Eratic and Rough Idle

I had a leaky gas filter replaced in my 1982 Ford Granada (3.8L V6). The filter is located under the carburetor air filter housing. When I picked up the car at the mechanic, it was already running but idling rough. He said I can bring the car back for a complete carburetor cleaning. He couldn’t do it then because of time constraints and commitments to other customers. I noticed that when I start the car when cold, it doesn’t go into fast idle mode. I have to rev it to keep it from stalling. It goes into fast idle when I turn on the AC and rev it. I can’t get it out of fast idle without shutting the AC. Is it possible the mechanic re-connected some vacuum lines to the wrong locations?

Replacing the fuel filter on you car is fairly easy.

So, I don’t think it’s a disconnected vacuum hose.

Was the car running fine when you brought it in beside the leaky filter?


Yes, it was idling normally. He had to remove the air filter housing to access the fuel filter. Is it possible he also had to remove a vacuum line or two to access the fuel filter?

Vacuum lines may have been disconnected inadvertently. Pull off the air filter and look for unattached hoses. A hose might be cracked too.

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After 4 decades of use, I’d be surprised if a vacuum hose–or two–wasn’t cracked. Also, that old carb could act-up at any time, even if the mechanic did everything perfectly.

Here’s the vacuum hoses on the air filter housing.

See if the long vacuum hose got disconnected when the air filter housing was removed.


I own an early 70’s Ford truck w/5L v8 w/carb. I’m thinking one of these

  • A vacuum hose was inadvertently disconnected when the air cleaner was removed, and the engine is now running overly lean. If so, a mechanic’s quick visual inspection will likely discover the problem. If not, an engine vacuum system test should figure it out. Pretty good chance the mechanic is without blame in any event. The plastic connectors in the vacuum line system become brittle with age and will often crack when even slightly disturbed.

  • The choke plate (at the very top of the carb) was knocked out of alignment when the air filter housing was removed/reinstalled. I’ve had that happen, usually contributed to by the screw that holds the choke plate in place having worked loose.

As part of this fix, ask shop to lube the choke linkages.

It’s possible the problem is the carb’s passages are plugged as well. This appears to be what your mechanic is thinking. Whenever the air filter housing is removed, it is possible for dirt/gunk to get into carb. Esp likely if engine is run without air filter installed. And of course a carb will eventually gunk up just w/use. I had to give my carb’s idle passages a cleaning recently. This isn’t the same as a complete cleaning and rebuilding, just cleaned out the air bleeds and the mixture screw passages, replaced fuel filter, took maybe 15 minutes.

If the above ideas don’t pan out, and if your carb hasn’t had a complete rebuild in some time, probably makes sense to have that done now.

Something is telling me that the OP is not capable/able to remove the air cleaner lid to look and see or we would not be having this conversation…

My 50 year old truck experienced a repeating series of rough idle problems about 10 years ago. One failure after another, a few months apart. They were all caused by the rubber diaphragms in vacuum actuators springing a leak. Ignition advance actuator, then ignition retard actuator, then the actuator that controls the intake air temperature, they failed one by one. I guess it was all the same diaphragm material, and had a fixed lifespan. I had another one fail not too long ago, the one that controls the amount of vacuum to the intake air temperature door actuator. That doesn’t have a diaphragm as far as I can tell, but it can still cause a vacuum leak. Ask me how I know? … lol …

OP, you need a shop with a tech who knows how to use a hand-held vacuum pump (Mity Vac) to check the entire vacuum system for anything that’s leaking. The intake manifold is the source of the vacuum, and the first test is to connect the Mity Vac to all the hoses, one by one, that originate at the intake manifold. None of them should leak vacuum. If one of them does leak, the shop will trace that vacuum circuit to find what exactly is causing the problem, a vacuum actuator, or just hose or connector is leaking. (Be sure to follow Mity Vac instructions, this is done w/engine off).

As part of replacing all those vacuum actuators I also replaced all the vacuum hoses and all the vacuum hose connectors. OP should probably do that too.