Knock sensor missing from a 1985 Toyota Supra

A while ago I purchased a replacement engine that was a couple years older and was missing the spot for the knock sensor. The car runs like crap and has little take off power. I have been told by many that the knock sensor missing does not matter. I did rig up something to install the knock sensor, but it is not in the location Toyota put it originally. I took a rectangle of metal and drilled and tapped a hole and bolted that down to the block and screwed the knock sensor in.

So far I have checked that the exhaust is not a problem by disconnecting before the catalytic converter. I tested vacuum pressure. It was within the normal limits. I can not find the paper with my recordings, but it was steady and a good reading on the lower side of normal. 35 with vacuum hose disconnected and 30 with connected. I tested fuel pressure. It was also within normal limits. I did a compression test. I was also within normal limits. the first 4 were either 145, 150, or 155 with 3 different tests. The last 2 were a bit lower at 135 and 125. I did a wet test, putting a bit of oil in and the 135 one went up to 150 and the 125 one had little to no change. I tested all grounds and cleaned up. I may have done a couple other things, but do not recall. I started working on this car a few months ago.

Any suggestions on what the problem could be? Any suggestions on what else to try? I have a parts car, and was suggested to try changing distributors. I will likely try that next. Is it possible that it could be the knock sensor causing the problem? I talked to a Toyota mechanic and he said yes, but I have had maybe a dozen people say no.

I was thinking I could install it in the correct position by removing all the intake stuff in the way and using JB weld somehow to either stick something to the block or just drilling and tapping right into the JB weld itself. Do not tell me that JB weld will not hold up because I once used it to seal up a cracked block and ran it for 10’s of thousands of miles afterwards. It is still holding to this day.

The compression numbers are low. I think you’re fighting a losing battle with numbers and cylinder pressure variances above 10%.

I agree with knfenimore because you are fighting a losing battle here.

I think we’ve been over this before.

We have been here before . . .

I know it may sound mean, but I think OP should have bought a healthier engine . . .and it should have been compatible for the model year

This one doesn’t meet either of those criteria

I don’t know why your car runs poorly (you’re sure it’s firing all cylinders, right?), but I’m fascinated with the epoxied engine block. What part of the block is bonded, and how did it break?


Why did you open a new discussion?

We still remember you from a few days ago . . . we could have kept the old discussion going

By the way, don’t take this the wrong way, but your “vacuum pressure” numbers make absolutely no sense

30 or 35 what . . . ?!

A healthy engine might have 20" of vacuum at idle, for example

Therefore, I don’t know what your numbers are supposed to mean

Perhaps you meant that your fuel pressure was 30psi with the vacuum hose connected . . . ?

No offense intended, but if that’s what you meant, it was kind of hard to interpret

To measure fuel pressure, can I assume you somehow teed in, because I’m 99.9% certain that your engine does not have a fuel pressure test port

I may have asked, but I’ll do so again . . .

Is your cam timing correct?

Is your ignition timing correct?

As was asked above, are you certain that all cylinders are getting proper spark?

I forget . . . was the old engine also low on power? And why did you replace it?

I’d focus on the ignition timing measurement first. It may be the lack of knock sensor signal it expects confuses the ECM (the engine computer), so it retards the timing just in case there’s knocking going on which it isn’t seeing (due to what it thinks is a defective knock sensor).

It seems like your method to install a knock sensor would work. But maybe the spot you are installing it, somehow the sound isn’t getting through like it would if it were installed at the correct spot, wherever that is.

Have you contacted a Toyota dealership shop about this, or a pro mechanic who specializes in Toyotas? There may a software change you could install in your ECM to make it forget about the knock sensor signal. Alternately, you might can find a used ECM which is exactly compatible with your replacement engine. Have you tried that? Ever since computers got mixed up with cars, I guess we’ll forever be dealing with these version compatibility problems.

When you accelerate, normally the timing is supposed to advance; i.e. during acceleration the spark occurs earlier in the compression stroke. Maybe your ECM is not doing this, due to the missing knock sensor signal. You’d have to measure this during acceleration not just at idle I expect to know if you have a problem or not.

As an aside, and I don’t think this is the main cause of your poor performance, but those compression numbers are a bit on the low side, especially for what is probably a higher than normal compression ratio engine as I’d expect in a Supra. I think engines are rated by a number which is the ratio of the volume in the cylinder (including the head space) at the bottom of the piston divided by the volume to at the top. It’s like 6-8 to one for most engines I think. I forget what that term is called, or is it just “compression ratio”? Anyway, do you know what your engine compression spec is supposed to be?

Edit: One more idea. There’s a hobby some folks have of turning Toyota engines into high output power plants. And there’s at least one magazine devoted to this topic I’ve seen at the bookstore. It’s called “Toyota Mod” “Performance Toyota” or something like that. There may be advertisers in that magazine that have the knowledge to help you. That magazine is stocked at my local Barnes and Noble bookstore, so you might take a visit there for a browse. Best of luck.


“find a used ECM which is exactly compatible with your replacement engine.”

I like your way of thinking. I also think it might be worth a shot, as long as that module was very cheap

And as long as you could 100% verify that it will specifically work on that 1982 engine and harness

The engine is on the way out but could loiter around for a while longer at those compression figures.
The engine management is pretty crude compared to modern systems and I doubt the knock sensor or any makeshift location is the cause of the problem.

Do you even have a timing light and is timing being set by the underhood decal or by guessing? Retarded ignition timing will cause sluggish running and even overheating. If retarded too much the knock sensor (assuming it was even working) will likely have a negligible effect and will make things worse.

If you want to check the knock sensor circuit take a hammer and tap on the cylinder head firmly to see if the engine stumbles at idle. This means tap, not whale on.


“This means tap, not whale on.”

It’s not okay to crack the head . . . ?


It’s been done… :smiley:

Many years ago I saw a guy get mad at his old kick start Harley because it refused to fire up. He ran over, wrestled a 4" diameter fence post out of the ground, and worked the bike over with that…

In the shop, I’ve seen “antics” like this a few times

A guy had to be pulled off of a job. I believe he was supposed to replace a window regulator, and things weren’t going well. He was repeatedly stabbing the door panel with a knife . . .

Another guy threw 3 pound drilling hammer across the shop, “narrowly” missing the head of a guy that was walking by. By the way, the guy throwing the hammer purposely waited until the other guy walked by, and purposefully threw it right past his head, so that he would get scared. It worked

Another guy was doing some suspension work. I forgot what the problem was, but he repeatedly threw his air hammer down on the ground. Not surprisingly, it got damaged. He immediately realized this, got frustrated and smoked a cigarette


I remember disconnecting knock sensors on engines of that vintage, and they ran fine. So I suspect your engine drive-ability problems are from another problem.

A friend suggested changing the throttle sensor since I have a parts car same year. It did not help, however, had what i think is strange happen. I disconnected the wire going to the throttle sensor and the car ran better than it ever has since I got this engine. Idles like a charm, when under the hood and I rev it up it revs up fast and sounds good, like it has tons of power. But when I take it down the road it will only go about 50 and seems like its pulling something very heavy. From the very beginning it starts out pretty good but then lacks power.

Someone said something about overheating. I drove it for a good few miles and it ran for a while idling and it does not over heat a bit. Oil pressure, according to the gauge in the car is very high.

A friend of mine who has a timing light is going to check the timing.

The ECU may have a default mode that it uses if it determines the TPS is disconnected.

I do believe when I took it to the Toyota shops years ago, they attempted to change the ECU, I think the one from the original car this engine came out of, as I got the entire car with engine, and Toyota could not get the car running decent. You would think if anyone could Toyota could, however the guy at the Tech College seemed to do a better job than Toyota.

The junk yard told me it was compatible. And I have seen compression tests lower than that where the car ran pretty good.

Someone asked why I replaced the engine. Because the original engine blew up.

I do believe all 6 are getting good spark. I did also attempt removing one wire at a time to see if removing one would cause little change, but it ran much worse after doing all 6 like this. Could it be it is getting some spark but not enough? Is there some way to test this while the car is running and see exactly how strong the spark is?


To check spark, you use a spark tester, NOT the regular spark plug

The spark tester should have a very bright blue, almost white, spark

If you’ve just got a pathetic orange, that’s not good enough