then move on. with the info we have, the engine is a bit worn, but not to a catastrophic level.

reading a vac gauge at this point is useless in my mind. part of me thinks that the o/p thought a vacuum gauge would tell them if they had a leak...which it kind of would...if you knew what you were looking for.

I just re watched your vid....you have a clattery lifter from what I can pick up in the video...that needs to be addressed before moving forward. you should be close to 20 inhg at idle. the bounce is due to the lifter being collapsed

Wed Apr 10, 2019 7:18 am

Worn valve guides will cause a bouncy vacuum signal, often intermittent - bouncy for a few seconds, not for a few then bouncy for a few and on and on.

Wed Apr 10, 2019 7:27 am

Thanks so much everyone. You've calmed my mind a bit - I was just looking at those animations and mine looked like BURNED VALVE - BAD RINGS. I'll take it down to get smoke tested and report back.

And yeah, I was trying to diagnose a vacuum leak. Could a valve cover that isn't sealing 100% cause that? If so, I might need to double up on gaskets, as I noticed some oil on the exchanger last time I was down there.

I'm going to pull the chattery lifter completely and clean it out. It's been bleeding down constantly after just sitting overnight.

Wed Apr 10, 2019 8:41 am

oshima wrote: Thanks so much everyone. You've calmed my mind a bit - I was just looking at those animations and mine looked like BURNED VALVE - BAD RINGS. I'll take it down to get smoke tested and report back.

And yeah, I was trying to diagnose a vacuum leak. Could a valve cover that isn't sealing 100% cause that? If so, I might need to double up on gaskets, as I noticed some oil on the exchanger last time I was down there.

I'm going to pull the chattery lifter completely and clean it out. It's been bleeding down constantly after just sitting overnight.

Try adding some Marvel Mystery Oil to your crankcase to get the lifters to clean themselves up a bit. Lifters like thin oil, which almost be definition means they don't like cold oil and will leak down if the engine is shut off before the oil gets hot.

I let my 83 1/2 (2.1L WBXer) sit for eight months one time and when I fired it up the lifters were quiet. Only let it run for a few minutes and then shut it off. Restarted it after about a 1/2 hour and the lifters sounded like they were going to beat themselves to death as being shut down with cold oil had cause the lifters to leak down. Unless you have been getting yours out on the road and getting it hot before shutting it down, you can expect the same thing, noisy lifters at start up.

FWIW I have been running 5w40 synthetic in my engines for years at this point in time and am thinking of going to 0w40.