5.3 C10 Swap Idle Issue

[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.