4.3 Loses Power

[skier-24]

Hello all,
I have a 1990 Sunbird with the 4.3 175 horsepower engine. Last year it would run fine for 20 minutes or so then gradually lose rpm's until it would barely run. I worked on it with only partial success until I had to winterize it. New parts include the coil, spark plugs, points, condensor, cap, rotor, set timing, checked an reworked many wiring connections. I even found a small leak in my fuel line. Took it out Monday and still having issues. It will run fine on the trailer for hours. Tuesday I pulled the water separator/filter, took out the fuel sender to look in tank. All looked normal. I tried to remove the fuel pickup tube from the tank but could not get it out. Any ideas?

I am also having a shift issue but not worried about that right now.

 

[Maclin]

Re: 4.3 Loses Power

What you described is one symptom of the fuel tank not venting. As the fuel is drawn out it needs to be replaced with air, and if that cannot happen in good fashion then the fuel pump will vacuum lock. One thing to try when this has happened is remove the fill cap and listen for air rushing in. If that was the problem then it will run good for a slightly longer period of time.

For a test you could try running the boat from a separate portable tank (like from an outboard) by removing the line from the fuel tank and put it into the portable tank and see if that changes anything.

 

[skier-24]

Re: 4.3 Loses Power

I'll check the vent line.

Do you or anyone know how to remove the pickup tube from the tank? I heard it can collapse causing fuel starvation also.

 

[MikDee]

Re: 4.3 Loses Power

I'll check the vent line.

Do you or anyone know how to remove the pickup tube from the tank? I heard it can collapse causing fuel starvation also.

Usually, The fitting should just unscrew from the tank.

 

[skier-24]

Re: 4.3 Loses Power

The vent line is not plugged. I installed an electric fuel pump this weekend but didn't get to take it out and try it.

 

[Walt T]

Re: 4.3 Loses Power

Does it run badly when losing rpm or seems to run fine just loses power? If running badly I would tend to lean toward fuel issues. Carburetor float level is first thing I would check, then ignition module which you can check with a timing light on various plug wires see if the spark becomes irregular when it's doing it. If it runs smoothly just loses power, I'd lean toward exhaust or intake restriction somehow, Throttle plates somehow closing which I can't come up with a good way to happen but hey free and easy to check. I assume it's not overheating, verify with a little laser temp thingy from Harbor Freight. Ignition timing somehow changing, which you can check. You'll have to do these checks out on the water while it acts up. Probably should leave the beer at home.

 

[MikDee]

Re: 4.3 Loses Power

Walt T, he has no ignition module, it's points, condenser, & coil ignition.

This sounds like fuel starvation, it could be anything from a plugged anti-siphon valve, to carb issues? Check out the fuel system.

 

[skier-24]

Re: 4.3 Loses Power

When it starts losing power, it runs badly. It starts like a miss or two, then gets worse until the rpm's start gradually dropping. I was cruising about 3500rpm's the last time it acted up.

The carb was rebuilt about five years ago. The engine was stalling when giving it gas from idle. Rebuilding did not cure it. But up until last summer I could run wide open with no issues. The temp gauge does not indicate overheating.

One thing I have not mentioned: When running at high rpm's, if I push the power trim button to trim up, the engine will start to cut out and lose some rpm's. I don't know if this is related or not.

Thanks for the suggestions.

 

[Maclin]

Re: 4.3 Loses Power

n/a

 

[Walt T]

Re: 4.3 Loses Power

The timing light trick I mentioned above can tell you if the distributor internals are distributing like they're supposed to with points or module. I wasn't sure which it had though thanks for the correction. Points and condenser bring up all sorts of possibilities, the point settings and timing are directly related. It also sounds like your carburetor wasn't built properly and that could be the source of your problems. Just because it was "Rebuilt" doesn't mean it was done properly. I put my kids through kindergarten getting paid to fix supposedly rebuilt carburetors. At any rate the cut out while messing with the trim points to a voltage drop and I have found this to be a ground problem with the trim unit, and / or a big ground cable problem. It's time to verify connections, battery, where the cables are bolted to the motor and starter, Ground wires attached to bellhousing bolts, remove the bolts and check them, clean off corrosion. I recommend this because it's free and easy. Once you verify good grounds battery connections, then it's time to whip out the volt meter and find voltage drops. I've seen it over and over, guys experience problems like that, see a drop at the coil, throw a coil at it, and problem is still there. So they go looking for other parts to throw at it. Verify basics first. That's the foundation of diagnostics.

 

[MikDee]

Re: 4.3 Loses Power

Good very informative post Walt T.!

Check Distributer, Carburetor, Voltage drop issues,,, You took the words right out of my mouth!

Wait, but why can't he take the beer!? :lol:

 

[skier-24]

Re: 4.3 Loses Power

So many great suggestions. Thanks guys.
Maybe by the time I check them out, the rivers will be back to normal. All waterways are approaching flood stage now.

 

[skier-24]

Re: 4.3 Loses Power

I didn't need to go anywhere. It was running badly on the trailer yesterday. New electric fuel pump evidently didn't help. Everything in the distributor looks ok. Carb probably ok. It starts better and quicker than it ever has.

A buddy at work told me before the 4.3's are famous for cracked blocks between cylinders. Anybody else heard of this? If it has, how will I know? What would be the symptoms?

 

[MikDee]

Re: 4.3 Loses Power

When it starts losing power, it runs badly. It starts like a miss or two, then gets worse until the rpm's start gradually dropping. I was cruising about 3500rpm's the last time it acted up.

The carb was rebuilt about five years ago. The engine was stalling when giving it gas from idle. Rebuilding did not cure it. But up until last summer I could run wide open with no issues. The temp gauge does not indicate overheating.

One thing I have not mentioned: When running at high rpm's, if I push the power trim button to trim up, the engine will start to cut out and lose some rpm's. I don't know if this is related or not.

Thanks for the suggestions.

Yes, it maybe related,,, I had something like this happen to me. I had a bunch of strange issues besides. I found it was the battery cables connection! Corrosion could not really be seen, but I cleaned up every connection I could, and put dielectric grease on it all, and had no problems since. It makes sense now, because a bad connection will draw power away from the motor, and anything else requiring a heavy load.

 

[Walt T]

Re: 4.3 Loses Power

Could be vapor lock, symptoms sort of fit, but it would have hard start/ no start when hot.