Fuel Pressure Drop at 3k only?

[355chev]

Hey guys,

Haven't posted in a while. Here is the problem. My fuel pressure stays steady at 28-29 psi. Under WOT from dead stop stays at 28psi, then at 3k rpm it drops off down to 15 psi, truck falls on its face then in a half a second time it comes back alive, fuel pressure goes back up to 28psi. This is on a full tank of gas. What I found out though it only does the fuel pressure drop from a dead stop WOT. When you're rolling along at 5 mph or so and up, then stomp it its fine doesn't fall on it face at 3k. I hooked fuel pressure gauge up and routed it to right in front of windshield to I could watch it drop so its not tuning problems in my opinion. Any help would be much appreciated!
Here is a video, its hard to see gauge but you can hear it happen and watch needle go up and down.

Thanks
Max

[steveo]

in order of likelyhood,

-dead pump
-improper fuel pickup height or location causing insane fuel slosh (cavitation)
-clogged fuel filter
-clogged sock for the pump
-pinched or clogged fuel line
-shoddy or insufficient pump wiring or relay
-regulator that leaks down heavily but only when it's under very low vacuum (pretty unlikely)

[brian617]

Check the plastic baffle around the fuel pump in the tank. Mine came loose in my truck and was slamming back and fourth against the pump.

[lionelhutz]

That sounds too consistent to be a mechanical problem. I'd hook a test light to the fuel pump power and make sure its not cutting out.

[355chev]

The pump is brand new as well as sock and fuel filter. Had the box off and checked baffles, they are solid. I watched fuel pump voltage on computer and its 13.4 steady if I remember right. What I don't get is that it only does it from a dead stop, and doesn't do it when you do a rolling start.

[lionelhutz]

Pump voltage on computer?

I'd actually hook a light bulb or min/max recording meter between the pump power and pump ground and make sure the power isn't cutting out.

[steveo]

The pump is brand new as well as sock and fuel filter. Had the box off and checked baffles, they are solid. I watched fuel pump voltage on computer and its 13.4 steady if I remember right. What I don't get is that it only does it from a dead stop, and doesn't do it when you do a rolling start.

that screams pump cavitation. starting from a dead stop creates the most sloshing and might just be uncovering the pickup.

was it happening before the new pump was installed?

[355chev]

I still am struggling trying to figure this out. Put a full tank of gas in and tried it and it still did the hesitations. I checked relay, wiring, ground. Today checked the little air hole in the regulator housing as well as diaphragm both are good. Anybody?

[steveo]

why dont you try straight wiring the pump right to the battery, no relay, no anything, with some decent gauge wire, see if the symptoms persist

also new pump doesn't mean good pump. perhaps even the line from the pump to the top is cracked, or you botched the install and broke something or left a hose clamp loose.

i'd say try straight wiring the pump, if that fails, you're dropping the tank.

[alan91z28]

While this may not be your case I had something very similar in my iroc in that it was running great except at wot at around 4000 rpm where it would stumble

i was tuning and logging with data master and I could watch the 02 sensor go lean at this point and stumble Sometimes when cold it would not do it but I could get it to happen pretty quick then all the time during that drive. I could run all the way to redline at part throttle no trouble

i didn't have a pressure gauge long enough for the windshield so couldn't get a good reading while operating but assumed fuel pump and it is 25 years old so I just went for it with new high flow walboro (also old one and inside of tank actually looked almost new after 100k miles and 25 yr)

changed fp and about everything else in the fuel system including Bosch 3 inj and new fpr. After several weeks of down time due to work I got it done and took it out. Thought I had it licked as running good with rich 02 in power enrich at wot to redline. After 4 or 5 of these pulls in my road test same issue back!

did some quick thinking and decided to swap the core 165 ECM as maybe the fuel inj driver overheating at high duty wot??? Bingo, my swapped in spare fixed it and iroc drives overall like new in high heat and wot pull after pull this past month with no trouble and perfect data logs.

[355chev]

Would it matter that Im running 95 TBI sending unit with a vortec pump? Also would a fuel pulsator have anything to do with drop off? I am not running one of those.

[355chev]

Cured it with new Dorman sending unit and Quick Fuel Hot Wire Kit.

[suffolkauto01]

I dont think pump. pump is not rpm controlled look elsewhere.

[355chev]

IMO the pump either works or it doesnt. I fixed it with new sending unit, dedicated grounds to frame and hot wire kit.

[suffolkauto01]

IMO the pump either works or it doesnt. I fixed it with new sending unit, dedicated grounds to frame and hot wire kit.

so it was a ground issue