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Fuel Trim issues

posted Jun 20, 2016 20:17:25 by XxBlksnshnxX
I have one that is stumping me right now. It's an 81 G20 that we put an 6.0 in a couple years back. Customer brought it back last week reporting no power, backfire and smell of raw fuel.

I drove it and it ran fine for a while, then all of a sudden it started running horribly, backfiring, etc.... When it starts to run bad, The Bank 1 Sensor 1 o2 sensor shows about 46Mv and sticks there. At the same time the ST Fuel trim for bank 1 spikes to 52%. Bank 2 remains normal and LT always remains at 0 (maybe part of the tune, we outsourced the tune). I did try putting an o2 in with no difference.

After the van sit's, It will run good again for a while until it decides to act up again.
[Last edited Jun 20, 2016 20:19:08]
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16 replies
juergen scholl said Jun 21, 2016 01:46:42
Are you positive about the year 81?
XxBlksnshnxX said Jun 21, 2016 13:13:39
Yes, Positive.

It has a 6.0 liter engine from an 04 Silverado.
XxBlksnshnxX said Jun 28, 2016 20:43:35
After putting this one on the back burner I've started working on it again today.

Checked more thoroughly for vacuum leaks with no results. I'm beginning to wonder about the PCM.

Here is the Van, Don't think there are too many of these with Fuel injected 6.0L running about.

[Last edited Jun 28, 2016 20:44:21]
Tyler said Jun 29, 2016 02:04:08
Wow, nice! I would never expect to see that when I pulled the doghouse off that van, lol.

So, the B1S1 O2 flatlines lean and the fuel trims peg in response. I think I'd be most interested to know if this is a true lean condition, or if it's a false lean, and there's an O2 circuit fault.

A easy way to tell would be to pull a vacuum line and add fuel to the intake while the O2 signal is stuck lean. If you can't get the O2 to swing rich, then that supports a circuit problem. If it goes rich, then we can look into how one bank is suddenly going lean.

Next thing to do would be to unplug the B1S1 O2 with the signal voltage stuck low. On your average road-going GM, the signal voltage should go directly to 450mv, as this is the bias voltage provided by the PCM on the signal wire. This gets tricky, though, since this has been tuned, and may not play by the same rules.

Let us know how those tests go, and we can take it further.
PaulDanner said Jun 29, 2016 02:09:20
Couldn't have said it better myself. Thanks Tyler!
And XxBlksnshnxX welcome to the forum! Thanks for posting your question here. It's a lot easier to help with my friends having my back :-)
XxBlksnshnxX said Jun 29, 2016 13:07:07
Thanks for the response Tyler.

I have not tried adding fuel to the system yet, That will be my next step today.
I did unhook the o2 and voltage did return to about 450mv.

I think it's also worth noting that I can smell raw fuel when this problem is happening.

I agree about the tune, I have a feeling it is making this much more difficult to diagnose than if it were just in a untuned truck.

Thank you Paul for the welcome, I've been a long time viewer of your channel. Your videos have helped me learn a lot over the past few years.

Here's a pic of the interior for entertainment.

Tyler said Jun 29, 2016 17:17:52
No problem, sir! The fuel smell suggests a false lean, and that bank one is getting way over fueled. You could also pull a plug out of each bank and see if bank one looks darker.

Let us know what you find!
XxBlksnshnxX said Jun 29, 2016 19:44:37
Looks like I found the issue. The harness that connects right at the coils was getting a bad connection. I was moving things around to put some propane into a vacuum line and it started running better.

Looks like my biggest mistake was skipping the simple step of a wiggle test. I sometimes jump straight into the technical issues and miss something simple.

Thank you guys again for your assistance.
Tyler said Jun 30, 2016 01:25:11
Ahhhhhh, at the bulk connector for all the coils on one bank, right? That'd certainly explain your symptoms, then! Four ignition misfires would add up to all kinds of fuel trim issues.

I dunno if I'd consider not wiggling the wires a mistake, as I think it's too easy to 'fix' something inadvertently. Given how this issue was intermittent, and how the van is 'unique', it's easy to see why this one would be challenging.

Got any more photos of the engine for us?
XxBlksnshnxX said Jun 30, 2016 03:42:15
Another thing that threw me off was that the misfire counter was showing 0. So I'm guessing that was tuned out for some reason. I plan to start doing my own tuning after doing a lot of research and practicing on my own vehicles.

Yeah, I have some more pics, I'll upload them when I get to the shop in the morning.

This was really an interesting build, I couldn't find an instance online of anyone else doing it.

It started because the original 305 was burning some oil and the carb was leaking fuel and the customer hated the smell. I told him we should LS swap it and he wrote us a check for 8k to get started. Lol

It was his third build in the shop at the time. We also had his 02 wrangler and 75 vette for frame of restorations.
XxBlksnshnxX said Jun 30, 2016 13:41:00
Ditching the old 305

Just fit the A/C

Wiring still everywhere

Dylan said Jun 30, 2016 16:25:38
Awesome! Been following this topic. Nice job.
Belgium, Europe
XxBlksnshnxX said Jun 30, 2016 18:51:15
Thank you sir.
Tyler said Jul 01, 2016 01:01:35
That looks amazing, sir! Nice work with the bracket for the A/C compressor, wouldn't fit in the normal location due to the frame?

The old engine looks like it just *barely* came out of there, too.
XxBlksnshnxX said Jul 01, 2016 13:47:56
Yeah, there was no chance of the A/C fitting in the stock location. It's actually a totally different pump then what came with the van or the engine. We also built a crossmember to fit the trans, and a custom drive shaft going back to the disc brake rear end.

It was a tight fit but not too horrible to do. lol I really enjoy doing this type of work. I'm in Michigan so the majority of stuff I see is rusted out junk. So to work on this 30 + year old van with not one spot of rust on it was a pleasure.
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