81 rough idle

[Silver81Vette]

The other day im out driving my vette, and all of a sudden when I took my foot off the pedal, the RPM's dropped of course, and it started running choppy. I pulled it into a parking lot and stopped. It idled like crap. just under 500 RPM. When I gave 'er gas, it ran fine, but when it came back down to idle speed, its very very choppy, and loses power, so I drove it home carefully and put it back in the garage.

This has never happened before, and it still runs like a champ (just not at idle speed). I need a little help here.

Thanks,
-- G.J.

 

[Sensei]

It could be any number of things, but I'd check for a vacuum leak first. Look for a broken or disconnected hose. If you don't find anything, then take a 2 foot long peice of vacuum hose and put one end in your ear. Take the other end and move it around the carb and vacuum lines while the engine is running and listen for leaks. Let us know what you find. God bless, Sensei

 

[smithjw]

I had the very same symptom a few years ago. I also checked all over for vacuum leaks. Couldn't find any. Turned out to be the fuel pump went bad. Replaced it and life was good again.

 

[Silver81Vette]

What i've done so far... (ATTN: Sensei)

Ok, here's what i've done so far.

Replaced the fuel pump, checked for vacuum leaks, replaced fuel filter, cleaned carb, looked over carb (which was rebuilt one year ago and seems to be operating correctly), replaced fuel lines, vapor lines, dropped and cleaned gas tank, replaced the ECM, replaced the air diverter valves, replaced the A.I.R. smog pump (that little expensive pump thing in the front of the engine), it just got a complete tune up, new wires, plugs, distributor, etc... I also run nothing but high-test 93-94 octane fuel in it.

I notice that there is a "Check Engine" light that now illuminates when I put the new computer in the car. Hopefully I can take it somewhere to get a code read or whatever. But thats all I have so far. I have drove it today, and it idols very rough at about 300 RPM at a red light / stop sign, but when I "get on it" it runs just like it did before this problem occurred, which was awesome before this mishap. It also sounds like it is definately starving for something, and I dont know what, but when I take it out on the interstate, I have no problem getting up to speed, or exceeding the speed limit at all. No problems there.

But still having the rough idle problem.

Any takers?

Thanks!

 

[Silver81Vette]

?

Although... The fuel pump was replaced a few months back, maybe it went bad again???

 

[Peer81]

If it's a mixture thing I would also look at the O2 sensor. But as far as I know it would effect the entire rpm range and not only idle.

Groeten Peter

 

[Sensei]

If your check engine light is on, you can check codes with a metal paper clip. With the ignition on and the engine stopped, short the two end terminals ("D" and "E") on th ALDL (under the ash tray). Your check engine light will flash code 12 (diagnistic check: one flash, a pause then two flashes) 3 times. Then it will flash any trouble codes stored three times each, then repeat. God bless, Sensei

 

[big2bird]

Check the codes and correct them first. If all else fails, and you have the original one, change the Cat converter. If it clogs, it will act just like that.

 

[GerryLP]

G.J.,

My vette had the same symptoms as you described, and what was so telling was how I noticed it right away.

I did the checks under the hood, and then I noticed a slight "hiss" sound inside the cabin.

It seemed to come from the brake pedal area, and then I noticed how much more force I had to use to stop the car (it was a gradual increase in pressure required).

Fixed it, and the roughness went away (and the "hiss" as well.

 

[Silver81Vette]

Hiss...

I have noticed that a hiss is coming from the carb, when i take off the air cleaner, but I'm figuring that its the carb sucking air for the mixture. Who knows. I really havent seen a problem stopping the car, the brakes work fine to me. dont know... still at a loss here. i havent checked the codes yet, but I will ASAP.

-- G.J.

 

[Yoda]

Spray some WD40 around the base of the carb If the idle straightens out for a little bit then goes back to rough, My guess would be your carb sucked part of the gasget and you need a new carb gasget ...

I've had more than one Quad suck a gasget

Bud

(the WD40 provides a temporary seal and once it is all sucked into the intake the leak comes back & the rough idle)

 

[Peer81]

Spray some WD40 around the base of the carb If the idle straightens out for a little bit then goes back to rough, My guess would be your carb sucked part of the gasget and you need a new carb gasget ...

I second that!

Groeten Peter

 

[Silver81Vette]

I have noticed that when I start it, initially, it runs ok... then once the RPM's drop, the engine starts to shake and idle's about 300 RPM. I'm going to check out the carburetor gasket. Would a carburetor gasket do this also ??? I need to get this fixed. Not cruising in my vette is horrible !!!

Thanks again )

 

[Silver81Vette]

Also, I have sprayed carburetor cleaner in the carb, and that has happened (the correct idle and all that. I'm not Mr. Goodwrench, but im gonna bet its that damn carburetor gasket. But please, keep the help coming!

 

[big2bird]

Buy a set of rubber vacuum plugs. Disconnect every port on the carb and manifold except the map sensor. Plug em all. See if it runs better. If so, add them back one by one till you find the culprit. Cost you $3.00 at the Zone, NAPA, etc.

 

[22229stingray81]

Clogged EGR? I had the EGR, and TPS changed at the same time and it cured my at idle roughness. Just a thought...


-Tatortot

 

[big2bird]

Clogged EGR? I had the EGR, and TPS changed at the same time and it cured my at idle roughness. Just a thought...


-Tatortot

Good point. They even sell aftermarket screens to prevent clogging with carbon.

 

[Silver81Vette]

Update to my idling problem

I installed a new ECM (reconditioned) now the check engine light wont give a code, its continuously illuminated. The idle problem is gone. When I get on it on the highway, it goes up to about 4000 RPM or so and starts to backfire, but when I let off the gas, it shifts and runs fine, just cant pass anyone, or "get on it" when i want to play around. Along with the long list of things i mentioned i replaced before, I just replaced the EGR valve, checked for vacuum leaks, replaced the PCV valve, and replaced all the air check valves. I recently replaced that darn A.I.R. diverter valve thing (the big black thing with 14 air-ports on it). I tried that paper clip thing with the check engine light, and it still wont give a code.

I havent tried messing with the carb yet, or check the carb gasket... should I still check it if the bad idle is gone now?

Still at a loss here... Help.

 

[SIXFOOTER]

OK, so by process of elimination, clear the following:
Vac leaks, I use an unlit mapgas torch around the intake, carb base plate, vac hoses etc with the engine running. If the engine speeds up you have found a leak. You can also disconnect and cap all the vac hoses for all the various stuff to eliminate those systems.
Base time, check it with the 4 pin connector from the wire harness to the distributor disconnected, set it for 8* for a start
Choke, make sure it works, should be about 1/4" gap at the front of the blade with the engine cold and full open when warm.
Does the Accelerator pump work? squirts immediately when you move the throttle
Is the distributor cap good? No cracks, carbon tracks any kind of contamination inside
Same for rotor
Plugs look good?
Air Filter clean
I see you changed a bunch of stuff so that should be good
With all that good, you need to get the computer working. The light is on solid but no code? I'll look in the book I have and see what it says

 

[Silver81Vette]

MAP Sensor

Maybe a bad MAP sensor ????