I rarely post nowadays because 98% of what I need I find by searching or trial an error. I had to fly out for work and I'm 700 miles from home so bare with me but I'd like to have a list of things to tackle when I get back home. Since I'm getting into tuning I'd like to learn to diagnose properly when something goes wrong, I hate throwing parts at it hoping something finally fixes it.

The truck started acting up about two weeks ago. It only gets driven a few miles a week. Recently it developed a nasty stumble on acceleration/cruising ONLY when the engine is warm. These last few drives it finally triggered a p0101. Mostly everything I've read leads to problems with the tune. It ran fantastic up until two weeks ago.

Nothing has been changed or replaced other than fueling up when it gets to 1/2 tank. I replaced the MAF the day I got the code (lifetime warranty, aftermarket). It made absolutely no difference. I checked for codes again and p0101 came back. I did notice ltft were pegged lean on both banks. +25%. I replaced the fuel filter (corvette style with reg), and though it was dirty it made no difference either.

Figured it was time to look for a vacuum leak. Brought it to work the next day and smoke tested the intake. Found a very small leak at the pcv connection, tightened the clamp and made no difference.
I managed to get a log right before I had to fly out and e-mailed it to myself.

I'm thinking its a problem with the 02s although they seem to be switching fine and at the same rate. I've also read it could be caused by a bad map sensor. 0 misfires across the board so I really doubt its an ignition problem? I did unplug the maf in my driveway and it ran worse.

The truck is a 91 s10 blazer with a 5.3. ls6 intake/injectors, shorty headers, asa cam, mild stall, frost mail order tune. I've added the tune and log files

Any help or ideas will be appreciated!

 

Total shot in the dark here, I had a friend with similar symptom and his flexfuel 2012 NNBS (without a real flexfuel sensor, the ECU "guesses") but it had a bug in the OS where it thinks it's running on E85. Maybe you're E85 and it thinks it's gas...?

Giveway would be the Ethenol PID showing like 40% when it should be single digits or vise versa. Was visible in Torque Pro.

Both banks I think were rich at -25% I think for him, throwing codes, running bad. Happened right after a fillup (which you mentioned), when the ECU does its guesstimate of the ethanol content. I think the opposite direction rich/lean than you but maybe the bug works both ways?

Not sure. Worth a check. Fix for him was to use a Tech 2 and do an ethanol relearn, which fixed for a while, then he paid a tuner to disable flex fuel and fixed permanently. Again, no idea if this relates to you but other than the flip of the fuel it sounded like it figured I'd throw it out there.

Edit: this is what my friend's vehicle was doing:

 

Thanks for the insight. I don't think my OS even supports e85 although the symptoms do seem very similar. I should also mention that if I go straight to half throttle from a stop it runs great although with it running lean I've only done it a few times.

One other thing I forgot to mention that I did do recently was add A/C. I hooked up the compressor to the stock wiring but I did run the green a/c wire to my fan controller to kick on the fans on high and also up to the a/c request wire on the pcm. It may be irrelevant but that is the one recent change I did.

 

Are you getting power to the MAF? With it running, is the MAF putting out any voltage on the signal wire?

 

It would be helpful if you would log MAF frequency. You say it's an aftermarket MAF? can you explain?
Also, at times it's behaving normal, then out of the blue it's adding fuel like crazy. It can't add fuel that isn't there. Knowing what the fuel pressure is while driving would be very helpful.
I looked at your log file. noticed it died once as well.

Ron

 

Quick update since I've been back.
​​​​​I refused to just throw parts at it but this mid throttle stumble is driving me nuts. With a fresh paycheck and lifetime warranties on most everything I replaced a bunch of things.

I've followed the diagnostic tree for p0101 to the t.

​​​​​​Despite seeing good readings i've changed
the map sensor with another gm map.
O2 sensors with denso sensors.
Replaced maf with a Delphi unit.

I checked continuity between pcm and all 5 maf wires.
I have good battery voltage on ignition wire, good ground. Yellow signal wire and iat wires all ohm good.

I thought my cheap intake elbow might be collapsing so I connected the maf to it.
​​​​​​
Fuel pressure is good. I replaced fuel filter and In tank fuel pump just in case.

Still exactly the same.

0 misfires across the board. I don't think it's ignition related.

As soon as I unplug the maf it runs way better, just very rich.

I'm at a loss! I added a screenshot of my "lean spike" stumble. Does anyone see anything abnormal? I added the Hz channel to the maf if anyone is interested in seeing the log.

​​​​​​

 

If you fail the maf and run off just the ve see how your O2's are working then. Are you sure you don't have a vacuum leak? Or an exhaust leak near the 02 sensors.

 

Can you just post the actual log file?

 

A few months later but I'm back. I went ahead and installed a wideband to be able to monitor what's going on better. I quickly found out that the stumble is a lean misfire for a few seconds. I cannot figure it out.

To summarize what I have done so far

New plugs
replaced fuel pump with AC delco
new fuel filter (corvette style) - I backflushed the filter and the fuel was nasty and a little rusty, but has not made a difference
three MAF sensors with the current one a brand new Delphi unit
Brand new Denso o2 sensors
Two MAP sensors I borrowed from my other vehicles and RTVd in place
New exhaust gaskets
I have smoke tested the intake system cold/hot. Found some pinhole leaks but none have made a difference.
Checked continuity and resistance on all MAF and MAP wires. Resistance is very low on all and no short to ground.
Voltage drop tested all power wires

Tonight I removed all injectors and cleaned them with a neat contraption I found on amazon. The spray patterns looked good even before cleaning. No dirt on the screens




When the lean stumble happens it triggers a p0101 and it switches to speed density and stops stumbling but still leans out.

If I hammer the throttle and stay above 50% tps my fuel trims look pretty decent . I don't think I'm losing fuel pressure but I can't measure it I only have a handheld fuel pressure gauge

I'm out of ideas!! I've attached tonight's log if anyone has any suggestions

 

This is going to sound really odd, but I would take a look at your fuel pressure regulator. If it's degraded or partially failed, it can hit a point where it hits a harmonic and drops in pressure, then kind of recovers and works semi-normally.

May be entirely wrong. It seems to be the only parameter your system had no sensor for.

 

So... what is the latest symptom/issue after all the parts swap? Mid throttle stumble lean issue?

 

I hope you find the 1 bad fuel injector that caused all these problems. A thermal camera would be of use in this situation.

 

Funny you should mention fuel pressure. I started working my way back and took off my "new fuel filter". full of crap again. took out my "new fuel pump" and this time took a harder look at the tank. Last time I swapped them it was almost midnight. I hope I found the problem.

I have a new tank, filter and pump on order. I'll report back as soon as I put them on!

 

Mucho gunkitude....

 

Looks like a tank that's had diesel pumped in to it at some point...

 

I would also get your injectors cleaned in a real injector machine after seeing the inside of your tank. I have the deal you posted a picture of and also a top of the line injector machine, cant compare the two.

 

Ever come thru the fix?

 

I never found the root cause. I ended up buying hp tuners, putting it in speed density and been driving it like that since. It still leans out in open loop and runs great once it warms up.
I'll be pulling the motor to fix an oil leak in the next few months.

I'm hoping for an aha moment once I take everything apart