1990 TBI 5.7 Idle surging,stalling

Truck runs great, but recently when the engine comes off the enriching circuit. At idle it slowly drops idle speed and stalls. It is not plugs, cap, rotor, TBI is new last year. No codes are coming up. I have
turned up the idle screw in the base plate of the TBI, which does not seem to help, it just goes back to low idle and stalling again, is the a way to get that back to the original factory setting? I am presuming that screws up the main computer. I open to suggestions, put will start with replacing Throttle Position Sensor, tighten the 3 screws on the TBI, check for vacuum leaks. Should I be looking at the IAC sensor, like I said I am not getting any codes. I only have this problem when sitting at idle for a couple of minutes, then it starts to drop slowly then stall.
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On 12 Oct 2006 08:21:25 -0700, " snipped-for-privacy@yahoo.ca"

Before you go too far you migh pull TBI and check think gasket between it on intake as they are prone to fail. Mine did a few years ago and I had all kinds of weird idle problems from it. ----------------- TheSnoMan.com
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Id look into the EGR valve. When they go bad they can cause surging/stalling symptoms. When mine went bad it did show up in code though...
Please note Im no certified mechanic, but have been under the shade tree long enough...
Charles
snipped-for-privacy@yahoo.ca wrote:

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I agree with this... Plus, you shouldn't be playing with any screw in TBI. Computer controls everything. The only thing that you can adjust is the fuel pressure on the fuel pres regulator, and it is pre set at the factory. Don't play with that without a fuel pres gauge. Should be at about 10 to 13 PSI
HDS
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Normally you would not have to adjust the idle screw but I have seen times that it had to be done. the EGR might be a cause but the gasket between TBI and intake is know to fail with age, Also nrmal fuel presue from pump is about 12 to 14 PSI and pressure after regulaotr is 9.5 to 10 PSI but I do not think this is a fuel pressure problem. ----------------- TheSnoMan.com
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Take a look at the IAC, Idle Air Control. It is the large round device screwed into the side of the TB. Clean the pintle off with spray carb cleaner and clean out the air passage where it mounts. The IAC controls idle speed by admitting more or less air under the throttle blade. It may need to be replaced. Put the throttle stop screw back where it was and leave it alone.
Also, it might be a fuel pressure regulator going bad. Fuel filter? Fuel pump? These three things need to be checked before you do anything else.
If you replaced the TB last year, you surely put a new gasket on then, right?

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ok.. start with fuel pump pressure..then check your fuel filter, might be clogged. always start with small stuff that you can check. after all that. you narrowred it down to injecters, and throttle body, and throttle postion sensor. good luck.
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