Start and Stall on LS3 Performance

Matthew Owner/Technician Oregon Posted   Latest   Edited  

I'll be the first to admit I'm not a performance guy, but this one has me pondering a bit and I'd like some feedback. This is a year old install of a LS3 non Erod from Chevrolet Performance Kit number … This shop is a well known and very meticulous restorer and has done dozens of these things before. This one was put in about a year ago and has been to the customer and driven around without issue. It came back to the shop and they put in a window regulator (mechanical) and waxed the car, did a 10 mile test drive and parked it. Next day it's a start/stall just like a security issue. 

The Tech2 has a option for performance part installs and that's what I've been using for diagnostics. No codes. I found the MAF walking a little bit KOEO so replaced the MAF with no difference, but it no longer walks voltage. No diagnostic codes of any kind. Sensors all look good, Map, Baro, IAT, MAF, Stays in open loop, 450mv O2 activity (or none rather). I have checked and checked and there is no security on this install or vehicle and it has worked well for a year. Then I went to verifying powers and ground, and see what it's losing. When I supply propane the vehicle will continue to run. I put a scope on the Injector amperage, voltage and coils to see what was dropping out and As soon as it starts the injector pulse width gets smaller every time until it stalls. It stalls very fast, just like a security shut of would be like for reference.

I can disconnect the IAT (MAF) and coolant temp sensor and it will fatten out the mixture enough to run, but very badly. It appears it's being commanded to narrow the injector pulse width but I can't imagine why or what's doing it. I don't have it run long enough to have O2 sensor activity. The only anomaly I found there was when I connected my leads to the front O2's the voltage dropped from 450mv to 250 mv, but never saw any activity (I thought if there was voltage leaking from the coils or injectors into the O2 the PCM might be picking that up and seeing O2 activity and leaning it out that way, but didn't see any activity at all.

I've done a fuel sample, flow and pressure. It stays high all the time and flow continues after the vehicle stalls for half a second. Powers and grounds all appear good. There are multiple redundant grounds because of the build of this and I see no voltage drop across any of them. 

Does anyone have any insight they can give me before I call a PCM on this thing for logic lock issue? I've cleared adaptives, Done a hard reset on the PCM shorting the cables together, and so far, nothing has changed. Start and stall immediately. The restorer has a fantastic reputation and has assured me he didn't jump start it, had no issue at all until this out of the blue. 

Thank you everyone for any input. 

Edit: I've also scoped the cam and crank sensors. I haven't check correlation but don't see any drop outs there and everything looks good as far as I remember these reluctors being. 

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Interesting
Harm Technician
Ontario
Harm
 

What is the load calculation? Sticking torque convertor?

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Matthew Owner/Technician
Oregon
Matthew
 

Load calculation is good. Adjust load looks high to me but I'm not sure how that is calculated on a starting engine. Would the torque converter account for the narrowing of the pulse width?

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Harm Technician
Ontario
Harm
 

It all depends on how the whole setup has been tuned. Narrowing the pulse width would be trying to richen up the fuel mixture to keep running. I would try to unbolt the torque convertor and see what happens.

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Matthew Owner/Technician
Oregon
Matthew
   

Wouldn't you want a wider pulse width for a richer mixture? Longer on time= more fuel. The tune is nothing fancy. No HP tune or anything like that. It's the bone stock performance tune out of the box.

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Richard Instructor
Florida
Richard
 

Has it been like this “ever since” the window regulator repair? Is it possible a ground, plug or door jamb switch got disturbed?

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Matthew Owner/Technician
Oregon
Matthew
 

Unfortunately no. Window repair. Wax. 10 mile test drive. All good. Next morning it started this.

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Paul Mobile Technician
United Kingdom
Paul
   

Is there something not being mentioned here - why would you take a car on a 10 mile test drive after a window repair and wax ? Other than MAF the only thing I can think of is the fuel pressure sensor reporting high. - Did they wax the engine bay…

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Matthew Owner/Technician
Oregon
Matthew
 

No fuel pressure sensor that I know of. It's a 60ish psi system. I don't see one on the wiring pinout supplied. Test drive was the final step on the “check out the car and fix things please"

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Scott Manager
California
Scott
 

Hi Matthew, I have quite a bit of experience with these systems and have found many times that the specific instructions for MAF placement were not followed properly. If the MAF is not placed in the tube GM engineering has outlined then you'll have trouble in most cases. There's also this note in the instructions: If the under-hood configuration cannot support this configuration then you may…

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Matthew Owner/Technician
Oregon
Matthew
 

Scott, Thank you so much for the comprehensive reply. I'll look into this today. Is this something that is initial install or could this problem arise after it's been driven 2k miles? Curious. It wouldn't be the first time a car woke up and decided to be “unhappy” with something that worked fine previously.

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Scott Manager
California
Scott
 

Hi Matt, I would be suspect that there was a battery disconnect somewhere along the way and that the adaptives are showing how hard they are needed. It’s very possible that the cylinder air flow calculations measurements are so far off that it has taken quite a bit of long term adaptive memory to compensate. Can you record RPM, MAF, MAP, LTFT & STFT and share that with us?

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Matthew Owner/Technician
Oregon
Matthew
 

I'll do that this morning. It will be a bit as it's a little drive to it. I'll probably have to do a phone recording off of the tech 2, unless it has a way to record I haven't used. I've disabled the Map yesterday to make sure it wasn't going into fuel cut, but nothing new. Thank you for the help.

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Nick Service Manager
Indiana
Nick
 

Have you considered a large vacuum leak causing this? Is this car running vacuum brakes? The shortened injector pulse width could be due to an inaccurate MAF reading. It would also explain why the car will run with the fuel trims forced rich, albeit poorly.

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Matthew Owner/Technician
Oregon
Matthew
 

The car doesn't have vacuum brakes. A separate brake pump and controller so no vacuum there. I would think a vacuum leak would show the pulse width widening to account with the STFT changing, but it doesn't hit closed loop. So perhaps it's too quick an event. Supplying alternate fuel it should pick up and fatten the fuel to run when it's been running for a few seconds tho and that's not what…

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Nick Service Manager
Indiana
Nick
   

If it will run long enough to use the o2 feedback to adjust fuel trims, then it will shorten them if you are running it on alternative fuel. This engine should be able to run off the MAP input if the MAF is unplugged. Does the throttle body respond to accelerator pedal position? Pcm failure on these isn't unheard of either, but I've been burned on a very similar issue before.

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Caleb Diagnostician
Indiana
Caleb
 

Dont see anything on ECT. Is it stuck at say normal operating temp? Around 200 degrees for example?

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Matthew Owner/Technician
Oregon
Matthew
 

Ect is at ambient temperature when cold. Goes up a few degrees after started a few times. Goes to - 40 when unplugged

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Scott Owner
California
Scott
 

Hi Matthew, In addition to what Scott posted here's a link to the instructions - chevrolet​.​com/content/dam/ch…​.​pdf

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Eric Owner/Technician
Wisconsin
Eric
 

I agree with what Scott posted. I've worked on a few dozen of these installs and if the MAF isn't installed correctly you will have poor running/start die problems. I have also seen a large number of the MAF installs just done wrong, the attaching screws left loose, trying to glue the MAF in place with silicone, the MAF installed backwards in the air tube, the air tube installed backwards and…

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Matthew Owner/Technician
Oregon
Matthew
 

Thank you so much for your input. I'll double-check the MAF sensor. As we were placed it initially due to the reading Walking with key on engine off. This vehicle has started and ran and has over 2,000 miles on it. This problem just popped up out of the blue. It has worked previously up until the morning it did not. I agree the injector waveforms look like garbage. That is the proper setup. I…

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William Owner/Technician
Pennsylvania
William
 

Matt, Assuming this is an E67 PCM, if you disconnect the MAF it should run on the VVE tables as long as the MAP sensor is reading correctly. Scan the MAP PID, KOEO, and make sure it is reading somewhere close to the KPA you should see at the altitude you are at.

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Matthew Owner/Technician
Oregon
Matthew
 

William, I agree completely. This should do exactly that and even tho the numbers are correct it does not run with the MAF unplugged.

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Matthew Owner/Technician
Oregon
Matthew Resolution
 

So…This vehicle is fixed. Or as fixed as my customer is allowing me to call it fixed. After I got it running he wouldn't let me touch anything to take another set of readings off anything incase something went wrong… Alas. Thus begins my tale of woe, and I'm open to all thoughts and ideas of what I could have/should have done differently. I have a list myself. The MAF was installed…

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Jamey Owner/Technician
California
Jamey
 

No flogging needed here. Just live and learn. I will agree that allowing the customer to become involved in the repair process is a huge mistake. That is one reason why I will not diagnose cars for the customer to take home and repair himself. When you or I diagnose and repair, we are still diagnosing during the repair. We may see or find something that changes our minds on the original…

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Thanks