Ok, ill start from the beginning. I have a 1994 gmc s15 jimmy (square body style) with a 4.3L vin code "w" vortec engine. Oil pump went out july 2009 and engine shredded the main and rod bearings, so i decided to rebuild it with a little more kick... I bought a second engine, same style 4.3 out of a 1993 jimmy, stripped down the block and had the local machine shop clean the block, bore it .030" over, install cam bearings, head job, ect. Then i bought a comp cam (266/270 advertized duration, 210/215 duration at .050" lift, .500"/.500" lift) & keith black performance .030" over hypereutectic pistons. I checked with several techs to make sure all of these mods will work with factory parts an computer before-hand. What i'm running into is that the engine seems like it is misfiring at idle, and idles up and down a little, and when I drive it, it will only accelarate at wide open throttle and will not hold a steady speed, it decellerates about 1mph per second. I pulled the intake tuner valve when i made it home, and the inside of the intake plenum had a mixture of gas and carbon standing in it and everywhere was wet. So, i replaced the injector assembly and supply/return fuel lines, and cleaned the intake really well with brake cleaner. It ran great for about fifty miles then when i went to leave work the next day after it had sat for about 6 hours it started running like crap again. Pulled the tuner valve and the inside of the intake was wet, but didnt have gas standing in it... Any ideas?

 

I would do a leak down test on the fuel line. Having a fuel pressure gauge is IMO a standard tool for all " W " vin engines. On a side note, it seems the re-man spider units can be faulty, on my second re-man unit. Good thing they are under warranty, just sucks once you buy junk new the warranty only replaces it with junk again.

 

Yea, i was kinda afraid that being a cheapskate on the injectors would bite me in the ***, but i guess we'll see... Fortunately i work at a toyota dealership, so i have access to most of the specialty tools i need, but that will have to wait till monday. That would explain why it feels why it is misfiring at idle, but do u think that because the intake has a fuel leak that i would only be able to accelerate at wide open throttle though?

 

From what I have read the early CPI units either spray 2 or 3 times per crank revolution. I say 2 or 3 because I have read articles claiming one or the other, never the less, the poppet injectors have to work perfectly or the fuel injection system goes to sheet. Fuel pressure is what drives these early CPI systems and if there is any drop in system pressure do to weak fuel pump or leaky components performance is downgraded.

I hate to rain on the parade, but IMO, these early CPI systems are not worth the money to maintain them. Looks like you spent some bank on rebuilding the engine, where you might run into problems is tuning the fuel mixture and timing. Everything is computer controlled as you probably already know and the only way to modify anything is to re-burn the chip. The stock chip can only re-learn so much from the stored data tables. So it might be worth the extra money to either find a local guy or send out for a chip burn. There is a site, can't remember right now, that will loan the tools needed to data log your driving and then burn the chip to your engine. But, this is still lipstick on a pig, keeping the CPI.

These 4.3's came with a carbureted manifold on the marine option either 2 or 4 barrel. I have read many of forum posts where guys have made the swap and claim it was worth it. The drawback is once you make the move to carbureted the computer system is out the window. You would have to lose the stock distributor and go aftermarket along with other mods. And this all means more money, but that's why we do this right, to spend money.

Sorry for being so long winded but my truck and its CPI have been a thorn in my butt. To answer your question, it's hard to say what is causing your performance issue since you rebuilt beyond stock, so it's best to take it "one" diagnostic at a time.

 

The carb marine intake swap isnt really something i want to look into b/c i want to keep the factory engine and computer system. I think that the vehicle won't have as good of a resale value if i make the swap, and im just not a fan of carburators. My old '85 chevy pickup is carburated, and it seems like i cant ever get it fine tuned. Also, im sure that this will wind up being something simple, b/c there have been several times that this engine will run like a champ. Also, there is a company called motorvation chips that will custom burn chips which will work with aftermarket cams and any other mods for $425. I just have to be patient i guess and perform the necessary diag tests monday. Im hoping its the fuel filter, but it was replaced like 4k-5k miles ago, but u never know.... I keep u posted monday. Let me know wha tests you think i need to perform.

 

Ok, made a new discovery today, no fuel leak, but there is oil getting into my intake plenum via the egr system!!! WTF!!! What would cause this? I can only guess That my intake manifold has a crack in it where the ports for te egr run? Anyone else had this problem before? There is about half an inch of oil standing in the bottom of my intake.

 

There should be 2 return ports in the lower plenum, one on each side. You can see this one, drivers side, next to the CPI unit. It looks like a half circle covering a hole. I would check to make sure these ports are not blocked.

 

Ok, so i have a new update. I had a bad pcv valve which was filling my intake with oil. But at least it was simple to diagnose and i now have peace of mind with the injectors being replaced. I wonder if it would be better to replace the pcv valve and reconnect it to th intake, or just put a breather on top of th valve cover and cap off the port on the intake? I installed a mellings high volume high pressure oil pump that cranks out anywhere from 40-80 psi oil pressure. Could that cause my pcv valve to go bad? But now arises a new issue. When i drive roughly 10 miles down the highway to town, it wants to overheat and has a rough idle at stop lights, and on occasion will die. But going down the highway it runs steady at 200 degrees. Also i installed a heavy duty waterpump with a bigger impeller, would that make a difference? Any suggestions?