Question: rough idle........vacuum??

[Dunzvett81]

Hi Gents, I am trying to pin down my idle problem. When cqar is initally started up and is warm it idles not to bad. However, when car is driven about 2 mile or so the idle seems to be worse. It idles at about 750-800 rpm but it just sounds and feels a bit off almost like it is missing. I have replaced spark plugs, rotor, cap, wires, timing is 10 BTDC with a new cam, lifters, balancer, chain etc.. I just repalced all vacuum lines for the headlight system and i am getting 20 off the manifold. I know pretty much anything could cause a bad idle but would a vacuum leak cause a rough idle as i described. The headlight actuators are new however the relay valves are not and I just ordered new ones, if they are leaking could it cause the problem? or would it be most likely linked to the carb?
thanks

 

[73shark]

A vacuum leak can cause poor idle and it could be the carb. First let me be sure I understand what you are saying. You start the car and with it completely warmed up, it idles fine. But after driving two miles, the idle goes south. Not sure what's happening here.

Couple of ways to check for vacuum leaks. One is to direct an unlit propane torch around areas such as manifold and carb gaskets while the engine is running and when the idle changes, then the propane is being sucked in which changes the idle. Another is to use something like WD-40 instead of propane. Just a little messier.

 

[82ColEd]

Might not be vacuum either, though of course it can be..

You replaced a lot of parts already, so i dont want to suggest you do more without knowing whats actually wrong. Do you have the Shop Manual? If not get one, and trouble shoot this.

My first thought was the CTS (Cooleant Temp Sensor), but thats a guess. I am using WinALDL to check on the sensor info now, so that i can see what they are telling the ECM. Check to see if you can get a readout fro the sensor, if not, that might be your issue (Temp changing, and the ECM doing something to 'correct' the situation).

But again, check a Shop Manual for a 81 and go through their troubleshooting to find the cause of this one.


-Stefan

 

[Bills money pit]

Consider checking the butterfly shaft on the quad. My car ran rough until I put bushings on the shaft. That fixed the problem. This repair is often overlooked during a carb rebuild. The repair kit can be obtained from Cliffs Carb Service.
Good luck

 

[Mr. Chuck]

Hi Gents, I am trying to pin down my idle problem. When cqar is initally started up and is warm it idles not to bad. However, when car is driven about 2 mile or so the idle seems to be worse. It idles at about 750-800 rpm but it just sounds and feels a bit off almost like it is missing. I have replaced spark plugs, rotor, cap, wires, timing is 10 BTDC with a new cam, lifters, balancer, chain etc.. I just repalced all vacuum lines for the headlight system and i am getting 20 off the manifold. I know pretty much anything could cause a bad idle but would a vacuum leak cause a rough idle as i described. The headlight actuators are new however the relay valves are not and I just ordered new ones, if they are leaking could it cause the problem? or would it be most likely linked to the carb?
thanks

Had similar problem; had my OEM carb (100K+ miles) rebuilt by old GM service guy, he knew what to do and what to check, found most of the electrically activated in carb controls were worn-out, replaced same; fine tuned all setting/adjustments, etc. Car now starts 1,2,3; set the choke (once), 2 - 3 short cranks (depends on how long between uses) on the starter, starts, idle (~900) smooth cold and hot (~700). Passes CA SMOG at a walk. You may not have all the extra SMOG required adds to make life more interesting in keep the Vet running SoCal.

 

[killain]

Might not be vacuum either, though of course it can be..

You replaced a lot of parts already, so i dont want to suggest you do more without knowing whats actually wrong. Do you have the Shop Manual? If not get one, and trouble shoot this.

My first thought was the CTS (Cooleant Temp Sensor), but thats a guess. I am using WinALDL to check on the sensor info now, so that i can see what they are telling the ECM. Check to see if you can get a readout fro the sensor, if not, that might be your issue (Temp changing, and the ECM doing something to 'correct' the situation).

But again, check a Shop Manual for a 81 and go through their troubleshooting to find the cause of this one.


-Stefan

This would be my guess, and since my 79 had the vacume sensor that's located in the thermostat housing, they often after twenty years develope cracks, another to me would be to check the vacume line from the manifold to the brake booster for cracks and lastly, the EGR valve for holes in the valve itself. If you have some doubt as to any of the different vacume lines, the above propane trick is the best way to find a leak, you'll notice right away if you hit a leak as the engine idle speed will jump right up on the new fuel source, another is to clamp off any vacume lines that go to different things like the vacume tank inside the front bumper and or the headlight actuators ! When i restored my 79 I found a dozen leaks !