2003 Tahoe 5.3 Flex Fuel Surging at 2000 RPM uphill

So here is my issue. My engine light came on about one month ago while I was traveling about 45mph up a hill around 2000 rpms. The rpms would hit 2000 and then bounce back down to 1500 rpms. This happened about 3 or 4 times and then the engine light went on. I gave it more gas and it went fine over 2000 rpms and took off up the hill. I checked the code and it came back to a knock sensor. I replaced both knock sensors and intake gaskets. I cleared the code and it has not come back on. However.... I am having the same issue with the rpms surging at about 2000 rpms up hill. It will bounce if I leave the throttle in place, but the engine light will not come back on. The engine idols perfect and drives perfect other than the surge. It seems to get crappy gas mileage as well. I pulled about an 8000lbs trailer while moving, but it never surged or lost power. I have checked and cleaned the MAF and air filter (K&N). I have also replaced the fuel filter. I really don't want to throw any more parts at this thing, especially replacing the dreaded fuel pump. I don't think it is the fuel pump because it only happens at a specific time and only when the engine is warm. I am about to sale this thing and buy a Prius...lol. Just kidding...

so with all the fuel pump issues with these vehicles you "THINK" its not the fuel pump.

these forums have much info about all this ! much info !


no guessing just get a fuel pressure guage and measure the fuel pressure.

If its below 53psi engine running replace it.

that would not be throwing parts at IT . that would be logical troubleshooting steps to determine the engine issues of power loss.

with 170,ooomi or running this type vehicle with low fuel levels this would be what to expect to fail.

Thanks jcat, you sure like to make a guy feel SPECIAL! I failed to mention the pressure was checked by dealership. They said it was fine, but wanted to charge me a but load of more money to diagnose the actual problem. Thought I would try here first, but can't find any posts exactly like mine.

Thanks jcat, you sure like to make a guy feel SPECIAL! I failed to mention the pressure was checked by dealership. They said it was fine, but wanted to charge me a but load of more money to diagnose the actual problem. Thought I would try here first, but can't find any posts exactly like mine.
You now tell us.:rofl:
I failed to mention the pressure was checked by dealership.
But did they check it on the road when acting up?:grinno:

As Jcat said get a fuel pressure gauge on it.
Tape gauge to outside windshield or outside mirrow.
Drive it untill it acts up and check fuel pressure when it is acting up.
If low replace pump.

Thanks jcat, you sure like to make a guy feel SPECIAL! I failed to mention the pressure was checked by dealership. They said it was fine, but wanted to charge me a but load of more money to diagnose the actual problem. Thought I would try here first, but can't find any posts exactly like mine.

the dealership is trying to crank up this repair cost.

do not have a dealership repair a vehicle this old. they will try to get you to purchase a new vehicle by jacking up the repair cost. they make money selling cars not repairing old obsolete vehicles.

fuel pressure testing is no big deal. I would call an autozone near you to see if they have a pressure gauge to use /help you hook it up . takes about 5min total to do the checking.

my guess is you are not into working vehicles if so perhaps you should get a new vehicle.

you did come to the right place to get info. the info I give is test then replace. vehicle repair is much different today than back in the 60's.

TESTING IS REQUIRED. also you need a repair manual to do the tests and know how you vehicles maintenance should be performed.

So here is my issue. My engine light came on about one month ago while I was traveling about 45mph up a hill around 2000 rpms. The rpms would hit 2000 and then bounce back down to 1500 rpms. This happened about 3 or 4 times and then the engine light went on. I gave it more gas and it went fine over 2000 rpms and took off up the hill. I checked the code and it came back to a knock sensor. I replaced both knock sensors and intake gaskets. I cleared the code and it has not come back on. However.... I am having the same issue with the rpms surging at about 2000 rpms up hill. It will bounce if I leave the throttle in place, but the engine light will not come back on. The engine idols perfect and drives perfect other than the surge. It seems to get crappy gas mileage as well. I pulled about an 8000lbs trailer while moving, but it never surged or lost power. I have checked and cleaned the MAF and air filter (K&N). I have also replaced the fuel filter. I really don't want to throw any more parts at this thing, especially replacing the dreaded fuel pump. I don't think it is the fuel pump because it only happens at a specific time and only when the engine is warm. I am about to sale this thing and buy a Prius...lol. Just kidding...

the knock sensor failure is common when the engine is cleaned. care must be taken not to get any water under the upper intake manifold. this is how this occurs as these sensors get corroded.

the foam filler fabricated by GM is a design failure. should have been a solid water proof flexable material. I used dow 795 sealer this sticks well to metal and can handle the temps at that location .

rodents do not like eating it, like that GM foam material.

Jcat, thanks for your feedback and sharing your knowledge. I did pressure wash the engine and the knock sensor went out shortly after. I replaced both and used sealant to prevent future problems on both sensors. I took your advice and went to Autozone to check the fuel pressure myself. I am somewhat mechanically inclined, but managed to get fuel all over me while trying to get this done (don't push pressure release valve button while hooked up and engine is running). The first attempt showed pressure solid around 45 psi. I thought it maybe because there was air in the line, so I pushed the release button and all hell broke loose in the Autozone parking lot. Luckily me, my truck, and Autozone didn't go up in flames. The second attempt I did take video and published on YouTube. http://youtu.be/nk7NIPS0uvI The pressure gauge was freaking out around 40-47 psi. So, after I go kick the technician in the balls who did my fuel pressure test at the dealership, I am going to need to change my fuel pump after all. I watched a video on this, and it looks kinda hard. Also, Autozone has an OEM (they claim) fuel pump for $210 US Currency. I will search online in the mean time, but does anyone have any suggestions on fuel pumps for this truck.

well it is what I suspected ! the dealership was trying to screw you out of your hard earned money . needing more testing at the dealership means get the anal lube out.


the tahoe fuel tank drop sucks. If you had fuel squirting all over you best get someone who knows what the proceedure is.

I don't want you to get burned up!

on pumps using your vin # you can get the correct ac delco or delphi fuel pump for around 200.oo....

with the pressure around 50 psi this is how the engine runs. sometimes just enough, sometimes not. shortly this will fall more and you will be stuck.

siphon the fuel out into a 5 gallon container. then place in your other vehicle. when done replacing the pump install the 5 gallons in the tank and see how the pressures look.

with the fuel at 1/8 indication the tank should be 50 lbs. 8lbs/gallon.

the fuel pump is re-designed so the connectors to the pump assy will need be soldered and sealed to adapt to the new style pump. unless this was done before .

DO NOT USE AIRTEX EVEN IF IT IS OFFERED FOR FREE !

After reviewing everything online and reading your post, I guess AC Delco would be the way to go. Autozone will get me one tomorrow morning for $170. They did offer the Airtex at $210 and said it was OEM and in stock now. I will give it a whirl and post the outcome tomorrow. I don't think I will get fuel all over me this time, just never used a fuel pressure gauge before. Thanks for your informative replies.

After reviewing everything online and reading your post, I guess AC Delco would be the way to go. Autozone will get me one tomorrow morning for $170. They did offer the Airtex at $210 and said it was OEM and in stock now. I will give it a whirl and post the outcome tomorrow. I don't think I will get fuel all over me this time, just never used a fuel pressure gauge before. Thanks for your informative replies.

post back how it you made out.

you may need the FUEL quick disconnect release tools. 5/16/ 3/8 INCH

Well, that was a fun way to spend my Friday morning. I decided to drop the tank early this morning so when my part came in at 11:00am at Autozone, I would be all ready to put it in. First, I tried to siphon the gas out of my tank and put it in my other car, but there was a screen on the tank that would not let me. So, I flexed my muscles and said screw it, lets do this with just over a half tank. This is not a good idea if you can drive and burn it all, but if your pump dies, you really have no choice.

I tried to disconnect the black evap flex fuel plastic box thingy, and that did not work out to well. The bolts were easy, but the three lines sucked. There was not enough room for the quick connect tool on the two little lines. I decided to just remove the bolts and leave everything connected since it looked like it would stretch far enough to get the tank out from under the truck for the pump removal. I used jack stands to hold up the tank and put my oil pan underneath to catch fuel if it came out, and boy did it come out. I disconnected the fill line and vent line in the back of the tank. and removed the two metal straps holding the tank. Well, I started to let down the back of the tank and the fuel started pouring out the back (of course). At least the pan caught most of the fuel, but it was filling up fast, so I hurried and grabbed some rubber bands and two plastic sandwich bags and wrapped them around the two holes. This was about the only smart thing I did. I would recommend doing this before letting down the tank. ;)

I then reached up front of the tank and was able to get one fuel line off by just pinching the plastic clip. The other line was similar to the evap black box lines and called for the quick disconnect. Fortunately this one I could easily get with the tool. I was now ready to drop the back of the tank and remove the fuel pump.

As I lowered the back of the tank, I was trying to keep an eye on the evap lines I left connected. All was good until I started pulling the tank out from under the truck on the driver side. I heard the horrifying snap of plastic.... I managed to snap the plastic nipple off the evap black box :( Hoping the line would stretch didn't work out very well. I will now probably need to buy a new black evap box, but super glue would have to do for the time being.

I then disconnected the wiring harness. That was a pain since they were rotted with brittle plastic. I was able to get the fuel pump out fairly easy (I would recommend cleaning the top of the tank before removing the old pump to refrain from getting crap in the gas tank). I then cleaned the rest of the tank and went and picked up my new AC Delco fuel pump (J Cat's recommendation). I get to Autozone and they ring it up at $270???? I was sooooo freaking ticked off. After arguing for a few minutes because I was quoted $170, I handed over my credit card (makes me mad just typing about it now because I could have got this same pump online for $160).

Anyway, I went home and dropped the new pump in. I could not get the ring locked on for the life of me. I lubed the crap out of the gasket, but still no luck. I then had a bright idea and slid the tank back under the truck. I lined up the top of the pump with the frame of the truck. I used a metal spacer and my Tahoe's jack to press the fuel pump down while I twisted on the ring. It quickly went on with the help of channel locks. I then put the tank back up the way I got it out. I got to the black evap box for the last challenge. I tried to repair the nipple with JB weld, but it will not hold for very long, so I will need to drop more money on this thing and buy a new evap box, or whatever it is called. I decided to go test drive the truck with the broken evap box (not sure if this is safe, but I couldn't wait to see if the pump worked and no leaks).

I primed the pump and lines by just turning the key to the on position twice. The engine fired up and runs good. I took it for a test spin and have no surging. The truck actually feels allot more responsive then before. I can feel the power increase when going up hills.

A big shout out to Jcat and your mountain of knowledge. If I learned one thing out of this big pain in the butt, it would be to never trust a dealership. Befriend a mechanic if you really want honest answers. Now all I need to do is buy another evap box. Anyone have an idea what the part name for this is actually called?

The evap box would be the charcoal cannister. this has 3 ports for containing the vapors. when the engine is started this part is purged and the fuel tank is tested by way of a pressure sensor to report to the computer that you have no vapor leaks.

I am not aware of any screen in the fuel filler hose that would prevent siphoning the fuel out. this is basically a small hose 1.25in dia in a larger 2.75 in dia hose.

lowering the tank with 12 gallons in it is too heavy for one person.

the cannister connects come off fairly easy . using some lube on the fittings and the tool the come off . on the tahoe the tank has to be lowered a little to get at them so using a jack or blocking wood to gradually drop the tank is best.


you could have blocked the tank, droped the straps, then jack up the vehicle to get at all those lines/connectors.
to do that you would need some large bottle jacks etc..

go to the scrap yard and get a cannister.

So went on a long road trip today and noticed the original problem is not gone. The rpms are dropping from 2000 to 1500 again when going up incline. It is not as frequent, but still happening. I hooked up a fuel pressure gauge with results showing 45 psi and holding. At least it's not bouncing like crazy. I don't know what to do now. I'm sure I needed a new fuel pump because I can feel the difference, however, I am discouraged the problem didn't go away. What would cause poor fuel pressure that is not the pump?

So went on a long road trip today and noticed the original problem is not gone. The rpms are dropping from 2000 to 1500 again when going up incline. It is not as frequent, but still happening. I hooked up a fuel pressure gauge with results showing 45 psi and holding. At least it's not bouncing like crazy. I don't know what to do now. I'm sure I needed a new fuel pump because I can feel the difference, however, I am discouraged the problem didn't go away. What would cause poor fuel pressure that is not the pump?

the fuel pressure regulator could cause a lower pressure. this is possible . since this is 10yrs old it is a good component to replace .If you restrict the return line at the engine [squeeze it] see if the pump pressure rises to 80psi. do this for a very short time.

replacing the fuel pump relay is what I would do whenever replacing the pump . also make sure the pump is grounded secured and this frame rail connection is clean and secure. bad ground will cause the pump to run slower , lower pressure. connections at the pump usually the splices. these should be soldered and heat shrunk sealed.


what was the pressure when you installed the pump ? before you put everything back always check the pressures.

IT COULD BE A BAD PUMP. this would not be the first time .

the pressure has to be 53psi to 57psi with the engine running.


disconnect the vacuum line to the regulator. see if the pressure rises to 60-65psi. see if the regulator vacuum line has fuel in it.

Sorry, it has taken so long to re-post on this thread. I was out of town for a few days. So, Jcat, I did some more diagnostics with the fuel pressure gauge while driving. I did tape the gauge to the window and went for a drive so you can see what I am seeing and see if you can shed any light no this. The fuel pressure is holding solid at 45 psi in the on position and while running. I unplugged the fuel pressure regulator line and there was no fuel in the hose. The PSI did go up to 52 psi while running. My brother has the same motor, but on a 2005 GMC 2500. His fuel pressure is at 63 psi while running, but he does not have a flex fuel like I have. Someone mentioned in another forum that the fuel pressure should be between 42 psi and 48 psi with flex fuel engines. I do not have a Chilton to confirm this spec. In the video's the fuel pressure will only move when I step on the gas or take my foot off the gas pedal. It will not move when my rpm's bounce at 1800 and 2000. So, I took three video's while driving, and here they are. Please help...
Part 1- http://youtu.be/8_P9-zY2XuM
Part 2- http://youtu.be/qVagof6rEoI
Part 3- http://youtu.be/y65zDlvmH3U

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...

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.

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.

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 ?

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.

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.

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.

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 ...