Low Idle Issue 5.3L LM7/4L60e

[66LSx]

Alright guys... a bit of an issue in my 66' C10... but first the layout of the land

2004 5.3L LM7 / 4L60e
DBW, stock internals/stock tune
CAI/Full Long Tube Headers (w/ front o2's)
Fuel Pressure gauge is showing 58 PSI without deviation (Stealth Aeromotive 340)
Feels like it runs out/drives fine... don't have enough miles on it to get a grasp on MPG (300 miles only so far)

Upon coming to a stop, RPM's drop 400 or even below. Occasional sound of backfiring (when I simply let off the gas...stops after brake pedal input to TQ)

VSS is working (says my speedo) but I had to add a pickup resistor wire (10k ohm) like I did with the tach signal from the ECM

No MIL codes being thrown or presently being stored

o2 voltage looks good via tech2 same with MAF and injectors

Seems to improve after a long temp increase... but is still even present in both open and closed loop system. o2's are brand new delphi's... along with a new delphi MAF.

Using a vette filter/regulator combo after the Stealth 340 to the stock returnless system.

Issue came about 100 miles after first drive.

Have a vacuum gauge installed in the truck also... idling @ 17-19"hg @ idle.
Carb cleaner couldn't sniff anything out so far (haven't pulled the manifold cover off to check EVAP solenoid cap or the old booster cap.)

Not sure what the truck LM7 5.3L should be idling at in regards to "hg

It hasn't stalled... but it has given a shimmy and touched that 250-400 RPM range

clueless

Ideas?

[ls1nova71]

Anything been done to the tune other than VATs removal and emission deletes? Also, is your speedometer hooked into the VSS directly or from the speed signal out from the PCM?

If you have a Tech2, you can do a snapshot of it when it happens and go back later and look to see if anything is off.

[66LSx]

Originally Posted by ls1nova71

Anything been done to the tune other than VATs removal and emission deletes? Also, is your speedometer hooked into the VSS directly or from the speed signal out from the PCM?

If you have a Tech2, you can do a snapshot of it when it happens and go back later and look to see if anything is off.

Nothing besides just VATS/emission deletes from LT1swaps.com (brenden)
(ECM fan temps were also lowered but not related to problem)

Speedometer is hooked from speed signal out from the PCM which is why I think it needed the pickup resistor (speedhut gauges might not be able tow work on such a low voltage pulse)

(Harness is a PSI conversion HAR-1014)

I'm leaning toward potential vacuum leak... cant be exhaust leak except if its at the header/cylinder-head location. Collector extensions are post o2 as the longtubes have built in o2 bungs in them and thus there is no other exhaust 'connection' before o2

tech2 was borrowed... not sure if i can steal it for an evening to do some joy riding for a snapshot

[BR3W CITY]

My thoughts would be ; problem with VSS signal, possible issue with defuel (I'd need you to look at a wideband), small vacuum leak, or possibly a small issue with the tune.

If u were to try a 0-60-0 test, would it bog on heavy braking?

[66LSx]

Originally Posted by BR3W CITY

My thoughts would be ; problem with VSS signal, possible issue with defuel (I'd need you to look at a wideband), small vacuum leak, or possibly a small issue with the tune.

If u were to try a 0-60-0 test, would it bog on heavy braking?

I think the tune can be ruled out considering I had 100-150 miles of clean operation.

I ran the first 100 miles without a VSS signal pickup resistor (I went 100 miles without a speedo).

I suppose I could unplug the speedo pickup resistor and see if it goes away. (Maybe I'm backfeeding voltage into the ECM?) (I have my main gauge panel wired with Wago Blocks so its only a 15 minute process to remove the main gauge panel and pull the pickup resistor from the wago block)




I'm leaning toward back or side of of engine vacuum leak or Speedo ECM output pickup resistor.


on the 0-60-0 bog would result after sitting for a period of time at 0 mph.

can't try now (rain in Green Bay)

[BR3W CITY]

You'd be surprised, you can have tune issues that don't creep up until a change occurs elsewhere in the system. For example; my tune was fine for 1200 miles, until we made some changes to the brake system and found out that our TCC and idle settings were done the "wrong way" to achieve the result. We didn't find out, until we dug into unrelated issues and troubleshot backwards....know what i mean?

Specific to the problem of letting off the gas and popping; when you let off the gas, the ecm handles defuel. This setting can result in a very rich or very lean condition depending how it was setup, and can be made worse when a lockup issue is present. The only reason I come back to this is that you mention it improves with the heat.

If you were going rich on defuel, the unburned fuel would result in the backfire...however if the engine temps increased, more of that leftover fuel would be burned. I'd have to doublecheck, but I dont' recall defuel being temp based, so it would explain why that occurs in such a specific range. That and air leakin in the manifolds would be the main causes of decel backfiring etc.

[66LSx]

Will be checking all my vacuum plugs tonight along with re-torquing my header bolts to see if I have some loose culprits.

Hoping its just a vacuum leak maybe near the capped EVAP solenoid etc.