Loss of power

ltbbfd · June 15, 2005

Hi, I have a 1999 chevy K1500 5.7 litre with approx. 88000 miles on it. The problem I'm having is that under a load such as when I am pulling a trailer and climbing a hill the truck begins to lose power and and vibrate and shake. The check engine light comes on and reads a misfire code according to my mechanic. The truck runs fine on the highway with no load and the fuel pump I'm told is putting out the right pressure. I would appreciate any thoughts on this my mechanic is at a loss. Thank you very much

GM-Tech · June 15, 2005

I would suspect ignition.... plug, wire, etc.

71GMC_3/4T · June 15, 2005

Pressure can be good and the flow be too low. Can they check the flow rate? Maybe your tank pickup tube is partially clogged.

ltbbfd · June 16, 2005

Pressure can be good and the flow be too low. Can they check the flow rate? Maybe your tank pickup tube is partially clogged.

<{POST_SNAPBACK}>

Thanks for the replies. As far as the wires and other ignition componants they are all new within the last 6 months. My mechanic and I discussed the very issue you mention about the flow today. He said the pressure is good even under a load but that there could be a possible flow issue. The truck is due for its yearly fuel filter change and we will keep on it for now. Thanks for the input

GM-Tech · June 16, 2005

I've never bought the, "pressure is good but there's not enough flow" theory.

Perhaps some mechanical/hydraulic engineer can chime in here if they see this.

71GMC_3/4T · June 16, 2005

I'm not a mech or hydro engineer, just an EE.

However, I'm fairly mech inclined, so I'll try to explain it to you.

Take a garden hose. Put a restriction in the hose. Now put your sprayer on the end of it and turn on the water. The hose will pressurize to city water pressure of 50 psi +/-. When you first pull the trigger on the sprayer it will blast out of there at 50 psi, but then it will slow to a trickle (if your restriction is small). If you let off the trigger, the pressure will build back up.

Now back to the truck. When you put a pressure gauge on it, you are only measuring the pressure at no flow. It might be able to build that pressure if the pump is good. However, if the pickup sock is nearly plugged, it can't maintain that pressure at any reasonable flow rate and the pressure will drop out. To check the flow, you'd want to see how fast it can fill up a bucket. If the pickup tube is restricted, you won't fill that bucket very fast.

Make sense?

GM-Tech · June 16, 2005

When you first pull the trigger on the sprayer it will blast out of there at 50 psi, but then it will slow to a trickle (if your restriction is small). If you let off the trigger, the pressure will build back up.

What you said is that one can have good pressure and low flow. That would presumably mean AT THE SAME TIME.

I understand that if one checks fuel pressure static, pressure could be good, but when you drive and fuel is being injected, returned to the tank, etc., flow might not be able to keep up.. and pressure would drop.

If we have doubts, we drive with the pressure gauge taped to the windshield to see real time pressure.

Stump Jumper · September 28, 2006

When you first pull the trigger on the sprayer it will blast out of there at 50 psi, but then it will slow to a trickle (if your restriction is small). If you let off the trigger, the pressure will build back up.

What you said is that one can have good pressure and low flow. That would presumably mean AT THE SAME TIME.

I understand that if one checks fuel pressure static, pressure could be good, but when you drive and fuel is being injected, returned to the tank, etc., flow might not be able to keep up.. and pressure would drop.

If we have doubts, we drive with the pressure gauge taped to the windshield to see real time pressure.

<{POST_SNAPBACK}>

Fluid flow rate at the individual poppets is impossible to be sure of. Pressure readings indicated with the gauge only indicate static pressure as you noted. Not flow.
You are correct in assuming that the flow of fuel through the injectors creates the drop in fuel pressure. However it does not accuratly indicate if it is flowing simularly to all poppets. Only testing them individually will find a problem. If your test gauge could be placed between the injector and poppet....then you'ld have something. The only other indicator that is easy is the tried and true "what color is the plug"(lean or rich).