I have a 2002 Toyota Sienna van with 153k on the car and 81k on the engine, which had been rebuilt at 72k under warranty. I had been getting P0171 & P0174 codes on it, so I replaced the gas cap as a possible solution, but it didn't fix it. So I put the old one back on.
I did notice the engine would miss sometimes when accelerating. What was odd was that the CEL would come on and then after a few days it would turn off. I think this happened three times where all the codes turned off, but only for a day or less while driving around.
However, one night the CEL finally it stayed on and added more codes: P0100, P0101, & P0110. I also got a P0306 misfire code and it was driving horrible with the CEL flashing. I drove slow, but got home safely. When it threw the misfire code it was a rapid putt-putt-putt-putt noise when it was running rought and had very little power to accelerate.
I did some checking and thought that the MAF Sensor might be bad and I put a new one in. That cleared the P0100, P0101, and P110 codes. They have not come back as a result. I then moved the coil pack from cylinder 6 to 2 and then got P0306 and P0302 errors showing misfires on both cylinders. So I moved the coil backs back and put a new one on cylinder 6. It seemed to run fine for a short while, but while my wife was out, the flashing CEL came back on and it started running rough again as before with the rapid putt-putt sound in the engine.
I had an Auto repair shop run scans on it and check for leaks, but couldn’t find the issue. They did find that the remanufactured MAF sensor wasn’t working consistently, but only randomly, so that got replaced with another one. They said the misfire though might be related to the O2 Sensors, so we replaced all three of them. All the codes left for about 30 minutes of driving where the car seemed to drive fine for my wife, but then the P0171, P0174, and P0306 codes all came back with the car running rough again as before. I later found out that replacing O2 sensors is a common wrong fix for lean codes.
I hooked up my vacuum gauge and let the car run for about 10 minutes. The reading was very steady and didn’t change after warming up, so it didn’t appear to have any vacuum leaks as I read that sometimes a leak can come from the intake manifold gaskets. I also used a mechanics stethescope to listen to the front fuel injectors as I read that these lean codes could be caused by a faulty or clogged fuel injector. I also read that these vacuum readings should indicate that it’s not a mechanic problem.
I then took it to another Auto Repair shop. They couldn’t find either the vacuum leak or the misfire. They performed a leak down test, which it passed. They also did a compression test on the cylinders. Then they swapped out one fuel injector with another, but it didn’t fix anything. The only thing that they did find was that the #6 cylinder with the misfire had 130 lbs compression versus 150 lbs on all of the other cylinders. To me the 20 lbs less compression wasn’t significant enough to account for codes, but I’m guessing. Their analysis was that it was a mechanical engine problem, like piston rings, flat camshaft lobe, or value issue.
After a friend and I looked at it longer, he checked for piston blow by putting his hand over the engine where the oil filler cap goes after he took it off. He didn’t feel any blow by. He then discovered the issue with the P0171 and P0174 codes, which was an exhaust leak, not a vacuum leak. He explained that the computer can’t tell the difference. Since the exhaust pipes appeared new we think it was replaced just before I bought the car this year. Where the two pipes met the previous repair shop had stuck two washers going to the spring bolts in-between the two pipe flanges by mistake. When I put my hand near that spot I felt lots of air blowing out at one spot. Then I put the washers where they were supposed to go and started the car, the P0171 and P0174 codes left and haven’t come back. That explains the random nature of the lean codes as movement, heat, and expansion would sometimes seal the leak there.
Now I was left with the P0306 code. I swapped the two front O2 sensors with each other, ran a noid test light on the #6 cylinder, and swapped out the #6 coil pack with a new one, but nothing changed. I looked at the #6 spark plug and it appeared normal, no excess fuel being present, but I swapped it out anyway. Again, nothing changed with the misfire as a result. At idle, the engine will miss, but it gets worse when accelerating and the CEL will flash. I read that this is indicating excess fuel is going to the catalytic converter, but it doesn’t appear to be the case as the plugs are not fouled.
I am at a loss on this as from all the testing it doesn’t appear to be a mechanical issue with the engine itself, while it only is on the #6 cylinder. Generally that indicates an issue with the spark plug, coil pack, fuel injector, or mechanical issue on that specific cylinder.
Any ideas as to what to try next?