Ok, here's my story...
I bought my car March 2013 with 165K miles.
Since then I drove it 17K miles... mostly highway.
When I first got it I realized it needed to warm up before it would shift into 3rd and then into OD. So I always let it warm up before getting on the road and it was never an issue. A couple over times over the last 5 months I noticed that when I'd start it after it had sat for about an hour, it would not idle properly. I would shut it off then restart it and it ran perfectly.
About 7 weeks ago I made a quick stop and jumped back in and it was completely dead. Turned out the neg battery terminal was loose, and it started right up. During that time I noticed a little miss when driving from time to time. But it was barely noticable. Then about 5 weeks ago it began running rough. As long as I kept the gas on it was okay but the moment I took my foot off the gas it started running rough again. I took it to a local cottman transmission where I know the mechanic. He looked at it and said the sensors indicated it was clogged catalytic converters. He took the exhaust down and discover the catalytic converters had been hollowed out. Apparently the previous owner's mechanic did it. But I drove the car for 17K miles without cats so that wasn't the problem. Cottman ran diagnostics, replaced the plugs which were fouled saying that was the problem.
I drove the car and within 4 miles it started having the same running rough problem. I decided to get it home which is about 60 miles and it really ran rough with the CEL starting to blink a couple miles from home. But it stopped blinking when I stepped on the gas. The next day I drove it 20 miles to a mechanic who is a friend. He had the car for 3 weeks, ran diagnostics, checked the timing, removing sluge from the valves and thought on 2 occasions he thought he fixed the problem until test driving it several miles and it would start running rough again. He checked the cylinder compression and told me the back ones were puting out about 80 psi while the front ones were at 120 psi and that had to be the problem. Said it would cost $1500 to fix but that might not even work because of the ollowed out cats. He said the last owner may have put oil additives in the engine as a temporary fix. But it ran beautifully for 17K miles. He suggested I get rid of the car. So I drove the car home and it really ran rough and didn't shift into OD.
I picked up M1 Mobil synthetic oil and ring compression fix additive and changed the oil, filter and added the additive and put 93 Octane gas with fuel injector cleaner in the tank. I drove it 65 miles on a highway to work and with the exception of a couple misses it ran great. On the way home i got caught in traffic and it ran really rough and the trans continually would shift out of third and back into third gear. CEL would blink and stop randomly but mostly when I was at a slower speed.
I read the cause could be the IACV so I removed it, cleaned it with CRC and reinstalled. Today I drove it 15 miles and it ran beautifully. I made a stop for 15 mins, jumped back in and noticed it wasn't going into OD. Made a couple more stops before driving home. With the exception of no OD it ran fine until I stopped at my driveway, put it in reverse and it started running rough again. I turned it off, waited a minute and started it, drove around the block and the running rough and high idling happening. The CEL was not blinking but I can only think it wouId have if I continued to drive the car. I started to think this is a computer problem. I should mention the CEL lit up 150 miles after I bought the car.
I would love to know if anybody has any idea what is going on!! Sorry for the long story!
