Chevy 350 TBI stumble

[FrankieD61]

On my truck, (91 Chevy k1500 l, 350, TBI, not cat, no EGR) if I drive it when it's a little cold, at about 2-3000 rpm, it has a slight hesitation. Only when cold.

 

[Hipster]

EGR helps recirculate warm air back through a cold engine. It's been deleted. Go figure.

 

[FrankieD61]

EGR helps recirculate warm air back through a cold engine. It's been deleted. Go figure.

Yes, it has been deleted. The cat has been too. Is it possible that this could be a contributing factor?

 

[Schurkey]

Connect a scan tool, find out what the computer is trying to tell you.

Verify fuel pressure after the fuel filter. How old is the fuel filter?

How old are the usual "tune-up" parts--cap, rotor, plug wires, plugs, PCV valve, vacuum hoses?

FIX THE EGR. Which may mean putting fresh cat(s) into the exhaust. No EGR means detonation, detonation means knock sensor going crazy, which means retarded timing and poor power/fuel economy.

 

[FrankieD61]

Connect a scan tool, find out what the computer is trying to tell you.

I have a Snap-On MT2500 that I love, but I use the What's Up Display. Read the very last paragraph for more about that.

Verify fuel pressure after the fuel filter. How old is the fuel filter?

Fuel filter is brand new. It's a WIX.

How old are the usual "tune-up" parts--cap, rotor, plug wires, plugs, PCV valve, vacuum hoses?

Tune up parts are all brand new. All of it.

FIX THE EGR. Which may mean putting fresh cat(s) into the exhaust. No EGR means detonation, detonation means knock sensor going crazy, which means retarded timing and poor power/fuel economy.

The truck is not pulling timing; my knock counts are 0 (my knock sensor checks out). As far as the computer knows, the EGR system never existed and knows how to operate the engine within it's perimeters with out it. If there is anything else about this, that I should know please let me know.


Okay, after reading through this entire thread, I'm realizing how stupid I really am...

So, some background: the truck has a 350 TBI. It originally had a 4.3l in it. I bought it with that but it was blown, so I swapped a 350 into it. I then realized that I needed to do something about the computer. So, I sent the old computer to "Bobr" over at DynamicEFI and he did his magic. I've been doing all of the tweaking with his built in "What's up Display." Which is basically the best "scan tool" you can get for these systems, IMO.

 

[AuroraGirl]

EGR helps recirculate warm air back through a cold engine. It's been deleted. Go figure.

that is not at all what EGR is for lol!

EGR is for reducing the temperature of the combustion chamber by displacing air/gas with inert exhaust gases which allow the engine to make less bang so it has less heat. this is to control knock most of the time. other "benefits" are 2nd to that as far as this era of TBI / fuel injection goes

 

[AuroraGirl]

I have a Snap-On MT2500 that I love, but I use the What's Up Display. Read the very last paragraph for more about that.




Fuel filter is brand new. It's a WIX.



Tune up parts are all brand new. All of it.




The truck is not pulling timing; my knock counts are 0 (my knock sensor checks out). As far as the computer knows, the EGR system never existed and knows how to operate the engine within it's perimeters with out it. If there is anything else about this, that I should know please let me know.


Okay, after reading through this entire thread, I'm realizing how stupid I really am...

So, some background: the truck has a 350 TBI. It originally had a 4.3l in it. I bought it with that but it was blown, so I swapped a 350 into it. I then realized that I needed to do something about the computer. So, I sent the old computer to "Bobr" over at DynamicEFI and he did his magic. I've been doing all of the tweaking with his built in "What's up Display." Which is basically the best "scan tool" you can get for these systems, IMO.

Have you made your exhaust match what a 350 would have come with? something tells me a 4.3 doesnt need as much exhaust but im just guessing. Also you only needed to swap your PROM (I assume, maybe pcm too) to get the right control maps and tables.

also no offense but you sweapped from a v6 to a v8 and "did some magic" and deleted cats and EGR at the same time.. were you expecting it to run like a champ lol?

 

[Dropped88]

I've EGR deleted quite a few of these trucks and I haven't had the first one cause knock nor kill fuel economy.

I'm not saying it can't but I find it interesting that some have had that sorta problem

 

[AuroraGirl]

I've EGR deleted quite a few of these trucks and I haven't had the first one cause knock nor kill fuel economy.

I'm not saying it can't but I find it interesting that some have had that sorta problem

Your truck either has the best operating conditions that meant it never had much to handle Or you never noticed the truck pulling timing but the factory knock system possibly kept it in safe range( a good thing)

To note, not all EGR systems these years were the same behavior and not all situations would ultimately need it.

Also, EGR is supposed to be off-idle cruising almost exclusively.

I dont know how gm small blocks with TBI dealt with EGR and what if it didnt have EGR , but depending on the vehicle no EGR when its expecting it could cause a AFR mixup and code, once code fault for those systems, vehicle wouldnt continue to calculate as if it had EGR, thus it would run normal. Some vehicles simply cannot tell and may just correct itself with more fuel and air, and some may be pretty noisy about it.


Being that its exclusive purpose here is to reduce temperatures in the combustion chamber, the mileage difference is hard to gauge. Most new vehicles dont even do EGR anymore at least gasoline wise.

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Id be curious if you ever had a loaded trailer, a hot hot day, a factory 195(I think?) thermostat, and a hill you ever had to climb?

I've EGR deleted quite a few of these trucks and I haven't had the first one cause knock nor kill fuel economy.

I'm not saying it can't but I find it interesting that some have had that sorta problem

Not bagging on you just wondering. some climates, some uses may see conditions that wouldnt need EGR lol.
EGR is one thing on an old carbureted small block, but when using a PCM and TBI, youre basically wanting to allow the PCM to use as much advance and as lean (well, the stoichiometric) mixture as possible, and to do that you give it the best gasoline, the cleanest, coolest, least restricted air, the proper, sealed exhaust (with cat), and you give it good, non poisoned O2 sensors, no vacuum leaks, functional EGR, proper calibrated idle(mechanically and electronically), a learned computer with driving values under various operating conditions, and clean functional TBI with clean, mechanically sound engine with the cleanest and properly grounded/wired electrical system.

If you give it everything it needs, it can give you the most power and very reliably. I didnt list every factor above, but you get the idea

 

[Ruff Idol]

FIX THE EGR. Which may mean putting fresh cat(s) into the exhaust. No EGR means detonation, detonation means knock sensor going crazy, which means retarded timing and poor power/fuel economy.

I second that. Walker makes a great hi-flow cat available on RockAuto - I don't feel my truck breathes much less at all than when it was straight piped w/ headers!!