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I’ve had an oil leak coming from the valve cover gasket for about 5 years now. Every time a mechanic changes it, it always ends up leaking. I’ve had 2 mechanic shops change it but no matter what, it always leaks. Any causes what it could be? Could it be that they’re just not putting it on right? I’d do it myself but I don’t have the tools for it and I don’t know what to look for. I notice the oil leaks down the front side of the engine on the passengers side and pools up near a little corner.
 

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The only thing I can guess that the shops might have missed is adding the rtv sealant at the timing cover edges. Here's a video describing the gasket and tube seal replacement process. I don't know if you have a GT or GT-S, the vid is for a 2zz. If you do this right and it still leaks then you likely have other damages/issues.

 
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2 spots on the valve cover gasket will leak without adding RTV Silicone gasket paste. They are the interfaces between the valve cover, valve head and front timing cover.

The gasket has 2 bumps identifying areas that need RTV. Just smear RTV on those spots.
 

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Besides what these guys said about properly using RTV silicone sealant on those important spots, I'd say check your timing cover gasket. I replaced my valve cover gasket and had leaks still on the passenger back side of my engine for a while not knowing where it came from exactly. Turns out I had a good leak coming from my timing cover gasket. My 03 GTS has 188k miles so I also could hear the timing chain start to make some chatter, so I decided to take it into a shop to replace the timing chain and gasket. Had my drive belt and water pump replaced at the same time. Now my celica doesn't leak a single drop. It's amazing. So check your timing cover gasket. I had oil leaking all over the place on the back left side of the engine (towards passenger) where the drive belt is.
 

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High-mileage 2ZZ engines, or any engine, have a few potential leaks in the following probable order:

1. Front timing chain cover
2. Valve cover
3. Timing chain tensioner.
4. Crank shaft seal.
5. rear transmission seal
6. Oil pan seal
7. OCV solenoid seal

Best thing to do is to bring the car to a reputable shop where they thoroughly degrease the entire engine then check for leaks and fix the correct ones. Guessing and fixing the wrong leaks will likely cost a lot more time and money.

The rest of the leaks can be fixed fairly easily, except the front and rear seals which is very labor intensive due to tight clearances with the chassis. The front timing cover seals requires removal of all pulleys, about 10 hours labor by experienced mechanics with the right tools due to tight clearances.

My 2ZZ engine was losing about 1 quart of oil every 5,000 miles. It was not that bad. I just laid a carton sheet in the garage under the engine to catch a few drops everyday. It was probably leaking more running at high RPMs. Dealer wanted $1,200, independent shops wanted $400; I finally had a shop do it for $130. Engine is dry now.

Unless you can find a shop that does good work cheap, just live with it. Oil is cheap. Many 1ZZ engines has far worse oil consumption and sludge problem, like a quart every 500 miles . Yike! They could avoid sludge problem simply by flushing engine with Motor Medic's GUNK, dissolving carbon deposits, keeping engine clean and oil return holes on pistons unclogged.
 

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Mechanic resealed the front timing cover then forgot to tighten the 10 bolts on top of the valve cover. Bolts were totally loose, not even hand tight.

No oil leaks out of the cover's edge, but the 4 plugs grommets were loose and oil got down into cyl 1 and 2 plugs' wells. The plugs had about 150K miles, drenched in oil and badly worn out, gap out to 1.5 - 2.00 mm. Damn. But car was still running strong and smooth.

I replaced the plugs and carefully tightened down the 10 top bolts to 8 ft-pounds with a torque wrench. Very easy to strip hole on the aluminum cover, head and block. So far no more oil leak anywhere out the cover or into the plugs.
 
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