What Happens At 1400 Rpm?!

CanadaCraig · 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!! :)

alsalih · 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.

CanadaCraig · 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!! :)

SKperformance · 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.