LQ4 Swap Not Running Right

[72longhornTX]

Alright everyone, I need help. I am at a loss as what to do next.

I have been searching here, google, ShopKeyPro, and anything else I can.

Here is what I have.
71 C20
2001 Silverado LQ4 6.0/4L80E.
Computer done by Brendan and LT1Swap
Wiring harness cleaned up per his website.

My problem I am having, short of the couple oil leaks which I can deal with easy.

The truck starts fine when cold and idles great for about 30 seconds to a minute. Right around 600 RPM. I am using a Snap-on Modis Ultra to view live data. However, after that time the idle starts going low and the engine starts chugging and shaking. If I let it keep doing that, it'll die. If I press the gas it takes a minute and then catches up but once I let off, it goes to the same thing and dies.

If I try to drive the truck, once I let off the gas it dies. Common problem here is, it dies.

I have pulled the throttle body, cleaned it using a parts washer, pulled the IAC and cleaned it. Put new AC Delco plugs and wire on the truck and I am not getting a single misfire.

Just can't seem to figure it out.

As for codes, the only ones I get are when I unplug a sensor to see if something changes. No MAF code, no IAC code, infant the readings on the IAC are on spot with what it is requesting and what it is giving. I have checked the fuel pressure, 55psi at idle and 58psi when I unplug the vacuum regulator on the rail.

This is a drive by cable engine as well.

Any advice is greatly appreciated and if anyone who knows a lot about this on the west side of Houston, feel free to invite yourself over, I can provide beer and BBQ. I just need to figure this out by Sunday night, This is suppose to become my daily driver.

[LeesTruk]

Watch your O2 sensors on live data stream and see what type of activity you have. They should be toggling all over the scale. If they're "flat-lined", then that's your problem. On my first start-up, I had a similar situation, where it would idle good, then start to load up and die, with no codes. I found that my bank 1 sensor was dead. I replaced it and that took care of my idle quality. Ultimately, I had to replace my PCM, but that's a whole 'nuther story

[72longhornTX]

My O2 sensors are running between 200-900. Both Bank 1 and Bank 2. So not sure thats it.

[akdg87]

whats your fuel system setup like? Never mind i see it seems to check out okay when you tested it

[BR3W CITY]

Can you monitor IAC% on that scanner? Do you see the IAC% change/chase idle as it stalls?

Honestly its kinda a vacuum system leak type of vibe to me...its just soooo easy to have that cause idle hunt and dying.

Of course its also possible that the idle/iac settings aren't correct in the tune, but thats the last thing to prove out after the mechanical stuff.

[65standard]

Can't you raise the idle in the tune? Maybe it is just set to low in your tune? Shouldn't your cold start idle be higher than 600?

[BR3W CITY]

He's got a mail order tune, so I'm guessing he can't write to the file, only monitor it via the scanner he's got.

Yes, you can set idle window in the tune. If he's got an NSS setup, it should also see the trans load and adjust for that. If the vehicle is already commanding a high idle and not getting it, thats a different story.

[72longhornTX]

The IAC was giving the same the IAC was requesting.

I did put some exhaust on the truck because someone said the O2 sensors too close to the end of the pipe could cause issues. I did that and also pulled the IAC again and really cleaned it and pulled it apart and lubed it. Now it is running smooth but at 515RPM. A little low, but it is running none the less. I suspect the PCM just needs to relearn the idle over the next few run times.

Now just to find out why it isn't cooling all the way.

[Custom 68]

glad you got it running right. As far as cooling these motors are problematic for trapping air. You will need to make sure you put coolant in the top hose into the motor. It seems otherwise they will just hold that air bubble. Try that if you didnt add your coolant or water that way.

[GMC-YA68]

Originally Posted by Custom 68

glad you got it running right. As far as cooling these motors are problematic for trapping air. You will need to make sure you put coolant in the top hose into the motor. It seems otherwise they will just hold that air bubble. Try that if you didnt add your coolant or water that way.

I had the exact problem until a fellow board member told me the same thing. Problem solved.

[Andy4639]

Cooling issues are most likely from the air pockets. Take the steam ports loose and fill through the hose. Let the water come out the steam ports. Oh and lifting the front up helps to get the air out. I had to put my 71 front end on a lift and get it up high enough. that was before I learned about the steam ports though.

[Robznob11]

I had that happen one time on my truck as well where it would load and die. I did find that though I had 55psi ideling that hitting the throttle made it drop to 40psi. Changed the pump. Also how is your fuel tank rigged. Since I use no emissions I had to drill the gas cap as well for air flow. I'm running a camaro tank designed for closed system and purge. Mine left as quickly as it came. I'm still not convinced it was not something else like a sensor freezing up from setting to long....

[72longhornTX]

I will try that on the cooling system. I have been warming it up and letting it cool and slowly topping off the radiator. So that seems to be helping too.

As for the fuel, I custom made a sending unit using a Walbro 225 put that fits in the stock tank. I am not sure if the filler neck is vented. If I have issues with that I will run without a fuel cap for a while and see what happens. They I can deal with that when the time comes. I will also plug in the pressure tester and drive the truck too and see what happens.

But now that I have a full exhaust put on there, true duals dumped with flow master 40s. And since I cleaned the IAC more, it is running great.

[Liltroy]

Originally Posted by 72longhornTX

I will try that on the cooling system. I have been warming it up and letting it cool and slowly topping off the radiator. So that seems to be helping too.

This is what I had to do. Make sure you pull the top radiator hose and fill the engine first, then the radiator. Let the engine come up to operating temp them shut it off and let it cool for about 30 minutes. After that I was able to put almost another half gallon into the radiator and everything was good.

[Custom 68]

yep they are tricky. I don't know that it will work without filling thru the top hose.

[72longhornTX]

Seems I got it filled up after 4-5 times of topping off once cooled down. Never breaks 200. Highest I saw was 199 after getting off the highway and sitting at a stoplight.