BigDav782 Posted September 5, 2019 Share Posted September 5, 2019 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. Link to comment Share on other sites More sharing options...
Dorsai Posted September 5, 2019 Share Posted September 5, 2019 Does this help? https://www.onallcylinders.com/2015/05/08/quick-tech-how-to-read-a-vacuum-gauge-to-pinpoint-engine-problems/ Link to comment Share on other sites More sharing options...
Gary Lewis Posted September 5, 2019 Share Posted September 5, 2019 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. Link to comment Share on other sites More sharing options...
BigDav782 Posted September 5, 2019 Author Share Posted September 5, 2019 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. 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. Link to comment Share on other sites More sharing options...
Gary Lewis Posted September 5, 2019 Share Posted September 5, 2019 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. 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. Link to comment Share on other sites More sharing options...
BigDav782 Posted September 12, 2019 Author Share Posted September 12, 2019 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. 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 Link to comment Share on other sites More sharing options...
Dorsai Posted September 12, 2019 Share Posted September 12, 2019 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 Link to comment Share on other sites More sharing options...
1986F150Six Posted September 12, 2019 Share Posted September 12, 2019 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 Or, a new clone YFA on eBay for <$75, including freight. Link to comment Share on other sites More sharing options...
BigDav782 Posted September 12, 2019 Author Share Posted September 12, 2019 Or, a new clone YFA on eBay for <$75, including freight. 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... Link to comment Share on other sites More sharing options...
Gary Lewis Posted September 12, 2019 Share Posted September 12, 2019 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... 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. Link to comment Share on other sites More sharing options...
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