5.3 FI swap... acting carbish?
 

Not sure that makes sense, but I'll explain...

When I fire it up it starts right up for about a second or two, then dies. I try to fire it again and it struggles a little to start, but will fire up... then dies again after about a second or so.

When it fires up and I give it gas, it will stay running but I have to keep pressure on the throttle for 20-30 seconds for it to finally idle. I haven't checked fuel pressure lately, but initially it was at like 57-58 psi IIRC. I've driven it several times and don't have problems with it bogging down at WOT or in between, so I'm not sure if it's fuel pressure, possible something else?? It's been like this from the start, just moved on from it to get some other things dialed in for testing...

Any ideas as to what the problem is?

Specs:
5.3/4l60e ( Motor had about 120K, opened up and all "looked" good )
zo6 cam, new springs, stock replacement pushrods 7.4's
Fuel Pump - Walbro 255 brand new

Any help is appreciated!!
Justin

Re: 5.3 FI swap... acting carbish?
 

If you have to keep pressure on the gas I'd look at things like; fuel pump not keeping up (maybe a bad ground or weak power source), losing fuel pressure, or a problem with the tune, if the idle speeds were messed with at all due to the cam etc

Re: 5.3 FI swap... acting carbish?
 

BR3W - Ok, when I check my fuel pressure, will it read the same when key is turned to crank/start engine? (hope that makes sense)... Edit - Fuel pressure would stay the same throughout, correct?

I am running a harness from PSI, so should be a "stock tune", however I did let them know I am running a z06 cam.

I will check voltage for my power source. Hmmm, I have the Fuel Pump Load wire running through my truck harness (EZ21 - fused) the to the relay for the fuel pump. I could try bypassing the truck harness and go straight to the FP relay... Could that make a difference?

Re: 5.3 FI swap... acting carbish?
 

Is your idle air control plugged in and functioning? Has the screw for the throttle blade stop been messed with at all?

Re: 5.3 FI swap... acting carbish?
 

As far as I know, the IAC is working properly, no codes are showing if that's what you mean.

The throttle body has been ported and the "stop bracket" on the body itself was milled down for a Wider Open Throttle, if you will... The screw for the throttle blade stop, i don't believe has been messed with...?

The throttle blade should be fully "closed" at idle, correct?

Re: 5.3 FI swap... acting carbish?
 

Should the throttle blade be fully closed at idle??

After thinking about it an what you brought up, doesn't seem like the blade should be fully closed...?

Re: 5.3 FI swap... acting carbish?
 

NO

It should be JUST a hair open. Like I'm guessing you could probably put a .040 feeler gauge in there if I had to pull a number out my a$$. If its all the way close its choking the motor out at idle.

You need to barely adjust that throttle screw to let enough air in to hold a 8xxish rpm at idle, assuming that the idle settings are all stock.

A caveat to that, and maybe wasted or someone can chime in; I do not know if you can adjust the TB screw on a cold engine, or if that IAC would throw it off. You may have to get the engine warm before attempting to level the idle.

Right now the IAC is probably fighting to get the thing to idle.

Pull the battery off and disconnect the TPS, then adjust the little screw a tiny bit to see if you can get it just offset. Hook it back up and start her, see if it changes.

I should also mention that a ported tb may have an effect on the tune as well, including MAF and idle settings, considering you're not really on a stock setup anymore with that and the cam etc.


Take a look at this thread over on YB. Its pertaining to fbodies but much of it should be similar, unless there has been a drastic change in the way the trucks handle these in the tune.
http://www.yellowbullet.com/forum/sh...d.php?t=482495

Re: 5.3 FI swap... acting carbish?
 

I had the same problem with mine and it was my tune.
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Re: 5.3 FI swap... acting carbish?
 

knowing psi...its prob the tune....

Re: 5.3 FI swap... acting carbish?
 

Ok... I'm gonna fiddle with the throttle body to see if it's that simple.

If that doesn't work, I'm going to check fuel pressure to eliminate that. If it is the tune, I just need to take it down and have it "adjusted"?

Re: 5.3 FI swap... acting carbish?
 

ideally yes. You should be able to let them know whats going on, and make sure your clear about the mods on the truck, if they weren't initially setup for all your running.

The problem with any mail order tune is that the little things in the functionality and driveability of the car can only be ballparked, vs a dyno or in car tune in which things like part throttle response etc is more closely examined.

Re: 5.3 FI swap... acting carbish?
 

You didn't mention whether its DBC or DBW, but since you say your IAC is working ill assume its cable. A cable TB will be fully closed at idle, the IAC is what keeps it from bogging down. On a DBW the blade will be slightly open since there is no IAC its controlled by the throttle blade.

Does it always do this? I had one do this when it would sit for extended periods of time with the battery unhooked. Once it was driven for a while it would straighten out and wouldnt do it again until the battery was again unhooked for a while.
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Re: 5.3 FI swap... acting carbish?
 

It is DBC. I looked at my old TB, which was the original TB for this engine and it is a DBW. The blade is slightly open as you stated.

Yes, it always does this, however, as you mentioned about the battery, I unhook it almost all the time as I have a load that's draining the battery when truck is off and haven't tracked it down yet. I believe it is my air compressors, but just haven't made it a priority to find out where it's coming from... until now!

I tried adjusting the set screw for the blade and it's doing the same thing, so I reset it back to it's original height.

I've also noticed some TB's have a hole drilled in them...? Mine doesn't have one, which I'm going to assume doesn't need to be there?

Re: 5.3 FI swap... acting carbish?
 

Every stock DBC TB i've seen has a hole drilled in it. You need to have SOME base airflow other than the IAC. The IAC is more for adding air for cold start, and to stabilize idle with changing engine loads such as the transition into gear from P/N, AC kicking on and off, electric fan kicking on and off.

Sounds like maybe the TB porter swapped the DBC blade for a DBW blade?

Re: 5.3 FI swap... acting carbish?
 

Wasted... Any idea what the hole size in the TB blade should be?

I re-read the link that BR3W provided and it was suggested that the hole should be there. But no specifications on size... 1/16? 1/8? 3/16?

Re: 5.3 FI swap... acting carbish?
 

Yep, on a dbc the hole needs to be there to let that little bit of air bypass the throttle blades. iirc its called an air bleed.

For the ls1's its 70125 is .143" and 79651 is .163" depending which part # throttle blade it had. I couldn't find the info on the trucks.

Re: 5.3 FI swap... acting carbish?
 

Thanks fellas!!! I'm going to look around, see if I can find the proper measurement! I hope that is what's causing the problem, guess I'll find out!

I'll let you guys know how it goes...

Re: 5.3 FI swap... acting carbish?
 

Just don't drill it out.

Re: 5.3 FI swap... acting carbish?
 

Not sure what you mean?

Re: 5.3 FI swap... acting carbish?
 

Why can't I drill it out??

Re: 5.3 FI swap... acting carbish?
 

I removed the TB blade last night and looked on the back part of the blade. The two holes that are usually present were blocked off? Not sure why...? I couldn't tell initially when I got the TB because the blade had been smoothed off as well.

So I drilled them out. (after researching that most 4.8,5.3,6.0 TB blades have two holes). fired it up this morning, and it still was doing the same thing. However, I tried it one last time and just tapped the accelerator and it stayed at idle... Sounds like it could be the tune as Moose and Smoky kindly stated :)

I left the battery connected and will try it when I get to the crib and see if she fires up without any assistance...

Re: 5.3 FI swap... acting carbish?
 

Far too many people use regular bits and drill it out to "What looks good", but this can create its own set of idle issues. It can be done but its the kind of thing that should happen 1/64 at a time with machine bits, not really a powerdrill kinda job.

That being said, if yours were blocked, re-opening them isn't a problem, provided you can do so without boring the hole over.

Re: 5.3 FI swap... acting carbish?
 

I gotcha. I contemplated buying a new blade, but since the holes were there and I had a bit that "looked close" lol, I gave it a shot. Didn't have to bore out the holes at all.

Re: 5.3 FI swap... acting carbish?
 

As stated before by LS1NOVA71, the battery could be your problem. When I got my 2000 Z71, it had been sitting for about 6 months with a dead battery and did the exact same thing for about a week after me getting it up and running again.

Re: 5.3 FI swap... acting carbish?
 

Ya, going to keep the battery connected to see if it works itself out.