sputtering down the road

[87GMC2WD]

hey guys,
Ive got an 87 v20 with a 350. Now i bought the truck a few months ago and someone had hacked in a carbed motor. This motor was very tired and was burning about two quarts of oil  a week. so i got a used motor from a local junk yard that is really relaible, they run all their motors and give a 6 month warrenty. the motor came out of a 1990 truck and was obviously throttle body. now when the guy put the carbed motor in he tucked all the wires for the old throttle body back into the firewall, thats why i wanted to try and go back to the throttle body because i like that a lot more than the carb. ive got it all in and it runs good at idle and revving it up in the shop. but when i put it in drive and try to take off down the road it sputters and almost dies. i know that it isnt losing spark because i set up a spark tester and watched it as i tried to drive down the road, never skips a beat. from what i can see in the shop with the air cleaner off it is getting plenty of fuel. BUT when i unplug the map sensor it runs better under slight acceleration and breaks up under more than 1/4 throttle. so i think that it would be running lean than if when i unplug the map it runs better. i have a new oxygen sensor and looking at the data it is switching from lean to rich. so i would love to hear some more imput becuase im gettin lost.

PLUS i did get the computer that was in the 1990 truck, i tried the new computer still does it. i tried the old computer with the new computers PROM and it still does it. Hopefully im missing something

Thanks in advance,
Kyle

[87GMC2WD]

ONe more thing that may be important. The truck has an aluminum tank in the bed. So when i put in this motor i put in a Carter P74017 fuel pump. The specs are 10.3-14 psi and 23 gal/hour. from what all data says you need 9-13 psi at the filter and 15 gal/hour. i figured this was suitable, but may be costing me im not sure.

[Lt.Del]

did you check the Idle Air control valve?

let me get this right, you have an 87 and went from Tbi (being an 87) to carb, then back to TBI w/ the new engine.  What kind of fuel pump?  (nevermind, you answered that on a follow-up)

[87GMC2WD]

i did not check the iac. it is functional because running in the shop it will high idle then come down off. i dont know how else to check it. and yes it was stock TBI, i bought it with a carb, and now im trying to make the TBI work again

[VileZambonie]

You don't have the map sensor to ported vacuum do you? Check for vacuum leaks & proper fuel pressure. If everything is in good working order rebuild the throttle body.

[Irish_Alley]

i had a 92 sable due that, due to the fact the map sensor was on facing the wrong way. but i would unplug one sensor at a time and see if the problem doesn't change then go from there

[jaredts]

Doesn't unplugging a sensor just put it into open-loop mode--ignoring all sensors?  Do you have a working check engine light?  Seems like this thread would have started with whether or not trouble codes are present.

[87GMC2WD]

thats the weird thing is no codes. i wish ther were so i could have something to go on. i have the map sensor on the port on the back of the throttle body which im pretty sure is where they go stock, could be wrong. i have found no vacuum leaks. the fuel pump is putting out 11.5 psi, i havent actually checked the gph i was just going by specs. but i failed to mentioned that it is an inline fuel pump, i dont know if anyone has had any luck with these or no.

[jaredts]

The check engine light did come on when you unplugged the map sensor, though, right?  It does come on when you turn the key on, right?  So if that all works and no codes, I can only say that I'm not that experienced with tbi but had the exact same symptoms with no codes.  It happened for me after installing a junkyard throttle body.  I rebuilt the throttle body and it ran great.  This was a 305 Caprice, but that is irrelevant.  Just another confirmation of Vile's comment:

If everything is in good working order rebuild the throttle body.

[87GMC2WD]

the light does not work and i assume it did set a code after i unplugged the map sensor but i have not yet scanned it since. how far of a rebuild should i get int, like new injectors or just rebuild the regulator?

[jaredts]

So you have an OBD I scanner that will give you codes without using a check engine light for the codes?  Take my advice with a grain of salt, but I think I would just start with a throttle body rebuild kit.  O-rings, injector filters, gaskets, regulator diaphragm, are all included.  There may be some other odds and ends in the kit (this was 11 years ago for me).  I think you could pretty easily change the injectors later if this didn't fix things.  Vile and some of the others on here are light years ahead of me, so.....

[Josh454]

Sounds like it might be lean to me too.  Are you getting enough volume out of the sender in this aluminum tank set up?  Change the fuel filter?

[VileZambonie]

Make sure there is vacuum at the map sensor hose. I'm not a fan of inline pumps but if your fuel pressure is consistent rule it out

[Irish_Alley]

if you unplug a sensor the engine will change how its running more than likely if you unplug lets say the map sensor and its the maf sensor thats bad it will stall the engine but if you unplug the maf it shouldn't change how its running

[87GMC2WD]

i will check for vaccuum tonight. its a brand new pump with a brand new filter. im going to put it up on the lift tonight and see if i can make it brake down here while watching the fuel pressure. last i check it was 11.5 running.