4.9L Flat Spot Near WOT and Integrator Shoots Above 150 (Page 1/2)
Warlordsix OCT 28, 07:56 AM
OK, guys, I've encountered a tuning issue. Engine runs great, but when I mash the pedal to near WOT the power flattens a little and my integrator shoots above 150 (as seen real-time on my ALDLdroid on the dash). Take my foot out of it, and integrator drops back down to its typical range and power returns to normal. I'm suspecting the fuel pump. Pressure at the rail is around 32 psi at idle (which is low) and has been since I installed a brand new Delco EP378 a few months ago. Installed a brand new FPR, sock and fuel filter when I did the pump. Pulled the fuel filter yesterday, and she was clean as a whistle.

My thoughts are that when the integrator shoots high the ECU is trying to add fuel to compensate for a sudden lean condition....which leads me to suspect the fuel pump isn't keeping up to a sudden demand. BTW if I roll the gas on instead of mashing the pedal I don't feel the flat spot, and integrator remains normal.

I'm tempted to pull the Delco EP378 and install a Delco EP381.

Thoughts? Suggestions?

Thanks.

Ernie

------------------
'87 Fiero 4.9 5-speed

Will OCT 28, 08:03 AM
Are you running into a power enrichment tuning issue?

Can you put a fuel pressure gauge somewhere a helper in the passenger seat could read it while you do this?
Warlordsix OCT 28, 08:24 AM
That's a good suggestion, but right now my fuel pressure gauge is hard-mounted to the rail. Best I can do for the moment is watch that gauge while I twist the throttle cam on the throttle body....which I will do next.

Ernie
Dennis LaGrua OCT 28, 10:15 AM
Sounds like the fuel pump but that can be easily found by monitoring fuel pressure. A reading of 32 psi of fuel pressure at idle should indicate a defective pump or fuel pressure regulator. Even if you used a new AC Delco pump they have been heavily counterfeited. AC Delco has also shifted production of these to Communist China so the quality may not be quite as good. The last item is the hose that connects the pump to the fuel line. It should be J30R10 rated submersible fuel line. Standard fuel injection hose will not hold up.

------------------
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

Warlordsix OCT 28, 10:45 AM
Thanks, Dennis, yeah the hose i used is submersible and came with the pump. I agree with you on the likely defective pump, but do you think I should just upgrade to the EP381 while I have the tank out? That should supply more than enough pressure for this engine, so long as the new pump isn't defective like the old one. Any downside to running the higher pressure EP381? I can't think of any, except maybe a little more current draw.

Ernie

------------------
'87 Fiero 4.9 5-speed

pmbrunelle OCT 28, 11:05 AM
A few things you can try:
Camera under the hood to record the gauge
Extend the gauge with a flexible hose to bring it to the passenger window area (visible). Keep the gauge and hose outside, so as not to flood the cabin with fuel in case of a problem
Check on the ALDL if the TPS is reading as it should. If the ECM doesn't see sudden throttle movements, it may not be giving the engine acceleration enrichment when it needs it.


quote
Originally posted by Warlordsix:
Any downside to running the higher pressure EP381? I can't think of any, except maybe a little more current draw.



The fuel pump doesn't determine the fuel pressure; that's the job of the pressure regulator.

Potential downside (not sure how likely this is): your existing pressure regulator is incapable of returning enough fuel to the tank, causing fuel pressure to rise above the setpoint. You'll see this on the gauge if this happens. Someone having experience with this combo could chime in.

Darth Fiero's hot wire fuel pump mod is suggested to deal with the increased current draw of an uprated fuel pump:
http://www.gmtuners.com/fiero/hot_wire.htm
Warlordsix OCT 28, 11:44 AM
"The fuel pump doesn't determine the fuel pressure; that's the job of the pressure regulator."

Thanks, pmbrunelle. Roger...I meant increased volume.

Good suggestion to check my tps voltage when I next drive the car. I have it displayed on the ALDLdroid. Should be somewhere between 0.5 and 0.9 volts, right?

Ernie

------------------
'87 Fiero 4.9 5-speed

pmbrunelle OCT 28, 11:56 AM
Yes, something along those lines for the TPS.

For acceleration enrichment, the most important is that the value rises smoothly (without hiccups) until WOT.
Will OCT 28, 02:33 PM
Note that power enrichment and accel enrichment are two different things.
Warlordsix OCT 29, 09:08 PM
"...for the TPS. For acceleration enrichment, the most important is that the value rises smoothly (without hiccups) until WOT."

I drove the vehicle all over town today while monitoring the real-time TPS voltage on my ALDLdroid. The readings were smooth and within range. So, that reinforces my suspicion of a weak fuel pump.

Ernie

------------------
'87 Fiero 4.9 5-speed