I've read the 100 posts on HG replacements already on this board and I guess this will be # 101!

1995 3.0 ATM 151k 2WD

I've always wanted a Toyota since working in Yellowstone for a few years and those 87-89 trucks saved me on many cold winter mornings. Bought this runner at a great price, knowing it would be a project. Had some minor problems, (it seemed) but took it to my mechanic and he felt strong about the quality of the engine.

Right after I bought it, fixed a couple of issues...

- Replaced Brakes
- Replaced EGR Temp Sensor (CEL 71)
- Refurbished Brass Radiator
- Coolant System Flush
- Replaced Valve Cover Gaskets (leaking)
- Replaced Fuel Injector Wire, (Rat had chewed some wires)
- Replaced Thermostat & Housing

Compression test showed 50psi in Cyl #1 and after replacing the fuel injector wire & valve cover gasket, it bumped back to 120.

After all this, and having JUST installed a thermostat, (there wasn't a thermostat in my truck) I get 20 yards away and I'm hit with CEL 52, Knock Sensor Fault.

My first thought was that somehow, the previous owner removed the thermostat for some reason. When I bought it and drove it home, I noticed the O/D wouldn't engage and that was cause for concern. After reading some posts on here, I learned that a faulty thermostat, (or in my case, a missing thermostat) won't allow the O/D to kick in. It was obviously pulled for some reason and sure enough, O/D is working perfectly since the thermostat was installed. The truck isn't overheating with or without the thermostat so I'm left to wonder why it was pulled...

Took it back to my mechanic for a diagnostic and he told me there was, "excessive resistance in the knock sensor sub harness" and that the pigtail needed to be replaced. Using OEM, $620 to do the job.

I just have a hard time believing this is a coincidence... The truck was running fine until I installed the thermostat and really, not even 20 yards of driving after the install until I got the knock sensor CEL.

I wonder if the CEL is telling me the knock sensor is no good or is it telling me its picking up a knock? I'm not clear on that point.

As for the Head Gaskets... My truck wasn't in the recall, (production 6/95) and Toyota told me my HG are not defective. I have no service records for this truck and I'm under the assumption the HG were never replaced. Although I don't really show the signs of HG issues, (milky oil, white smoke, overheating, exhaust in coolant) the truck still runs a bit off. Even before the knock sensor CEL, (which retards the engine) it was running a bit sluggish and horrible MPG. I'd fill up and get around 150 miles, so about 10mpg. My radiator was full of brown sludge which some on this forum account as HG issues, oil mixing with the coolant, but the radiator shop said it was nasty rust and possible mixing of coolant. They didn't think it was oil.

The big question... If I get the Knock Sensor & pigtail replaced, would it be wise to do it all at this time? HG, water pump, timing, and so forth. I also have a slight RMS leak it seems. Maybe 2 or 3 drops a day. The engine has 151k miles and I'd hate to replace the engine knowing it has life left but should I be looking at a new engine or overhaul what I have?

Your input and advice is greatly appreciated. I just don't want to make a mistake of replacing the KS and missing the chance to replace some parts that might fail. If you were in my shoes, what would you do?

I love this forum and thank you in advance for your wisdom!

*UPDATE*

After a few months of driving around with the CEL, finally decided to bring it to the shop to have the knock sensor and sub harness replaced. Cost about $700 which included the OEM parts and intake manifold set. Also found that the timing marks were off on the cams, both a tooth off. Set the timing correctly and now the old 3.0 purrs like a kitten. Only problem now is a gas leak from the first fuel rail injector. Leaking at the connector so... towed it back to the shop. If they have to remove the intake plenum again to fix the leak, think I'm going to drop the cash for the timing belt, water pump, idler pulley, tensioner pulley, hydraulic tensioner, cam seals and crank shaft seal. Truck has about 152k and I doubt the belt and pump have been replaced. Looks like another $800... Thought I was just going to drive it till it busted but now that the timing is fixed, (no more knocking in engine) it seems worth it. Who knows, might get another 100k out of her.


Pictures added of the old Knock Sensor and sub harness in case anyone is interested. You can see the ks had melted a bit and the sub harness is in pretty rough shape. Ks part cost $215 and sub harness was $15, OEM

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First a bit of advice... most of the guys here don't read so well, so the longer the story, the less responses you're bound to get. Usually short, concise questions get better responses.

As far as your issues, I'll start with this:

1) the knock sensor pigtails are known to go bad. The heat these 3.0's produce cook all the engine wiring. Replacing the FI harness probably cracked the pigtail and its probably shorting now. Its definitely not a $600 part.

2) you can't fix compression on a cylinder by either valve cover gasket or FI wiring. Something was wrong with the numbers or methodology used. Cylinder #1 is not usually one to pop a HG so its hard to say.

Personally, I wouldn't start looking for a new motor just yet. More than likely, you just have a lot of little things adding up to a poor running motor. I'd run compression tests again, replace Knock pigtail, verify the coolant is clean now (reflush again if not), and start with normal maintenance stuff if you haven't yet (plugs, wires, dist cap and rotor, fuel filter, etc).

Thanks Crash... I will tighten it up in the future and keep it short. The chewed FI wire was causing a misfire, making the truck run horrible. After the replacement, no more misfire and compression came back up in Cyl # 1. Not to argue against your point but thats what happened.

I meant to say that my mechanic will charge $620 to replace the pigtail and knock sensor, using toyota parts. This job is a bit over my pay grade.

Should have mentioned that I put in new plugs, oil change, dist cap, wires, and just flushed coolant system.

Just wondering if I'm going to pay $620 to replace KS & pigtail, is there anything else that I should do while we have it open?

Knock sensor sits below the throttle body so not much you can do in there while you're at it. Other than cleaning out the TB a little while its off, not sure it would be worth what a mechanic might charge you to do it.

As far as compression, I'm just letting you know that either one of the tests was a mistake. I would just verify that the higher number is in fact, the valid one, especially if you continue to have issues.

If you and your mechanic(I hope you trust em) come to the conclusion that its the knock sensor than that's what ya gotta go with. I agree with crash that you don't need to be looking for a new motor yet. Usually the CEL for a knock sensor means that the sensor itself isn't working and has thrown the computer into rich mode making you get bad gas mileage and run poorly.

Hope this helps and bummer on the $620 bill

If your comfortable doing plugs,wires and cap. I think with an fsm and going step by step you could remove upper manifold which contains one gasket and lots of vacuum hoses. Then examine knock pigtail,then see if you can remove knock without pulling lower intake

I have the fsm and I've been looking all over the the right section. Can you let me know where I can find instructions on the knock sensor? I understand it is a big pain to access knock sensor without pulling the manifold but I'm willing to give it a try After all, this was why I bought the darn 4runner

I've pulled the throttle body to check for carbon buildup and its clean as a whistle. It was a bit disappointing as I was hoping to find a mound of crud but its good news the engine is running clean. Also, no carbon buildup in EGR Valve or vacuums lines.

I'm a bit confused about the compression test now. I thought originally that I had a burnt valve so the mechanic pulled the heads to check and thats when he found the FI wires had been chewed. Valves looked fine. I'm not arguing your point but after replacing the wires, the misfire stopped and compression came back to normal, (if 120 is considered normal for a 19 year old truck.) Maybe it was just a coincidence or maybe there was an error in the testing. You bring up a good point that maybe I should check the compression again.

I don't have my FSM in front of me, but look under head removal


best to double check before spending the cash and being bent when it is a misdiagnosis. Sad part is if he had the heads off the knock was right there and he should have noticed if it was brittle ,because he had to disconnect it ! I am not saying your guy is bad or BS'ing you for $$$, maybe just busy and over looked it.

I can't offer any specific advice because I don't know the 3.0, but I vote you do it yourself too. You cannot own one of these trucks and rely on a mechanic to keep it running and expect to have any money left over to play with - old trucks need love to keep on the road, and you're going to need to learn how to give her what she needs or you're going to kill your bank account.

Get busy and yank that manifold. Use stickers or string tags to label your wires and vacuum connections as you pull them, and put all your nuts and bolts in sandwich bags with the name of the part scribbled on them. If you want to practice, take your toolbag down to a pick and pull yard and cut a pigtail from a junk truck. It all may seem intimidating now, but you'll quickly get the hang of it.

If he pulled the head, its very possible that there was previously a HG leak or the valves where not in proper timing. Either of those could easily been fixed with a removal of the head which is significantly more than valve cover gasket fix. Just trying to help you understand how all these play into each other so you have more information when diagnosing or even talking with your mechanic.

This is great advice. I can't tell you how much I've learned by taking cars apart for free at the pick and pull.