I apologize in advance for this long post.
I have a 1990 F150 Lariat with 5.8L EFI and it was built in Canada. I bought it because I had a piece of property in BC a few years ago. I moved back to California and took the truck.
This truck was running great and then it started to run bad intermittently. I would drive on the highway for about 20 minutes and when I got off the highway I could hardly keep it running. The tach was going up and down eradictly and then the engine would just die. If I waited awhile for it to cool down it would start back up and run ok. Driving around town it was ok but I noticed that after it warmed up the oil pressure would go to Zero when I came to a stop and was at idle. If I gave a bit of gas to raise the RPM from 800 to about 1000 the oil pressure would come up to normal. Then one day on the highway while pulling a trailer with my 1968 Cougar body, the engine just stopped and would not start. I had a lot of smoke coming from some area around the top end of the motor maybe from the intake manifold??? After sitting for about an hour it did start but died again within 1/2 mile. I needed to get towed in but the shop did not get to it until after the weekend when it had really cooled down and could not duplicate the problem, it has not happened since and was running ok but not as it should. Since the break down, I had a new single muffler and tailpipe installed to replace the original muffler and double tail pipe and I do not know if this caused any problem.
For about 6 months now, when I start the truck, the engine RPM goes up to about 1500 and then down to 800 and it does this 4-5 times and then settles down to about 800 RPM when it gets warm. After it warms up and I re-start this RPM only goes up to 1500 one time. I also noticed that when the truck shifted from 2nd to 3rd the engine hesitated as it tried to accelerate. It seemed to run better with the Over Drive Off. Now the truck is running very bad. It takes so long to accelerate it is dangerous in traffic so I can't drive it. Prior to it really running bad I took it to get a smog cert. and it failed. I was told that the Cats needed to be replaced. I understand that there are two on this vehicle. Do you think the cats are the problem? Do I need special cats for California?
I am hoping not to have it in the shop where they just start changing things on a trial and error basis and I end up with a $2000 bill. I can buy a re-manufactured long block for $1200 which I may do if I can't find the problem. Has anyone had this kind of engine performance before and solved the problem? The Smog Technician indicated that I may have blown head gaskets. My experience is that I would see oil in my radiator or water/foam in my oil. I have neither right now but it is hard to tell about the radiator fluid as it is an old truck and the antifreeze tends to get brownish anyway.
Just a side note in the event that the transmission or torque converter can be contributing to the problem. The transmission was rebuilt by Aamco two years ago. I was charged $3900 and I ended up paying $2600 after complaining to the main headquarters. Yeah, I know I should have just walked away from the truck but my wife was driving back from Canada and got stuck in WA state in the middle of the night pulling a trailer.
Any help would be appreciated.