Engine idling rough.

[Bonnewagon]

It sound like the choke is set wrong. It should be totally closed when cold, crack open a set amount when the motor first starts, then open completely when at operating temp. That set amount is critical, older carbs had a fractional gap of between 1/8" and 1/4". As a rule of thumb, stalling means too much air, stumbling means too little. Once hot, the vacuum should be around 20" and steady at 650 to 750 rpm idle.

 

[Buford T. JuSStice]

That almost sounds like a bad thermal vacuum switch (TVS) or mis-routed or leaking vacuum lines; like you're porting vacuum to something when it's cold that should not get vacuum supplied until the engine is at operating temp.

Have you verified that all of the vacuum lines are properly connected and that any or all thermal vacuum switches are working properly?

This is very likely part of the problem as the Vacuum timing on the distributor is routed to ported vacuum on the Carburetor. A suggestion made by the shop help (I guess he technically qualifies as a teacher though he is not our head instructor) as we did not have a TVS handy at the time, and since I didn't (and still don't) know which TVS to get. The quick-fix we did there did help with performance, but if it is now causing the car to idle like this, it's time to fix it properly.

To be honest, I don't know what happened to it, all I know is that the switch is broken off on the manifold and it could have been Pep Boys, or some other random shop that broke it off and never said anything. I've owned the car almost 10 years now and as lame as an excuse as it is, I've never been too observant of work done to it, but now that I'm going to school for automotive, I'm quickly becoming much more observant of other people's work (mostly their mistakes).

The same problem exists for my EGR as the vacuum switch there was broken off as well. At this point, I don't even run an EGR, I've been planning on deleting the EGR Valve, but I'll be more than willing to not do that if it is a mistake waiting to happen.

The two previous owners of my car were my Uncle and Grandfather, with my Uncle owning it last. He has very good memory and according to him, when I took over the car it did indeed have all the vacuum equipment. Somebody somewhere decided the car would be better off without it... (I suspect the best place to point a finger at would be the place that installed the rebuilt engine, but ultimately it comes down to being my fault for not noticing sooner)

When you rebuilt the carb, did you pull out the idle tubes to clean them and the passageway under them? Just soaking a carb won't clean them, it's a very common mistake and a big cause of rebuild failures.

BTW, all US G bodies were computer controlled starting 1981. Many Canadian G bodies never got computer control even though the 80s. So either you have a Canadian or someone swapped carbs or your car is a 1980 model.

Yes, the car is a 1980 year model. When we rebuilt the carburetor we cleaned everything thoroughly. Even after we soaked parts we took the time to scrub them out and make sure they were clean. Also, I believe this car qualifies as an A-Body though it does share a lot in common with G-Body cars.

It sound like the choke is set wrong. It should be totally closed when cold, crack open a set amount when the motor first starts, then open completely when at operating temp. That set amount is critical, older carbs had a fractional gap of between 1/8" and 1/4". As a rule of thumb, stalling means too much air, stumbling means too little. Once hot, the vacuum should be around 20" and steady at 650 to 750 rpm idle.

I will most definitely crack out my Sears engine analyzer and check RPM and vacuum against one another! As far as the choke goes, I can definitely see what it is doing when to make sure it is doing what you're describing.

 

[Clone TIE Pilot]

At the very least you should use .030 wire to push though the idle tubes to make sure they are not plugged. The best thing to do thought is use a self tapping drywall screw and wire cutters to pry the idle tubes out of their bores in the main body for cleaning.

 

[Bonnewagon]

A stuck open EGR would cause a very rough idle, and disappear at speed.

 

[Buford T. JuSStice]

A stuck open EGR would cause a very rough idle, and disappear at speed.

True. I have connected the EGR to vacuum on the carburetor but I took it off immediately because it would cause the car to idle so poorly and so rough that it made the way it idles now look almost smooth.

The funny thing about all of this is that while my car does in fact idle a little rough, my grandmother's car, a 1983 Buick Regal which also has a 3.8L V6, idles very similarly, though the engine rocking is not quite as dramatic as it is in my Wagon. Another thing is that it seems, going through the checklists of everything that could be causing rough idle, that it could be a compound problem that doesn't have a single solution. The exhaust system is in rough shape, and the TVS for the vacuum advance is basically gone, replaced by ported vacuum.

I'm sure I could get it to run smoothly without the EGR, or the AIR injector pump, and I'm certain at this point that none of the cylinders have low compression, and all of them are hitting on spark, so none of them are misfiring for either of those reasons, if there is a misfire at all.

I guess my order of repairs should be...
Exhaust system *remove catalytic converter and use a smoke machine to see if there are any more clogs. If it is clear, then patch it, if not then suck it up and buy a new exhaust*
Thermal Vacuum Switch for the vacuum advance
EGR Delete
Thorough carburetor tuning to make sure the choke is set correctly.

 

[Bonnewagon]

A bad Cat would not cause a rough idle, it would choke up at high speed. Deleting EGR requires adjusting timing down to pre-EGR specs. TVS switch that only allows timing advance after a certain temp is reached would not cause rough idle. I would look for vacuum leaks, slightly flooding carb, and ignition leaks like the cap and wires and a fouled plug or two. I once had a fuel pump that would overflow the carb ever so slightly. Pulled strong, ran great, but the idle sucked. Plugs would get black, idle would slowly get worse, and after I cleaned the plugs it ran great for weeks and then slowly degrade. Drove me nuts. The cause was a fuel pump meant for a return line used without a return line. Too much fuel.

 

[jmt455]

I once had a fuel pump that would overflow the carb ever so slightly. Pulled strong, ran great, but the idle sucked. Plugs would get black, idle would slowly get worse, and after I cleaned the plugs it ran great for weeks and then slowly degrade. Drove me nuts. The cause was a fuel pump meant for a return line used without a return line. Too much fuel.

Man,I bet THAT was tough to find!

 

[Buford T. JuSStice]

Man,I bet THAT was tough to find!

No kidding!

Eventually I want to have a little fun with this car, and put something a little bigger/better in it for an Engine/Transmission combo, but for now I need the car to at least run reliably and efficiently so I can use it as a winter car (it's amazing how well it handles in the snow). Plus learning how to trouble shoot problems whenever possible definitely helps me learn, and this car just seems to love having problems. It's like the car knows it is in no danger of being junked so it tries its best to test my patience :lol:

 

[DRIVEN]

You keep bringing up the exhaust system and the cat. Don't let that distract you. Like Bonnewagon said, that will not effect idle without doing the same (likely way more) at cruise. Also ignore the A.I.R. system. It is downstream of the engine and only effects emissions readings. These areas may need attention but they won't smooth out your idle.
-The 3.8 is not a silky smooth engine. First you need to determine whether you're chasing unicorns or not. I'll assume that it has run smoother in the past and that's why you're focusing on it. If it's really never idled any smoother this may just be as good as it gets.
-I would suspect a vacuum leak. It may likely be the EGR valve not sealing completely in the closed position. It would be pretty easy to replace the valve with a plate to rule it out. If there's no improvement you can always reinstall the valve. You can pull the PCV out of the engine and put your finger over the hole in the engine. A slight amount of pressure (blow-by) is typical. If there is any suction at all you have an internal vacuum leak. Suspect a bad intake gasket. I have also seen the bottom of an intake cracked and cause the same symptoms.
-You can test fuel mixture by making a small vacuum leak at the PCV hose or introducing propane in the carb throat.
-Use a vacuum gauge to check valve condition. Just because it has decent compression doesn't mean the valves are sealing properly. Really, any shaking of the needle is an indication of either a burnt valve or worn guides. Yes, it's possible on a fresh engine. You cannot "tune away" a mechanical problem.

Please keep us posted.

 

[Buford T. JuSStice]

You keep bringing up the exhaust system and the cat. Don't let that distract you. Like Bonnewagon said, that will not effect idle without doing the same (likely way more) at cruise. Also ignore the A.I.R. system. It is downstream of the engine and only effects emissions readings. These areas may need attention but they won't smooth out your idle.
-The 3.8 is not a silky smooth engine. First you need to determine whether you're chasing unicorns or not. I'll assume that it has run smoother in the past and that's why you're focusing on it. If it's really never idled any smoother this may just be as good as it gets.
-I would suspect a vacuum leak. It may likely be the EGR valve not sealing completely in the closed position. It would be pretty easy to replace the valve with a plate to rule it out. If there's no improvement you can always reinstall the valve. You can pull the PCV out of the engine and put your finger over the hole in the engine. A slight amount of pressure (blow-by) is typical. If there is any suction at all you have an internal vacuum leak. Suspect a bad intake gasket. I have also seen the bottom of an intake cracked and cause the same symptoms.
-You can test fuel mixture by making a small vacuum leak at the PCV hose or introducing propane in the carb throat.
-Use a vacuum gauge to check valve condition. Just because it has decent compression doesn't mean the valves are sealing properly. Really, any shaking of the needle is an indication of either a burnt valve or worn guides. Yes, it's possible on a fresh engine. You cannot "tune away" a mechanical problem.

Please keep us posted.

Thanks for the advice. I have been suspecting that I may be chasing unicorns, and to be honest, it hasn't really ever run any better in the past... in fact, right now this is the best the car has ever run since I've owned it. My teacher seemed so convinced that it was skipping, and/or idling too rough though that I've been trying to figure out if there is indeed any problem with it, but I guess if you start looking for a problem, you'll eventually find one whether there is one or not.

To be fair, I don't want to make it seem as though my teacher doesn't know what he's doing or what he's talking about; I wouldn't be going through so much trouble if I didn't think he was at least somewhat competent. Albeit he isn't as by the book as the head instructor is, he's a little bit backyard, but he's good.