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Troubleshooting TBI, recommendations for Fuel Pressure Gauge?

RockBottomRacing

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Put some work in on my 91 V2500 Burb tonight trying to troubleshoot my TBI 350, currently the only way it runs right is with the IAC unplugged, or if I leave it plugged in but unplug the PCV hose from the front of the throttle body. It’s a new Napa part, I did the relearn be process, twice. I have now come to my MAP sensor, and I have a known good sensor off another truck so I’ll be swapping it out tomorrow to test. Truck has 225k on the clock, I’m actually planning on swapping to a Holley sniper set up one day, but I’m thinking about rebuilding this TB for now. I have already cleaned it prior to switching out the IAC (made a noticeable difference). My injectors don’t appear to be leaking, wondering if fuel pressure is issue so looking to plumb in a gauge, full time, so if I ever need to I just pop the hood and look at my gauge. Looking for recommendations for a T fitting that’ll fit my TB on my Suburban without fitment issues with my fuel line. I’ve seen a few but I wanna find a quality piece that one leak. Any recommendations? Thanks fellas!
 

RockBottomRacing

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If it runs right with the IAC unplugged, the fuel pressure is fine. Sounds like you got the wrong IAC model from Napa, maybe bad wiring, or clogged PCV valve.

I replaced the IAC with a known good spare with the same results. Replaced the PCV already. Been messing with the wiring haven’t found anything wrong yet
 

bp71k5

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I replaced the IAC with a known good spare with the same results. Replaced the PCV already. Been messing with the wiring haven’t found anything wrong yet
If it runs good when the pcv is unplugged, but not when the IAC is plugged in, the IAC must be choking the idle air from coming in. That’s not a fuel pressure issue. If you are sure you have the right IAC (there are different versions from different throttle body types that are wired differently) it’s gotta be calibrated incorrectly or a computer malfunction (unlikely).

one potential is that the big block IAC is wired backwards from the small block and so if you get the wrong one, you need to swap wires around.
Did it just stop working on it own or did something else happen before this?
 

RockBottomRacing

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It’s done this as long as I’ve owned it (3 years). It’s annoying but i daily drove it for like 8 months and she did fine, but it always stalled idling in reverse or idling and turning the wheel, and I have stock tires for now
 

bp71k5

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It’s done this as long as I’ve owned it (3 years). It’s annoying but i daily drove it for like 8 months and she did fine, but it always stalled idling in reverse or idling and turning the wheel, and I have stock tires for now
Is this a stock motor or has someone swapped parts into it? Of course you can put a fuel pressure gauge on it, but your issue doesn’t sound like fuel pressure. I think I’d look into vacuum leaks and maybe the EGR (only cause I don’t know how that works but people always seem to have trouble with them).
 

RockBottomRacing

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Is this a stock motor or has someone swapped parts into it? Of course you can put a fuel pressure gauge on it, but your issue doesn’t sound like fuel pressure. I think I’d look into vacuum leaks and maybe the EGR (only cause I don’t know how that works but people always seem to have trouble with them).

this is a stock motor, with 225k on the clock. I ended up doing a new IAC, did the relearn, replaced the map sensor, now the opposite condition exists. Park neutral or gear she idles super high, tries to drag through my brake pedal at stop signs lol. I took some starting fluid to check for vacuum leaks a 3rd time, have not found anything. Already replaced the tbi gasket. Sprayed around intake manifold just in case and no change. After discussing with a friend who owns an auto shop, I tested my TPS and found it was out of spec, so I just ordered a new one. I also have decided to rebuild my throttle body since it’s got so many miles, so I ordered a rebuild seal kit as well as a 14 psi regulator spring and injector pod spacer. The thought process is it needs to be gone through anyways, and if the TPS doesn’t fix it, I’m back at vacuum leak, and nothing external is showing signs of a leak so my next place to look is inside the throttle body. For laughs I got her temp and unplugged the CTS next to the thermostat housing and no change their either. So I shall continue on with my investigation!
 

bp71k5

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this is a stock motor, with 225k on the clock. I ended up doing a new IAC, did the relearn, replaced the map sensor, now the opposite condition exists. Park neutral or gear she idles super high, tries to drag through my brake pedal at stop signs lol. I took some starting fluid to check for vacuum leaks a 3rd time, have not found anything. Already replaced the tbi gasket. Sprayed around intake manifold just in case and no change. After discussing with a friend who owns an auto shop, I tested my TPS and found it was out of spec, so I just ordered a new one. I also have decided to rebuild my throttle body since it’s got so many miles, so I ordered a rebuild seal kit as well as a 14 psi regulator spring and injector pod spacer. The thought process is it needs to be gone through anyways, and if the TPS doesn’t fix it, I’m back at vacuum leak, and nothing external is showing signs of a leak so my next place to look is inside the throttle body. For laughs I got her temp and unplugged the CTS next to the thermostat housing and no change their either. So I shall continue on with my investigation!
The TPS can be adjusted on the motor with a torx driver. Probably don’t need a new one unless the voltage doesn’t change smoothly when actuating the throttle.

If the new IAC changed the behavior that much I’d have to look at that more closely. You need to have the TPS adjusted right for the IAC to be calibrated as well so there might be something to the TPS issue.
 
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