5.3 C10 Swap Idle Issue

[shortboxin]

Originally Posted by layinrocker65

I’d still wonder if the tune is not correct for the fuel pressure. You don’t have a standard regulator you can throw on?

This is more along my lines of thinking. According to the tuner, it's correct, but I'm with you.

What do you mean by standard? Do you mean adjustable?

[layinrocker65]

If I’m reading the thread right, your using the stock regulator in the stock early year LS truck rails. The regulator for flex fuel is different than the standard(non flex fuel) ones per online info stating the flex fuel runs at a lower pressure. So changing the regulator or rails to a regular(non flex fuel) setup will test if my theory is right. It will(or should) put the fuel pressure to the standard LS 58psi and if the truck runs normal, it’s an issue with the tune.

[layinrocker65]

Do you have a set of non flex fuel rails with a regulator that you can put on?

[shortboxin]

Originally Posted by layinrocker65

If I’m reading the thread right, your using the stock regulator in the stock early year LS truck rails. The regulator for flex fuel is different than the standard(non flex fuel) ones per online info stating the flex fuel runs at a lower pressure. So changing the regulator or rails to a regular(non flex fuel) setup will test if my theory is right. It will(or should) put the fuel pressure to the standard LS 58psi and if the truck runs normal, it’s an issue with the tune.

Per Bad Luck Garage on youtube, the return style systems are only around 42-50PSI at idle. Granted, the returnless non-flex runs higher than the flex systems, but at idle should still max out around 50PSI. The only way I could test your theory would be to have a returnless system altogether. Assuming that the in tank pump can drive to 58PSI, it would require a swap to a returnless fuel rail and a return filter inline.

Thanks for the idea, but I think there's too much required to test out. I'm thinking of having someone else reprogram a second computer to the flex specs and see how it runs.

Another question for anyone interested... Could there be a problem with the computer not getting any feedback about the type of fuel in the system thus causing an issue like this?

[layinrocker65]

The early LS truck motors were return style with a 58psi regulator at the rail.

[shortboxin]

Originally Posted by layinrocker65

The early LS truck motors were return style with a 58psi regulator at the rail.

I have no experience with the Chevy LS stuff. This is my first. I'm a 30 year Ford guy, so I only have feedback online to work with. You say the return style non-flex is 58PSI and this guy on Bad Luck says only the returnless hit that high.

When I disconnect the vacuum line, I get 54PSI at idle. This should be close enough to resolve the issue, but it still idles very low until the computer bumps up to fast idle after 20-30 seconds. Maybe that's an issue with computer needing to learn? Don't know.

[layinrocker65]

I have no experience with stock flex fuel LSs but I can tell you that the early style LS truck rails with the regulator in them sets the fuel pressure to 58psi. I’ve done more than 10 swaps with most of them being off the stock truck return rails. Even the corvette fuel filter regulators set the pressure to 58psi.

It’s a super easy thing to swap in another regulator or complete rails and check to see if that’s the issue. Hard to tell if the tune is the issue without seeing it.

[shortboxin]

Originally Posted by layinrocker65

I have no experience with stock flex fuel LSs but I can tell you that the early style LS truck rails with the regulator in them sets the fuel pressure to 58psi. I’ve done more than 10 swaps with most of them being off the stock truck return rails. Even the corvette fuel filter regulators set the pressure to 58psi.

This is where it's a problem for me because some guys say one thing and others say another. I haven't personally experienced any of this, so I don't know who's right. The Corvette would be the "returnless" system from my understanding, so I would expect 58PSI.

Originally Posted by layinrocker65

It’s a super easy thing to swap in another regulator or complete rails and check to see if that’s the issue. Hard to tell if the tune is the issue without seeing it.

I did already test another flex fuel regulator, so I am aware that's easy. It's just that I can't see spending a bunch on one just to test when I won't use it after that. Is a generic $10 unit reasonable to test with?

[shortboxin]

Originally Posted by layinrocker65

I have no experience with stock flex fuel LSs but I can tell you that the early style LS truck rails with the regulator in them sets the fuel pressure to 58psi.

Swapped in a 12 bolt with lower gears on my days off, so I didn't get back to this idling issue. Found some comments straight from the Ford repair manual at the links below that reinforces your statement, "Non flex fuel engines, fuel pressure spec is 55 to 62 psi with vacuum line disconnected, fuel pump running." That would put it about 58PSI mid range with vacuum line disconnected on return style, non-flex systems.

https://ls1tech.com/forums/fueling-i...injectors.html
http://www.lt1swap.com/identify_vortec_engine.htm

Also found that the flex injectors are rated at 8PSI less than non-flex injectors, "25326903 - 5.3L L59 Vin Z, flex fuel 5.3L - flows 4.164 gm/sec or 33lb/hr @ 50 psi".

Thing is that the low end of acceptable range for non-flex systems is 55PSI and my flex is running 54PSI with vacuum disconnected. Assuming my gauge is 100% spot on, I doubt 1PSI is going to make a lot of difference with the idle. I'm still thinking tune or something outside of the fuel system altogether.

I will look further into the throttle body for vacuum leaks this weekend. Thanks for your continued help!

[shortboxin]

Sprayed brake cleaner all over the throttle body, no change in idle at all. Sent a message to a tuner via ebay the other day. He asked also about the cam and told me to plug in a good scanner to verify if the IAC counts were between 50-70. If they were out, adjust the idle screw to dial it in (didn't know there was an idle adjustment).

Neither of my scanners check counts, so I looked into a scanner software package for my bluetooth OBD scanner that would read them and decided money might be better spent on another PCM with the idle bumped up.

In the meantime, I decided to just rotate the idle screw 1/2 turn. Started engine, still almost died and then improved significantly after 15-20 seconds. Put in gear and it still was iffy. Turned screw another 1/2 turn (1 turn total). Started up and it seemed pretty good, but idle was still fairly low and unstable.

Turned screw another full turn (2 turns total). Idle sounded great. Took the truck for a drive on surface streets, on freeway, and back to surface streets. That was when I found out I'd gone too far. Idle took a little too long to drop back down and was around 725RPM in park/neutral. Also found that I now had a P0507 code, IAC count high.

Backed out 1/2 turn (1 1/2 turns total). This brought the idle down to 650RPM. Took the truck for two drives, one yesterday and one today. Starts great, idles great, drives great and idle isn't too high even when warm. Just checked codes and nothing. I'd say for now, I'm good.

[skyphix]

Good news! IAC Stepping can definitely be a pain, I just don't have as much experience with it (all my stuff is DBW.)

Glad you got it worked out - though I'd still keep an eye on that fuel pressure but as long as its working for you at this point, enjoy driving!

[Rich84]

Crazy question here...Did you find that it idled worse in reverse then in drive?

Mine will idle well in drive , but in reverse it wants to die. I can even feel it in the power steering.

[shortboxin]

Originally Posted by Rich84

Crazy question here...Did you find that it idled worse in reverse then in drive?

Mine will idle well in drive , but in reverse it wants to die. I can even feel it in the power steering.

I don't think it was any worse in either gear. I had it stall both in D and R, but it would stall in P as well for the first 1-2 minutes. So far, no stalling at all post the adjustment.

[jjzepplin]

Subbed for longevity details or other possible issues.