5.3 Idles at 2500-3000 RPM

[Zachalanche]

Hi,
I'm firing up a rebuilt 5.3 for the first time, the engine starts no problem but insists on running at 2500+ rpm.

This is a mostly stock gen 4 5.3 with a stage 1 truck cam, and holly terminator X max. throttle is DBW.

When I pull the data log, the throttle is all the way shut at 5%. I pulled the intake tube off and checked this visually because I didn't believe it, and it appears to be completely shut.
I have sprayed every inch of this intake manifold with brake cleaner, and can't seem to see evidence of a vacuum leak.
I also removed all the vacuum lines except the MAP sensor line, and capped the ports - still no change.
I removed the manifold and found the ports to be warped, so I trued them up with a sanding block and reinstalled the manifold. - still no change.
The only way to reduce engine RPM is to physically block the throttlebody.

I'm confused. how is it possible for the engine to run this fast without any additional airflow? am I missing something?

The only other thing that stands out in the data log is that the AFR is around 10, but the expected is around 14.5, is too much fuel finding its way in? and can this cause the super high idle without additional air flow?

thanks in advance if anyone has any ideas for me to look into next.

Cheers.

[Hotwire]

Sounds like you have multiple problems starting out.

Have you physically verified the throttle is closed even though the computer says it is? Are there any calibrations you can perform to make sure the throttle blade is doing what the ecm states?

Have you checked fuel pressure?

[dhutton]

What are you doing for crankcase ventilation. Too much and unmetered and it looks like a big vacuum leak. At least that is my amateur opinion.

[TheJDMan]

Might be time to contact Holley support.

[Zachalanche]

Sounds like you have multiple problems starting out.

Have you physically verified the throttle is closed even though the computer says it is? Are there any calibrations you can perform to make sure the throttle blade is doing what the ecm states?

Have you checked fuel pressure?

yes. there is a throttle calibration that I performed. I also took a video of the throttle blade with the engine not running, and it appears to respond to throttle input correctly. And, I looked at the throttle blade with the engine running, and it its completely closed. fuel pressure is right at 60 but I am curious if the gauge isn't accurate. I'm planning on replacing it with a pressure sensor since the terminator is wired for one.

[Zachalanche]

What are you doing for crankcase ventilation. Too much and unmetered and it looks like a big vacuum leak. At least that is my amateur opinion.

Fresh air from intake tube to passenger valve cover, inline pcv valve between driver valve cover and intake manifold. there is no MAF sensor, so it's not an issue of unmetered air, and I have tried removing the pcv line and capping the vacuum port, and that didn't reduce engine speed at all.

- - - Updated - - -


Might be time to contact Holley support.
that's on the agenda for today.

[Zachalanche]

Holley says the only possibility is an air leak somewhere.

So i tried to see if the engine would run while covering the TB. I put a piece of cardboard in front of the TB and it stalled the engine. more interestingly it held vacuum for quite some time after the engine stopped. so maybe I don't have a vacuum leak. I then tried pushing the throttle blade more closed (despite it already appearing almost fully closed), and i was able to momentarily reduce engine speed before the TB began to resist this action. so now i am thinking maybe this is a TB issue.

[robbbc]

I don't have any experience with the Holley stuff but if it operates like OEM ECM's, you may have too much Airflow in that rpm range. Have you tried adjusting the idle rpm with the handheld controller? For example with HP tuners, you can lower base running airflow in the 500-2500rpm range and it would allow the idle to operate where it should. When adding a cam, it lowers the efficiency of the engine at lower rpm, so, it doesn't need as much airflow.

[Zachalanche]

I figured it out! Long story short the throttle body was the issue.

Short story long, I found that if I choked the engine to a stall with a piece of cardboard, the intake would hold vacuum for a surprising amount of time after the engine stopped. based on this I ruled out the possibility of an air leak. So, I started pressing on the throttle blade with the engine running and was able to briefly get the engine speed down before the throttle body started to fight me. whatever it thinks 5% is was apparently too much. I had an extra throttle body for an LS3, so I decided to try that one. the engine idles perfectly now. If i didn't have the spare, I would still swear the original throttle body was good based on the way it functioned, it only needs to be opened a hair to really get the engine wound up.

thanks to all of you who offered up advice, hopefully this thread helps someone in the future.

[KEGS73]

out of curiosity, did somebody drill the throttle blade hole for that cam?

[Zachalanche]

out of curiosity, did somebody drill the throttle blade hole for that cam?

No.
210800

[Vimes]

It might just be the picture, but the lower right side on the right hand throttle body looks like someone took a file to it. Might be enough to cause the problem.