2003 Tahoe 5.3 Flex Fuel Surging at 2000 RPM uphill

[miverson332]

According to this website: http://www.lt1swap.com/identify_vortec_engine.htm It states, "5.3L Flex Fuel engine 2004 and back, 48-54 psi with vacuum line disconnected, fuel pump running." My fuel pressure goes up to 52 psi with vacuum line disconnected.

"25326903 - 5.3L L59 Vin Z, flex fuel 5.3L - flows 4.164 gm/sec or 33lb/hr @ 50 psi"

This site never says what the pressure should be at when the vacuum hose is connected??? I wish my engine would throw a trouble code to help me diagnose this problem. I am about to change the plugs and see if that helps...

[j cAT]

I just looked up the vin Z 2002-2004 flex fuel 5.3L fuel spec.

key on engine off the pressure should be 56-58psi. engine on pressure must be above 48psi.

remove the vac line should go up to 56-58 psi.

If the pressure is not between 48-54psi engine running ,then I would next replace the regulator. the regulator is returning too much fuel.

not sure what fuel pump , for the E85 fuel. many numbers...

check that the regulator is for the vin Z engine when you purchase it.

replace the plugs with the ac delco irridium plugs.

do not attempt to gap plugs ..if the gap looks off they are defective so check plugs out before leaving the store.

this flex 5.3L is rare around here. fuel pumps and these regulators with the E85 is hard on these parts.

[miverson332]

Well, replaced plugs with ac delco iridium plugs today while I wait for the fpr to come in. The tahoe's engine is running much quieter and way smoother than before. The old plugs were autolite iridium plugs that appeared to be in decent shape, but they were very worn and has some brown on the porcelain. I took it for a test run, but the rpm loss of power is still there when going uphill. I will post again once the new fpr is installed. Thanks again jcat!! Hope this thread helps someone in the future.

[j cAT]

Originally Posted by miverson332

Well, replaced plugs with ac delco iridium plugs today while I wait for the fpr to come in. The tahoe's engine is running much quieter and way smoother than before. The old plugs were autolite iridium plugs that appeared to be in decent shape, but they were very worn and has some brown on the porcelain. I took it for a test run, but the rpm loss of power is still there when going uphill. I will post again once the new fpr is installed. Thanks again jcat!! Hope this thread helps someone in the future.

what is the pump part number ?

[miverson332]

Fuel pump- AC Delco Part Number: MU1378
Fuel pressure regulator- GP Sorensen part Number: 800-399
Just picked up the fpr and will be installing later today.

[miverson332]

I put in the new fpr and the original problem still exists. If it is the fuel pressure that is causing the engine to fail, then I am not sure where to go from here? I can check fuel pressure before the filter, check the wiring harness, but it sounds like I may need to take it to a local shop. I did most of the diagnosing myself, so that should save some money and time.

[j cAT]

Originally Posted by miverson332

I put in the new fpr and the original problem still exists. If it is the fuel pressure that is causing the engine to fail, then I am not sure where to go from here? I can check fuel pressure before the filter, check the wiring harness, but it sounds like I may need to take it to a local shop. I did most of the diagnosing myself, so that should save some money and time.

when you started this you had fuel pressure jumping 40-47 psi engine running. then the fuel pump replaced and the pressure increased and was steady around 45 psi.

with the fpr replaced what is the pressure ?

I mentioned before at the fuel rail area squeeze the return line with the engine running and the pressure Gage connected see if it surges up to 80 psi area.

or connect to the fuel pump filter the pressure gage and ignition on engine off see how high it goes and if it holds pressure. should go high .75-90psi.

[j cAT]

explain the surge again.

going up a hill @45MPH is at the shift point of OD 4th gear. this would cause the transmission to down shift. this all is up to the hill grade and how you throttle the engine.

with lower fuel pressure you would have lower power to keep the vehicle moving at the same speed so more throttle than normal would be required this would then cause the tranny to down shift more often.

how many miles on the last transmission fluid/filter change ? did you use the new GM approved fluid DEXRON VI ? DEXRON III is not the approved fluid anymore. DEXRON VI is a synthetic blend ...