'98Q Plenum job/knock sensors done

[duck3986]

I've completed putting my new knock sensors in and after a 100 miles over 3 days, I believe it has solved my intermittant, weather related sluggish performance issue.

There is one problem however. In park or neutral the engine idles smoothly as can be. In drive with my foot on the brake, the car has a loping idle, like I have a performace cam in a big block.......well maybe not quite that bad, but the body of the car "shakes" or "bumps" and it's a seat of the pants kind of feeling. I don't feel it at all on the steering wheel.

The idle in gear, w/ foot on the brake, is about 600 rpm. If I raise the idle by giving it some gas the bump goes away. Also, I can accelerate slowly or wide open and it's strong and smooth.

Any ideas?

Thanks,Don

[Q45tech]

Assuming you cleaned the throttle body and plenum and IAC when you did the other jobs just turn the IAC bypass screw out one turn to raise idle speed to 650-675 rpm.

[duck3986]

Actually my guess is that the idle speed is at 650-675. It's hard to tell exactly on the analog gauge.

I did try turning that screw and found the idle didn't change noticeably. I turned in both directions.

If I had a bad injector and were running on 7 cylinders, would I notice it at idle in park or neutral? I can't remember what a v-8 engine feels like minus one cylinder. If it would be rough at idle and a noticeable loss of power driving, then I can eliminate that as a possiblility.

If sometimes the power is only slightly less and the engine can still accelerate smoothly, then maybe I have an injector that's failed and when there's a load on the engine at idle, then it's noticeable.

Don

[DAEDALUS]

What if you drive forward while idling (no brakes)? Or in neutral with the brake on? Trying to eliminate a vacuum leak in the booster.

[duck3986]

driving forward while idling there is no surging or bumping. coasting in neutral with the brakes applied is also smooth