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Discussion Starter #1
ok guys, beater truck 1991 2500 chevy pickup 350 tbi. When idling at a stop light the truck wants to die and runs like it has a cam in it, other than that runs like a champ. No check engine lights, new plugs wires dist. cap and rotor, checked for vacuum leak, checked egr seems to be fine. new idle air control valve (old one would not move). swapped out map sensor on diff. truck that runs fine and did not change the way it ran. The injectors seem to be fine as i checked those also, ran a can of sea foam through it hoping for good luck. also recently replaced is fuel filter and pcv valve. ANY ideas:help i am at a loss. may be forgetting something else i replaced....:help
 

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check timing? fuel filter?
 

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I had one that would do that and it ended up being the fuel pump. It started out just doing it at idle and eventually did it all the time.
 

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Timing and fuel pump would be your best bet... have had that happen before with a plugged pick up tube also which then led to the fuel pump causing problems next
 

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Discussion Starter #6
Ok thanks for the feedback. Yesterday i retimed it on a hunch, fuel filter is also new. Dont really want to try replacing the pump as of now anything else i could check out?
 

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Did you clean the tbi?
 

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Well if you don't want to mess with the fuel pump then you need to check your fuel pressure at the throttle body I can't remember what fuel pressure is supposed to be but a chilton book or your parts store could tell ya
 

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clean your iac valve you prolly have carbon in there
its on the passenger side takes a 7/8 wrench mine did it to just my 2 cents i have only had 4 of those trucks
 

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I bet your timing is off. Check it. The down and dirty way is to loosen the hold down for the dist. and rotate it slowly in one direction or the other and see if it runs better or worse.

Fuel pressure should be 9-12psi.

Another thing to check is to OHM test the coolant temp sensor that is in the front of the intake manifold, up by the thermostat kind of. A haynes manual will have the correct ohm readings for the sensor at a given temp.

I struggled with an identical problem on my 454 tbi truck. Turned out the timing was off by over 20 degrees, and the fuel pump wasn't making enough pressure. Fixed both of those at the same time, and she's a powerhouse now! I currently own 2 tbi trucks, a 350tbi and a 454tbi so I know a thing or two as well ;)
 

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Discussion Starter #11
Thanks for the help guys! Retimed it again jsut to make sure, i did already replace the iac valve. Still not running right but now the guage is telling me the truck is overheating and its not. I have an old fuel pump for my vortec 350 with a bad sending unit does anyone know if this will work to test out the fuel pressure?
 

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No, the vortec pump has way to much pressure for a TBI motor, CEL on? Maybe MAP sensor. If you want to check fuel pressure, You'll need a gauge and a tee to put where your fuel filter goes.
 

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check egr valve , make sure it is not stuck open, check coolant sensor with scanner if that is bad it will mess everyting up,and check dist
 

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No the pump off the vortec will not work. It pushes 60 psi and you only need 15 max. How are you setting the timing? Are you unhooking the brown wire with the white stripe? If you did and set it to zero then that should not be your problem.

How big of a cam is in it. It does not take much to throw the TBI off. There is no mass air meter on them. If you start adding or taking air away it throws the mixture off. You might need a custom chip.


Also like already mentioned the coolant temp sensor on the front of the intake can cause all kinds of problems. It is the one that tells the computer engine temp to control timing and fuel.
 

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Sounds like tps to me. I have a 91 with the 350 tbi. I know quite a bit about this motor since I built mine bottom up. Get a scan tool and watch your tps. Its probably all over the place
 

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I didn't see you already traded MAP sensors, the vaccum hose/ wiring harness that hooks up to it is ok?
 

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Discussion Starter #17
Yea vaccum lines are fine...We've been up and down this truck and cannot solve this problem! But now its been doing something new the temp guage goes well over 210 but the truck is not this hot! We are going to replace the temp sensor see how things go
 
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