BNastee

1999 LS400. 205,000 miles. CEL for both spark knock sensors. After a reset, the CEL comes back on after the engine is warm and goes over 40 mph. It's never one of them, always both at the same time. I can drive under 40 mph all day and it won't come back on. Before I go through the trouble of replacing these I watched the timing using BT OBDII connector and an old cell phone.

Watching the timing, the knock sensors seem to be working. The knock sensors adjust timing as they should. Makes me think the knock sensors are working and something else is wrong. I've also HEARD spark knock when driving in a parking lot and it goes away very quickly as the ECU reacts. When the CEL is on, the power is cut but the timing seems to go up and down to the same levels and the same timing as before. Looks like knock sensors still operate after the CEL is on. Acts exactly the same on cool (55*) mornings.

What I've done:
- Tried 91 octane, 93 octane and 93 octane with 2 gallons of Turbo Blue race gas. no change.
- base timing is around 8 degrees. around 12 if A/C is on.
- OBD told me the car runs at 199.4 * F. So I replaced radiator, thermostat (180*F) and coolant sensor. Still runs exactly at 199.4 after warming up. Confirmed that the electric fan comes on. Weird.
- replaced spark plugs. zero effect. Have a colder set on deck.
- two sessions of seafoam. First time I've used the stuff. Did the spray as directed. Then a few days later did the liquid in to the air bleeds under the plate and let it soak. This made it cut timing sooner (watching via OBD).

Looking for ideas on chronic spark knocking cars because I think the knock sensors are doing their job, they just can only go so far. What do you all think?


Rich

djamps

Lazy O2 sensors and it's leaner than it thinks?

BNastee

After reading your post I watched the O2 sensors via OBD. They act the same from L to R. Very different between sensor 1 and 2, which makes sense. I wouldn't guess they'd both go bad at the same time but stranger things have happened!

I'm hoping someone has had some experience with a spark knocking LS400 otherwise I'll change the knock sensors and see what happens. That's a lot of work for a 'maybe' right?!?

djamps

Maybe the timing belt is off a tooth.

RA40

iTrader: (6)

Those are the areas I'd have checked too. May want to put a bore scope in there and see what the cylinder/s looks like. How are the plugs? Does it get long drives? May be some other upstream sensor related to fuel delivery. Possibly the ECT if this has not been changed in those 200K miles.

BNastee

Great idea on the scope. I picked one up online that connects to your phone. I'll try it when I change to the cooler plugs.

We have a weekend trip in our 2000 Lexus this weekend, so I'll hook the BT OBD up and watch it and compare to the 99's activity. I wish I could watch the knock sensor activity. I'm using Torque Lite.


Rich

BNastee

New plugs put in last night. One heat range colder. No change!!! Strangest thing I've ever seen.

We didn't take the 2000 LS on the trip this past weekend, so I don't have any comparison OBDII data.

I used the wifi boroscope. It works real easy and was cheap! Very dark and dirty in the combustion chamber. So I poured the rest of the seafoam (half a can) in the gas tank. Was about 3/4 full of gas.

I'd like to see a clean combustion chamber before I resort to replacing the knock sensors.

Car gets nothing but long trips and fully warmed up. My commute is 34 miles of 70 MPH roads. But I picked it up this spring. Based on where I bought it, I suspect it was exclusively short trips.

BNastee

When I took the intake off to replace the knock sensors, I found a real nice mouse house and missing wiring. Copper and all was gone. I feel like original equipment stuff is better than rock auto stuff, so I only replaced the knock sensor that had visible damage and fixed all the wiring. After this I had a CEL for only one sensor. Progress?

This past weekend I took the intake off again (not so bad on 98-00 models) and replaced the second knock sensor and confirmed the wiring. Back to two CELs, one for each knock sensor. Maddening!

Reminder 1 - when I watch the timing live via OBDII, you can see it retarding at high load, sometimes even as far as *after* TDC. This is with the CEL on for both knock sensors. Seems like the knock sensors are working despite the codes.
Reminder 2 - car runs ok but is +/-25% down on power, and 25% down on fuel economy.
Reminder 3 - If I clear the code, the car runs perfectly strong and just like my other 98-00 models. When the knock sensor codes turn on, the car is down on power.

BNastee

I'm getting good at removing the intake! Had the dreaded click-click when starting. Got progressively worse quickly!

I replaced the starter and triple checked the knock sensor wiring.

As usual, the car runs like a champ around town. It's not until its warmed up *and* you go over ~50mph that the knock sensor codes pop. Then it feels like it is at half power.


Rich

Areyes610

Hey op I was wondering if your issue was ever resolved. I am currently having the same issues you speak of.

BNastee

Same exact symptoms unfortunately. The car is for a teenager, so the reduced power isn't such a bad thing.

I should change my original subject in hindsight. The car never spark knocks, it just pops the knock sensor code.

One update - My 2000 LS400 started showing the driver side knock sensor code recently. Same symptoms. So I hard wired both to the passenger sensor and now it pops codes for both sensors. I wish there was a hack for eliminating the sensors, but I think the ECU is looking for something more than a resistance. Next step is to remove the intake and confirm the wiring. I might replace the sensors, but I really don't think its the sensors. Who knows.

aptoslexus

The ECU is looking for something quite specific regarding the sensors. I'm not sure what exactly that is, but non-factory knock sensors trigger a code, where factory sensors don't, in my experience. The ecu is looking for it's signal between 1700-5400 rpms, which might explain why you don't get a code in low rpm around town driving. When I was getting a knock sensor code, it almost always showed up in my driveway, because it's a steep driveway and I put the car in low with increased revs to climb the hill. That put the revs in the "knock sensor zone" where the ecu could check it and trip a code. It all cleared up with new factory sensors.

BNastee

I sold this 99 to a family friend and worked on it while they owned it. Their kids are in college now and I bought the 99 back from them. I can't bear to have this beautiful car get too far from me.

Anyway, it was running the whole time in reduced power mode. Unrelated, it lost coolant for some reason and got overheated. I removed the head and had it resurfaced. Combustion chambers did not have carbon build up on that side. I put in some used (tested to OK to SPEC) original Lexus knock sensors and no change to how it pops the code.

I have a better scanner now and will try to save the Live Data and post it here.

@aptoslexus maybe i should just drive around under 1700 rpms! I wish I knew what the ECU was looking for and just defeat the knock sensors temporarily.

BNastee

Also should mention that the rockauto starter is going bad now at 218,000 miles. Geesh.

I bought the contacts and plunger and will rebuild an original starter I have from a junkyard motor.


Rich

aptoslexus

That would be the best way to check knock sensor integrity. The fact that you're changing the sensors and still getting the code suggests a wiring or ECM fault.