Tach goes nuts when the truck is under load.

[Edward Huntley]

Not really sure how to explain this one. I have a 94 K1500 4x4 350 TBI automatic. Has about 280k miles on it and still runs just fine until you really give it some gas. When I first got the truck, sitting in park if you stomped it the motor would get up to maybe 2k rpm then act like it had launch control or something and would just bounce at 2k snapping and popping. It came out of it for a while but its back to doing it whenever you give it a lil too much gas going down the road. The whole truck shakes and the motor snaps and pops. I replaced the plugs, coil, wires, cap/rotor air and fuel filter. Nothing has helped I figured maybe it was starving or choking out. I'm wondering if there could be an electrical issue as the temp gauge is almost as nuts as the tach cause one minute it will be running at like 110 then 260 then back to 150.

 

https://youtu.be/7uJp_0KwD2k

 

My dad was thinking maybe a stretched timing chain but as violent as that is, and it does this every time you accelerate unless you practically let it idle up to speed. I would think it would have jumped timing or something by now but it runs near mint normally just idling or cruising around.

 

 

 

 

[rusty503]

Sounds like you have a broken motor mount.

[Edward Huntley]

I thought that myself for a bit but the motor mounts are pretty solid. Even then though I don't see how that could make the tach go nuts or the motor bounce in RPM or blow all the exhaust gaskets. It even expanded the muffler. The sound in the video is not the motor moving around that's the exhaust. Its blown out both manifold gaskets and expanded the muffler. Ive been tempted to unhook it and see if maybe the exhaust is clogged but I would think that would just make it bog rather than go completely insane.

[CamGTP]

Check fuel pressure while is running and under load, make sure it's within spec all the time.

 

This will be hard to check but the TBI injectors could also either not be sending out enough fuel or there are just like a fire hose causing you these issues. At idle the injectors should have a steady spray of fuel, if there are any solid drops of fuel then they are leaking and bad.

 

Check all your engine grounds for the battery and harness too. I would also check the distributor cap/rotor for corrosion and maybe even the gear on the end to make sure it's not worn out. Something is making the tach wig out and it likely deals with the way it gets it's tach signal.

[Edward Huntley]

Ive been considering tossing a 4bbl on it as that is what I am more used to. Truck will be getting a 454 next spring so I just need it to make one more Maine winter lol. I will do what I can to check the fuel pressure and stuff.