Need more ideas for P/U

Last week I posted on this pick up, 87 GMC 4x4 3/4 ton, and a 350, and thought I got it fixed. I spoke way too quick. Put in new plugs, Dist cap, rotor, modual, idle control valve, TPS, and retimed it. Bob said to block the EGR off. Pulled the hose off, plugged it, and it ran good. Plowed the drive way out. The next day I must have had a brain f art, took another EGR valve off another motor I have, installed it, back to chit. Idles good, give it a little gas and it will pop back through the throttle body. took the hose off and plugged it, no different. Took the EGR valve back off, made a plate, and bolted that on, no different. I replaced the fuel fliter, no diff. I stuck a vacum gauge in it, and it sets steady at 18.5. Open the throttle a little, and it drops down a little, 17, open it a quarter of an inch, it will drop to 5, a little more and it will go to 0 and shoot a flame up through the throttle body. Checked the timing again, right on.
Bob said it was hard to believe a fuel filter could rust on in 6000 miles. Here's a picture. I would have taken a torch and warmed it up to get it off, but wanted to send this picture to him. I just cut the line off and ran a rubber hose around it. SO..... I'am wanting more ideas, before I give up all togather. Thanks Jack

Forgot to add, this time the check engine light came on. It blinks 12. The code sheet I have start at 13. ?????????

My Google search say that a code 12 means that you computer is in diagnostic mode. I guess that it should give you a code 12 to tell you that you are in the right mode, then give you the code you are hunting for.

www.troublecodes.net/GM/

Once had a 91 chevy with a 454,same thing backfiring and all, check the $3.50 gasket under the throttle body.

The gasket under the throttle body and check your intake bolts and see if they are loose.Ive seen Chevy intake bolts loose before cause something like that.Its kind of strange it ran while you bladed the driveway.Might want to check and see if there is some way the distributer cap can hit something and break,like a broken motor mount letting the motor rise way up and break the distributor or cab floating around and if its rusty and get in contact with the cap.If that is alright it makes me wonder if putting that EGR valve on maybe.Like a line cracked or left off in that area.

Might not want to be warming up a FUEL FILTER with a torch. That could turn out bad.

Your best bet is to get the repair manual to cover your truck if you haven't already.
I had a lot of similar problems with my old 89 F250 that I would not have been able to fix without the book.
I did disconnect/plug all of my emissions stuff since it was too deteriorated to work.

Camshaft worn lobe seen it before

What do you have for fuel pressure ? Did you unplug timing connector when you set the timing ?

distributor wire going thru the bottom plate of the distributor has bare spot, shorting out against plate. Wiggle wire around when its running. Hit a bump or get engine hot & wire moves & shorts against plate. Cools & moves away, allowing it to run ok til gets hot or hits bump again.

when you checked the timing did you give it part throtle to see if it moved? I think you have a worn dist shaft and at just over idle it starts jumping around.

Grab the TB by hand and twist they have been known to blow the center gasket too.

What is the fuel pressure?

Restricted exhaust? Cat converter on my 98 Dodge broke apart internally and acted alot like this. Wound up with the can empty and the muffler packed full.

Drop the exhaust at the manifolds and crank it up. If it runs ok then the exhaust is blocked. Probably the cat converter. Just changed them on my 03 GMC.

HAPPY NEW YEAR