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High Idle Speed Too High During Cold Start


gtolds
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I have experience with two LS400 cars - a 1990 that ran perfect and a 1996 that idles too high when cold.

I know that they are similar but they do have differences.

I'm going to try to solve the 1996 problem and want your opinion before I touch anything.

I know there are atleast three temp sending units: one for the ecu, one for the guage, and one for the radiator fan.

I have measured the functionality of the sending unit for the ECU as follows:

4 ohms resistance at 43 degrees

3.8 at 47

2.75 at 61

1.83 at 81

1.37 at 98

1.05 at 110

.88 at 120

.57 at 145

.45 at 160

.30 at 195

I have only had the 1996 during this winter so the outside air temp has never been over 60 degrees.

Even at 60 degrees however, the cold idle is still 1500 rpm - too high for me - I would rather have 1000 or 1100.

How to get there s the question. Do I just jump the wires leading to my temp sender for the ecu with a 2 ohm or so resister?

I don't even know if that would work. I'm a simple man. Do I have to go after my Idle air control valve?

Do I have to do something else? I checked my TPS and it was tripping at just the right spot. The TRAC position sender was OK too - not that it has squat to do with the problem.

Let me know what you think of jumping the temp sending wires with a 1.5 ohm resister to make the car think it is never colder than 88 degrees or so and thus idle lower.

Thanks

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What does your temp guage read after warm up?

The bottom of the temp guage has a fat white line and when the car is cold, the needle sits just below that fat white line. There are thin white lines above the bottom line and a fat white line at the top. When the car is warm, the needle points just a hair above the 2nd thin line above the bottom fat white line.

The thing is that this guage gets its signal from the temp sending unit that sits right next to the temp sending unit for the ECU. I'm not aware of how the guage sending unit could affect idle speed but I'll cross my fingers that it can...

Thanks for the help.

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GT, I've reread your opening post and think I misunderstood it first time around. Idle at 1200 to 1400 is normal until coolant warms up. Yours is at 1500 which is a little high. Nevertheless, after starting, your idle should start descending in a few minutes, depending on what the ambient temp is, to finally reach a resting idle of ~650. I wouldn't worry about the intial slightly high idle as long as you end up at ~650.

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Anybody?

amc;

Yeah, sure......some will say that replacing the motor mounts does wonders to solve this. I haven't done it, but I 've read enough other posts to take stock in it.

E/U/B/T; When you say 650 RPM, you mean while in gear, with brake on, right?

Also, I've seen you make numerous comments about "cleaning" the IACV. I've only ever replaced them. Have you been successful with cleaning them and if so, how much longer did they operate before finally packing it in?

Thx,

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Anybody?

amc;

Yeah, sure......some will say that replacing the motor mounts does wonders to solve this. I haven't done it, but I 've read enough other posts to take stock in it.

E/U/B/T; When you say 650 RPM, you mean while in gear, with brake on, right?

Also, I've seen you make numerous comments about "cleaning" the IACV. I've only ever replaced them. Have you been successful with cleaning them and if so, how much longer did they operate before finally packing it in?

Thx,

The problem with the high idle is that it drives itself to about 40 mph when cold and you have to be on the brakes just to do 25 in town for a full 5 minutes which is way too long. No other car I have ever owned idles this high this long. It was a rolling hazard when it was snowing and icy. I'm going to throw a two ohm resister on the colant temp sender wires and see if it idles lower and let you all know what happens...

wish me luck

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Anybody?

amc;

Yeah, sure......some will say that replacing the motor mounts does wonders to solve this. I haven't done it, but I 've read enough other posts to take stock in it.

E/U/B/T; When you say 650 RPM, you mean while in gear, with brake on, right?

Also, I've seen you make numerous comments about "cleaning" the IACV. I've only ever replaced them. Have you been successful with cleaning them and if so, how much longer did they operate before finally packing it in?

Thx,

The problem with the high idle is that it drives itself to about 40 mph when cold and you have to be on the brakes just to do 25 in town for a full 5 minutes which is way too long. No other car I have ever owned idles this high this long. It was a rolling hazard when it was snowing and icy. I'm going to throw a two ohm resister on the colant temp sender wires and see if it idles lower and let you all know what happens...

wish me luck

gt:

I've got a '95 (1UZFE) & it idles @ 1500 RPM when cold & ambient is 35 deg F, or so. Once warm (at least 5 minutes later) it comes down to 750 RPM in neutral, AC on. Try turning off your AC if you haven't already done so. The ECU does indeed control the idle speed & RPM vs temperature seem pretty linear. Engine warm, AC off, in gear (Drive), stopped, foot on brake, it idles at about 650 to 670 RPM.

I know what you mean about the 40 MPH comment. I hate having to put it in gear right after starting it (because I'm usually running late).

I'd suggest not altering any of the control circuits. It sounds like a part needs to be replaced or something is out of adjustment. If you start changing resistors & whatnot, your just masking the problem, not fixing it. Depending on where you are in WA, there is a good Indie in Bellevue. You're welcome to PM me and I'll give you their info. They just did my starter & there were no surprises. They weren't cheap, but I really wasn't interested in doing it this time around.

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Anybody?

amc;

Yeah, sure......some will say that replacing the motor mounts does wonders to solve this. I haven't done it, but I 've read enough other posts to take stock in it.

E/U/B/T; When you say 650 RPM, you mean while in gear, with brake on, right?

Also, I've seen you make numerous comments about "cleaning" the IACV. I've only ever replaced them. Have you been successful with cleaning them and if so, how much longer did they operate before finally packing it in?

Thx,

Hey threader, sorry taking so long to get back, been busy. ~650 in P/N. With the iacv, if you're operating a shop, replacing gives you a warranty to fall back on and less likely to get a return visit. If you're a diy, it's less expensive to clean, if the valve and seat are in good condition. Don't know how much longer they last. As for the isc motor, if the bearings are drying out, they can be replaced. I've heard you can inject oil into the bearings for some of the stepper motors if you can get past the seals without damaging them. Don't know how well that works. If the coils are fried or the rotor is siezing, then you have to replace. I believe the newer units are sealed so there's no option.

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