Vortecs, fuel trim and o2 sensors (1 Viewer)

[GetItBilly]

I've had an '88 with a 2003 5.3L Vortec for a couple years now, and as the seller was very open about, it runs fine (at least it runs smoothly and never stumbles) but does throw p0135 and p0155 codes for the front O2 sensors consistently. And sometimes I get p0171 and p0174 for a lean system, too. I've done some reading and now I need any pro tips on where to go from here.

-O2 sensors check out fine on the usual voltmeter and ohm tests
-Hooked up bluetooth OBD2 scanner to my phone and can see closed loop being reached, both sensors oscillating between about .1v and .9v at idle.
-I did notice that at idle, long term fuel trim is up around 20%, but it comes down closer to 5% around 1500rpm. This truck does have a cold air intake.
-All the above is after cleaning the MAF sensor, which didn't change any behavior.

Should I be focused on looking for leaks? I've read some people think the O2 sensor location matters and others that say as long as you make it to closed loop you are good....

 

[Elbert]

I'm no expert but I think its very clear that location of the 02 sensors in the exhaust stream do matter relative to the proximity of the exhaust manifold and other components. Also that one needs to replicate as close as possible the OEM setup on the overall exhaust system.

 

[GetItBilly]

Here's where mine are....too far back?

 

[White Stripe]

Try a new maf sensor. If you know your oxygen sensors are good and your maf is good then you can work from their. The MAF and oxygen sensors work together to determine fuel trim. So make sure they are good first.

 

[GetItBilly]

After reading about o2 sensors, I think I understand correctly now that the placement of the sensors shouldn't matter as the p0135/p0155 codes are only checked/thrown at ignition and indicate something is wrong with the heater circuit (I mean it may matter for o2 readings I suppose, but shouldn't matter in terms of 135/155 codes). Mine only throws the codes on average around the 5th-10th startup.

 

[AggiePE]

My O2 sensors are located right in the exhaust manifolds. Not sure how far back is too far, but as was stated above, the close the better. I was having trouble with a high idle trouble code and Found the intake manifold leaking on the front DS of the manifold.. checked it with a can of B-12 carb cleaner... might be worth a shot....

 

[workingdog]

Fuel trim is up? That means it's adding gas? Is it both banks evenly, or is it mostly one bank? Yes, spray for leaks - that is one explanation. Is this the stock tune in the ECM that came with the motor? Was the tune ever looked at?

 

[NCFJ]

The intake manifold gaskets are a common fail on these engines and certainly can be part of your issue. Pretty easy to replace. You may want to swap out the knock sensors too while you have the intake off, they are located under the intake.

 

[GetItBilly]

Fuel trim is up on both banks. Fuel Injection Specialties did the ECU and harness when the Vortec conversion was done.

Yeah, I started on a homemade smoke tester that kind of fizzled...I'll prob just take it to my mechanic and have them smoke test it.



Thanks, guys.

 

[workingdog]

Did FIS check the tune? Is it possible it just has the wrong tune?

 

[GetItBilly]

The FIS stuff is 10 years old now, I just bought the truck 2 years ago.

 

[workingdog]

I have someone hook up to the EMC with something real like HP Tuners or EFI Live and see what's happening. It may not be mechanical - it may have always been this way.

 

[AggiePE]

You can also call/email brendan over at lt1swap.com - good guy and only charged me $75 to retune my ecm after my LS swap.

 

[MANUCHAO]

^^^ X2 on LT1SWAP....

FIS stuff was crappy at best.... at least that was my experience back when.....

 

[GetItBilly]

Thanks for the link to lt1swap. Looks like I need to figure out if the ECU was original to the engine before I mail it out...I'll save that for the winter when the truck is parked. But considering the age of the swap and me thinking not much has been touched under the hood since it was done, there's a good chance there is a vacuum leak, too. But for $75, why not get the tune checked, too.

 

[GetItBilly]

I took the winter off from this problem but am back at it with a diy smoke tester off of eBay. Found a very small leak at the end of a hose back by the brake booster, fixed it, but long term fuel trims were still up both at idle and 2500 rpms. My fuel pressure is right where it needs to be around 60psi at idle.

LTFT was around 8 at idle. I unplugged the maf sensor while the truck was running and it made a very slight stumble but kept running fine. Idle LTFT went down to .8 on one side and about 1.6 on the other, which I think is where it should be. Trims also looked good at 2500rpm with the maf sensor sensors unplugged. So I'm off to order a new MAF sensor, but also keeping in mind that the Sceptre cold air intake may be playing a part in this as well.

 

[Beams37]

I would consider reaching out to @FJ60Cam ... He knows his stuff.

 

[GetItBilly]

Hmm, reading some more about this, and you guys may be right about the tune being wrong. I looked up my ECU # and it appears to have been most commonly used with a W3500 Van with an 8.1L. Then I found this on another thread on ih8mud:

That being said the ECU was programmed for the wrong Mass Air Sensor which gave me some troubles untill i hooked it up to a scanner and figured out that it was trying to richen the mixture because the Mass air sensor was telling it to run too lean. Basically they can either be programmed to run a truck mas air meter (silverado) or one out of a van. I had the truck mass air meter and it was programmed to run on the other.... Willie at FIS fixed me up though and reprogrammed it fo rthe one i had (which also came with my motor).

So perhaps the ECU is programmed for a Van MAF and O2 sensors, which wouldn't play well w/ my truck parts.

 

[GetItBilly]

New MAF sensor didn't change any behavior. Still at around +19 on bank 1 and +16 on bank 2, then comes down a bit to +9 and +6 at 1500 and 2500 rpm. Still seems like a vacuum leak but I can't find one.

 

[scrapdaddy]

I have a 2004 5.3 in my 55 and was throwing those 02 codes, couldn't get rid of them. I found a company that does reprogramming and wiring and they just eliminated the heater circuit in the computer. He said I didn't need the heater circuit anyway, if that simple explanation makes any sense. My codes are gone and she's running fine.