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Ryan Schott
Ryan Schott,
Category: GMC
Satisfied Customers: 83
Experience:  technician at Demarois Buick Gmc
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I have a customer with a 2010 GMC Acadia. Codes have been

Customer Question

I have a customer with a 2010 GMC Acadia.
Codes have been set for misfire on cylinders 1, 3, and 5 (essentially bank 1) and P0300 random/multiple misfire. Codes also set for P2097 (post cat fuel too rich). LT fuel trim is showing about -20%, more negative under load than at idle. Engine runs rough with obvious misfire.
Historical data shows tens of thousands of misfires on cylinder 3, with about a thousand on 1 and 5. My thoughts are cylinder three is my true misfire and 1 and 5 are either registering wrong or being affected by the performance of cylinder three. I am able to command cylinder 3 off with my scan tool and the rate of misfires on cylinder three increase, so I do not believe cylinder 3 is a "dead cylinder", but it does misfire with great frequency. Cylinder 3 has good compression (~180 PSI) that matches it's neighboring cylinders.
02 sensors all show appropriate voltage and voltage gently moves between .1-.9 V as expected. All 02 sensors respond appropriately when I induce rich or lean condition with starter fluid or causing vacuum leak.
So, cylinder 3 misfire with tons of unburned fuel on that bank. Dealership changed spark plugs and recommended catalytic converter replacement (horrible idea, obviously cat inefficiency is result of misfire and raw fuel). I swapped ignition coils and plugs but the misfire did not change cylinders. Good spark present at cylinder 3 at spark plug. So...I replaced fuel injector on cylinder 3, with absolutely no change in any codes or live data. Also, the resistance on the old fuel injector was within spec and matched all other 5 injectors. I have perfect 60 PSI on fuel Schrader valve.
I read up some on direct fuel injectors and found a TSB for the intake valves being dirty on the intake side due to fuel not being present to "clean" the valves and causing misfires due to "tumbling of air". I cleaned the back of the intake valves thoroughly, but to no avail.
Reading my scan tool all of the bank 1 cylinders have a narrowed pulse injector voltage, as to be expected by the fuel trim numbers. Honestly, it seems to me like cylinder three fuel injector is just "dumping gas" like crazy, but the strange thing is I've replace it, and also the fact that I can command it off with a scan tool which makes me think wiring is at least (partially?) functioning.
My next thought it to backprobe cylinder 3 fuel injector with oscilloscope, capture what it looks like, and compare it to a backprobe of cylinder 1 or 5 and see if the suspect fuel injector (cylinder 3) is actually receiving the appropriate pulse width/voltage. If not, maybe wiring issue or PCM not actually sending appropriate signal? Am I missing something I should do first, because this is getting kind of drastic and my customer is unhappy with diagnostic costs (I don't feel like I've screwed the pooch or been sloppy in diagnosing, though).
I'm not very familiar with modern direct fuel injection, so I am hoping to find a GM guru to help me out with this one. I will pay for you to help enlighten me.
Submitted: 1 year ago.
Category: GMC
Expert:  cloe0626 replied 1 year ago.

honestly i do believe you have covered all the bases and the only thing that makes since at this point is the pcm not sending the correct signal but you havent missed anything you have covered everything i can think of

Expert:  Ryan Schott replied 1 year ago.

Ground wire at the back of bank one cylinder head. Its the ground for your coils on that bank. You have spark but its insufficient under compression and leaving fuel unburned therein playing hell with your fuel trim. Ground 112.

Customer: replied 1 year ago.
Ryan that sounds like a good bet, I'll get back to you after I take a look-see tomorrow...
Expert:  edhunt0r replied 1 year ago.

Hello. Welcome to the site. Thanks for coming. I'm ***** *****.
I'm here to provide as much information and insight as I can, to best answer your question.

Can you give me your VIN# ***** I can see if there are any Service Bulletins?

Thank you for posting your question.

Your business is very valuable to us.
Thank You
***** *****

Customer: replied 1 year ago.
VIN # *****I visually inspected ground 112 today, does not look like the issue. With key off engine off I have no resistance between black wire (ground) on fuel injector 3 plug and battery negative. With engine running I have no resistance on brown wire to ground telling me the ECM is opening a ground as it should.Also, I pulled cylinder 3 spark plug and used test light to ground to see how strong arc was, I was able to pull an arc over an inch away from the spark plug tip. It looked similar to other cylinders spark plug too which could also pull an arc about an inch, so I'm doubting it's a spark issue (unless spark delivery timing is off).My guess is my fuel or spark delivery is occurring at the wrong time on cylinder 3. I can visually see I have good spark, and I know fuel is making it into cylinder because I have unburned fuel being read on that bank by 02 sensors.Maybe a bad crankshaft position sensor (like bad teeth not reading on tooth reluctor wheel) is giving ECU bad timing info for spark?
Expert:  Ryan Schott replied 1 year ago.

Any cam or crank issue in this will almost always after 08 throw a DTC. Chain issues are consistent in these but usually will throw a cam code( not always the right one but whatever). Try doing a leak down at low psi and let me know what you find. Sorry about the delay in response its the weekend so my dirt bike was calling.

Expert:  Ryan Schott replied 1 year ago.

Oddly enough today a 2010 Acadia. 3 is my main misfire spark is good but I have unburned fuel in there. I'm going to run the AFIT and test the injector then I'll do a leak down but I'm probably going after a valve I'll shoot you some more pics once I wrap up.

Customer: replied 1 year ago.
I don't have an AFIT...will do a leak down...when you say low PSI what are you talking?
Expert:  Ryan Schott replied 1 year ago.

I was just commenting on one that I was working on that was similar. Keep your psi at about 80 for the leak down. Look mostly for valve leakage and let me know what you find. I've replaced quite a few heads on 3.6l.

Customer: replied 1 year ago.
I ran a leak down compression test on cylinder three and it held 80 PSI perfectly. Uggg. One thing I saw in Alldata's explanation of the fuel system is the the ECU sends a high voltage (~65V) signal to the fuel injectors to open up high pressure to overcome cylinder compression. My thought is to back probe cylinder 3 at ECU (which I believe is super easy to get to) and see if I'm getting that high voltage peak regularly on o-scope. If it was only sending the low voltage it's never overcoming compression so there could still be raw fuel in exhaust and spark present and a cylinder misfire. Other ideas?
Expert:  Ryan Schott replied 1 year ago.

you can check if you have a good scope, you can switch the pins for injector one and three at the ecm then swap the plugs At the injectors to verify you have no drop in your circuit. if you weren't getting sufficient power to the injector though i would suspect you'd see a lean condition. did you get a look at your cam lobes when you did the leak down? That one i mentioned the other day needed a cyl head, luckily it didn't ring wash too bad. let me know if alldata doesn't have the connector views for the ecm and i will shoot them too you after you scope it if you want to swap those circuits. what is the mileage at on the vehicle i will call GM TAC and have them check all closed cases for cyl 3 misfires and see if there is something im overlooking.

Customer: replied 1 year ago.
Have you seen them hold good leak down compression and still need a head?
Customer: replied 1 year ago.
92,000 miles
Expert:  Ryan Schott replied 1 year ago.

yah if it had a physical cam issue,let me check resolved cases and ill regroup with you shortly.

Expert:  Ryan Schott replied 1 year ago.

called and checked same story there he said most of their cases for this end in a cylinder head assy. always a right bank too. do you have it together enough or a quick road test?

Customer: replied 1 year ago.
Customer has vehicle but I could get it back, what are you looking for in a road test? Also, how would a physical cam issue cause misfire but with good compression - are you thinking physical cam damage threw mechanical timing off?
Expert:  Ryan Schott replied 1 year ago.

a worn lobe will let you have compression but not open sufficiently, if you can monitor the misfires live with your scan tool at a pretty hard acceleration and it doesn't miss id say it still looks like a valve issue.

Customer: replied 1 year ago.
I recall hard acceleration did reduce misfire. Trying to get vehicle back and I'll tear the valve cover over and look at cam
Expert:  Juan Crespo replied 1 year ago.

After reading about all the troubles GM is having with "cam walking" and other such mishaps, I'm with Ryan in thinking this is most probably a cam issue and the cure will be none other than a new head - the customer also mentions a well known malady with these direct-injection systems; and that is carbon buildup. An issue worth discussing with the customer post-repairs.

So, how about performing a dynamic or "running" compression test on that No. 3 cylinder? At idle, compression should be about 50-60% of static or "cranking" compression. Snap-throttle compression should be about 80% of static compression - a lot less than 80% would indicate problems with the cam and/or carbon buidup.

Best Regards ***** *****

Expert:  Ryan Schott replied 1 year ago.

Let me know and try and monitor that live again too before you tear it down.

Customer: replied 1 year ago.
I test drove it and it did lessen misfires when under load. Took the valve cover off and observed a vertical gouge in #3 intake cam lobe (the intake lobe closer to back of engine). See picture (sorry for crappy quality). Also, I could feel a groove in the cam lobe (see picture where I am pointing). This groove was over 0.010". I'm pretty sure this is my issue, thoughts? Also, the cam lobe height was within spec.
Expert:  Waverly Lorello replied 1 year ago.

Have you been able to command the o2 sensors on that bank full rich with propane or full lean with o2 sensor

Expert:  Waverly Lorello replied 1 year ago.

Also what is you thought on pulling the engine down?

Have you found a reason other then a groove in cam lobe?

have you ever seen carbon build up in a direct injection engine?

I have pulled so many 3.6 gm apart but never for any of the problems you are thinking

timing chains, carbon build up, cam phasers

Never heard of cam walking in head

This question has been open for 20 days lets get this car fixed

Expert:  Waverly Lorello replied 1 year ago.

Also after looking at your first post you say the cyl 3 injector you replaced did you check to see if the injector is stuck on by a shorted driver in the pcm

Customer: replied 1 year ago.
Hey Waverly, thanks for your ideas. If you read my original posting I have verified 02 sensors by introducing lean and rich conditions and the voltages respond appropriately. Also, I found a TSB on carbon buildup (and read up more on the internet) on DFI systems and thoroughly cleaned the back of the intake valves. I was able to command the injector on and off with my birectional scan tool, and the cylinder became a "dead miss" when turned off. However, the condition has worsened and the cylinder is essentially a dead cylinder now. I am more concerned by the "groove" in the cam lobe than the gouge, although I must admit a little skepticism that is the root cause...I've seen much rougher cam lobes running better than this thing...however that was on older style fixed camshafts, so I'm left to wonder if the variable cam timing exasperates the cam lobe wear.
Expert:  Waverly Lorello replied 1 year ago.
What kind of scan tool are you using so i can send you some vvt information
Expert:  Waverly Lorello replied 1 year ago.
if it is snapon what version of software is it
Expert:  Waverly Lorello replied 1 year ago.

I think you need to focus on the high pressure fuel pump

Expert:  Waverly Lorello replied 1 year ago.

here is the picture of the pump

does that cam lob that is damaged correspond with where the pump bolts in the head

Expert:  Waverly Lorello replied 1 year ago.

And here is the fuel system overview go to the hi pressure pump description

Customer: replied 1 year ago.
The high pressure fuel pump is on the #2 bank so no it is not on the bad cam. I am using a MaxiDas 708. The fuel rail pressure sensor is reporting appropriate data, and bank 2 and cylinders 1 & 3 generally do not misfire. So I don't really see how the high pressure fuel pump could be issue and effect only 1 cylinder, and I know it's getting fuel as I have a rich condition in the tail pipe. The ECU could be not supplying the high voltage signal, or wiring could be bad. As Ryan said a bad ECU high voltage would most likely cause a lean condition, and I can command the fuel injector on and off on cylinder 3 so I know the wiring is good and the ECU can command on and off the cylinder.
Customer: replied 1 year ago.
Also,I thought I would note for general reference that the misfire occurs in open and closed loop. It starts immediately with the car and the ECU can't hold the RPM's steady, it run's +/- 75 RPM with the misfire although it does smooth out a little at higher speeds as we discussed.
Expert:  Ryan Schott replied 1 year ago.

Yep that's a valve sealing issue and not a fuel one if load improved it, a groove isn't good but shouldn't totally kill it if you can get in there with a mirror or boroscope pry the springs around on the valves as sometimes they fracture and still hold shape makes them very hard to spot. end o the day though the cam should not have a groove period, the head assy is an updated unit GM couldn't really specify the change when i asked though but obviously they have issues, might not be a terrible time to throw that leak down back on and manually turn it a few times and tap on some things and see if it decides to show some leakage. but most all fuel or spark management misfires will be static under load when monitored via scan tool, all misses 'feel' better at high rpm but when monitored will be consistent. but there should definitely not be a groove in the cam, i'd still service it as an assembly though, wouldn't look too hard at that fuel pump.