My 1989 Corvette's Rough Idle

Been having problems with my 89' Vette all year. This spring I pulled it into the garage for what sounded like a bad dual mass flywheel. Well lets just say that was a waste of money because there is no ticking sound, but the car still idles rough. Here's what I've found so far. I had the injectors checked at a shop to make sure that everything was running right. The injectors were all operating fine and the fuel pressure was right where it was supposed to be. I've changed the mass airflow relays. I've disconnected the mass air flow sensor like it said in the service manual and the engine still ran rough. I checked all the plug wires to ensure they had the correct resistance and they all checked out. I changed the cap and rotor on the distributor. I pulled the plugs on the car and they were carbon coated. The car has 20 thousand miles on it and has had the plugs changed at least once, so I replaced them with the correct delco plugs. Everytime the car is out it always gets up to operating temperature because I drive it for at least a 1/2 hour so I don't think they are carbon coated from short distance driving. Now here's where the problem gets tricky it idles rough, yet as soon as I put it in gear and pull away it is completely fine. I do not notice any stumbles at high rpms and no loss of power. If it wasn't for the rough idle I'd think the car was running perfectly. I'm thinking of replacing the 02 sensor next, but I'm trying not to throw parts at the car like I already have. Oh yeah one last thing the engine does what the manual calls hunting. It goes from 700-900 rpms while idling. Has anyone experienced a similar problem like this and if so how did you fix it?

[redvett]

Have you had a leak down test done to see if your engine is healthy? A miss at idle will cancel out somewhat as RPM's increase. Have you checked for vaccum leaks at the runners & intake manifold along with any vaccum hoses?

I'm having the exact same problem with my 88 right now. Runs great but once in a while it will "hunt" Have you cleaned the IAC and reset the TPS yet? It's easy enough to do. The TPS can and usually will move out of position and mess the idle. If you had carbon fouled plugs, it's a good chance the IAC and throttle body need clecning as well. I'd start with a can of throtle body cleaner, (NOT CARB CLEANER) and give the TB and IAC a good cleaning then do a reset. Won't cost a thing except for the cleaner. At least you'll be able to ryle those items out before doing the O2 sensor which is what I'm about to buy.

Keep us (me) posted.....

Len

[redvett]

I wouldnt waste mony on an O2 sensor the problem isnt there. When you are idleing on the earlier year vehicles they go into open loop and the O2 sensor isnt doing anything.

I'm thinking that I should take the egr valve off the car and clean it along with the tb and anything else that has carbon on it in the intake plenum. If the plugs are carbon coated something else in the engine is probably as well. Got some other things to look into that are mentioned in the service manual. I have not checked the vacuum hoses, but I will do that when I get a chance. Pretty sure with the low mileage that they are in pretty good shape and none of them look dry rotted.

[Mika]

I have same kind of problem which we have tried to solve for a VERY long time. Now we have located this into distributor...so maybe my rough idle is because distributor insn`t working all the time so some spark plugs don`t work all the time. This because my car recently also had problems with starting up so that smell of gas came in but engine didn`t even start when cold.
This in just one thing that you can check there is inside the distributor is some controller which might cause this. On friday we change my whole distributor so maybe after couple of weeks I can tell did this help or not.

Here is a list which parts has been already been changed in my car and these didn`t resolve this problem.

Spark plugs+wires, altenator,distributor gap, O2-sensor, battery, EGR valve+solenoid, TPS sensor, injectors, valve o-rings, fuel pressure checked, all filters inc.fuel filter

[Mika]

I forgot one, also IAC is changed recently.

[vetteboy86]

Originally Posted by redvett

I wouldnt waste mony on an O2 sensor the problem isnt there. When you are idleing on the earlier year vehicles they go into open loop and the O2 sensor isnt doing anything.

I dont think that is entirely accurate. When the car warms up to operating temps it goes into closed loop, and stays there whether you are at idle or not. I have never seen my car come out of clsd loop at idle when it is at or above operating temps. Please inform me if I am wrong.

[RWDsmoke]

It may or may not drop out of closed loop after idleing for a while. Not designed to but sometimes the o2 sensor cools off and it drops out, which is considered normal. But if it runs smooth in open loop and rough as soon as it hits closed loop then you could change the o2 sensor. I had the same problem with rough idle on my s-10 with a 91 TPI camaro motor. New injectors, new distributer and wires, new coil nothing helped. And the new motor in my 86 Vette does the same thing. Only part not new is the distributer. It was a rebuilt unit from Oreillys. I'm thinking about getting a new new GM unit from SDPC. Data logging shows nothing, a missfire can show rich because the fuel in that cylinder was not burned. Hope you find it and let us all know.

[G Winter]

I had a rough idle. Kind of a miss yet I couldn't locate it by unhooking plug wires.Funny thing I notice is that it idled better cold then it did at operating temp. Chased this for months. I also had the gear clatter at idle, was thinking I had bad dual mass flywheel.The change in idle from cold to warm clued me to injectors or vacumm leak.I finally checked the ohms on the injectors. I found two that had low ohm reading. Changed those and it smoothed out nicely and the gear clatter is also gone.Don't know if it applies to your problem but might be worth a check. 16.2 is what the ohm reading should be.

Glenn

[Achilles]

Check the butterfly shaft on the throttle body. Mine has worn out the aluminum bore on the throttle body & is sucking air through it making the car idle rough.