Another P0300 issue which only happens when the MAF sensor is connected

[BillM1965]

Hey GM truck fans I have been a member for a few years now but have only posted once prior to now. I have a strange issue which I am hoping someone can help me with my truck is an 07 Sierra with 300,000 miles and two months ago my AFM lifters failed on cylinders 4 and 7. To make a long story short lifters were replaced cam shaft inspected to reveal no damage and the truck put back together. I have not been able to drive it though due to multiple random misfire codes being set. Here is the problem.

 

Misfires only occur with the MAF Sensor connected if you disconnect the MAF Sensor the truck runs very well and only throws codes associated with the MAF sensor.

 

What I have replaced to date:

 

Plugs and wires

All Lifters (yeah I know should have done the DOD delete but decided to leave it original since it has 300,000 miles on it)

lifter trays

VLOM

MAF Sensor

O2 sensors all 4

Cleaned the Throttle body and relearned the idle 

attempted the crank variation relearn

 

Vacuum reading = 19.5 inHg

Map reading = 10.5 inHg

Compression across all cylinder varied no more that 5 psi with an average of 172

Fuel pressure KOEO = 60 psi

                        KOER= 57 - 59 psi

 

With the MAF Sensor connected the truck idles real rough and misfires and the O2 sensors seem to go nuts and the fuel system goes to OPEN LOOP - Fault. Once I disconnect the MAF the O2 sensors operate as though they are in an engine in excellent condition and the fuel system goes back to CLOSED LOOP operation the truck does not miss a beat it runs real smooth.

 

I am currently at my wits' end with this it has been two months and I just want my truck back any help would be greatly appreciated

 

[BillM1965]

Forgot to mention that I have also changed the fuel injectors because I found three leaking into the intake manifold.

[CamGTP]

Did you use a OEM MAF?

[BillM1965]

I thought about buying one from an after market supplier but changed my mind even though as usual the one I got from the dealer cost 2.5 X more. So to answer your question Yes.

[CamGTP]

My guess was an aftermarket MAF or it's installed backwards.

[BillM1965]

There is a big arrow on the side of the housing to indicate flow direction, how could anyone install it backwards (but I have heard of stranger things I guess).

[richard wysong]

does the injector pulse width increase or decrease or no change when you connect it? I guess the best way to go about it is to find out what's causing the misfire. I'm thinking possibly timing problem  or maybe AFM if the fuel looks ok. timing really doesn't make sense where it runs normal disconnected but you may want to dig out the timing light and see what it does when connected v disconnected. could be a bum ecm but that's usually a last ditch cross your fingers and fire the cannon. miss the days where you could swap them without reprogramming. no problems doing the lifters?

[richard wysong]

the MAF is a reference sensor, it doesn't control anything it lets the ecm know the air going into the engine so the ecm can adjust the control outputs for maximum efficiancy'  so determining what the ecm is adjusting to cause the misfire should help with diagnosing. is there any chance a ground got left off or there was any wiring that may be was damaged doing the lifters? 300k things get brittle. you could start by checking the wiring to the MAF for correct voltages, ground continuity and continuity of the signal to the ecm, there may be a relay for the MAF as well. I'm betting there's a bad ground somewhere

 

[BillM1965]

Completed all of the electrical testing last night and did not find any issues. Attempted a different test removed the O2 sensor on Bank 2 upstream and I am beginning to think it may have a clogged cat causing the issue but I need to do further testing. I am going to pick up a back pressure tester on my way home from work to see if that will tell me. As of right now I have done the IR temperature check on both sides of the Catalytic converters the one on bank 1 reads 250 top side and 450-500 degrees on the bottom, the one on bank 2 reads about 290 top side and 650-700 degrees on bottom (this seems a little hot to me).

 

A little more information regarding what I have tried:

Injector balance test before and after changing fuel injectors

Changed all 8 coils with MSD coils

 

Richard - thanks for your reply as it just confirms some of what I have already done but helps me realize I am not wasting my time chasing this issue. The truck is currently undriveable and yes I learned the hard way to be very careful about where you get your repair parts I had cheap sub-par lifters sent to me initially and decided to return them without using them. Unfortunately I did not notice that the head bolts were cheap Taiwan made ones and after putting everything back together the first time and trying to test drive the truck I blew a head gasket and had to restart the major part of the process including having to replace a head with a remanufactured one. The new head bolts I took out had stretched 2 mm so I decided to measure the original head bolts which only stretched 0.2 mm even after 300,000 miles of hard driving.

 

I found another post on this forum where someone was having a similar issue with their truck that was posted back in 2019 which helped a little the only problem was there was no solution mentioned the last post I saw was someone telling the originator to check his motor mounts as the cause of the misfires.

[richard wysong]

any progress"?

[BillM1965]

Completed the back pressure test Friday night to find right side Cat reads 4.5 psi at idle and 13 to 14 psi at 2000 rpm (way too high) the left side Cat is better but still high at 2 psi at idle and 6 psi at 2000 rpm. I am currently waiting for the new flow master Cats to arrive which is supposed to be today to see if that is the fix.

[richard wysong]

did the new cats cure the MAF problem?

[BillM1965]

Sorry had a heart attach recently that is why I have not posted anything the cats did fix the MAF issue but ran the truck for about 100 miles and the number 7 lifter came apart so I'm thinking it is time to junk this one and move on.

[richard wysong]

hope you are feeling better, that's a shame that things turned sour with the engine. hope your luck makes a turn for the better. thanks for posting the results, I'm sure your posts will help some one else with similar problems