Hi guys,

New to you, please be kind...

My buddy put a 350 TPI into his 76 Stepside. We have been diagnosing a rough idle/barely running condition. Truck won't idle, nor run without some throttle applied. It runs better at higher RPM's.

Last night, we put a Fuel pressure guage on the rail. Turn the key, pressure seems to spike around 80psi while the pump is running. Then, it settles down to the correct 42psi after the pump turns off. I applied proper idle vacuum to the Regulator, and the pressure dropped to around 33psi.

Start the truck, and the pressure bounces (non-damped pressure gauge) around 80psi. At this point the Mityvac is still hooked up to the regulator. Running, with no pressure, horribly. I apply vacuum (slowly) to the regulator, from 0psi up to around 10psi (vacuum). This results in no change in fuel pressure (still reading around 80psi).

The regulator will hold vacuum, no problem. It does not leak while vacuum is applied.

We pulled the return line. Fuel comes out when the pump is priming. We blew air through the return line to the tank, and are convinced that it is flowing good.

I think the mechanism inside the regulator may be stuck, is this possible? Is there a schematic of the regulator mechanism that can be found?

Any help would be much appreciated. We've been chasing this issue for about 4 different shop nights (finally got a fuel pressure gauage yesterday). All other parameters have been checked to be found okay (timing, spark, etc...), and no consistent codes have been thrown (though, since first start-up, we have received 1 code 34, and 1 code 44, but they have never repeated).

Thanks,

Parsko

 

When I installed mine, it came with a defective IAC. I chased my tail for weeks on it. You didnt mention if you checked it, or if it was a new item.

 

Dale,

yes, that is one thing we checked. There was no difference between in how the engine was running with the cable unplugged, versus plugged. I will admit, we never removed and checked, but my impression was that this would not cause this type of problem.

Also, when running last night, the TB vacuum went in the opposite way one would expect. Normally, one would see high vacuum at idle, and it would decrease when the throttle is opened. What we saw was no vacuum at part throttle, and the vacuum increased as the throttle increased...

-Parsko

 

just replace it>>>>??<>><(((**

regulator that is

 

Got it. My buddy routed the fuel return line into the vent on the gas tank. Once we put it in the ACTUAL return port, it has run fine. Whew!!!! Such a hard thing to diagnose. We spent many sessions finding that one!

Thanks for all the help, everyone.

-Parsko