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1979 10th Anniversary Trans Am
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Discussion Starter · #1 ·
Hi Guys- I inherited a 1979 10th Anniversary Trans Am with 22K with the 403 Olds engine, I replaced the entire fuel system and tank, also rebuilt the original carburetor I also replaced the ignition except for the coil pickup, car runs great but lately has had idle problems, I replaced the EGR valve and it fixed the problem.

Now after about two months I'm having the same idling problems so I thought maybe the EGR valve went bad, I changed it and I still have the same problem, I saw that the EGR valve connects to an EGR control valve (which I would love to know what it does and why is it mounted where it is) I was going to replace it but can't find a replacement so I blocked off the vacuum at the carburetor with no luck, I also noticed that there was no vacuum at the carburetor, is that normal?

Any suggestions would be greatly appreciated. Please ask any questions and thank you in advance. RON.
 

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The port your EGR uses on the carb should be a "ported" port.. no vacuum at idle. I would probably just get a block off plate and not worry about the EGR anymore. This is a pretty good vid if you decide to keep it.

 

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EGR control valve that sits on top of water pump? That one is coolant heat sensitive, only becomes a check valve until it heats up, EGR only operates when engine cold. And yes, easily blocked off to test if it's the issue of EGR and recycled exhaust through carb once warmed up or something else.
Did you have any factory manuals specific to 79 Firebird or 403?
 

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1979 10th Anniversary Trans Am
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Discussion Starter · #8 ·
Aus78Formula if I can ask you, I'm looking for a vacuum leak and I'm trying to find a diagram with all my hoses but I don't see one for a 403 with A/C and cruise, they are close but are not exact, one of the hoses from the back temperature sensor geos into the fire wall, why? Also the hose from the cruise goes through the firewall. If you can help it would be much appreciated. Thank you
 

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Does you car still have the white emissions decal on the radiator support? Or able to make out the 2-letter code in the box on it? There's different vacuum and emission setups for high and low altitude, smog, etc, although by 79 they were very similar, and 40 years on often incomplete or rerouted.

I'm just heading out the door to work but here's a quick cruise control diagram I had for Olds engine cars. There should also be a vacuum line and check-valve coming off intake rear through firewall that operates the air vent vacuum controls.

1979 403 Cruise Control vacuum diagram by Aus78Formula, on Flickr
 

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Discussion Starter · #10 ·
Here is a picture of it, I hope it helps, Is that release valve easy to get to? This is a perfect diagram of the cruise. Thanks so much for the help, please let me know if you need anything else.
Font Publication News Document Commemorative plaque
 

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I've never had to work on one of these cars with cruise control so can't help too much there. As with anything vacuum, block off a line if you suspect a leak, it may stop it from working altogether but also run better as no leak, depending on what it is! Old hoses can be replaced one at a time and their location confirmed as you go. Cruise Control servos dried out and the bellows cracked or splitting was common, but I'd suspect you'd notice that a long time before it ran rough, like not being able to sustain speed settings due to slow leak.

That diagram is a fairly standard one with the OD. I have a larger scan at home, or do a search on 79 403 OD label reproduction for larger pics or new decal. The vacuum and coolant sensor trees play up or get brittle over the years, often replaced with only similar parts, or not at all. Deleted, capped, redirected without checking route etc.
 

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The cruise control vacuum release valve is fitted to the brake pedal similar to a brake light switch. If you look up there, you’ll actually see a brake light switch, the vacuum dump valve and another electrical switch which kills power to the cruise transducer when the brake is applied.
IF you are seeking that out looking for a vacuum leak, that’s likely not it as that only has vacuum applied to it when cruise in engaged. While it COULD be possible that the cruise transducer solenoid is stuck in an “open” position thereby creating a vacuum leak it’s not likely. You’d probably know unless the cruise system is not intact.
 

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Discussion Starter · #14 ·
Has anyone had a leaking intake manifold gasket? If so, what test did you do to see if that was your problem? I've blocked off every vacuum port to no avail so the intake manifold gasket is next. Thanks for any help.
 

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To answer the question of how to locate an intake leak, there are several methods ranging from listening with a mechanics stethoscope to filling the system with smoke and looking for leaks. You can even mist water along the gasket (and vacuum hoses) to see if any is sucked in in a specific area, or you can spray a flammable liquid like carb cleaner or WD40 around the intake gasket. If the engine picks up RPM when you go over an area, that's the location of the leak. I AM NOT INSTRUCTING YOU TO PERFORM ANY OF THESE METHODS. I am simply telling you how it can be done. Which method you choose (and there are others if searching Google), and bad things that may result are a result of your actions, not my advice because I am not advising you. ...only explaining.

...but do back up a moment and respond to b_hill_86. There's a reason he replied in the way he did.
 
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Discussion Starter · #17 ·
Thanks for the reply’s, I can start the car cold easily after about a minute or two when I kick the idle out of fast idle it stalls, I start it back up and it runs but rough, when I close the choke about three quarters the idle is perfect but the engine is at operating temperature. I figure making it run leaner means vacuum leak. Please let me know your thoughts and if you need any other information. Thanks again.
 

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Ok. Just for the record a vacuum leak causes the lean condition. Closing the choke reduces the amount of air entering which effectively richens the mixture.
If you haven’t, I’d pull every vacuum line off the carb and cap all of the ports. Maybe leave the line to the distributor advance if it’s attached to manifold vacuum. Then see if you still have the issue. If so, you can move on to other things. If not, you know one of the lines you pulled is your leak source.
You mentioned you rebuilt your carb. What did that involve?
 

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Discussion Starter · #19 ·
Thanks b_hill_86!!! Yes I tried to all vacuum lines but nothing helped that is why I went to the manifold gasket. Thanks again for helping
 
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