http://www.cdxetextbook.com/toolsEquip/hpt/measuring/vacgauge.html here is a site to help show you how to diagnose an engine using a vac gauge
I don't have one. I could probably borrow one though. I never tested it when it was running well so I don't really have a baseline to compare my readings to anyway.
I had a similar issue. All of a sudden started running like ****. Would not idle but would go down the highway fine. Everything was new. After eliminating water in the gas possibility. Started at the carb checking the fuel lines, regulator, and filter. Long story short the filter had come apart inside and dumped all the trash into the carb. Rebuilt the carb and all was AOK.
That doesn't sound fun. But this is idling like you would expect for a car running on 7 cylinders. There's a definite miss, but only on the one cylinder.
Why would it affect all cylinders? If the leek is after the "divide" in the intake manifold shouldn't the leak only effect the cylinder(s) "south" of the leak?
A "vacuum" leak is obviously really an air leak in. So there is a lower pressure everywhere in the manifold, and all cylinders are exposed to it. Anywhere you open a hole to allow air in affects pressure in the entire manifold. If you have a true dual-plane manifold (divided all the way to the carb base) then a vacuum leak will only affect the 4 cylinders on that plane of the manifold (1, 4, 6, and 8, IIRC?). If the problem is confined to #6 hole, I'm doubting it's vacuum related. I'm wrong a lot, though...
Any one of these valve train issues would most definitely show a variance across the cylinders and without any doubt at all on #6 if its enough to kill it. OP stated "I tested the compression, it's even from one cylinder to another." Compression test is very telling and if done incorrectly will undoubtedly send you on a unicorn hunt.
Well, I found the problem. A lifter not collapsed but instead "expanded". The exhaust valve on 6 was not quite closing all the way. It's running fine now but it's making a lifter noise. I'll take tapping over missing any day. The lifter must be damaged because I can't adjust the lifter so it doesn't click with out it missing. Don't buy Melling lifters from Autozone!
Never saw such a thing as an expanded lifters. Valve hanging up, bent valve head, busted spring, foreign matter on seat, adjusted too tight and eventually collapsed a lifter, How is it compression was present with an exhaust valve open let alone even across cylinders?
I don't know. My only guess could be that oil pressure has something do do with it. I didn't properly test the compression i.e. all spark plugs out throttle open. I just wanted to see if there was any compression at all, at that point. Perhaps the lifter never properly expanded when I put the motor together, it may have taken running 85 MPH for a good long while to get pressure up. Who knows. I'm just happy it's running right now, except for the lifter click. Thanks to everyone here who posted. It's nice to be able to throw a strange problem out there and get the benefit of everyone's collective knowledge and experience.
well ive been reading and waiting to see what it was. i had my own ideas.but now im confused. what youve said there ay the last just dont make any sense. at least not to me/ im so confused!!!
Sounds like the lifter might not have pumped up from the start and was adjusted a little too tight. It would act like a solid lifter without enough valve lash. As long as everything is cool, the valve closes. As everything heated up, the valve wouldn't close anymore. I'm ***uming that it had cooled down by the time you did the compression test so the valve was closing. Maybe your set of lifters had one solid lifter in it or just one bad lifter. I usually submerge the lifters in oil and use a push rod to pump the air out of them and make sure they pump up.
The Jalopy Journal
