Author Topic: Intermittent vacuum loss..timing loss..misfire and clears up..  (Read 163 times)

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Offline Greg W.

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  • I own: a Motor Home
  • Year: 1987
  • Make: Winnebago
  • Model: Ela'ndan
  • Chassis: Chevrolet
  • Engine: 454
I have an 87 Chevrolet 454 carburated motor. most of the time, motor runs great,  but during the drive, I lose idle, sometimes backfire during load but clears up soon afterwards,  steady load most time fine, but occasionally you can hear the slight misfire as if timing is out..  ceramic plug caps and high grade wires installed along with new plugs,  distributor,  EGR, PCV, vacuum lines. Down to A.I.R. system with replacement parts not available.   Is this an A.I.R. system issue, or am I missing something else. All mechanics I have talked to are completely clueless. Help and any insight or information is greatly appreciated.
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Offline Oz

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  • Year: 1974
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  • Model: D24 Indian
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  • Engine: 440-3
Re: Intermittent vacuum loss..timing loss..misfire and clears up..
« Reply #1 on: January 19, 2021, 10:28 PM »
Many people have removed the AIR system due to issues.  Lots of info on that here.  Not saying that's the cause, but an option.
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Online TerryH

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  • Year: 1992
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  • Engine: 454 Dual Fuel
Re: Intermittent vacuum loss..timing loss..misfire and clears up..
« Reply #2 on: January 20, 2021, 02:02 AM »
Also, check ALL of your vacuum lines. Any sign of hardening or crumbling indicates an issue. Easy to check, relatively easy and inexpensive to replace all lines. Also won't be difficult  or costly to do so as often it may be not obvious when you have a vacuum leak.
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Online TerryH

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  • Chassis: GMC P30
  • Engine: 454 Dual Fuel
Re: Intermittent vacuum loss..timing loss..misfire and clears up..
« Reply #3 on: January 20, 2021, 02:36 AM »
Further, and since you appear to have a 1987 engine which is likely carbureted, should you check your vacuum lines, do not miss any 'nipples' that may be there to plug off vacuum sources. They are often overlooked.
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Offline Oz

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Re: Intermittent vacuum loss..timing loss..misfire and clears up..
« Reply #4 on: January 20, 2021, 09:15 PM »
Good advice, Terry.  I do recall someone at a jamboree who was having similar problems.  Same year,  Chevy P30.  When I saw all the vacuum lines under the doghouse, I was shocked.  They did exactly what you said.  Found 3 leaks and it cured the problem.
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Offline Eyez Open

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Re: Intermittent vacuum loss..timing loss..misfire and clears up..
« Reply #5 on: January 21, 2021, 05:29 PM »

Pull your vaccum line off the advance can in the distributor and check your timing does it change, if not there is your issue. Checking for a vacum leak can be done with a propane torch...unlit of course. Just point hold and wait at all vaccum line connections
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