My 80 Coup L48 has a tendency to idle rough at different times and I think it has something to do with a possible leak in one of the vacuum lines. Someone had said on the forum before that a vacuum gauge can be borrowed from a NAPA or other similar auto stores. My question is this: Where do I hook the vacuum gauge up and how much vacuum am I suppose to have? If there is a leak, do I just pinch a line off until I track it down? Thanks for any help. Kloppy80 :confused:

 

May be able to 'borrow' a gauge from Auto Zone or Advance Auto, but if you do much work on your car yourself, suggest you go out and buy one. These vacuum gauges are not very expensive, and you really need one to properly set your idle mixture screws, and gauge is very handy for giving you information regarding performance of your engine.

If your '80 is mostly stock, you should have around 16 to 18+ "s of vacuum at idle. Hook the gauge up at the rear intake manifold port just rear of carburator. This port feeds the automatic transmission modulator valve and your power brake booster. You should have a steady vacuum reading, one that decreases during engine acceleration, and increases dramatically when you decelerate the engine.

Once you have the gauge hooked up and warm engine ideling, suggest you start 'pinching' off selected vacuum hoses, one at a time with a vice grips and read the gauge each time you pinch a hose off, to see if it makes a change in the vacuum reading. Once you find a line near the engine that does make a difference, then you've got to start working from the engine outword to determine more precisely, where the vacuum leak is.

Hope this helps. Good luck.

 

Most service manuals like Haynes and Chiltons have a good breakdown on how to decipher readings. Also a common problem the EGR valve when the car is warm shows similar problems with idle.

 

If the idle mixture screws were adjusted with a headlight actuator seal boken, which lets vacuum escape, will that affect the idle when it has been changed? :eek:

 

An easy and effective way to find vacuum leaks is to take a long piece of vacuum hose and put one end to your ear and the other end you can move around the various lines until you hear a sucking sound. To get an idea of what it will sound like just move one end close to the primaries on the carb. The leak in the hose won't be as loud, but you'll get the idea of what to listen for. :)

 

81vette: You've haven't had to balance twin SU carbs on an MG or something, have you? :lol:

Before you start pinching hoses it's a good idea to check that the hose has a good airtight seal where it fits to the carb, with no splits or damage. It sounds like a really obvious thing to do, but it's easy to miss a very small split in a hose - especially if it is on the underside. Somebody could spend a lot of time trying to find a leak, when all the time it's right under their nose :bb . It's one way of forcing yourself to check out all the vacuum pipes though ;)
Paul

 

check your line going to vac canister for headlights .the vac canister gets holes and the headlight actuator's seals go bad, all causing vacum leaks.