Vacuum gauge reading help

dan1977p

New member
I'd like a little help reading this gauge. It's hooked up to my brake booster line and I'm at about 4400' ASL. I'm running it because I feel like it's running rough but every time I find and fix a leak in a vac line, it doesn't seem to change much.
Tuneup with Champion plugs, new cap, rotor, plug wires, NTK O2 sensor, fuel filter. Compression was high 120's and pretty even last I checked. Fuel pressure was good too after I replaced the regulator.
New injectors. Smoothed out significantly after that.
New motor and trans mount.

Started after new motor mounts. They were only thing installed when this happened so I'm sure the mounts are letting it shake, but I wonder if there is an underlying problem causing the shake. Unfortunately, this video isn't as bad as it has been lately. Usually I get enough shake that I think the downpipe rattles against the oil pan.

I'm thinking a valve job wouldn't be a bad idea here.
 

voudoux

New member
What is your reading in inches of mercury? Are you reading it on a vacuum pump,are you sure it is accurate? The reading should drop and return as you blip the throttle. Vacuum leaks usually can be heard at idle. Check the plug wires and plugs ,plugs will tell you if it is a dead skip.double check everything.Good luck
 

Ecomike

NAXJA# 2091
NAXJA Member
High 120's for engine compression is at the edge of needing a rebuild as I recall. But it should not cause any shaking.

If you have a new shake check your recent work, like cris-crossed plug wires?

Video does not work.
 

voudoux

New member
I reread your post and realizedI missed the motor mount part is there any way you disturbed a ground wire or strap? Sorry,Good Luck
 

Sidewinder CC

New member
That's a bad valve
Agreed.

Here's a vacuum diagnostics chart.

You can also look on Youtube for vacuum diagnostics.



If the spark plugs are oily, the valve stem seals may be worn.
.
You may want to pull the valve cover and inspect each valve spring.

If you have the tooling, you could do a compression loss test. If you hear air escaping from the TB it's a bad intake valve and if you hear air escaping from the exhaust it's a bad exhaust valve. Air escaping from the oil fill port on the valve cover it is worn/broken piston rings.
 

dan1977p

New member
What was the "bad valve" based on,I must have missed something.
The rapidly fluttering needle. I kind of already knew but wanted confirmation. I didn't mention that if I idle for too long, it will dump smoke out the tailpipe giving further confirmation of bad valve.
 

Ecomike

NAXJA# 2091
NAXJA Member
The rapidly fluttering needle. I kind of already knew but wanted confirmation. I didn't mention that if I idle for too long, it will dump smoke out the tailpipe giving further confirmation of bad valve.
Or valve spring?
 

dan1977p

New member
Or valve spring?
But, a spring should be a much slower change of the needle, right? I did use 'bad valve' as a general term when really, the valve is likely not seating completely for any number of reasons and if I feel it's a big enough issue, I will do a visual inspection and go from there.
 

Ecomike

NAXJA# 2091
NAXJA Member
But, a spring should be a much slower change of the needle, right? I did use 'bad valve' as a general term when really, the valve is likely not seating completely for any number of reasons and if I feel it's a big enough issue, I will do a visual inspection and go from there.

No clue here. Let us know what you find. I am always fascinated at how much you can learn from a $20 Vacuum gauge, in minutes. Way underused old-school tool
 

dan1977p

New member
No clue here. Let us know what you find. I am always fascinated at how much you can learn from a $20 Vacuum gauge, in minutes. Way underused old-school tool
Yeah, the kids born after 96 probably have no idea what we're talking about. "You mean I can't just plug my phone in to the dashboard and have it tell me what's wrong and how to fix it?"
 

Ecomike

NAXJA# 2091
NAXJA Member
Yeah, the kids born after 96 probably have no idea what we're talking about. "You mean I can't just plug my phone in to the dashboard and have it tell me what's wrong and how to fix it?"

LOL, just tell them hook up the I-Vac Gauge LMAO.
 

trippled

NAXJA Member
NAXJA Member
But, a spring should be a much slower change of the needle, right? I did use 'bad valve' as a general term when really, the valve is likely not seating completely for any number of reasons and if I feel it's a big enough issue, I will do a visual inspection and go from there.
Yes. I was a little quick in my reply, but an intake valve is not seating properly, so Everytime the cylinder moves up, a little air moves out, moving the needle. A broken spring could cause this. A weak spring would probably be more evident at higher rpm. stuck closed exhaust valve can cause some of these issues, but is usually more consistent thru the rpm range, at least in my experience. Sounds like you already know what you need to do. I might recheck your compression, see if it gives you an idea of what cylinder is at fault.
 

Ecomike

NAXJA# 2091
NAXJA Member
OP is at 4,400 feet, normalized to sea level thats about 137 psi. I think the FSM says 120 psi is on the low end.
I was wondering about that, but since the gauge is measuring gauge pressure, ie. the differential pressure between inside and out side the cylinder, would the gauge reading drop that low, from 175 to 137 for a new engine spec? And the 120 psi manual spec is at sea level, but would still hold unless the CPS/Ignition timing is changed to compensate for the altitude???? If altitude conpensated, could it be OK at a lower reading?

I know Renix had a high altitude CPS to compensate, but never really gave the altitude issue much thought, till now.

Interesting.

For those new to this, at higher altitude the air is less dense, and there is less air mass sucked in to compress, and thus less pressure generated at TDC of the stroke. I think I just answered my own question.
 
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md21722

NAXJA Member
NAXJA Member
Everything that uses air is downrated for altitude. I'm at 6500 so I know it well. Altitude 'corrections' are made for compression, vacuum, ... even how much air your air compressor can make. In some cases you need to downrate the pressure capability of the air compressor too. Need a 2000 kW generator? Here you buy 2500 kW.
 
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dan1977p

New member
I didn't even think of compression numbers being altered for altitude. I was concerned a rebuild was in store but can wait a while longer.
 

md21722

NAXJA Member
NAXJA Member
I didn't even think of compression numbers being altered for altitude. I was concerned a rebuild was in store but can wait a while longer.
I will tell you ever time I've pulled a head and had a valve job, compression goes up, after normalizing for altitude I can go from 135-145 to 160-170. Never could tell a difference in drivability though. I was always pulling the head for another reason, the compressor numbers were never the driving force. I'd pull the valve cover and have a look. Time, heat, and miles and I've since the valve seals dislodged and riding on the valve stems... on 4.0's... If you're burning oil you can try a dry vs wet test which will tell you about the rings. If it changes, MMO or Auto-RX treatment can get carbon out of the rings, help them seal better, and your compression will go up too. While the valve cover is off, if you chose to pull the rockers, you can look at the tops of the valve stems. You want to see a bulls-eye pattern which will tell you if the valves are rotating. If its a line, then there is a TSB to manually rotate the valves to try and fix it. If it doesn't work, pull the head and go for the valve job. A well maintained 4.0 bottom end will go the distance but they still need valve jobs from time to time... Last time I pulled the head on my 01 at 315,000 I was still around 160 or so after I dropped a new head on (to fix the dreaded 0331 crack).
 
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