Toyota Repair: 94 4runner engine, head gasket replacement, dual overhead cams

Question
I own a 94 4runner with a v-6 4-wd and lately after warming up (about 20 miles) it starts running like it is missing a couple of cylinders. If you keep your foot in the throttle it doesn't shake as bad as just going down the road. It will Quit shaking after 20 or 30 more miles but will start back up if you cut it off and start it back up. I found the Bi-metal vacuum switching valve broken and replace it along with any suspect vacuum hoses. I have changed fuel filter along with the regular service. It has 155,000 miles on it and has been a very dependable ride. any help will be appreciated. Thanks,  Walt

Answer
Hi Walt,

Your symptoms can mock small problems or very serious problems. You are correct in thinking that a vaccuum line can cause a rough idle/running engine. The solution to that problem is to do a complete visual inspection with a vaccuum line diagram. Electrical problems can also cause this. For example, if you have an "open" spark plug wire, it will ground to the nearest point. Just think of electricity as a "lazy" circuit. Electricity will look for the nearest ground, so if there's a damaged wire, it can and will misfire. I'm quite sure you have the 3.4ltr V-6 engine in your truck. You can tell if you do because the 3.4ltr V-6 has dual overhead cams and you will see "independent" coils on the top of the drivers side camshaft housing if I remember right. In addition, the 3.0ltr V-6 has a distributor located on the drivers side, the 3.4ltr V-6 does not have a distributor at all (DLI).

Unfortunately, Toyota's V-6 engine (the 3.0 and 3.4 model) both came with a poor head gasket design. The 3.4 did have improvements in their design, but still had handfulls of leaking issues. Due to this fact, Toyota launched a head gasket replacement campaign to warn all owners of this engine to bring their vehicles to a dealer of their choice for replacement of the head gaskets and knock sensor. In severe cases, the engines had to be short blocked. If your truck has a broken head gasket, you will notice additional symptoms such as: white smoke from tail pipe, and a sulfur smell from the exhaust. If you notice these symptoms, get your truck to a toyota dealer promptly! If you don't you could hydrolock/damage the entire bottom end of the engine. If you do notice white smoke or a sulfur smell, pull the spark plugs with a spark plug socket and look for wetness (coolant) on the plugs.

I hope this helped, please keep me informed.