CanadaCraig Posted August 14, 2006 Share Posted August 14, 2006 Hi Guys!! :) I hope you're OK. Here's what happens. I drive my '93 LS for awhile so it's nice and warm. I park somewhere and turn off the engine. After a little while [say 10 minutes or so] I will start the car and slowly rev it up to about 2,000 RPM. [in my quest to 'see' if the 'problem' still exists] I will then VERY slowly ease off the gas until the revs are down to 1,400 RPM. Why 1,400? Because THAT is when the engine is particularly rough. So much so - it shakes the car a bit. [and sometimes more than just a 'bit] So what I'd like to know is this. Is something happening at 1,400 RPM? Does the timing change? Does something switch from idle mode to running mode? [if that makes sense] The engine is also rough [but not quite as obvious] at about 1,800 RPM. If none of this makes any sense - please let me know and I'll try to make it clearer. [Just so you know.... this isn't a 'new thing'. It's just another 'yet to be resolved thing'] THANKS!! Craig!! :) Quote Link to comment Share on other sites More sharing options...
alsalih Posted August 14, 2006 Share Posted August 14, 2006 All engines have a resonance frequency which occurs at a certain RPM. The LS400 does appear to have one occur around 1800 RPM as mine does and many others have reported. It’s also possible to occur again at a higher RPM. With new front engine mounts, this vibration is nearly undetectable, so that’s one thing you might consider. The vibration you’re having at 1400 is however a bit strange. I would look closely at the crankshaft pulley (harmonic balancer) for deterioration of the rubber isolator or general looseness or the pulley. Quote Link to comment Share on other sites More sharing options...
CanadaCraig Posted August 16, 2006 Author Share Posted August 16, 2006 Hi alsalih!! :) Thanks for the response. Good to hear that the vibration at around 1800 RPM can be accounted for - at least. The one at 1400 make me wonder [which is why I asked] if it's a symptom of a problem my car often has with acceleration. Perhaps something is causing the engine to hiccup [just enough] at 1400 RPM to cause performance from that point on to NOT flow as smoothly [or as powerfully] as it should. But I'll certainly talk to my dealer about the crankshaft pulley and have them check that out. [including looking for abnormal wearing of the rubber isolator, etc.] Craig!! :) Quote Link to comment Share on other sites More sharing options...
SKperformance Posted August 17, 2006 Share Posted August 17, 2006 I agree with the mounts as well as the balancer. Another thing to look at is if the fan clutch is seizing up. I would not suspect an ignition problem as it goes away higher in the rpms which is where they only get worse especially distributor caps. Quote Link to comment Share on other sites More sharing options...
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