I haven't seen that before. What you can try is to find the two rubber fuel hoses where they cross over from the body to the engine, then, with the engine running, use a hose pinch-off pliers to pinch the smaller hose. That's the return hose for fuel going back to the tank after it squeezed through the spring-loaded pressure regulator. With it pinched, fuel pressure will go up to the maximum the fuel pump can develop. That could be as high as 70 psi. Don't do that any longer than necessary to see the results.
If the pressure remains steady with the hose pinched, remove the pliers, then unplug the vacuum hose from the regulator on the fuel rail, and plug that hose. No vacuum to the regulator equates to hard acceleration. There's two forces acting on a molecule of gas as it leaves the tip of an injector. Manifold vacuum is pulling on it and fuel pressure is pushing on it. Low vacuum during hard acceleration reduces one of those forces, so the regulator adjusts to make fuel pressure higher. As far as that molecule of gas is concerned, the total of the forces acting on it remains constant. You should see fuel pressure go up again. If pressure is smooth or steady, I'd question what is happening with that vacuum source. If pressure is still bouncing around wildly, but it wasn't with the return hose pinched, it would suggest the valve in the regulator is sticking. That would really be unusual, and I'd like to know more about that defect. GM did have a big problem with their regulators leaking fuel into the vacuum hose, but other than that, they have been very trouble-free on all brands and engines.
This might also be an issue with the gauge itself. Most pressure gauges have some sort of damping control to smooth out those confusing pulses. Often that is in the form of a restriction in the hose or the inlet port to the gauge, so it takes a fraction of a second for the gauge to respond to pressure or vacuum changes. That restriction might be missing, but for what we're looking for, that isn't a concern. We want to see if fuel pressure drops when a running problem occurs.
By the way, you will see fuel pressure drop noticeably when you're coasting from highway speed. Opposite as before, when you're coasting, manifold vacuum goes way up and that pulls the gas out of the injector harder. That by itself would result in a very rich coast-down condition. The vacuum applied to the regulator tugs on the spring-loaded valve to help it open sooner to maintain a lower pressure. Again, the total of the two forces remain constant. What took me a while to understand with my van was since the regulator was holding a lower pressure, it was much easier for the gas to push through the valve and go back into the tank. That is why volume goes way up during coasting, and if that volume can't get through the strainer in the tank, vacuum develops between the strainer and the pump. That vacuum offsets some of the pressure the pump is trying to develop. That is what results in the pressure on the gauge dropping too low during coasting if the strainer is plugged.
Tuesday, March 23rd, 2021 AT 9:13 PM
