michael200

Can someone please shed light on the easiest way to replace the knock sensor(s) on my '90 LS400, looks like I need 24-inch long needle-nose pliers to take off the connector, and a long extension and swivel socket? Not sure which sensor has gone bad, but most of the clattering (just off idle) seems to be on the right side. No error codes, but the engine timing rattles when accelerating from idle. Octane booster helps the problem a little, but it seems that the sensor is failing more as time goes on. Surely the sensor(s) can be replaced without removing the manifold, right? Talk to me....

CUMan

My LS is a 93 model. I cannot be certain that yours would have the same engine layout, but I'll give you the info from the service manual for my car.

According to the manual, the following steps are necessary to check the knock sensors:

1. Remove air intake chamber;
2. Disconnect fuel inlet hose from LH delivery pipe;
3. Disconnect fuel return hose from fuel return pipe;
4. Disconnect engine wire from intake manifold;
5. Disconnect engine wire from delivery pipes, rear water by-pass joint and RH cylinder head;
6. Remove fuel return pipe;
7. Remove delivery pipes and intake manifold assembly;
8. Remove knock sensor;
9. Inspect knock sensor (this is done by checking to make sure there is no continuity between
terminal and body).

The manual also indicates that a SST number 09816-30010 is used to remove the knock sensors; however, it appears to me that a deep well socket would fit it.

LILEVO

iTrader: (1)

Wait a minute,

It sounds like you think replacing the knock sensors will get rid of knock/clattering??

The knock sensors are there to detect the pinging/knocking and retard timing which is caused by bad gas/lifters/or rod bearings on their way out.

michael200

Thanks for the replies. PART ONE:
I thought that was what most guys would say - Take all the guts off the top of the engine to get to the knock sensors. It seems like that shouldn't be necessary, since you can see the sensors thru the openings (although small) between the manifold and the head. It's a little like orthoscopic surgery, but it looks like you should be able to disconnect the connector using a couple of long (modified as required) screwdrivers, and unscrew the sensor with a socket on an extension. Looks like you might have to remove a couple of pieces such as the EGR valve hardware (to get to the right passenger side sensor). and possibly remove the throttle body on the right side of the engine to get to the left (driver side) sensor. I assume you can tell which sensor is bad by removing the sensor wires (one at a time) and driving the car to see if the pinging gets worse. Has anybody successfully done this?
PART TWO:
I'm fairly sure that a bad knock sensor is the cause of the excessive pinging in my engine. When the sensor doesn't work, the ignition system does not get a signal to retard the timing. I'm sure there are no problems with the mechanical components, rods, valves, pistons, etc.

LILEVO

iTrader: (1)

Why would the ignition retard timing in the first place? It runs at a set amount of timing to begin with and doesn't retard unless the knock sensor tells it to. Your theory is that "the knock sensor is broke, so now the engine is advancing timing." When you run advanced timing is when you will knock/ping NOT the other way around. The engine wouldn't magically advance timing to create knock.

LILEVO

iTrader: (1)

And lastly, all a knock sensor is, is a microphone.

michael200

Hello, my friend. Thanks for your comments.

The good news is that I just finished changing the right-side knock sensor, and the problem is fixed (no more pinging). It was fairly easy to change the sensor. The whole job including test drives took about 1 1/2 hours. I removed a couple of the plastic cover pieces between the manifold and the valve cover, and with a long screwdriver, gently removed the connector on the sensor (accessed on the right side of the engine thru the small openings around the manifold). I removed the knock sensor using a long 3/8 extension with a universal joint and a 1 1/16" sensor socket taped together to hold the tools together. I installed the new sensor using tape on the socket to hold it in place. FYI - The sensors are very tight in the block.
Regarding the timing; Many ignition control systems (such as this LS400) have an agressive timing (advanced when accelerating) programmed into the ECU for max power. The control system relies on the Knock sensors to trim back the advance just to the point that the engine starts to "knock". When the control system is working correctly, you won't hear the engine pinging (maybe barely to the trained ear). Thanks again. P.S It looks like changing the Knock Sensor on the left side would be a little harder, but I think it can be done in the same way, without removing the manifold, etc. More - Actually, my "theory" is that the knock sensor is not working, and does not "tell" the ECU to cut back on the timing, thus the pinging.

Freeport

I just replaced the knock sensors and knock sensor wiring harness in my '91 LS400 -

It was relatively easy w/ the help of my son who's got small hands. . .

We removed the Air Intake Chamber (lots of tutorials around on that) and after that you can see both sensors and the harness undereneath.

Taking the bolts out of the wiring channel that runs down the center of the intake manifolds gave us a bit more room to work.

We used a 27mm deep socket with a 3/8" u-joint and a long 3/8th extension - that protected the new sensors (they're too spendy to risk) A regular 27mm or 1 1/16th socket would work too, but be careful.

With his small hands he was able to pull all three wires and remove/install the sensors and get the connectors back on.

It's worth getting the new knock harness as there was considerable corrosion on the sensors and the harness ends - the connectors came apart easily from the old harness.

It sure beat taking that manifold off as there are so many old/brittle wires involved I was afraid to give it a go.

Good luck!