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98 LS400 extremely low idle


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Idles fine when cold, I believe it is around 1,100 RPM, once it warms up, it idles between 200-400 in drive, if it is a little over temp it’ll idle at 200 in park and die in drive. No major vac leaks and no CEL. Injectors recently replaced but the issue existed beforehand. Also noticed what I assume to be the throttle body motor making a loud humming with the ignition on and the car off.

(UPDATE)

Car shows a CEL and “CHECK VSC” appears

  in the dot matrix screen, idles up to 800 sometimes, other times it does not. Have not checked for codes.

image.jpg

Edited by grandpa_spec98
Unwanted files, could not delete.
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So the throttle body wasn’t all that dirty, got a small cleaning and the motor was still acting up. Upon cleaning I noticed a connector where the throttle cable connects to the throttle body. (Shown Below)9A5A89BC-BDF7-4765-B9EE-4FBED656C4DE.thumb.jpeg.ec25ed2637b23f355e49b31c033d3897.jpeg

This connector was loose upon inspection and it turns out the clip is broken off. I would assume this is the throttle position sensor. Out of curiosity I started the engine with this connector unplugged and some strange but expected behavior followed. The idle was where it should be, on the dot if not a little high. As expected, there was no throttle response unless the pedal was mashed to the floor. (Shown below)

 

So my question is, could the throttle position sensor have to do with my low idle, or did it trigger a failsafe somewhere in the ECM that told it to just keep running, no matter what.

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Read codes, came up with six, including a Current and Pending code for system too lean bank 1, as well as knock sensor bank one. Bank one injectors just replaced less than 100 miles ago. Video attached shows codes and data stream data, fuel trims do indicate that it is running lean.

 

 

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On 5/18/2022 at 2:49 AM, grandpa_spec98 said:

So my question is, could the throttle position sensor have to do with my low idle, or did it trigger a failsafe somewhere in the ECM that told it to just keep running, no matter what.

The ECU would have a default preset to keep the engine idling (albeit low or high speed over the standard RPM) 

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Sometimes an EGR system loaded with soot can cause the darndest things. 

In the old days before the car had all kinds of sensors the old vacuum checker at various points in the system could point out where vacuum is out of spec leading the mechanic to know if it's the exhaust valves out of spec, the EGR clogging, low compression etc. 

I had all kinds of idle issues from a Honda once and it turned out a small leak in the head gasket had just enough coolant get into the exhaust pipe (undetected) and caused extra back pressure to give me a bunch of random grief like hard to start, shut off after releasing the throttle, low idle etc. Eventually the leak was bad enough to show up in the form of noticeable coolant loss but until then man it was so aggrevating. 

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Your intake is carboned up.  Pull the EGR valve and clean both passages.  A screwdriver will work.  Same with the idle air control.

Will bet your injectors and even the cold start is clogged.

Threads talk about bad capacitors in the ECM.  Located below the glove box.  Pull the ECM cover and look for leaky capacitors.  If you don't see that they are physically leaky, THEY ARE NOT LEAKY.  Forums mislead on this.

Digikey for replacement caps.  A creaky website but really good prices and quick shipping.

Hope this helps etc.

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Thanks, I’ve been looking into the process and I’ll probably be able to figure it out sometime tomorrow. As for the injectors, I have new OEM Denso injectors, not used reman Ebay injectors. Sure they are destined to fail on me at some point but for the time being they’ll work.

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  • 2 weeks later...

Thought I might add something to this, cleaned intake but it looked like someone was already in there, didn’t help. All of my codes are gone after reset but the idle is still low. Can’t tell if it idles up with A/C on as I am missing an e-fan at the moment, but the car idles up slightly with the headlights on, not sure if that means anything but I’m fine with driving around with my headlights on all the time if that’s what it takes to keep it running.

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Aaahh ...

Missing an E fan.....

Could be that the missing fan completes a circuit.

I have the shop manual for the 1990, official $300 manual, and the diagnostic for the Idle valve is VERY interesting.  On this 1990, you apply positive voltage to the center two terminals of the valve itself (after disconnecting the plug), then apply positive voltage to the remaining plugs (S4, S3, S2, S1) in that 4 to 1 sequence.

Imagine the number of jumpers you need.  Positive voltage to S4, keeping it positive, adding a positive jumper to S3, etc.  At the end of this sequence, the valve will open.

Then, pull the S1 to S4 jumpers.  Then add positive voltage to S1, add to S2, add to S3, add to S4.  Reversing basically.  Then the valve will shut.

4 idle positions.  Once S1 is okay it would open a calibrated amount, then continue to S2, etc for an additional amount, etc, up to S4.

Quite clever indeed.  Some complex little circuitry.

Either the idle air valve is bad, or the missing fan is a culprit.

Just a guess, but imagine the 98 follows the basic logic of the 90.

Hope this helps somewhat.

IMG_20220610_205725712.jpg

IMG_20220610_205808104.jpg

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Or could it be 164 different idle values?

Up 4 different values, then each adjusted downward by one of 4 different values? 

My brain hurts.

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  • 2 weeks later...

Looking back on this I might want to try testing it, I found a factory e-fan on ebay that I’ll use until I find an OE replacement. If the kickup for A/C works and it idles nicely I’ll assume that was my problem. Now that I realize, it idles fine until you get to the temp the e-fan is regulating you at and then it starts losing consistency in the idle. I will check the valve as mentioned if possible, but the e fan may solve 3 of my problems, A/C not working, running slightly hot but it manages to stay within operating temp range, and the low idle. My next thing would be motor mounts, then drive the wheels off it until timing belt service interval comes into range. Coincidentally at 300k miles 🙂

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Quick reply concerning timing belts:

 

Had a mechanic who I thought was super good replace the timing belt on my LS400.  This was one month before I bought the car.  Found the receipt in the glovebox.

Motor was a bit rough; cleaned and flowed injectors took care of that.  Small noise from the P.S. pump.  New seals took care of that.  Small grinding noise at idler pulley.  New bearing (available separately at Whisler bearing, give them the part number on the bearing itself.  Ran about $10.  Now I could reuse the original metal pulley.  Old bearing pressed out and in fairly easily.  Side note:  the same bearing as the idler bearing in my 98 Saturn SL twin.  Very common bearing.)

So.  More grinding noise at the front of the engine.  Blah.  Took the front of the engine off and found that whizmo mechanic reused the original tensioner bearing for the timing belt.

Bad words.  Since he charged $400 for the complete kit.  Available for $100 from Rock. 

Mechanic later fired and came to me wanting a reference.  Sometimes I wonder ..............

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Sorry, I was rereading your original post concerning injectors.

Denso no longer makes injectors for this engine.

My business is rebuilding fuel injectors.  We have found that Chinese injectors are being made with DENSO and the original part number printed on the side of the injector.  Printed, not cast into the body.

These are fakes.  We've seen these on Tacomas and other Toyota products.  Just tested a set of this junk where, out of 8 injectors, 2 had terrible spray patterns, and one was already dead.  Plus, the mechanic reported the cat almost melted because the engine was running so rich.

Besides the pattern problems, these "factory" replacements were 15% bigger than stock.

Here's a warning sign:  these fakes cost $50.  For a set of 6. 😖

I need to get on the forum under fuel or whatever.  I just finished a perfect set of LS 400 injectors, fit 1990 - 1994.  Factory official, etc.  Don't get any better.  $50 each.

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I know these cheap “Denso” injectors run very horribly, I had a set previously and immediately switched to a Bosch replacement, worth the money. I was having cylinders run lean wherever the cheap injectors were. I’ve been relaying the info of these bad injectors, it should have its own forum though as a warning, or to come up on google when someone searches “Toyota replacement injectors” so people don’t buy them.

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How they get away with it is beyond me.

Bosch won't allow fake injectors with the Bosch symbol.  They do so much biz with the Chinese, that the Chinese government will shut down the fakesters.

Denso?  Probably not. 

Chinese philosophy is that the Americans will buy cheap.  Period.  Quality?  Not so much.

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  • 4 weeks later...
3 hours ago, hmmmhaa3 said:

I need to get on the forum under fuel or whatever.  I just finished a perfect set of LS 400 injectors, fit 1990 - 1994.  Factory official, etc.  Don't get any better.  $50 each.

Pics, flows, etc?

IMG_20220520_174435610_HDR.jpg

IMG_20220520_174907042.jpg

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For fun I picked up a set of those “Denso” but not actually OEM injectors up, only 6 were functional and the others weren’t equal in flow. Great advice Chuck I hope word gets out about these bad injectors.

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