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Mike S.
Mike S., ASE Certified Technician
Category: GMC
Satisfied Customers: 4990
Experience:  GMC N.A.I.S.E. Certified Auto-Mechanic
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GMC Sierra 2500 HD Classic SLE: I rebuilt my 1995 GMC Sierra

Resolved Question:

I rebuilt my 1995 GMC Sierra 2500 5.7 L. engine into a 383 stroker, would I need to
change the fuel injector size, if so what size? The engine jerks when I pull my 5th wheel but runs
good when not towing. The exhaust donut melted the last trip out. Would the fuel
being too lean cause this.
Submitted: 2 years ago.
Category: GMC
Expert:  Mike S. replied 2 years ago.

What horsepower is it rated at?

I would read this,

http://www.tbichips.com/truckmods.htm

About 1/2 way down the page where it says,

So what size injector or fuel pressure combo to use? Well here is a basic guideline.

Stock 210hp 350 V8 up to 250hp the stock 55lb 350 truck injectors at the factory 9-12psi is fine.

from 250hp to 260hp the fuel pressure should be bumped up to 13-14psi on stock 350 truck injectors which will change their flow rating to 60lb/hr

From 270hp to 340hp you need to change the fuel pump as the stock pump can't maintain pressure over 270hp and you need to run 18psi on the 350 truck injectors for a flow rate of 72lb/hr

From 340-360hp this is where you should be looking at the 454 injectors and run them at the 13-15psi for their 82lb/hr flow rate. If your closer to the 340hp, you can run them at the stock 9-12psi and still be fine for this power range.

From 360-400hp you need to bump the fuel pressure up on the 454 injectors to the 15-18psi to increase their flow rate to 96lb/hr.

For over 400hp you really should be looking at alternative injection system but for up to 450hp you would need to run 24psi of fuel pressure to increase the lb/hr rating on 454 injectors to 112lb/hr and that is as much pressure as I recommend applying to a set of TBI injectors.

Customer: replied 2 years ago.

I forgot to mention that this jerking was there before the engine was rebuilt. The engine

was rebuilt into a 383 stroker for the purpose of towing. I do not know the specifics of

all thats changed in it. It is not a high performance 383. I have tried changing the

computer(not knowing that all the original info goes back into it) coil, distributor cap

and rotor, exhaust heat sensor, plugs and wires, lifted the throttle body and it still

jerks. My mechanic thinks the engine is running too lean because of the exhaust

donut melting. I thought the original motor had 195 hp, but I don't know for sure and

do not know what it is now. The truck is a '95 GMC K2500 4x4 -6 stud wheel not HD.

Do you think I would need to change the fuel pressure spring? I have also ordered a

performance & economy chip for it. I'm getting frustrated and am retired and can't

afford to trade it in after spending so much money on it. The truck has about 90 K

miles on it with only about 40K on the new engine. Is there any way to put a cold air

intake on the motor? Thank you.

 

Ken

Expert:  Mike S. replied 2 years ago.
What fuel pressure does it have now?
Customer: replied 2 years ago.

I don't know what pressure it has now. How do I find this out?

 

Ken

Expert:  Mike S. replied 2 years ago.
Do you have a hand held fuel pressure gage? If not you will need one to check the fuel pressure.
Customer: replied 2 years ago.

No, I do not have a fuel pressure gauge, I'll have to wait until monday when my

mechanic is working so that he can check it. I will let you know then what he says

it is.

 

Ken

Expert:  Mike S. replied 2 years ago.
ok
Customer: replied 2 years ago.

I finally got my fuel pressure tested, its 13 psi at idle and when gas pedal is revved

quickly, it drops to 12.5 psi. My mechanic suggests an 18 lb. spring so that he can

advance the timing. What do you suggest? The performance chip I ordered was

not for my truck and they do not make them for my truck. I still don't know what the

horsepower is, but the engine was rebuilt to a 383 stroker about 8 years ago, the

problem with the jerking under tow was there before the engine was rebuilt and

never changed after it was rebuilt, it also seems to hesitate a bit on take-off. With

all these things going on that is why the mechanic feels a bigger spring may help.

Would I need to change anything else if I went to a bigger spring?

 

Ken

Expert:  Mike S. replied 2 years ago.

Ok, first of all the jerking. Is that a positraction rear end? If so, I would check the fluid level and if it's good, you might want to go ahead and drain the fluid and and add new fluid and the proper positraction additive, if it requires it.

Although, I suspect that the rearend may be causing the jerking, is it possible for you to describe the jerking more or if not does it appear to be the rearend?

If not, like you said it has been there before the engine rebuild, so other than the rear the only thing left would be the transmission. I assume it's an automatic, most 4wd's are. If so, do you have a stall converter?

Now, to the exhaust donut melting. Was there an exhaust leak there? Is it a dual exhaust? If so, start it up, let it warm up and go back to the tailpipes and feel and compare both exhaust gases. Put the palm of your hand out about 6 inches from the tailpipe. Don't get burnt so if it's too hot move your hand further away. Also, smell for a rich (gas smell) or lean (rotten egg smell) condition at each tailpipe. I usually just let the exhaust blow onto the palm of my hand and then smell my hand. Also, if the exhaust gas burns your eyes, that indicates a lean mixture, too.

Ok, now to the fuel pressure. I don;t know if that will effect the jerking, simply because that problem was there before, but it could be possibly the cause of the lean fuel mixture but it would effect both banks if you ask me.

The hesitation could be a ignition timing issue, so if you haven't checked that by alls means do.

A bigger spring? Is he talking about the mechanical advance of the distributor? I would think you would want more advance and therefore go with a weaker spring so the advance weights throw out sooner.

Customer: replied 2 years ago.

First of all the jerking is more like the engine is missing when towing in 3rd gear. The

truck is a 5 speed standard transmission. I do not have dual exhaust. There was no

exhaust leak, the manifolds were brand new as were the donuts. The truck feels like

it needs more gas when towing, it loses power. At times

when you put the gas pedal to the floor the jerking (missing) seems to stop for a while,

but still no power. I was talking about the fuel pressure spring in the TBI. The timing is

set as far as it will go without pinging, thats why the mechanic wants to put a bigger

pressure spring in the TBI so he can advance the timing for more fuel pressure.

 

Ken

Expert:  Mike S. replied 2 years ago.

Sorry, I was napping.

You said, "My mechanic suggests an 18 lb. spring so that he can advance the timing."

Yes, I would go ahead and increase the fuel pressure, but I don't know why that is required in order to advance the timing.

You could consider and mechanical advance curve kit. They come with different pressure springs and weights and you have to test drive each while trying to find the correct advance. Also, perhaps an adjustable vacuum advance unit, if that does have a vacuum advance.

First of all though, you might even want to take a look at those advance springs and weights and make sure they are in good condition and they open and close and no springs are broken or have fallen off.

How much does your 5th wheel weight?

Are you towing in 2 or 4wd?

Customer: replied 2 years ago.
The fifth wheel weighs 7100 lbs. empty and I tow in 2 wd.
Expert:  Mike S. replied 2 years ago.
Open your glovebox door and look at the Truck-Camper Loading Information and the Cargo Weight Rating. What is it?
Expert:  Mike S. replied 2 years ago.

Actually, from what I read the weight should be ok. The weight-to-horsepower ratio is about 35:1, which means that the 7,000 lb trailer divided by 35 = 200 horsepower which I am sure the stroker has more than.

Another thing I did read though is don't tow in 5th gear.

Ok, now back to everything. Were there any codes stored in the ECM's memory? Is or was the check engine light on?

Customer: replied 2 years ago.

The truck -camper loading info says 1412 lbs. for a slide in camper. I am pulling a 27 ft.

5th wheel trailer. When I purchased the truck the weight of the 5th wheel was in the

acceptable range. The truck does not run like it should considering it now has a 383

stroker in it. I should be able to pull the trailer in 4th gear uphills not need to shift to

2nd. It seems to be starving for fuel under load and on take-off when not under load.

It has my mechanic and I stumped. That is why we are thinking to change the fuel

pressure spring and advance the timing. The check engine light has never come on.

I don't know if any codes were stored in the computer. It seems to me that the

computer memory should be replaced. When I changed the computer, the original

chip was taken out of the old computer and put into the new one, it seems to me

that I would have exactly the same outcome as with the original computer.

Expert:  Mike S. replied 2 years ago.
Actually, you may need a specific chip. The ecm's are basically for several models and the chip is programmed for a specific engine, transmission, rear axle ratio, etc.
Customer: replied 2 years ago.

Sorry for not getting to rating yet. I am still experimenting with my truck. I have now flipped the air cleaner lid and found that

the baffle in the air intake part of the air

cleaner assembly seemed to be stuck

in the closed position. I have taken off the whole air intake assembly. The truck seems

to have more power when I need it, but

I am just going out with my 5th wheel

trailer on friday and will see if I have

more power when towing. I have also

bought new fuel injectors, but am going

to see if the lid flip will help before I change

the injectors. I'll let you know on monday

how things work out. Thanks for waiting.

 

Ken

Expert:  Mike S. replied 2 years ago.

You flipped the air cleaner lid for what reason? Trying to get the engine to run leaner?

The baffle for the exhaust to help with cold start? So, it's stuck on outside air? It shouldn't matter now though if you flipped the lid over to lean it out.

Customer: replied 2 years ago.

I flipped the air cleaner lid in hopes that I would get more air, which it seems

to be getting. Yes, the baffle to help with the cold start seemed to be stuck.

I'll let you know if it makes any difference when I tow my trailer, which is my

main problem. Otherwise the truck seems to run good when not towing.

 

Ken

Expert:  Mike S. replied 2 years ago.

Was the baffle stuck so that the air from the exhaust would constantly be going up to and in the throttle body? If so, that there may be the reason that donut burnt up.

Customer: replied 2 years ago.

The truck did not run well at all when I was pulling my 5th wheel. The jerking in 3rd gear

was worse than ever. I did seem to have more power with the air filter lid flipped over.

I am going to change the fuel injectors and

see if that helps. Once I stopped the truck

I had a hard time starting it again. It was

like it wasn't getting fuel, because if it was

flooding itself you would be smelling gas

and I didn't smell anything. It did finally

start and then I shut it a few more times

and it started. After the fuel injector change

I am thinking of cutting off my catalytic converter. After that I'm lost as what to do

next.

 

Ken

Expert:  Mike S. replied 2 years ago.
Is this an OBDI or OBDII? Did you get a prom chip or flash the ecm specifically for a stroker?
Customer: replied 2 years ago.

I don't know what an OBDI or OBDII is. I didn't get a prom chip or flash the ecm

because I was told that my computer was compatible with the stroker. The problem

with the truck was there when the truck was brand new with the original motor and

did not change when it was rebuilt, it is just getting worse. I am wondering if the

catalytic converter is plugging up. It happened once before and the truck would

not run at all, I had it changed about 4 years ago.

 

Ken

Expert:  Mike S. replied 2 years ago.

OBDI or OBDII Is the difference between the old type and new type computer systems which changed over on most vehicles right around 1995.

Is it possible that the torque converter clutch is malfunctioning pr slipping when towing that 5th wheel?

Perhaps pulling the fuse for it or disconnecting the tcc wiring will tell if it is slipping or not.

Customer: replied 2 years ago.

The jerking or missing seems to be that the engine is getting either too much gas or not enough. My mechanic connected his diagnostic tool to what I guess must be the OBDI and there is no reading at all to tell

him what may be going on. The check engine light has never come on to warn of a problem.

I am still concerned that it may be the catalytic

converter plugging up. How long can I keep

asking questions before my time is up, and I

need to rate your answers? I can let you know

what is happening after I change the injectors.

I am going to ask my mechanic if there is a way to check the converter.

 

Ken

Expert:  Mike S. replied 2 years ago.
If you think the converter is clogged, just drop the exhaust system in front of the converter, although you won't be able to test drive it.
Mike S., ASE Certified Technician
Category: GMC
Satisfied Customers: 4990
Experience: GMC N.A.I.S.E. Certified Auto-Mechanic
Mike S. and 8 other GMC Specialists are ready to help you

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