Seemingly randomly won't idle

prizm
chevrolet

#1

Hi everyone, it’s really a 1987 Nova, but the drop down menu didn’t list that option. I have been working on trying to figure out the cause of this problem for a pretty long time. During the last several years it had been running deteriatingly worse until I did an ignition tune up and replaced the spark plugs, plug wires, and the distributor cap (and rotor). That restored operation back to just the original problem of random but often poor or inability to idle. I have spent a lot of time on the carburetor and think that isn’t the cause of the trouble. I’m pretty shy about doing this post, but here goes. Thank you.


#2

Have you rebuilt the carb? I have an 87 truck that ran great for years, but the ethanol ruined the carb. It wouldn’t idle correctly and would bog down at times.


#3

I didn’t rebuild the carburetor, and I haven’t replaced it with a rebuilt one either, yet I did put a new accelerator pump in it. Also, I did put another used carburetor on. Even though I didn’t think the carb is the likely culprit, after your suggestion and explanation, I think that it now sounds like it could likely be the problem.


#4

You could try removing the idle mix screws and squirting carb cleaner through them. I think a new carb would be better if you can afford it.


#5

Taking out the idle screw and spraying out the passageway with carburetor cleaner sounds like a good idea. I could try that first and then think about getting the carburetor later. For quite a while, it was hard determining what was going on because I was getting sidetracked by fuel pumps that kept failing. When I installed an electric fuel pump, that variable got removed.


#6

I have an early 90’s Corolla, so somewhat similar to your car. Fuel injection for my car though. But I have a carb’d Ford truck, and I recently rebuilt the carb.

hmmm … Your new electric fuel pump is carb compatible, right? Carbs use much lower fuel pressures than fuel injections systems. So swapping to an electric fuel pump, you have to use a version that is carb fuel pressure compatible. Double check on that.

After fixing up the ignition system with new parts you still have a poor idle quality, eh? … What about the rest of the routine maintenance? Air filter, fuel filter, valve clearances, etc , that’s all up to date? When an engine idles poorly but seems to work ok when driving at normal speed, that could be a lean mixture. Shows up at idle but as more gas gets injected at higher speeds the extra air into the engine that is causing lean operation at idle is swamped out by the extra air coming in b/c you have your foot on the accelerator pedal. So checking for something causing lean operation is a must. Could be too much air, or not enough gas. First step if I had that problem, do a complete vacuum system test, make sure everything is holding vacuum that should be, using a hand-held vacuum pump. If you don’t know how to do this, ask a shop to do it for you. It’s a common thing shops do.

Since another carb didn’t fixed the problem, its hard to see how rebuilding your carb will do any good. I’d wait on that. Instead do the vacuum test above, then double check the fuel pressure at the car input, check the valve clearances, and do a compression test.

You’ve checked for diagnostic codes, right?


#7

Sometimes the idle is perfect, but I haven’t detected particular circumstances when that happens. It just seems random. When the carburetor was suggested to be the problem, especially the idle circuit, it made sense as it got me wondering again if some dirt was shifting position in there kind of randomly. I did clean out the float bowl when I had the carb off the car and had the top off of it to replace the accelerator pump. The overall vacuum leak-down test sounds like something I would be interested in checking out, though I have been checking vacuum things by pinching off lines and other ways like pulling and plugging things for test purposes. I still could direct a bit of propane around possible suspected areas to further check for leaks. Carb stays full of gas, which tends to make me think it is seeing good fuel pressure.

Air filter and fuel filter are new. The electric fuel pump is the low pressure type that works with carburetors. The replacement carb, I have tried a couple of other ones as replacements for the one on there which was rebuilt about ten years ago when it was put on there, rebuilt by one of the places that does that. Yet the other ones I tried later were just from the junk yard.

I agree about the fuel flow difference that happens when idling and with the throttle pressed, and about the swamping effect as other internal fuel circuits kick in and the engine then runs good.

The engine is a strong tight engine otherwise, I suspect that it could be a professional rebuild in there. I have a repair book on the car and it doesn’t mention checking diagnostic codes like if it doesn’t keep any I guess.


#8

Check for air leaking into the intake manifold through the throttle shafts. That’s a common wear point on well used carbs and causes lean operation when it happens. I check that by removing the carb and shining a flashlight from below, and seeing how much light gets through into the lower venturi area. Propane-test should work for that too. If you find you have that problem, you can often just install some new bushings to fix it.

The idle circuit fuel path in the carb could be the problem too. It’s best for addressing that to remove the carb, clean the gunk off it, take it apart, and soak the whole thing (but nothing with plastic or rubber) in a big can of carb cleaner for a couple hours. Then blow out all the passages in both directions with compressed air (no more than about 30 psi or so). That’s the only way to be sure you are actually getting the passages clean. I’ve used the spray can of carb cleaner method, works sometimes, but not always.

Some advise against this next part, but as part of my carb cleaning process I also run a small diameter wire through the idle passages from the idle port at the throttle valve all the way to the upper idle air bleed port. And through the main venturi emulsion tube. The concern w/this is it might enlarge the path’s diameter, which would be no good. I use a 30 awg wire that is coated in plastic insulation, not bare copper, to minimize that possibility. Before doing a carb clean job, good idea to read a book on how carbs work, makes the job more understandable.

hmmm … what else? ok, one idea, air leaks at the intake manifold/engine interface are a common cause for lean operation in older cars also. Idle quality problems are a tough nut to crack. You pretty much have to go through each possibility one by one, so be prepared for it to be somewhat time consuming. But eventually you’ll figure it out. Best of luck.

BTW: I’d be surprised in your car doesn’t use an engine management computer which provides diagnostic codes. Maybe somebody here can check on that one way or another.