93 Jimmy misses, backfires, stalls when hot

I have a 93 S15 Jimmy with a 4.3 that will suddenly lose power at highway speeds after driving for about 60 miles. This will also happen in town when the outside temperature is real warm and I have been driving, stopping, and starting the engine several times. It also recently happened while trying to tow a small boat (16') in cool temperatures at highway speeds. The main factors seem to be heat of the engine and torque on the engine.

When the problem happens, if I push down on the accelerator the problem gets worse and the engine will backfire repeatedly and then stall. If I let back on the gas the problem subsides and I then need to slowly feather the accelerator in order to regain or maintain speed.

My mechanic tipped me off that it might be the EGR valve. I replaced the valve and took it out for a test drive and found it had the same problem immediately. The old EGR was very clean and the pintle moved freely.

This problem first happened about 9 months ago but was almost unnoticeable during the cooler months with in town driving (I am in Phoenix). Now with it warming up it seems worse than before. Any suggestions are appreciated.

Ok, I just went through this same problem. The 4.3 central point injection system works well but was designed to have problems that make most people send the truck to the junkyard prematurely. Remember this is GM here, we gotta sell new cars, and can't do that if the old ones last too long.
Heres the scoop. The 4.3 cpi system has 2 major flaws that cause the engine to misbehave. One is the fuel pressure regulator, located inside the upper intake manifold. This location gets hot, very hot sometimes, especially when you shut off the vehicle after towing a trailer on a hot summer day, or after any sort of overheating episode. The fuel pressure regulator has a rubber diaphram inside, that degrades over time, and when it gets hot it degrades faster. It also is not resistant to alcohol fuels. When the diaphram fails, it leaks fuel into the intake manifold, flooding out the engine, making it stall, run rough, lose power, smoke, stink, you get the picture.
The regulator is relatively easy to replace. GM won't sell the regulator seperate, only as an assembly. $600 later, you get porked. You can get the regulator for about $48 online if you look around. Get a manual, take off the upper manifold, and the regulator can be replaced by removing the two screws holding the regulator retainer in place. You don't even have to remove the CPI unit, be patient work slowly and it will take about 10 minutes. I just did one and it was not too difficult. You will need a special torx security tool, which is basically a regular torx bit with a hole drilled in the center. One came with the regulator I bought, for free.
There is only one other problem. The fuel lines that supply the CPI unit are also plastic. They are as likely to break as is the regulator. If the feed line breaks it will squirt raw fuel anytime the key is on. If the return line is broken( they usually get a pinhole or crack), the engine will usually start and run fine until it gets warm, and then it will leak when the fuel regulator is returning fuel to the fuel tank. This causes more erratic operation, and it drives you crazy, eg just go trade it in for a new one.
Ah the logic of GM!
Replacing the fuel lines is a pain and Gm makes a kit to do it with. It is called a nut kit, of all things. I got quoted $88. Some aftermarket suppliers also sell it for about $50. It involves removing a clip from the injector unit that holds the hoses in place, gently twisting and pulling out the hoses, and then you have to remove the metal fuel lines and metal nuts at the back of the manifold and remove the hoses. There is a retainer clip and bolt holding it all in place on the back of the manifold where you can't see it, but you can feel it and get a wrench on it. The metal pass through pieces are sealed with an o-ring. If you are not mechanical, go get a friend or mechanic to do this for you. I think GM will rip you about $800 for this trick, and maybe you can get a garage to do it for less. Remember, the dealers aren't selling many cars lately, because no one can afford to buy them, so they kill you on parts and labor. The independent garages, if you can find an honest one, are good enough to do a repair of this kind. It should take no more that 4 hours, and a good mechanic can do it in about 2 hours. I did it in about 2 hours doing both repairs.
Also if you are good with your hands and can find the tubing, you can replace the fuel lines without buying the expensive kit. With some careful teasing and heating, the old plastic lines can be removed from the fittings and replaced with new lines and reused. You gotta be good to do this though, and the lines are special high temperature plastic, and hard to find. I am still looking for a source.
Good luck, Fastcarface

First for info I neglected to mention in my initial post: My engine is 4.3 Vin W 4WD. 140,000 miles. Recent replacements: fuel pump & sending unit 30 months ago, fuel filter 8 months ago.

Thanks for the reply. When I first read your post, I had the truck tore apart. I decided to bite the bullet and open up the lower intake manifold to clean out the carbon. To my surprise there was almost none there, so my mechanic was dead wrong. I also believe your answer is dead on because I noticed a lot of fuel and gunk in the upper intake manifold this time around (I've replaced the injectors twice before). The manifold wasn't washed like in the photos in the FAQ section though. Just wet and dirty.

So anyway, I replaced the coil, distributor cap, rotor, spark plugs, and wires. I removed the EGR and put my old one back on which was cleaner than the one I just put on 6 months ago. I installed one of the screened gaskets to catch the carbon before it hits the EGR. I also put back in some injectors that I pulled out about a year ago. They were only about a year old and still good; I had replaced them because of a misdiagnosis. I then put in a new nut kit due to your advice.
NOTE: ANYONE REPLACING THEIR CPI, DON’T SKIP THIS NEXT STEP: After installing the fuel lines, nut kit, and injectors I turned on the ignition to energize the fuel pump and noticed it was squirting fuel from the pressure regulator. The diaphragm must have dried out from sitting in my Arizona garage for a year. I purchased and installed a new regulator from Auto Zone and put everything back together. It started out running really rough, the computer eventually compensated for a lot of it, but it still was drastically low on power. I then realized I forgot to reconnect the vacuum hose at the very rear of the manifold next to the distributor. This made it smooth out quite a bit more, but it still lacked power and backfired upon hard acceleration.

I did some more reading up on it and figured it had to be the timing. I could not find a timing light to rent. Or anyone to do the timing for me seeing it is Sunday. So I decided to try and time it by ear. It took me two attempts, but now it’s running great! It will actually through you back in your seat. Something this old truck hasn’t done in a long time.

This was a lot of work and a little scary as I have never dug this deep into an engine before, but it was well worth it. I am estimating it would have cost me between $800 and $1200 to have a shop do all of the work that I did, and they would not have taken the time and care to clean the manifolds like I did (a real pain). It cost me less than $200 in parts, and more time than I care to admit. I also came out of this with a much better understanding of how this engine works. I was planning on dumping the truck in about a year, but if it keeps running this good I may as well keep it.

I just had another thought. You said the diaphragm is not resistant to alcohol fuels. I wonder if putting in fuel additives like injector cleaners will actually do more harm than good? I was going to add some Sea Foam to my gas for one tank, followed by a tank of just gas, and then follow it with a tank with Marvelous Mystery Oil added. Maybe I should skip the Sea Foam? It smells like it is alcohol based.