Engine idling rough.

[Buford T. JuSStice]

As the title says... my engine, a 3.8L Buick V6 is idling fairly rough. I had it at school, we tuned the carburetor and the choke on it, then replaced the spark plugs and wires. Still ran rough, and the shop assistant seemed to think it was being caused by a misfire, so we did a compression test on the cylinders, and turned up about 130lbs or so which he said was fine. Next I did a spark test at home using an ignition tester, and after setting the gap, and firing up the engine, it seemed fine. So I've got good spark, and good compression... I know my exhaust system is old and worn out, and I noticed the Catalytic converter had a hole in the side of it when we were underneath doing the oil change. Perhaps that's causing the rough idle?

 

[jmt455]

Couple of ideas:
Could be an ignition module going bad, plugged fuel filter, vacuum leak...I'd look at this simple stuff first.

I don't remember if the 1981 V6 had a computer controlled carb, but if it has an Engine Control Module and fuel injection or computer controlled carb, a bad temperature sensor can cause what you are experiencing.

Could be a stretched timing chain. If that's the problem you will probably see the timing mark wander when you tried to set the timing.

 

[Buford T. JuSStice]

I don't think it's an ignition module, that would have shown up in the ignition spark test... if the spark is strong enough to arc across a half-inch in open air, it can arc the 0.060" spark plug inside the cylinder and provide enough spark to ignite fuel.

The carburetor on my car is an electric choke carb, but other than that, it's completely free of computers. Fuel filter is also brand new, as I rebuilt the carburetor, and changed the filter in it just recently. Everything in the carb as far as I know, is working fine. The idle mixture screws are adjusted, the choke is adjusted, and idle is adjusted... I put my hand over the top of the carburetor and it started to sputter and die out so I don't think there are any (major) vacuum leaks.

I could check the timing tomorrow, couldn't hurt. The engine is rebuilt and has less than 5,000 miles on it though... I wasn't really surprised when I didn't find anything off with the compression tests, though I guess it couldn't hurt to try again with the equipment I have here at home (we used three different dials at school, and two of them didn't work properly... who's to say the third one wasn't broken too?)

I suppose that it is possible that the engine is not misfiring at all, and there is in fact nothing wrong with it. My grandmother owns an '83 Regal Limited with a nearly identical engine in it (it has some more emissions equipment on it than mine does) and it too has a tendency to idle a little rough. Both of them like to rock back and forth a bit.

*EDIT*
Now that you mention vacuum, it might be possible that the timing is off because the distributor is vacuum advance and rather than going to a vacuum switch, it goes directly to ported vacuum on the carburetor (It's a long story). An easy test there would be to take the vacuum hose off of the vacuum advance, and plug the hose up with my finger and see if it makes a difference.

 

[jmt455]

Also watch it running when it's dark under the hood...you'll see the arcing if there are any crossfires across adjacent plug wires.

It's not common, but you might have a cracked plug insulator or a bad wire, even though they are new.

How is it running in non-idle operation?

 

[Buford T. JuSStice]

Also watch it running when it's dark under the hood...you'll see the arcing if there are any crossfires across adjacent plug wires.

It's not common, but you might have a cracked plug insulator or a bad wire, even though they are new.

How is it running in non-idle operation?

Seems to run fine non-idle. Accelerates fairly quickly (for a 3.8L V6) and the only issue I have is on the highway maintaining speed, but that I attribute to aerodynamics, and the transmission only being a 3 speed.

 

[docrocket1967]

I dont believe it would be a vaccuum issue in my opinion if it is running rough at idle. You tend to create less vaccum under acceleration than when you are idling. Now as for the fuel filter being and catylitic those are good possibilitys. More fuel pressure under acceleration may not be any issue if theres a little bit of a clog cuz your fuel is capable of working its way past that easier and same as the cat

 

[Bonnewagon]

Did you use a vacuum gauge to adjust the carb? What was the best vacuum you achieved and was it steady?

 

[Buford T. JuSStice]

I dont believe it would be a vaccuum issue in my opinion if it is running rough at idle. You tend to create less vaccum under acceleration than when you are idling. Now as for the fuel filter being and catylitic those are good possibilitys. More fuel pressure under acceleration may not be any issue if theres a little bit of a clog cuz your fuel is capable of working its way past that easier and same as the cat

The fuel filter is brand new, but I suppose it would be wise to double check it anyway. A new fuel line from the fuel pump to the carburetor was made by somebody I know, and it was a few days between when he made it to when I put it in, so who's to say something like a spider didn't crawl in there and call it home. The catalytic converter in my opinion would be the #1 suspect since, as I mentioned before, I noticed a nice size hole in the side of it while under the car. What I should really do is have a new exhaust system put in there, but I'm trying to keep repairs as cheap as possible, so what I'll probably try to do is cut out the cat and patch up that section with a length of exhaust pipe. In my state I don't even have to get the tailpipe sniffed as it is a 32 year old car, so I don't think there will be any trouble.

Did you use a vacuum gauge to adjust the carb? What was the best vacuum you achieved and was it steady?

There's a vacuum gauge on the dashboard, and when it's idling it's steady between 17 and 19 inHg at first, and as it warms up the vacuum picks up as well, up to around 22-23 inHg. It fluctuates a bit at idle but never more than 2-3 inHg. Before we tuned the choke, it was obvious the engine was smothering itself since it would start off around 15-17 inHg, and then quickly die down to 12 or so and then stall unless you held the throttle plate open slightly to allow the car to warm up.

 

[jmt455]

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?

 

[Clone TIE Pilot]

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.