Surging idle help

[Flip Smiley]

I am a longtime member and have not posted for a long time. I monitor this sight daily to see whatÂ’s going on. I built my MKII Roadster in 2002 and have driven it over 30K trouble free miles. This was a donor build utilizing a 1990 LX, stock 5.0, With smog delete, 5sp, 3:55 4 Link. I have done a few upgrades over the years but have left it as built for the most part.
The last few times I have had it out I have noticed a surge at cursing speed. Almost like a miss but not as abrupt. It feels like I have moved the pedal slightly when I am actually holding it steady. At idle the tach shows the RPM moving between 800 and 1000 about two or three times a minute. Sometimes at idle the Tach moves to 1000 RPM and hangs there. Back in 2015 I experienced similar symptoms and replaced the Throttle Position Sensor and it fixed the issue. I will run the TP Sensor test tomorrow. Does this sound like I am on the right track.
Thanks, Flip Smiley

[David Williamson]

yes, and if that is OK it could be a sticking or failed ISC but check vacuum hoses first including the ones under the intake manifold. Could be 30 years old!
David W

[Flip Smiley]

Thanks David. I'll try the TPS test first and see what's up. Check ISC next.
Phil

[Flip Smiley]

OK David, thanks for the heads up on the ISC. I looked at that first. Removed, cleaned the carbon out with some cab cleaner and re-installed. Went for a test drive and all seems to be well.
Thanks, Phil