Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

91 Buick Century Very High Idle

I have a 1991 Buick Century that has a very high idle speed. It started a few days ago out of the blue. The Check Engine light comes and goes randomly.

So far I have replaced the TPS (Throttle Position Sensor), IACV (Idle Air Control Valve), and cleaned the EGR Valve.

I’m not sure what else would make the car do this. Could this be affected by the O2 Sensor? I don’t think its related to the coolant temperature sensor as it seems to be idling higher than expected in cold weather.

Any suggestions are greatly appreciated. I need to get this back on the road ASAP.

A vacuum leak is a very likely culprit. This could be from a worn out vacuum hose (certainly possible on an 18 year old car), or from a leaking intake manifold gasket.

The first thing you should do is get the trouble codes read to see what they say. If you have a service light, a trouble code will be stored. Autozone will read them free of charge.

If the computer sees an issue with the IAC, such as no reading or incorrect reading, it will set the SEL light and a code. The idle will then go into a default setting which is very high (1200-1500) and it will remain high until the engine is shut off and the fault is not seen again. If you replaced the IAC and still get the problem, the other possibilities are a bad connector at the IAC, problem wires from the IAC through the several connectors to the engine computer, or the computer itself. You could also have a terminal connector problem at the computer. You’re going to need the factory manual and a fair amount of electrical work to find the fault testing at the various connectors. If it is intermittant, dang near impossible without just replacing wires.

A defective coolant temperature sensor for the computer can cause this problem. If the sensor is telling the computer that coolant temperature never reaches operating temperature, the idle will remain high.

If you want to pull codes to determine why the Check Engine light is coming on, here’s how and the code definitions


Thank you all for your suggestions.

I used the paperclip in the OBD1 connector as described above and got code 35 which is Idle Air Control Valve. However as I stated, the valve is new and I believe it is working.

When first started the engine will rev high but eventually (sometimes upwards of 4 minutes) comes down to a normal idle. Once the idle levels out the car will run fine. I have found that sometimes (but not always) when I turn on the air conditioning the idle will go high again and will return to normal if I turn the AC off.

This led me to think that the issue may be related to a vacuum leak however the guy working at Advanced said it could possibly be a sticking fuel injector.


I think you’ve got a bad IACV. You’re describing a lazy IACV. I would remove that new IACV, bring it back and get a replacement.


Sorry, I meant throttle position sensor TPS, not the IAC. My bad.

I doubt that Advanced will return the part. I will still try this when I get a chance though because I’m out of ideas.

Would this also make the car stall? It started stalling when idling in gear because the idle is too low when it isn’t too high. Ugh…

Yes. A faulty IACV will cause the engine to stall when idling in gear.


Replacing the computer fixed the problem. Luckily I didn’t even have to fix it myself as shortly after it the problem started I was out of town and my father in law fixed it.