LS swap run away RPM upon start up

[john55blaze]

6.0 LS swap, factory modified harness and reprogramed computer (per lt1swap.com) Starting the engine for the first time, the motor starts right up and then the RPM races really high right away. Things I have double checked: fuel pressure (56 lbs at the rail) all sensors are hooked up and all grounds are good. disconnected TPS and IAC with no change, changed throttle body gasket with no change, clamped off vacuum lines and checked for vacuum leaks, nothing obvious. Please help this is the getaway vehicle for my upcoming wedding, hence the emergency. Thank you in advance for any and all help

[andrewb70]

6.0 LS swap, factory modified harness and reprogramed computer (per lt1swap.com) Starting the engine for the first time, the motor starts right up and then the RPM races really high right away. Things I have double checked: fuel pressure (56 lbs at the rail) all sensors are hooked up and all grounds are good. disconnected TPS and IAC with no change, changed throttle body gasket with no change, clamped off vacuum lines and checked for vacuum leaks, nothing obvious. Please help this is the getaway vehicle for my upcoming wedding, hence the emergency. Thank you in advance for any and all help

You'll make a get-a-way alright!!! LOL

Has the intake been off? if so, there could be a vacuum leak there.

Is the IAC working? Is the throttle blade cracked too far so that the IAC is fully closed and not able to do what it is supposed to do?

Andrew

[john55blaze]

"You'll make a get-a-way alright!!! LOL" Now that's funny


The intake manifold has not been off since I heard the engine run before I bought it. I am going to steal the throttle body off of my dads 2002 silverado here in a few minutes and bolt it on to see if that solves the problem. also no throttle cable is hooked up so I am positive that the throttle blade has been all the way closed. Also I have an extra manifold laying around that I might swap on as a last ditch effort this evening

[Z06vet]

"You'll make a get-a-way alright!!! LOL" Now that's funny


The intake manifold has not been off since I heard the engine run before I bought it. I am going to steal the throttle body off of my dads 2002 silverado here in a few minutes and bolt it on to see if that solves the problem. also no throttle cable is hooked up so I am positive that the throttle blade has been all the way closed. Also I have an extra manifold laying around that I might swap on as a last ditch effort this evening


I had an IAC motor that was bad do that recently. It would start & fast idle for about 5 sec, then it would hang around 2500 rpm & never come down. How high is it revving?

[srh3trinity]

I was going to say check the fuel pressure at the rail at first, but the IMAC would be next on my list.

[Jameson]

How high is it revving to?

[john55blaze]

Sorry, I have been away from the internet for a while. So new update, hooked up a tach and high reving is 5000 rpm. Hooked up the OBD2 port and no codes thrown except the MAF when I unhooked it for a minute. I stole the IAC off of my dads truck and bolted that on and didn't change anything; Had the super scientific idea of having my dad stick his finger in the passage way that the IAC controls and what do you know? he can control the idle speed with his finger, the thing idles like a champ and sounds good. So that leads me to believe that the computer is bad??? or a bad program? I am trying to get Brendan (lt1swap.com guy) on the phone to talk to him because he tuned the computer but haven't had any luck yet. And time is of the essence, so I'm thinking of ordering another computer that I found on PSIconversions.com for $200 ($$$ I don't really have or want to spend). Any ideas??

[ADiCarlo]

Is there anyone local you can call to check the pcm?

[john55blaze]

For some reason Southern Oregon seems to be a black hole for "high tech" hot rodders. I've lived here for years and haven't had much luck finding people, I wish I had the money to buy the cables and software to give it a go myself but that's just not in the cards right now.

Does anybody know what other sensors tell the computer where to position the IAC? Temp. sensors or, TPS or? I'm just not convinced that the computer is bad and after switching out the IAC and watching it move around by turning the key off and on I'm not convinced that it is even bad. And I hate the idea of throwing money and parts at this just to see if something works

[andrewb70]

Are you sure you have the right IAC? If the travel is not long enough it won't be able to close the air passage. Also double check the throttle blade and maybe try closing it a little.

Andrew

[john55blaze]

The IAC and TPS came with the throttle body from the wrecking yard so I assume that they are the ones that go with that throttle body, but I'm wondering if it might be worth the $70 for a new IAC that I know goes to that throttle body? Also I forgot to mention that this throttle body is off of a 454 (when I bought it I wanted the largest diameter stock throttle body available) and I'm wondering if there is any way that this is affecting the idle because it was designed for 100 more cubic inches? I have 18 days until my wedding and this is my only big hurdle left to finish the truck! Are there any other sensors that could be giving the computer bad data?

Thank you guys for all your help and suggestions, I'm positive this will get solved by some "ah-ha" moment!

[john55blaze]

Problem solved! It was the throttle body which was off a different vehicle than the manifold and had a big port that allowed unmetered air into the motor, made a block off plate and JB welded it in and problem solved. Now I just have to troubleshoot and erratic idle (searching idle?) and it's running really rich