Loss of power/stalling/bucking under load (1 Viewer)

[pumbaatx]

My truck has started to behave strangely on long trips. If I drive for less than 20-30 minutes, I never seem to have a problem. After the engine has been running for a while (usually at highway speeds), it seems to have problems when I try to increase the load. Usually this presents itself as stalling on acceleration as if the throttle just is not responding, but when it gets really bad the whole truck bucks like in the video below. Sometimes I can resolve the problem by just letting off the throttle and other times it persists.

I have been through a list of things that have not seemed to resolve the problem. I have rebuilt the charcoal canister according to this post. I have replaced all of the spark plugs and the distributor has been adjusted. The carburetor has been rebuilt.

Releasing the gas cap does not solve the issue, through I do still get a big whoosh even after the charcoal canister was rebuilt (I may need to clean out the lines between the tank and the firewall?).

My other ideas so far are that I have a vacuum leak or that my fuel pump/filter are bad. I'm not sure about why either of these would only present themselves after I have been driving at highway speeds for ~1 hour. I am still a noob when it comes to engine mechanics, so any Mud posts you can link me to for diagnosis/repair are much appreciated!

Video of problem

 

[Seth S]

Sounds a lot like fuel starvation. Could be something simple like a dirty fuel filter or a poorly routed fuel line that gets a little too close to a hot object. Could be time for a new fuel pump. If you remove the gas cap and continue driving does the problem persist?

Alternatively sometimes coils work fine when cold but can fail when they get hot leading to spark degradation.

 

[OSS]

My vote is fuel starvation as Seth mentioned. I’ve experienced it myself. Super maddening.
Bottom line: engine is not getting enough fuel.

Possible causes:
1. Dying fuel pump
2. Debris floating around in the carburetor float bowl which intermittently blocks a jet
3. Clogged fuel filter
4. Debris in tank occasionally blocking inlet.

If the fuel pump is over 20 years old - replace it now regardless if it’s the problem or not.
If the carburetor is over 20 years old without a rebuild, rebuild it regardless if it’s the problem or not.

Junk in the gas tank might be harder to pinpoint, so take care of the things you can (cheapest & easiest first) and maybe it won’t come to that.

Unless a particular problem consistently manifests under repeatable circumstances- the event that seemingly triggered the problem may only be random and not indicative of anything

 

[LandoNick]

You can check the Fuel pump by disconnecting at the carb measure how much is coming out when cranking. Accelerator Pump? get a good spray if you look down the carb?

 

[Chad717]

@pumbaatx I am having the exact same problem. i could take a video that looks just like yours. going up a hill under load. I have a rebuilt engine. Changed plugs, cap and wires, new coil. Desmogged. Rebuilt carb. New fuel pump when I had engine rebuilt. I’m not quite at my wits end, but like @OSS said, it can be maddening.
im gonna try another one toyota fuel pump to rule out a bad pump, but mine pumps out the right amount of gas when turning over the engine on the starter, I think it’s a half a cup every 15 sec or so. I’ll let you know if I solve it.

 

[Michael Hanson]

Hi, Have you run the valves?

 

[klinetime574]

I’d check engine vacuum. What I’m thinking is fuel starvation OR too much air? Vacuum may not tell you that but if you have crazy numbers (thinking higher under load) then you would investigate further. I know when I floor it my vacuum gauge goes down under load.

Do you have stock intake/exhaust manifold? Maybe a crack under the carb? Common issue.

Definitely check what @Seth S and @OSS stated first. If you have a wild hair and feel like pulling the carburetor then go ahead and check my theory.

Side note; These rigs can be set up so many different ways. What works for you may not work for others, etc. For example some have gone to FI, some have headers, some don’t etc. Case in point; I’m FI (Sniper) and have a header.

 

[g-man]

Driving at highway speeds for over an hour could cause some electrical parts to over heat. My igniter recently when out on me but in my case I was driving fine then about an hour into the trip total failure. I guess it could be possible that your's is sorta dying? I did see in the FAQs under "igniter repair for cheapskates" that someone mentioned a similar condition when his igniter started to overheat. Anyway there are a few tests you can do that are in the 2F engine FSM under ignition section to test the coil and igniter. Also if you take it off the coil and see the silicone paste is gone between the module and the heat sink, that can cause overheating of the igniter. You'll see in the "igniter repairs for cheapskates" that I did replace with the GM module.

 

[LandoNick]

I had a really bad drivability problem. It would idle fine in the driveway. But under load it would buck and surge. Turned out that my timing was off. but I was for sure it was Fuel related