Fuel trim

Jesseyourcalipers · Apr 7, 2022

1999 Sierra 1500 4.3 v6 single cab

Negative 30 fuel trim on bank 2. Also get a cylinder 2 misfire. I changed the maf twice both ac delcos. Change plugs. Cables changed not too long ago. Air filter new 02 sensors new. TPS and iac replaced and tight. Most likely a Vacuum leak? I can’t find anything after days of looking at hoses and connections. Intake gasket?

Trav · Apr 7, 2022

It is taking fuel away, a vacuum leak would cause it to add fuel, it has symptoms of a bad spider injector.
These are poppet style that tend to stick when they get older.

Edit: replacing the injectors with a multi port unit is best, do not use reman units on these. You will have to look around for an ac delco or standard unit but they are out there.

ACDelco 89060439 ACDelco Replacement Fuel Injectors | Summit Racing

Free Shipping - ACDelco Replacement Fuel Injectors with qualifying orders of $109. Shop Fuel Injectors at Summit Racing.

proshop.summitracing.com

DemoFly · Apr 7, 2022

-30 is the computer trying to remove just about as much fuel as it knows how to. It is getting way too much fuel in the mixture.

Jesseyourcalipers · Apr 8, 2022

Trav said:
It is taking fuel away, a vacuum leak would cause it to add fuel, it has symptoms of a bad spider injector.
These are poppet style that tend to stick when they get older.

Edit: replacing the injectors with a multi port unit is best, do not use reman units on these. You will have to look around for an ac delco or standard unit but they are out there.

ACDelco 89060439 ACDelco Replacement Fuel Injectors | Summit Racing

Free Shipping - ACDelco Replacement Fuel Injectors with qualifying orders of $109. Shop Fuel Injectors at Summit Racing.

proshop.summitracing.com

It has injectors from a newer 4.3. It’s a 99 truck with the engine of a newer Silverado but with the 99 intake to have the egr

Trav · Apr 8, 2022

Then it may still have a sticking injector, pull the plug and see if its wet with fuel. It is helpful to provide info about modifications like a later model engine right from the get go. What year are the injectors?

Jesseyourcalipers · Apr 8, 2022

Yeah my bad you’re right. That probably would’ve been helpful. it’s 2003 injectors so they are different. I did have the truck running without the maf for a while probably should’ve mentioned that also. So the plugs would get wet but I have never gotten a misfire code and truck ran fine and the plugs would be wet. I pulled the new plug after running it and tip is bone dry except a tiny bit on the thread I’m assuming from taking out the old plugs because it left the spark plug threads wet on the heads.

The_Eric · Apr 8, 2022

What's number 2 plug look like? Better, all the plugs?

I'm with trav on this.. Not sure the exact difference between the 99 and 03 but injectors, or regulator leaking?

Trav · Apr 8, 2022

Okay these injectors on the 03 are still a spider but with electrically triggered injectors not the poppet type. The MAF sensor effects the whole system not just one bank, your issue is on bank 2 ifrc.

Look at this...

How To Test The 'Spider' Fuel Injector Assembly (4.3L, 5.0L, 5.7L)

How To Test The Spider Fuel Injector. Leaking Spider Fuel Injector Test. Leaking Fuel Pressure Regulator Test. GM 4.3L, 5.0L, 5.7L (CPI, CSFI). Spider Fuel Injector and Fuel Pressure Regulator Test

troubleshootmyvehicle.com

Jesseyourcalipers · Apr 8, 2022

Trav said:
Okay these injectors on the 03 are still a spider but with electrically triggered injectors not the poppet type. The MAF sensor effects the whole system not just one bank, your issue is on bank 2 ifrc.

Look at this...

How To Test The 'Spider' Fuel Injector Assembly (4.3L, 5.0L, 5.7L)

How To Test The Spider Fuel Injector. Leaking Spider Fuel Injector Test. Leaking Fuel Pressure Regulator Test. GM 4.3L, 5.0L, 5.7L (CPI, CSFI). Spider Fuel Injector and Fuel Pressure Regulator Test

troubleshootmyvehicle.com

Wouldnt the plugs be wet if the injector was leaking? I also have p0300 sometimes and sometimes it’ll just be p0302 and I’ve also gotten p0174. Maf disconnected I won’t get misfire codes.

Trav · Apr 9, 2022

Here is the thing, there are many stages of leaking injectors everything from a slight drip to an all out blow out where it flows wide open.
If it was just a slight drip that would richen it enough for the ECM to pull fuel in various degrees eg a very small drip say 1 a second may make it pull -5 a worse drip may well force it to pull -30, wide open would cause a huge plume of grey smoke and may hydrolock the engine.
Pulling the MAF connector off with cause a MAF code.

A cold engine does not use MAF input it is on open loop and may well be able to deal with a dripping injector as it needs to run richer cold anyway, in that case you would not be seeing a misfire until it went into closed loop.
It may even be something as simple as an ignition issue, bad plug, wire, coil, etc anything that would prevent the fuel being fired. If it has been running with a lot of unburned fuel entering the exhaust it may have damaged the cat also but you need to diagnose this deeper.
I cant add much to this, I don't have the vehicle here.

Jesseyourcalipers · Apr 9, 2022

Trav said:
Here is the thing, there are many stages of leaking injectors everything from a slight drip to an all out blow out where it flows wide open.
If it was just a slight drip that would richen it enough for the ECM to pull fuel in various degrees eg a very small drip say 1 a second may make it pull -5 a worse drip may well force it to pull -30, wide open would cause a huge plume of grey smoke and may hydrolock the engine.
Pulling the MAF connector off with cause a MAF code.

A cold engine does not use MAF input it is on open loop and may well be able to deal with a dripping injector as it needs to run richer cold anyway, in that case you would not be seeing a misfire until it went into closed loop.
It may even be something as simple as an ignition issue, bad plug, wire, coil, etc anything that would prevent the fuel being fired. If it has been running with a lot of unburned fuel entering the exhaust it may have damaged the cat also but you need to diagnose this deeper.
I cant add much to this, I don't have the vehicle here.

I have seen on multiple other posts people commenting to the thread starters to pm a username that just so happens to be you. You’re a serious expert according to a lot of people lol I read through the link you sent me and it’s literally all lining up to what I’m experiencing. I am going to be following that to the T now that I have a few days off work and see what happens. Thank you so much for your help. I am just a amateur diy’er getting lost in forums and getting mixed information.

Trav · Apr 9, 2022

Start with the basics, plugs, wires, ignition. even if an injector is not leaking the fact it isnt firing will have the same effect of dumping excess fuel into the exhaust.

Jesseyourcalipers · Apr 10, 2022

Trav said:
Start with the basics, plugs, wires, ignition. even if an injector is not leaking the fact it isnt firing will have the same effect of dumping excess fuel into the exhaust.

Plugs wires timing rotor cap coil everything new and ac delco. I have changed I believe everything. 02 sensors cats bunch of stuff I can’t even remember. Exhaust pipe is ridiculously super black though

repairman54 · Apr 10, 2022

Tip off to a leaky injector is having to open throttle slightly on hot starts to reduce excessive cranking.
Had that on my old '99 5.7 and had to replace the spider injector assy. due to leaky injector.
As prev. said never excessive cranking on cold starts just hot restarts after sitting for a little while. Cold motor doesn't mind excessive fuel.
It ran a lot better with the updated spider assy. .

Fuel trim

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