Inconsistent Idle And "reving"

[sgretchko]

'97LS w/ 180k, timing belt and major tuneup scheduled for next week.

The LS was parked outside at the airport for 4 days, temperature was low 40's at night and around 65 during the day. The last day was rainy. The normally flawless LS started fine but revved up to 20k rpm, then dropped to about 2k. It then proceeded to bounce around in a less than smooth fashion. It continued until the car was warmed up, then it seemed to run ok, maybe a little rich. It seemed to start ok when cold and ran fine today. The check engine light did not come on. Any ideas? I would like to figure out what it was before the major tune and timing belt. Thank you in advance.

[steve2006]

If your engine revved up to 20K RPM it should have thrown its internals all over the road!

If it has been OK since the incident I wouldn't worry, it could have been a slightly low battery creating ECU sensor problems,low fuel pressure or maybe condensation in the inlet manifold all due to being stood for some time.

[curiousB]

Could be a dirty throttle body and/or dirty/sticky IACV. Might want to have these cleaned when you do the timing belt. Erratic idle is common in these cars as they age due to carbon buildup. It is completely resolved with a good cleaning.

[sgretchko]

Thank you for responding. Throttle body is a good thought but it was cleaned about a year ago. The other reasons are all plausible, it is running fine so I am less concerned. Exactly what is a IACV?

[lexfourcam]

Idle air control valve. So after the car warms up it runs fine and doesn't stutter? If it was the only time it happened the answers above are very plausible but ever since i've owned my ls it's never on a cold start revved up to 2k rpm.

[Hanging In There]

'97LS w/ 180k, timing belt and major tuneup scheduled for next week.

The LS was parked outside at the airport for 4 days, temperature was low 40's at night and around 65 during the day. The last day was rainy. The normally flawless LS started fine but revved up to 20k rpm, then dropped to about 2k. It then proceeded to bounce around in a less than smooth fashion. It continued until the car was warmed up, then it seemed to run ok, maybe a little rich. It seemed to start ok when cold and ran fine today. The check engine light did not come on. Any ideas? I would like to figure out what it was before the major tune and timing belt. Thank you in advance.

Can't diagnose your engine issue but just add a bit of information on OBDII: A cold engine operates in "open circuit" which means that the engine's sensors are not providing information to the ECU to control the engine, hence the engine is operating on default settings stored in the ECU. When the O2 sensors, coolant temperature, etc.,reach operating temperature the ECU switches to closed circuit making it possible for the ECU to correct for issues not sensed during cold operation. In some cases components can fail to operate within design parameters and cause poor engine operation in open circuit operation but the issues go away after warmup. Not implying that this is your engine's issue, just sayin...

[curiousB]

Can't diagnose your engine issue but just add a bit of information on OBDII: A cold engine operates in "open circuit" .... When the O2 sensors, ... reach operating temperature the ECU switches to closed circuit ....

Yes this is intentional by design. Open loop is basically the way carburetors worked 100% back in the day. They didn't derive any feedback from the engine to trim the settings. Now with stricter emission standards numerous feedback loops are created to optimize emissions and performance.

The IACV is in operation during warm up though. It is a bypass air valve that lets air pass by a closed throttle plate. It opens and closes as needed to achieve a steady idle speed as dictated by the ECU. Idle is driven higher right after start up of cold engine and then trims back to normal idle speed as engine warms up.

If this valve is dirty or sticking it could result in erratic idle speed. In fact since it is for idle speed it really won’t have any effect at driving speeds. Since recycled crankcase gases are coming through the intake these devices do get carbon buildup just like the throttle plate. Particularly so for an older car with more blow by. So hopefully when your throttle plate was cleaned they did the IACV as well. If not then that would be my first thing to look at.

[fsuguy]

Hi listers,

It sounds like you know whereof you speak, so I am hoping one of you may be able to help diagnose my problem - the car is a '92LS400, and it has very poor throttle response once it warms up to normal operating temperature. The CEL indicates 25 (lean) but I am only getting about 12mpg(?)

When cold, the engine is smooth and sounds fine, but when warmed up it is miserable. I had the plugs, wires, distributor caps and rotors with original parts from Lexus (from Sewell) and Toyota, replaced about 3 to 4 months ago, but I think this may be a fuel pump or related problem. Just don't know where to start.

One other strange issue is that after warmed up, while driving, every so often the engine sort of loses power almost as if the fuel is cut off, or the spark is not getting to the engine, then it resumes. This "surge" is very noticeable and I am wondering if any of you have experienced it, and have any idea of what it may be due to. I had just changed my TPS because in addition to the "25", there was a CEL of "41" (TPS) which has since gone away following the replacement of the TPS.

I would appreciate any suggestions of what to look for, or how to resolve the issue.

Thanks in Advance

Mike

[curiousB]

Well poor MPG suggest unspent fuel and probably rich running. I would expect lean performance from weak fuel pump so I don’t think fuel pump is your first place to start. Strange the CEL code points to lean yet the MPG points to very rich....

I would check ignition. Common for these cars to get a failure in one bank of ignition (distributor, rotor, coil, or wiring) resulting in 4 cylinders losing spark. Fuel gets burnt in hot exhaust system instead.

Once car is warmed up try pulling a plug wire from one bank and then from the other. If one removed makes no difference on one side but quite noticeable on the other then you might have confirmed the dead side. If the bank is already dead unplugging a plug wire presumably wouldn't make it worse. On the other hand if you are running on 4 cylinders dropping from 4 to 3 should be very noticeable.

Many suggest changing the coolant temp sensor as if its faulty it could be telling ECU to run rich assuming car is still cold when it is not. A lot of changed temp sensor have been done needlessly but it’s a pretty cheap part and changing might be easier than a long drawn out diagnosis. If you have a DVM you can measure resistance of coolant sensor and compare it against a table for what it should be at that temperature. Thermistors (temperature dependent resistor used inside the sensor) typically fail to open circuit so a total blown sensor should read infinity or a very high resistance value (>100k Ohms).

If none of the above is faulty it could be damaged or dirty MAF sensor. These can be cleaned but they’re delicate instruments so read up before jumping in.

[killerFatty]

If none of the above is faulty it could be damaged or dirty MAF sensor. These can be cleaned but they’re delicate instruments so read up before jumping in.

Quick question, how does one go about "cleaning" a Karman Vortex MAF sensor? So long as there is no large objectec blocking air flow, the Karman Vortex should not be effected, right? The hot wire MAF sensor's can be cleaned by spraying special cleaner on the wire, but Karman Vortex reads pressure change against a pressure plate of sorts, so how does dirt affect that?

[sgretchko]

For what it's worth, the car now seems to idle better than before. Once it is warmed up, the tach sits consistently between 6 to 800 rpms. Before this happened, it bounced around a little between 400 and 1000 rpms. Lexus dealer couldn't find anything wrong, so I will just live with it and see what happens. Thank you for the input.

[curiousB]

Quick question, how does one go about "cleaning" a Karman Vortex MAF sensor? So long as there is no large objectec blocking air flow, the Karman Vortex should not be effected, right? The hot wire MAF sensor's can be cleaned by spraying special cleaner on the wire, but Karman Vortex reads pressure change against a pressure plate of sorts, so how does dirt affect that?

Agreed I thought this was a hot wire MAF which is most common.

[1UZ-FE LS400]

'97LS w/ 180k, timing belt and major tuneup scheduled for next week.

The LS was parked outside at the airport for 4 days, temperature was low 40's at night and around 65 during the day. The last day was rainy. The normally flawless LS started fine but revved up to 20k rpm, then dropped to about 2k. It then proceeded to bounce around in a less than smooth fashion. It continued until the car was warmed up, then it seemed to run ok, maybe a little rich. It seemed to start ok when cold and ran fine today. The check engine light did not come on. Any ideas? I would like to figure out what it was before the major tune and timing belt. Thank you in advance.

[1UZ-FE LS400]

I had the exact same problem with my LS400. I changed the throttle body sensor and took off the throttle body and plenuim attatched to it and found that i had MAJOR carbon build up covering the inside of it. Take carbourator cleanor or paint thinner and clean the carbon out of both of them. I haven't had the problem since. Try this and if it doesnt work then you might have to resort to an idle air control sensor, which will run you around $500. ouch. But doing this will most likely help alot and give you back your feul economy and precious horses you know youre not getting :)

[Quack11]

Had similar idle issue almost a year ago and tried everything in this forum but unfortunately nothing worked. Finally I changed the fuel pump and she has been strong since then.

[curiousB]

Had similar idle issue almost a year ago and tried everything in this forum but unfortunately nothing worked. Finally I changed the fuel pump and she has been strong since then.

Strange, a fuel pump would tend to be a problem at higher throttle positions when fuel demand is higher. OP was complaining about rough running of a cold engine. Fuel pump would tend to be issue cold or warm engine if it was the root cause.

[sgretchko]

Fuel pump was replaced with OEM part about 30,000 miles ago. It's possible but I would be really surprised. The alternator is original and at some point will probably need replacement but I don't think it is related.

[mann777]

The main culprit normally is the " Throttle Position Sensor for the EFI" P/N - 89452-33010" for the inordinate idle. I had similar problem, and I had done my max adjustments, I suppose the bushings within were not adjusting, Picked up a new one for 140/- bucks, And it has just too good since then, maybe you can check out.