So, recently I replaced my valve cover gaskets - both front and rear, because the rear one has been leaking for years, and it was finally time to do the maintenance needed on the path towards more power. While I was at it, it was time to relocate the knock sensors as well since the very first generation of the 1MZFE (the 1994 distributorless version) has a slightly different ecu from the other gen3s as well that requires a different (and apparently more constant and less forgiving) knock sensor signal than the later versions that results in frequent "knock" detection and bogging down of the car in a forced "safe mode".
So here are my tips from my experience:
Pick up an entire engine gasket kit
before you start. You will end up spending more money picking up gaskets separately, and for more specific gaskets such as the intake manifold gaskets, they are usually special order and will take time to get.
Don't be afraid to tackle this task, but it isn't too easy to do either. The rear valve cover is a bitch to get to, with the hardest part being removing the engine harness. Don't be afraid to cut off a couple of the tabs that hold it to the rear valve cover. Even without a couple, the harness will not move, and just breaking them or cutting them will make the harness a lot easier to remove as well as replace.
It is also a good idea to get some new valve cover bolts as well if you don't mind spending an extra couple of dollars, since they are designed with crush washers that don't crush very well after being torqued down the first time. As a personal experience, I snapped a couple even when torquing it with a torque wrench as specified, so after taking them out, I torqued all the other ones without the torque wrench and just put them back in as tight as they were taking out. Turn until stopped, and then another quarter turn and that's all.
The intake manifold is easy to get off. But
remember to drain the coolant from the block first. Then it is quite easy to just stick a piece of wood under it to prop it up while you're working.
At this point, you can see the coolant bypass hose (#1) that will have to be removed to get to the knock sensors. While you're at it, you may as well pick up a new hose (mine was in pretty bad shape), but make sure that you replace it with an OEM hose from the dealership. I tried to use another similar hose, but it leaked like a sieve AFTER I had already put everthing back together, and then I had to wait 3 days to get the hose since it was special order.
The top one is the toyota hose, the bottom one is the one that I tried to use The size difference looks like a lot in the pic, but it was the same size, just the shape didn't fit perfectly:
Tips on the actual knock sensor relocation part:
A number of people (those with and s/c) has relocated their knock sensors using a relocation plate made from aluminum that is grounded to the side of the front valve cover and will still pick up acoustics from the engine in case of knocking.
BUT (1994 1mzfe specific), the ground even through the bracket was not perfect enough for the picky ecu, and even with extra grounding straps to the bracket from the block, the signal the sensors sent back were still not enough to keep the ecu happy, so it kept throwing a CEL. At first I thought that it was a blown knock sensor, but even after splicing the signal wires together, the CEL didn't not go away.
Eventually, Tony helped me figure it out (since he had gone through the same problems in the past), and we moved the knock sensor to the bracket on the other side of the engine (and attached to the block so it was a perfect ground), and the ecu was finally happy.
After relocating the sensor(s), the car is finally feeling more consistent and there is more power at higher rpms since the car doesn't bog down anymore, and previously when 94octane made no difference and actually bogs down the car more on occasion, 94octane does make the car peppier than 91.
Hopefully this can help anybody who is about to tackle anything similar.
There is also other things you can/should do while you're in there, such as cleaning your intake plenum and intake manifold by soaking them in thinner, etc. etc.
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