Wild vacuum readings - ideas?

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Wild vacuum readings - ideas?

BigDav782
I'm still troubleshooting a rough idle, and am starting with the vacuum system.  When I hooked up a vacuum gauge to a port on the intake manifold, to read the vacuum, it was fluctuating wildly between about 14" and 20" of vacuum.  It's a very fast movement up and down, and doesn't go higher than 20".  

Anyone know what this could be an indication of?  Certainly I plan to keep looking for leaks, and I've already replaced the PCV and some hoses.  
'85 F150, I6, 4wd, 3 speed automatic
Hopkins, MN (Twin Cities)
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Re: Wild vacuum readings - ideas?

Dorsai
Does this help?

https://www.onallcylinders.com/2015/05/08/quick-tech-how-to-read-a-vacuum-gauge-to-pinpoint-engine-problems/
Matthew
1980 F-150 Custom 2wd longbed, 300-6, C6, 2.75 rear axle
Atlanta GA
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Re: Wild vacuum readings - ideas?

Gary Lewis
Administrator
In reply to this post by BigDav782
If the wildness is repetitive with each revolution that could be indicative of bad valves, a bad cam, head gasket problem, etc. Things that are specific to individual cylinders.

But if it is seemingly random then it could be a loose carb or something else causing an intermittent vacuum leak. Like a cracked hose that opens up as the engine shakes.
Gary, AKA "Gary fellow": Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker in front & 10.25 Spicer/Trutrac in back, & EEC-V MAF/SEFI
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Re: Wild vacuum readings - ideas?

BigDav782
Did the test again this afternoon, with warmer outside temperatures, and it's still fluctuating wildly and rapidly, though perhaps across a slightly lower range (11-17").  I gave it gas and it somewhat steadied out at around 15", though still shaky, and then it jumped to around 20" upon release of the gas.  

I also tested for vacuum leaks using an ultrasonic device that we happened to have at work, and couldn't find anything anywhere.  Not to say that I shouldn't keep looking, but...

In looking at the resource that Matthew linked to, plus this resource from Mityvac (pages 8 and 9), plus a random Google search, all seem to point to either valve guides, valve springs, and/or sticky valves.  

Is there a more definitive test to see if it is something with the valves?  I'm assuming it's not a small job to get to them and investigate.  

If it is valve-related, how big of a job are we talking?  I've never touched anything in any type of engine.  
'85 F150, I6, 4wd, 3 speed automatic
Hopkins, MN (Twin Cities)
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Re: Wild vacuum readings - ideas?

Gary Lewis
Administrator
The best way to tell is via a leak-down test.  Basically, you put a cylinder on TDC on the compression stroke and inject regulated compressed air into the cylinder via the spark plug hole.  There's a very small orifice in the line between two gauges, and you read the pressure on one gauge vs the other, which shows you the percentage leakage.

But, you also go around listening to the intake and the exhaust to determine if a valve is leaking.  An intake valve leaking will let you hear escaping air in the intake, and similarly a leaking exhaust valve will sound in the exhaust system.

We don't yet have a how-to Youtube on that, but hope to soon.  However, you should be able to find one pretty easily by searching.
Gary, AKA "Gary fellow": Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker in front & 10.25 Spicer/Trutrac in back, & EEC-V MAF/SEFI
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Re: Wild vacuum readings - ideas?

BigDav782
Thanks for the help everyone. 

Had a quote from a local shop that does a lot of machine work to diagnose the issue for $150, so figured that would be roughly the same cost as investing in tools for compression test or leak-by test.  They just called and said they need to rebuild the carburetor before they can do anything else - $700 at another local shop.  Trouble is, they didn't do a compression test or leak by test, though the head mechanic is 99% sure that the compression is good.  They said he introduced a vacuum leak and was able to get it running perfectly, so the internals are fine.  

Any of that make sense?  This is a well-respected place so I'm hesitant to pay for their time and take it somewhere else, but at the same time, I don't want to pay them to rebuild a carb, only to find out the valve guides were the original problem.  

Much appreciated,

Dave
'85 F150, I6, 4wd, 3 speed automatic
Hopkins, MN (Twin Cities)
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Re: Wild vacuum readings - ideas?

Dorsai
BigDav782 wrote
They just called and said they need to rebuild the carburetor before they can do anything else - $700 at another local shop. 
$700 to rebuild a Carter YFA?  

You can rebuild it yourself for about $40 and a weekend of effort.  Even buying a rebuilt one off of eBay would only cost you $220: Rebuilt feedback carburetor
Matthew
1980 F-150 Custom 2wd longbed, 300-6, C6, 2.75 rear axle
Atlanta GA
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Re: Wild vacuum readings - ideas?

1986F150Six
Or, a new clone YFA on eBay for <$75, including freight.
1986 F150 4.9L manual O.D. 3.08 Duraspark MSD ignition w/ no EGR
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Re: Wild vacuum readings - ideas?

BigDav782
Thanks - I was thinking the same thing, that it was a lot of money for a job that shouldn't be, plus still might not address the underlying issue.

Looked up the carb shop myself - even on their website they say $99 to rebuild most domestic carbs.  

Thinking it might be time to go get my truck...
'85 F150, I6, 4wd, 3 speed automatic
Hopkins, MN (Twin Cities)
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Re: Wild vacuum readings - ideas?

Gary Lewis
Administrator
Yes, it is time to go get your truck.

And the tools to do the testing are not that expensive:

Compression gauge: This one from Amazon is only $20.  But I never run a compression test anymore.  I'd much rather run a leak-down test.

Leak-down tester: Here's a bunch of leak-down testers from Amazon for prices ranging from $26 to $88.  The downside is that you have to have compressed air.  It doesn't take much, so if you don't have a compressor maybe a friend does?

As the others have said, you can rebuild the carb yourself for not all that much.  Or, buy a rebuilt one.  Assuming the engine checks out good.
Gary, AKA "Gary fellow": Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker in front & 10.25 Spicer/Trutrac in back, & EEC-V MAF/SEFI
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Re: Wild vacuum readings - ideas?

85lebaront2
Administrator
In reply to this post by BigDav782
When they quoted you $700 for the carburetor rebuild, was everything going to be gold plated? Was he holding a 6 gun, because that is highway robbery.

Even repairing the probably bad threads holding the throttle body to the float bowl and a few of the improvements I used to do on these, I would have to be charging in the neighborhood of $200/hr to justify that. I didn't even get near that on a 1951 Buick Carter WCD I did a while back for a customer in Utah, even the 1930 Dusenberg Shebler type S duplex was't in that price range!
Bill
"Getting old is inevitable, growing up is optional"
Darth Vader 1986 F-350 460 converted to MAF/SEFI, E4OD 12X3 1/2 rear brakes, traction loc 3:55 gear, 160 amp 3G alternator
Wife's 2011 Flex Limited
Daily Driver 1994 Taurus LX
Project car 1986 Chrysler LeBaron convertible 2.2L Turbo II, modified A413