2000 platinum 181K. I’ve been noticing a bit of rpm jumps at random speeds, whenever I hit the breaks in certain situations the rpm will jump down to around 1000, then shoot up to 1500-1700 even tho I’m slowing down going 20...... engine makes no noise when it does that. Another thing I’ve noticed is at highway speeds I’ll be around 2000 rpm at 60, then suddenly it will shoot down to 1500 but I will feel no difference in the car.... also I’ve attached some pics of what my rpm is in the gears, I have a higher RPM in park than drive, that doesn’t sound right to me but idk..... There’s also a bit of hesitation when sitting in park or neutral... I wouldn’t say it’s a shake but you can definitely feel the engine moving.
From my (limited) experience, here are things that come to mind:
- If idle is unstable or too high or too low when parked, the first thing to check is IACV (Idle Air Control Valve). That valve determines how much air goes into the engine when you are idling (and coasting I believe). On my car, 1990 LS, I can unscrew it and clean it from all the carbon deposits and once I do that, the car runs as new. Needs to be done every couple of years. I clean mine with B-12 chemtool and brake cleaner. Inside there are two bearings, and sometimes those go bad, I just replaced mine and it worked fine afterwards, but had similar symptoms to your before (strange idle, drops suddenly and then catches up and stuff like that which is all a sticking bearing). On most models of LS, IACV is not serviceable. They figured us out. So you need to buy a new part, or, I've been thinking - maybe there is a way to install the serviceable one in its place, because to me, they look identical. And, even if the valve inside is not identical, as far as I know, the ECU calibrates each time what the position of the valve should be (someone correct me if I am way off here).
- Other things in order of probability: spark plugs, wires, rotors and distributors. I've changed my rotor which looked like crap after 60k miles. (I was under impression that rotors last longer than that - they don't)
- Air intake leak. This would be more consistent type of problem - like consistently low idle, or something like that. If the air intake pipe (the round, accordion type hose that is between your MAF and throttle body) is damaged your engine won't run well - but it will run. (and sometimes you cannot see that it is actually broken! Aftermarket ones break between those accordion folds, so you can't see at all). Beware: aftermarket are junk. They last few months if that. Buy OEM only , even used OEM is better than aftermarket. I've had TWO aftermarket ones break within a year or two.
- Last would be ECU, but that usually brings up more gremlins than that. I have two, identical, perfectly working ECUs so every time something weird like that happens, I swap them to make sure.
2000 platinum 181K. I’ve been noticing a bit of rpm jumps at random speeds, whenever I hit the breaks in certain situations the rpm will jump down to around 1000, then shoot up to 1500-1700 even tho I’m slowing down going 20...... engine makes no noise when it does that. Another thing I’ve noticed is at highway speeds I’ll be around 2000 rpm at 60, then suddenly it will shoot down to 1500 but I will feel no difference in the car.... also I’ve attached some pics of what my rpm is in the gears, I have a higher RPM in park than drive, that doesn’t sound right to me but idk..... There’s also a bit of hesitation when sitting in park or neutral... I wouldn’t say it’s a shake but you can definitely feel the engine moving.
Do you notice only the indication change? I mean the tachometer is not working right. If so, I worry that the ECU or the tachometer's pulse circuit is malfunctioning.
A 2000 LS400 doesn't have any IACV nor distributor since it has a direct ignition system. The main throttle valve controls entire rpm range including the idle speed driven by the ECU.
I'd check the rpm signal from the ECU to the cluster board first.
