My 89 Camaro RS suffers from a severe loss of power once it has been warm for a bit. It's a gradual thing. When I start the car, it runs properly for a while throughout the entire throttle range. But after a while it starts to stumble when I go full throttle. I keep driving and the symptom gradually moves to half throttle then anything above 1/4 throttle etc, untill tonight when I could barely give any throttle without it loosing all of its guts.

When I give it gas and it starts stumbling then let off the throttle it has power again at any throttle position less than the point that it starts stumbling.

The car sat for about 7-8 years, and I just got it running a couple weeks ago. The old fuel pump hose was rotted so I put in a new Walbro 255, and fuel filter.

I don't have a fuel pressure gauge but am ordering one.

It's giving a code 44 meaning lean condition, so I'm thinking that it's starving for fuel, but why would it only do it when it is warm and progressively get worse?

I put a new O2 sensor in last night. Symptoms did not change at all.

Could the Walbro 255 be too big for the 305 TBI and too much fuel flowing through the bypass making bubbles in the tank?

Did I get a crappy fuel pump and it's dying already, and only showing symptoms once it's warmed up?

The fuel pump is very loud, I can clearly hear it running either in the car or standing outside.

Thank your for any suggestions,

Myles

 

Definitely sounds like the engine is losing fuel pressure. Code 44 is a lean O2 sensor malfunction.

There is a good possibility that the hose between the pump and sender split. Otherwise a 255 LPH pump will easily overwhelm the stock TBI unit return passages driving the fuel pressure through the ceiling.

RBob.

 

Thanks for your response RBob

I used the fuel line that came with the pump and it's been in the car for a couple months, but I've only been driving it for 2.5 weeks.

I put the 255lph pump in with the intention of having it work with future motor upgrades (ie 350). I was under the impression that it was common for people to use the 255lph pump with higher HP motors. Do people normally have to modify the return passages?

I filled the gas tank today (only took 8 gallons), and could not repeat these symptoms on my way to work. After driving home and another trip this evening it would just start to stumble a tiny bit at full throttle but seemed to clear itself up if I kept the throttle open for a few seconds. The symptoms are definitely related to the fuel tank level.

I guess I'll have to drop the tank on my next day off. Looking at pictures it seems that the return dumps the fuel right above the pump. If I can't find a split in the hose then the only other two options I could see would be to go to a smaller pump (seems counter intuitive when talking about a lean condition) or putting a piece of flexible hose on the return line that will direct the return to the bottom of the tank in a corner.

I've ordered a fuel pressure gauge and adapter, but I think it will take a couple weeks to get here. I guess I could just top up the tank every couple days to get by until then.

 

With the symptoms abating with a full tank, that points to the hose being split. Can check this once the fuel level is lowered, jump the pump to run it with the engine off, remove the gas cap and listen at the filler neck.

Should be able to hear the fuel raining down into the tank.

The folks that go with the 255 LPH pump are usually running a port injection system. Being run at a higher pressure reduces the fuel flow from the pump. Along with having larger return passages in the fuel rails versus the ones in a TBI unit.

The only time I've seen a 255 LPH pump work on a TBI set up is when the fuel pressure is North of 25 psi and an external FPR is used. Otherwise a 190 LPH pump or a stock Bosch brand replacement TPI pump is recommended.

RBob.

 

Well a little update tonight.

Topped up the tank on my way home yesterday, and today on my way home, still reading full and the loss of power got as bad as ever, so my tank level theory seems to be false. But it still only happens once the car has warmed up. I have totally normal use of my throttle when it is cold.

Came home and pulled the tank and sending unit (only took 1.5 hours). The rubber hose between the pump and sending unit is fine. I took it off and carefully inspected it and there are no cracks.







I took the filter off and blew through it, very little resistance.

I also blew through all the vent/return lines and they all flowed freely.

The little white filter on the pump was slightly discolored but would not restrict flow.

I'm still waiting on my fuel pressure adapter from RV Morse.

I went back and found where I got the idea for the 255lph pump. It came from the budget TBI build sticky at the top of the TBI section of this forum. Second post in:


I hate putting the tank back in without having fixed anything.

Any more suggestions on what to check are very welcome.

 

Swapped the fuel pressure regulator with one I had left over from a 350 truck motor. Didn't fix it.

I also disconnected the battery to erase the codes before I test drove it. I was able to make it stumble after a 30 minute drive of lugging it around in fifth gear at 30mph. When I checked the codes there are none stored. Maybe I just didn't make it stumble long enough. On Austin thirdgen code 44 says "Oxygen sensor voltage was under 0.25 volts for 50 seconds of closed loop operation". So there's a chance I just didn't keep my foot on it long enough when it was stumbling.

It's weird because the car has normal power until it warms up. I always takes 10+ minutes before I have any issues and then it gets progressively worse. Also sitting in traffic makes it way worse, on my way to work with little traffic and normal driving speeds it just barely hesitates at the top end. On my way home, stop and go traffic for 15-20 minutes and the car is barely drive-able by the time I get half way home.

 

Yesterday I pulled the thermostat and jumped the fan switch so it would stay on the whole time to see if the car would run better if it didn't warm up. No such luck, the problem still persisted.

Got by today with no stumbling by preemptively tuning the ignition off when I was sitting in traffic and a couple times while coasting down hills. Seems to point to a cause of either time that it is running or it just hates sitting in traffic. Normally if I turn the motor off and try to restart it right away after it has started running poorly it is hard to start and the problem persists, but a few minutes rest and it's back to normal.

I installed my fuel pressure gauge tonight and it said the walbro 255 was putting out 17 psi at idle and when I revved it, it would drop and hold at 12 psi. I guess the fpr is not capable of dumping enough fuel at idle, although it idles smoothly at 800-1000 rpm.

I will try to induce the bad running symptoms tomorrow on my way home from work and see if the fuel pressure is different.

 

Installed the fuel pressure gauge last thursday and haven't had the loss of power symptoms until my way home today. I pulled over and kept the car running, checked the pressure and sure enough it said 11 psi at idle but when I revved it it dropped to 2 psi. I shut the car off and let it sit for a minute then started it back up and drove the rest of the way home with no symptoms.

I wish I knew why this problem is happening. I have an AC Delco replacement TBI pump sitting here that I will put in tomorrow night.

I put a deposit on a vortec 350 out of a Tahoe. Will the TBI pump be enough to fuel it?

 

Buy a Ac delco replacement pump #EP241. This is a standard upgrade for stock and performance TBI as has been discussed many times over the past 12+ years on this board...

 

Thank you for saving me from installing the stock replacement tonight. Somehow I missed this part number as a good replacement. I called my part store and they will have an EP241 that I can swap for my stock replacement tomorrow. I should be able to install it later this week.

 

In case this thread shows up when someone searches here's my update.

I got by for the last couple months by shutting off the ignition for 5-10 seconds at a time when I could coast down a hill, or sitting at a traffic light. My best guess is that so much fuel being returned to the tank must have been causing turbulence/cavitation at or in the fuel pump and the brief shut shutdowns were enough to allow the pump to prime itself again.

I finally got a chance to replace the Walbro 255 with an EP241 a couple days ago and the car has been running normally since.

Thanks to everyone who made suggestions!

 

I think the confusion on my Walbro 255 recomendation is that I never run stock fuel pressure on my TBI builds. Most of my stuff runs 26-34 psi of fuel pressure. I have heard that Walbro's quality has tanked as of late. My last GM fuel pump upgrade was an EP381 in a TBI 350 truck.