High and erratic on my 2002 6 cylinder Honda Accord EX

The dealer recently replaced the front and side motor supports. Since then I’ve noticed that the car idles high when first cranked and I smelled excessive exhaust fumes. One time it idled really high and would not idle down when I hit the gas. Then it started idling up and down when my foot was not on the gas. Could there be any connection between this and the motor support installation? Could linkage to the gas pedal have been bent during installation?

More likely a vacuum leak. The dealer may have accidently damaged or disconnected a vacuum hose somewhere. They should be able to find and fix it quickly.

I’m a bit curious as to why they replaced the motor mounts. Were you in an accident?

In order to replace the mounts they had to shift the motor, which tweaked the connection between the exhaust manifold and the downpipe, causing an exhaust leak.
Air (with its oxygen) leaks into the exhaust, upsetting the operation of the oxygen sensor and affecting the idle.

Unlikely to be a bent throttle-pedal linkage.

They have to remove/disconnect a bunch of stuff to make room when working on mounts. I expect it is something to do with all that.

The O2 sensor/exhaust leak situation per circuitsmith would explain the idling up and down when the engine is warm, but not the high idle when first started. The O2 sensor is usually ignored by the ECM at first, until the sensor has a chance to warm up. But the exhaust fume odor is certainly consistent with an exhaust leak. Sometimes an odor from a too rich mixture out the tailpipe can be confused with an exhaust leak too.

I’d start by looking for any obvious problems, like pinched wire harnesses, disconnected chassis grounds, vacuum lines disconnected, or connected to the wrong place. There may be a vacuum hose routing diagram under the hood. If so, compare that diagram to how your car is hooked up. And problems like leaky connections, torn rubber boots, etc, between the air intake and the intake manifold could cause this symptom. Also a problem with the throttle position sensor could be a cause. That would throw a code usually. Do you have any diagnostic trouble codes in the ECM memory?

What else? If he ECM is confused about the coolant temperature, that could cause this symptom. Make sure the coolant temp sensor is connected I guess. I had a high idle problem develop on my Corolla one time, and it was a problematic idle air control valve, so that’s worth checking too. I expect when it is found, it will be something simple. Best of luck.