I checked the heat riser valve when hot and the flap was about half way between the stops but not much tension in the spring so I think the valve is ok.

 

I hooked up a vacuum gauge I could read while driving. I've got 17/18 inches at idle. There isn't a lot of flat road around me but I tried to read the gauge while maintaining an even speed of a level road at 45, 50 and 55 mph. I got about 8 inches in each case. I tried to get faster than 55 and most of the time vacuum was lower than 5 inches and I never reached 65 mph. Does that give anyone any clues to my problem?

 

Your exhaust is stopped up and the engine can't breathe. Pulling 17/18 inches at idle means the engine is solid. Somewhere in the exhaust is a stoppage. It can be a melted catalyst in the cat. converter, an internally rusted muffler (it it gets a lot of short drives, they rust inside fast) that has collapsed or, if you live near fields, you might have a rodent nest in the tailpipe.

With the exhaust cold, crawl underneath and thump (<-------fancy technical term) the bottom of the exhaust. You may hear a rattle of something inside. If, for instance, your muffler rattles, now would be a good time to put on a new exhaust with a nice rumble.

 

That’s not it. Earlier in my quest for a cure, I thought exactly what your saying so I disconnected the exhaust ahead of the cat so I was running straight out of the Y pipe and it made no difference.

 

have you replaced the timing chain?

 

Does the engine warm up completely or does it seem to run a little cool? Like others have said, have you checked the timing chain? 150K is time for the chain to be replaced if it hasn't already. Quite possibly the chain has a LOT of slop and is just bouncing around on timing.

 

It did have a lot of slack in the chain, about 12 degrees, so I replaced it but it made no change.

 

OK guys, I found the trouble and I am embarrassed to tell you, although in my defense when I bought the truck back in August it was already like this and the previous owner said he had the exact same problem since he got the truck from his father-in-laws estate. The father-in-law was the first owner . I wanted to check the compression one more time before I pulled the heads. The compression was 140-147 with the throttle plate closed so the real compression was higher. That is ok, not new but ok. Something in my mind made me look up the firing order and use that instead of putting the wires back where they came off. Here is the embarrassing part. After several hundred dollars in parts and scores of hours of my spare time, I found 4 and 6 sparkplug wires were reversed. I couldn't believe the engine ran as good as it did at low speed with the wires crossed but when they were wired up right, the truck went right up to 75 mph. I caught up with traffic at that point and turned around and went back home. How could a motor run well like that at slower speed with 2 wires crossed. I read a thread from about ten years ago that the problem sounded just like mine. A bunch of the same suggestions were tried and that guy sounded as frustrated as I was but he just abandoned the thread and disappeared off the site without a resolution. I figure he either never solved his problem and sold the truck, or he completely rebuilt the motor and got the wires right when he put it together or he found what I did and was too embarrassed to admit it and quietly disappeared. If he had fessed up and finished the thread it would of saved me a bunch of time and money. Anyone ever seen anything like this before?

 

One more question, what should the compression be on a new or rebuilt engine?

 

Depends on what the rebuilder did to it. 160-170 is generally in the ballpark.

I re-ran the wires three times on my 96 before I finally got them right. Don't feel too bad.