91 zeee

Stock 305 TPI. If I floor it, it pulls pretty strong to about 4000 rpm. Then it dies and the rpms start to fall and it'll just stay at about 3000 rpm and wont go any further.

I have already replaced all of the injectors, fuel filter, cap, rotor, plugs, module, coil, and wires. I just replaced the fuel pump as well (because the old one was loud and I thought that might have been the problem). I put a Walbro pump in and now, 200 miles later, that has gotten super loud too.


I'd welcome any input on what to look for.

scoflaw

iTrader: (2)

Sounds like a restriction in either the feed or return line. Pumps get loud when the fuel doesn't flow freely

ASE doc

Could very well be a restricted fuel supply. A fuel pressure check at the injector pod inlet should point that out. Another common cause of power loss at higher revs under load is restricted exhaust. A quick test is to remove the O2 sensor. That gives the exhaust a place to vent. If that isn't convenient, pulling the EGR valve and plugging the side that goes to the intake also works. Best test of all is to disconnect one of the exhaust down pipes from the manifold. Only run it this way for one quick trip up the street to see if the problem is gone. You don't want cold air reverting back into the exhaust valves.

91 zeee

well, I would agree on both points, but here is some more info (and why this is driving me nuts).....when I pulled the gas tank and had it cleaned I flushed the fuel lines. I installed a new AC Delco filter. Prior to pulling the tank I had just replaced the cat converter (about 200 miles ago) with a Random Tech high flow.
So....I have fairly new injectors, have flushed the fuel lines, cleaned and flushed the tank, new fuel pump and sending unit and new filter.
Factory cast iron exhaust manifolds, new cat converter and from there a fairly new cat back exhaust. It would seem that I have flushed out or replaced anything that could be stopped up.
I keep coming back to the fuel press regulator??? is there something else I'm missing?

Tuned Performance

iTrader: (92)

Did you install back the pulse dampener when you put the pump back in ?
Southbay08 are injector vendor here does have the best deal on tpi diaphragms IIRC $48 and the ride.

91 zeee

No, I read that the Walbro pump has a gerotor design and not to use the dampner.

ASE doc

Good. the dampeners are known for leaking. You've replaced all the right parts but have you verified fuel pressure under load, more specifically when this problem is occurring? One thing that could cause a symptom something like what you're experiencing is knock retard. You would need a scan tool that can access ECM data, something like tuner pro would work. Watch knock counts and knock retard degrees as the problem occurs. Knock retard can go as high as 18 degrees and really put a motor on its butt. If you do have high knock counts, you need to pay close attention to how the motor sounds. Is it making noise like lifters or ping or anything from the bottom end? If the motor is quiet, you need to replace the knock sensor.

The scan tool will also let you see total timing advance as commanded by the ECM. You can use a timing light to verify that actual timing is the same as commanded. Even without a scan tool, you can use a timing light to verify that timing reaches about 40 degrees on a snap of the throttle. If timing starts to advance and then pulls back, that's a sign of knock retard.

Another possibility is the distributor and/or coil. If they are original, they are very old and suspect for failure. You need to visually inspect the dist for rust around the timing core and pole piece. Heavy rust can interfere with generation of the PIP signal and tends to have more of an affect at higher revs.

Make sure your cap, rotor, wires and plugs are in good shape and perform a KV test of the ignition. You can buy a cheap KV tester at the local parts store. I have one that is basically an adjustable spark tester with a clip on it so that it can be attached to a good ground. Spark plug wire attaches to the other end and you can set it anywhere from 15 Kilivolts to 40 Kilivolts. The GM HEI E-core coil should produce 40 KV and the wires, cap and rotor should be capable of delivering this to the plug. If you get no spark at the tester at 40KV, try testing at the coil wire. If you still don't get 40KV, try a different coil and repeat the test. If you get 40 at the coil wire but not at the plug wire, replace cap and rotor. Replace wires too if they're questionable.

91 zeee

I checked the fuel pressure first. goes to zero at full throttle. at idle it holds at about 30 psi. brand new walbro pump, flushed the fuel lines, new filter. could the fuel press regulator cause this?

ASE doc

To check the regulator, pinch off the return hose and see if the pressure holds. The regulator shouldn't dump all of the pressure, but if it's wide open maybe. At least you're on the right track now. If pressure still drops with the return blocked, do a free flow test of the flow test of the pump by disconnecting the supply line and placing it in a bucket. Run the pump, using the prime connector, for 30 seconds and verify that it pumps at least 1 quart in 30 seconds. More importantly, make sure that flow doesn't drop off during the test. The pump needs to flow as much at the end of 30 seconds as it did at the beginning. If not, suspect the rubber sections of the line of collapsing, at the tank end and at the engine end.

GohanMint15

Soooooo what happened?

91 zeee

Sorry, its been a while. No answer yet. I did what ASE doc said with the return line. It holds. I replaced both rubber fuel lines up front. Same problem. I did not do the short rubber line at the tank; but I will when I drop the tank. I have another fuel pump and am going to change that in the next week or so. I will post back when I find the problem. It is not a daily driver (and actually runs and is driveable) so I haven't had to get it fixed quick. Too many other projects.....