BillyB Posted September 14, 2006 Share Posted September 14, 2006 91 LS400 idles up and down in park or neutral. Fine in drive (although a high idle ~900). Is this a vacuum leak, clog, sticky valve? I replaced EGR vacuum modulator filter - it was clogged beyond recognition. This didn't help. My mechanic put it on a "smoke machine" I guess to check for vacuum leaks but found none. There is no check engine light on. Has anyone had this problem? Where do I start? Thanks. Quote Link to comment Share on other sites More sharing options...
Threadcutter Posted September 14, 2006 Share Posted September 14, 2006 91 LS400 idles up and down in park or neutral. Fine in drive (although a high idle ~900). Is this a vacuum leak, clog, sticky valve? I replaced EGR vacuum modulator filter - it was clogged beyond recognition. This didn't help. My mechanic put it on a "smoke machine" I guess to check for vacuum leaks but found none. There is no check engine light on. Has anyone had this problem? Where do I start? Thanks. How old is the Air Control Valve (ACV) on the Power Steering Pump? It could be the culprit. If you get white smokey exhaust periodically, the ACV needs to be replaced. Quote Link to comment Share on other sites More sharing options...
drelegal Posted October 23, 2007 Share Posted October 23, 2007 91 LS400 idles up and down in park or neutral. Fine in drive (although a high idle ~900). Is this a vacuum leak, clog, sticky valve? I replaced EGR vacuum modulator filter - it was clogged beyond recognition. This didn't help. My mechanic put it on a "smoke machine" I guess to check for vacuum leaks but found none. There is no check engine light on. Has anyone had this problem? Where do I start? Thanks. did you ever discover the cause. I have exactly the same problem with my 400LS '91. Exactly. Been down all the same paths and am now looking at the computer. What did you find out? Quote Link to comment Share on other sites More sharing options...
pishta Posted October 23, 2007 Share Posted October 23, 2007 91 LS400 idles up and down in park or neutral. Fine in drive (although a high idle ~900). Is this a vacuum leak, clog, sticky valve? I replaced EGR vacuum modulator filter - it was clogged beyond recognition. This didn't help. My mechanic put it on a "smoke machine" I guess to check for vacuum leaks but found none. There is no check engine light on. Has anyone had this problem? Where do I start? Thanks. did you ever discover the cause. I have exactly the same problem with my 400LS '91. Exactly. Been down all the same paths and am now looking at the computer. What did you find out? My '06 21k mile Silverado 2500HD did that. I called it a bouncing idle. Garage put a whole can of Throttle body cleaner through it and its back to normal. I bet it is a clogged IAC passage and the motor compensates for it by increasing the IAC metering, then backing it down, hence the bounce. I had no codes stored either. Quote Link to comment Share on other sites More sharing options...
R.Holmes Posted October 24, 2007 Share Posted October 24, 2007 91 LS400 idles up and down in park or neutral. Fine in drive (although a high idle ~900). Is this a vacuum leak, clog, sticky valve? I replaced EGR vacuum modulator filter - it was clogged beyond recognition. This didn't help. My mechanic put it on a "smoke machine" I guess to check for vacuum leaks but found none. There is no check engine light on. Has anyone had this problem? Where do I start? Thanks. did you ever discover the cause. I have exactly the same problem with my 400LS '91. Exactly. Been down all the same paths and am now looking at the computer. What did you find out? My '06 21k mile Silverado 2500HD did that. I called it a bouncing idle. Garage put a whole can of Throttle body cleaner through it and its back to normal. I bet it is a clogged IAC passage and the motor compensates for it by increasing the IAC metering, then backing it down, hence the bounce. I had no codes stored either. Sounds like the IAC valve needs to be cleaned or replaced. Quote Link to comment Share on other sites More sharing options...
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