The Ranchero randomly cut out on me the other day, took a while to get it started again. Got back home, and checked over a few things. Found an air leak at the carb gasket, so sorted that. Anyway hooked up a vacuum gauge....... Initial fire up reading was 18, as the choke comes off it drops right down to around a steady 8/9. Mixture screws make no difference, idle is low and rough when choke turns off. I'm guessing there's still an air leak somewhere??
My guess is yes. Try spraying carb cleaner around base of carb while it's idling. If idle picks up, you've found your leaky area.
Could be worn engine,timing,leaking gaskets ect.I disconnect all vacuum ports/plug,connect vac. gauge and diagnose.
If you have a low reading after an initial high reading, I would suspect an exhaust blockage. Anything from a melted converter, collapsed muffler baffle, dead rat, mice food storage, could all cause a block. Loosen but do not unbolt the exhaust downpipes from the manifolds, then restart the car. If no change from initial startup, it's exhaust related.
Unfortunately the previous owner has welded the downpipes to the manifolds and flow masters are also welded..........Never easy, is it....
Finally getting somewhere... Embarrassed to say I hadn't disconnected all the vacuum ports, when testing, just relied on spray...... Plugged the tee to the vacuum tee as suggested. Low and behold..... Sat on 20, idled smooth, dropped to 5 on blipping the throttle..... Air leak search continues........
Developments....... Been tracking things down. Reconnected vacuum tee, and thru a process of elimination, it seems the problem lies with the egr valve. When connected at the tee, the engine dies/loses vacuum/ bad idle after warming up. If I pull off line at tee, immediately picks up, albeit with a big air leak, so have just capped the tee at that point.
The EGR valve can act like a vacuum leak at idle (exhaust gases diluting the idle mixture). It sounds like someone hooked manifold vacuum to it. It should be hooked to venturi vacuum on the front driver's corner of the carb (if you have the original 2150 carb), through a thermal vacuum switch on the manifold's coolant passage, then to the EGR valve.
The carb has that port, its capped at the minute. What I assume to be the thermal switch is by the waterpump. I'll hook up the line to the carb and see how it goes.... Ta.
I don't know what engine you have but I cut the EGR valve mount off the spacer under the carb on my '75 460 and stopped the two holes with soft plugs. Also, if applicable, check the rubber plugs on the intake manifold and vacuum T. Those miserable things want to get hard and leak. I finally permanently plugged all of those.
Yes, check those caps. More often than not, they're a leak when hardened and cracked. The thermal vacuum switch will have only two ports; the switch for vacuum advance will have three.
Clark..... It's a 400M. The egr valve is on a separate pedestal behind the carb. I had assumed it had gone bad, but from what Handy Andy said, it may have been hooked up wrong.
There's also the possibility, if you haven't replaced the both of them, that the upper carb gasket, and the lower carb spacer gasket, could be rotted, causing vacuum leaks. I've seen 2-barrel and 4-barrel lower gaskets rotted out from the moisture in the exhaust, combined with low engine run time. So make sure you spray on the passenger side, and underneath the EGR part of the spacer, for leaks.
Been thinking, it makes sense that it was hooked up wrong. The vacuum was fine for 2 or 3 minutes, then dropped, together with revs. Its always been like this and I assumed the revs dropping was the choke coming off....... but infact it was the thermal sensor opening the egr and throwing off the idle with an air leak. Since disconnecting the egr and plugging the lines it idles fine. Next step is connecting it back to ported vacuum at the drivers side front of carb. Happy days.......
It's nice to fix something just by moving a hose. And now as a bonus, I sort of know how it works. With the help of you lot, of course. Can't learn much when stuff works....... Finally I can start tuning the thing........
