350 TBI Boggs and surges under load

[1990Z71Swede]

Definitely not oil, just fouled from a rich mixture. Could that be because of the misfiring cylinder causing the computer to add more fuel? Or would that be the opposite?


I replaced the ICM. I don't think it's the distributor because when I unplugged the electronic timing advance and set base timing at around 15⁰ the truck ran much better, and vacuum was much higher. To me this really feels like a camshaft issue. Seems like the computer is feeding both the ignition system and fuel system bad information.

I know you replaced the ICM, and measured pickup coil resistance. But at this point I would consider trying a Delphi or Delco coil just to rule that one out. Or better yet, put a timing light on it and have someone rew the engine.

 

[Schurkey]

The problem is not the "lean" plugs, the problem is the black (oil-fouled?) plugs.

Definitely not oil, just fouled from a rich mixture.

It would be exceedingly unusual if not outright impossible for a TBI to "fuel-" foul four out of eight plugs in that pattern. There'd have to be severe mixture-distribution problems with the manifold.

Pretty sure that's oil, not fuel.

Could that be because of the misfiring cylinder causing the computer to add more fuel? Or would that be the opposite?

A misfiring cylinder on the O2 sensor side could cause a false-lean O2 sensor signal, which would cause the computer to go into rich-command.

Connect the scan tool, look at the fuel trims.

I replaced the ICM. I don't think it's the distributor because when I unplugged the electronic timing advance and set base timing at around 15⁰ the truck ran much better, and vacuum was much higher. To me this really feels like a camshaft issue. Seems like the computer is feeding both the ignition system and fuel system bad information.

Start by installing non-fouled plugs.

Verify everything with the scan tool.

I would consider trying a Delphi or Delco coil

Or TEST the coil with a spark tester calibrated for HEI. If the spark RELIABLY jumps the gap when the coil is hot and preferably misted with water from a spray-bottle, the coil is almost certainly good.

Or better yet, put a timing light on it and have someone rew the engine.

Not a bad plan to actually verify the electronic spark advance.

 

[1990Z71Swede]

It would be exceedingly unusual if not outright impossible for a TBI to "fuel-" foul four out of eight plugs in that pattern. There'd have to be severe mixture-distribution problems with the manifold.

Pretty sure that's oil, not fuel.



A misfiring cylinder on the O2 sensor side could cause a false-lean O2 sensor signal, which would cause the computer to go into rich-command.

Connect the scan tool, look at the fuel trims.


Start by installing non-fouled plugs.

Verify everything with the scan tool.


Or TEST the coil with a spark tester calibrated for HEI. If the spark RELIABLY jumps the gap when the coil is hot and preferably misted with water from a spray-bottle, the coil is almost certainly good.


Not a bad plan to actually verify the electronic spark advance.

I was talking about the pickup coil, I think everything else ignition wise have already been replaced.

 

[Schurkey]

I was talking about the pickup coil

My mistake. Sorry.

Also look at the magnet on the mainshaft next-door to the pickup coil.

 

[Thenamesducky]

I have a 1994 Chevy k1500 with a 350 TBI that is kicking my butt. Starts fine, idles fine other then a little shutter now and then, snaps the throttle fine, but if I hold the throttle in any position it starts surging and bogging. Definitely worse when warm.

What I have done so far:
Replaced: Plugs, Wires, Cap, Rotor, Coil, ICM, O2 sensor, Fuel Pump, Fuel Tank, Fuel Filter, PCV Valve and hose
Checked and confirmed good: TPS, MAP, Temp Sensor, 0° Ignition Timing

I believe I have fixed all vacuum leaks.

This truck has made my head and wallet hurt and I just want it to work. Everything I do seems to improve it a little but the main issues still stand.

I'm going to re-check the fuel pressure tomorrow with a more accurate 0-20PSI gauge. But other then that I'm at a loss. Any help is much appreciated.

Try rebuilding the tbi. Fuel pressure regulator was doing this to me

 

[Sean Buick 76]

Yes my vote is the fuel pressure regulator in the TBI. Been there after replacing the fuel pump and all the engine sensors