Did replacing my timing belt cause my knock sensors to fail? What is happening?

Hey, everyone. I have wanted a first gen Tacoma for a long time, and last May I purchased a 2004 Tacoma SV5 3.4L V6 4X4 with 77,000 miles on it. It was a one-owner vehicle for almost all of its life, then some guy bought it and put about 7,000 miles on it. He didn’t have any work done on it, and he didn’t have any maintenance records from the previous owner. I am trying to learn more about my truck, but I’m not really there yet. I recently left town for a week, and I decided to do some scheduled maintenance based on my review of posts on various forums here (THANK YOU). I found a shop that came highly recommended, and the guys there seem pretty legit and love my truck. That said, I have some questions, and I am hoping some kind and generous souls on this forum might help me learn a bit and help me understand what’s going on with my truck.

Anyway, the truck has 80K miles now, so I decided to have the timing belt and water pump replaced while I was out of town. I asked the shop to also inspect/change fluids, check LBJs, check CV axels, etc.—basically go through the thing with a fine-toothed comb and let me know what needs to be done. They did and told me the front shocks and rear struts needed to be replaced and that it needed new spark plugs. All in with timing belt and water pump was going to cost about $2,500 in parts and labor.

They just called me and told me that everything was done, but when they test drove it, the check engine light came on, and the knock sensors need to be replaced. I asked if if they were sure it wasn’t a problem with the work they had just done, and they said yes. They are convinced there is a bad sensor, and the timing of the sensor failure is just a coincidence. It’s going to be another $1,000 to replace both sensors, bringing the grand total up to $3,500!

Your thoughts? I absolutely adore this truck. It is cherry with 80K miles, and I want to keep it forever. Are the knock sensors related to the other work I just did, or is it really a coincidence they one or both failed now? Did the shop screw something up? Is it possible the sensor is not bad, and the engine is knocking? Should I just fork over the cash? I need some help.

Please don’t tell me I could have saved a lot of money doing some of this work myself, because I know that, but I’m not competent to do it yet, but I am committed to learning.

Thanks in advance for any sage wisdom and advice you can provide.

- Eric

 

If the shop is legitimate and comes with references, pay the bill. This is the same basic argument for and against changing the transmission fluid. If you touch something can something happen.

 

It may be just the wiring harness for the knock sensors. I've read where several others have had an issue with harness becoming brittle from age / heat, and after replacing just the harness there were no other issues.
I'm sure the harness got moved around during the repair, and they may have been enough to do damage to the harness.
Could be worth a try since the harness looks to be less than $40 on most sites and that's with it being OEM.
Good luck, and hope it works out.

 

In short, no I don’t think replacing the timing belt and water pump had anything to do with the knock sensor.
I don’t know how much is the knock sensor, but $1000 is a little much

 

As the guy above mentions, possibly the harness associated with those sensors as well. They ride in the valley of the V of the engine. The reason it costs so much is the labor required to get down to those sensors again. It's basically removing most of the stuff like doing a timing belt. So th bulk of that extra 1k they want will be for labor. That sucks especially since they had it open and the sensors and harness was right there. But what can you do? I don't think they are pulling the wool over your eyes BUT we have no idea on the DTC that turned up. There's a lot of assuming that they did the proper testing and got it diagnosed.
Only thing we can tell you is if that cost of that repair is in line with what others have paid. Hope that fixes your MIL. Other option would be to leave it until you have access to it again. If you were there, would you be doing the work instead of the shop?

 

Could easily be a coincidence. They aren't likely related.

Couple thoughts on the matter:

1) Did they tell you "your rear struts need to be replaced"? I only bring that up because Tacomas don't even have *front* struts... You'd hope a shop would know that, lol...

2) Honestly, $2500 for a timing belt and water pump plus some cheap shocks is pretty damn high. I was quoted about $900 for my timing belt and pump. I did it myself for like $250-300, lol...

Anyhoo, when in doubt ALWAYS go back there and ask them to "show me". Have them show you the parts, show you the OBDII scanner code.

If "they already cleared the code" that's BS and you need to walk away from that shop.

 

I was gonna say...I bought my own Toyota OEM timing belt kit (~$200) and got a quote for a local shop that can do the labor for $450. That includes the water pump (came with the kit).

$2500 sounds like something I'd expect a dealership to charge, crazy

Anyway what's done is done, I got reamed on a clutch install once when I first got my truck. It happens. We live and learn

I second having them show you what the code was...if they're charging that much for a timing belt job I'd want to be sure I wasn't getting taken for another ride

 

"If" it were mine, I'd pick the truck up and replace the knock sensor harness myself. Removing the upper plenum on the 3.4 is pretty easy. Even if you went by the manual and replaced the two plenum gaskets, you are still waayyyy better off than $1000.
If you go this route, check out Timmy's video's below. His videos provide a wealth of information, and he's very precise and goes slow enough for you to follow and make notes.

He's a member on this site that goes by
Timmah!
Well-Known Member

https://www.youtube.com/channel/UC9EMiD9KPy61kJ6hdXcbjxA

 

With that few miles the knock sensor should be ok.

The pigtail from the knock sensors actually is
right there were the timing belt work is done.

Have the knock sensor wire connector tested that are on the top front of the motor. Its probably those wires right there
going into the connector or the connector on either side on the top of the motor,
Or it's not plugged in correctly.

 

They are taking you for a ride my friend.

 

I would disconnect that knock sensor harness wire connector, put some Dielectric grease in there and reconnect it and see what happens.

 

before doing this, spray liberally with Electrical contact cleaner, let dry then reconnect with above. Its worked for me before

 

Just my opinion here.... I find it very unlikely that both knock sensors failed at the same time. I vote that the wiring / and or connector (s) is screwed up. It might be easiest to check from underneath the truck....

 

taking off the upper and lower intake to get to the knock sensors is cake. Your guy is too high unless you live somewhere really nice.

 

If you're not willing to do the work yourself call the shop back and ask them to work with you. Just be adamant that the problem wasnt there before and the item needing to be replaced is in the same vicinity as where they were working.
Most legitimate shops will work with you.

If they don't and it's a corporate shop call the customer hotline and go from there.

 

Yes, electric cleaner BEFORE di-electric grease.

 

Thank you everyone for your very helpful replies. The suggestions that it could be the wiring harness or a faulty connection are great. I talked to my shop, and they are going to order a new wiring harness with the new sensors. Once they have everything taken apart and accessible again, they are going to check the wiring and connections first before replacing any sensors. When I pick the truck up, I will ask them to show me the bad sensors.

I don't really have the time, or place, or tools to work on it myself yet, so I'm kind of at the shop's mercy at this point. I feel good about things, but I will post an update with a copy of my final bill. You can tell me then definitively whether I got screwed! Thanks again for all the helpful advice. Much appreciated.

 

I would go to another shop dealership if needed and pay them 50.00 to check that connection 1st. They can check it in 10 minutes or less

 

You are NOT at their mercy. That's what the shops want you to think.

Don't have them show you the parts AFTER they put the "new" ones in and charged you for the work. Make them convince you the parts are bad in the first place. In all likelihood you can't tell if some random sensor is "bad" just by looking at it, anyway. They could just as easily show you an old crapped out O2 censor from a Dodge.

The wire harness for example. Any good shop would have at LEAST tried the suggestions from above (making sure the connection is bad, testing connectivity in the harness, etc...) first before just ordering a new part and slapping it on.

We're trying to help you BEFORE you get screwed.

 

knock sensor runs under the intake manifold right? refresh my memory because its been a while