350 TBI Boggs and surges under load

[Madcrafter52]

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.

 

[scott2093]

Any codes?
If you can get a scanner, maybe something will stand out....

 

[Madcrafter52]

Any codes?
If you can get a scanner, maybe something will stand out....

Knew I forgot something. No Codes, even if I try to force the issue.
Don't have a scanner. I don't know where to get one, You need a laptop right?

 

[scott2093]

Don't have a scanner. I don't know where to get one, You need a laptop right?

I have an old scanner similar to the brick one people seem to like. But there are bluetooth aldl deals that work with your phone ....
Not sure how good they are in general. I have an older 1320 electronics bluetooth scanner and it's pretty good. Shows enough to get you most relevant stuff...o2 activity...iac counts ..map readings...etc...I think he went out of business .. But there must be plenty out there. Imagine they're similar...

Or find someone who will let you run a quick scan...

I see you didn't mention replacing the distributor...egr....
You should download your manual from the sticky if you haven't already and look over the symptoms to get some good ideas for tests you can start doing now..
I like the emissions manual. Very informative for troubleshooting....

 

[Schurkey]

1994 Chevy k1500 with a 350 TBI

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,

High-quality brand-name parts, or bottom-feeder eBay/Amazon junk?

Did you check the magnet on the distributor shaft for cracking?

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Is the spark powerful enough to jump the gap of a real spark tester, calibrated for HEI?

Fuel Pump, Fuel Tank, Fuel Filter,

Yes, you need to know the actual fuel pressure.

Checked and confirmed good: TPS, MAP, Temp Sensor,

How about the wire harness connected to those sensors?

Have you verified the knock sensor + EST module? Electronic spark advance? Air filter? Charcoal canister? Heated air intake?

0° Ignition Timing

With the timing connector disconnected?

I believe I have fixed all vacuum leaks.

How many were there? Where?

I'm going to re-check the fuel pressure tomorrow with a more accurate 0-20PSI gauge.

Good idea.

But other then that I'm at a loss. Any help is much appreciated.

What is the cranking compression pressure of all eight cylinders?

I have an old scanner similar to the brick one people seem to like. But there are bluetooth aldl deals that work with your phone .

A scan tool or computer/phone-based scan tool simulator is pretty-much essential. I have a strong preference for a real scan tool vs. the software/connector cable/dongle/laptop or 'phone.

Or find someone who will let you run a quick scan...

Yup. If you don't want to own the tool, find someone who does, and make arrangements.

I see you didn't mention replacing the distributor...egr.

Good ideas worth investigating. Test/inspect before replacement, though.

You should download your manual from the sticky if you haven't already and look over the symptoms to get some good ideas for tests you can start doing now..
I like the emissions manual. Very informative for troubleshooting.

ABSOLUTELY!!!

 

[Erik the Awful]

Definitely worse when warm.

Check for vacuum leaks. Your engine runs richer until it warms up, which hides vacuum leaks. Once the engine warms up the enrichment ends and you'll be running lean.

 

[8thNote]

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.

I had a 94 with 5.7 TBI that was doing the same thing. I replaced plugs, wires, rotor, cap, and icm (ignition control module). None of it helped. Then I happened to read, on some obscure and now forgotten corner of the web, that an overheating icm can cause this exact issue (runs fine when you first start it up, but bucks and dies when you give it throttle at normal operating temps). So I went to BestBuy and bought a tube of thermal paste, which is used to dissipate heat between components of high end computers. I removed my icm, coated the back side of it with the thermal paste, and reinstalled. I never had another issue again.

 

[Schurkey]

No Codes, even if I try to force the issue.

That in itself is a problem. HOW are you "forcing" codes? What tool are you using to retrieve them?

If the computer won't set codes, it's faulty.

 

[Madcrafter52]

High-quality brand-name parts, or bottom-feeder eBay/Amazon junk?

Quality

With the timing connector disconnected?

Yes

Air filter?

Freshly cleaned and oiled K&N, although for most testing I have the filter off

Check for vacuum leaks. Your engine runs richer until it warms up, which hides vacuum leaks. Once the engine warms up the enrichment ends and you'll be running lean.

How many were there? Where?

Vacuum leaks: Only about 2 or 3, just rotted lines. PCV, canister purge valve, might have been one more I don't remember. I believe I have curbed them all

an overheating icm can cause this exact issue. So I went to BestBuy and bought a tube of thermal paste. I removed my icm, coated the back side of it with the thermal paste, and reinstalled. I never had another issue again.

I've heard of the overheating ICM issue before and mine was the OG GM hence my replacement. When I replaced it I put a good coating of silicone grease on the back and I've heard that can do the trick. Never thought of using thermal paste before, good idea. However I don't think its my issue.

That in itself is a problem. HOW are you "forcing" codes? What tool are you using to retrieve them?

I try to force codes by holding the throttle in positions that really make the engine struggle. Never works. It will throw codes from me unplugging sensors and the like. I'm just using a jumper wire and counting blinks.

Is the spark powerful enough to jump the gap of a real spark tester, calibrated for HEI?
What is the cranking compression pressure of all eight cylinders?
A scan tool or computer/phone-based scan tool simulator is pretty-much essential.

I'll have to buy or borrow this stuff from somewhere

Current to do list:

check the magnet on the distributor shaft for cracking
check the actual fuel pressure.
check sensor wiring harnesses
verify the knock sensor + EST module? Electronic spark advance? Charcoal canister? Heated air intake?

 

[Madcrafter52]

Tested the fuel pressure, 12 PSI solid no matter what (as it should be with a brand new pump)

Checked the distributor and the magnet does look a little worse for wear. However the ohms on the pickup coil are okay ~1300

I double checked a few other sensors and everything seems fine.

I also took some videos of my truck running so maybe that will give you guys a better understanding of what's going on
Engine Bay and Interior (Yes the oil and temp gauges aren't working. got bigger fish to fry)

Darn electronics, Any input on a carb swap? lol