2008 Silverado 5.3 restricted exhaust
This Chevy truck with 209k on the clock had complaint of falling on its face under heavy load. I have a few questions about my captures at end of post if any one interested in conversation.
Fuel pump and FPCM were replaced due to low fuel pressure and a internal fault code in the FPCM. Fuel pressure was now within specification, however under heavy load will miss and can hear a slight popping in the intake. Complete code scan when I took a look revealed a P0300. Looking at the misfire data showed us that all cylinders on Bank 1 misfiring under heavy load. Note that this only happened at full WOT load, Scan tool would not count misfires at idle, higher RPM loaded or unloaded, misfires would only count and be felt at WOT loaded. Fuel trims in shop at idle very close to 0 , higher RPM load was not showing as much difference / opposite bank to bank as I have seen in past experiences with restricted exhaust. Bank 1 @ 2500 RPM loaded was reaching 8% total fuel trim and bank 2 was close to 0. Technician states that it did set a P0175 at one time.
At this point I did a clear flood crank as I have incorporated this as part of my SOP for any drivability concerns, as expected due to running smooth @ idle the cadence was normal. In days past seeing the misfires on one bank high rpm fuel trim difference I would have pulled the B1S1 O2 sensor and installed a back pressure gauge. NOT FUN ,original exhaust / sensors on a 12 year old vehicle in the rust belt. So looking to get to verify my assumptions as quickly I went in-cylinder, here are the results. diag.net/file/f34j4dehp…
Idle: #1 was averaging 1.4 PSI, # 2 was averaging < .6 PSI on exhaust flat
2500 RPM unloaded: #1 peaking 13.7 PSI, #2 < 1.5 on exhaust flat
WOT snap unloaded #1 peaking over 60 PSI, #2 peak 16.4 PSI
So we have a restricted exhaust, B1 for sure.
- Are the readings for B2 high? What would pressure gauge show at O2s? ( vehicle getting both Converters as assembly)
- How does the higher RPM breathing affect the compression for the affected bank. Cranking, and Idle compression for both banks are the same , and at higher RPM the restricted exhaust has lower peak pressure. I am trying to wrap my head around this, ( like a totally restricted exhaust will not show up on a RC test.)
- What steps would you do make sure that we are not fixing the symptom and not the root cause of the problem.
I hope this explanation helps others as I am learning to trust my gauges, I mean transducers and lap top LOL.
For problems like this, I always start with a scan tool and fuel trims. By using fuel trims, you can pick out low fuel supply, restricted exhaust, whether it is both banks or just one, cam timing problems and about any other low power causing problem. If when using the trims, and I'm not confident in nailing the real problem, I might then fire up the scope and take a look in cylinder.
Thanks Albin, I did look at fuel trims and scan data before going incylinder, as I said the trims were not nearly as bad as I usually see with a restricted cat on GM's I have seen a hand full over the past years with one bank + and one -. Talk to you soon.
Albin, You have helped me before with using fuel trims to diagnose just about anything in a engine. Have you made a book or a file of if/then kind of information? If so would love to buy it or ask for a copy. Or a good source to get the basics of this? Thanks, Craig
Hi Craig. Thanks!! It is always good to know I have been able to help out with problems. Over the years, fuel trim data has been good to me. I have written a lot of stuff on it over in iATN, and some here on DN, and have even written a few training classes over the years. Some of the best fuel system & fuel trim information I have ever found is from Ford Motor Company. You can go here…
The end pressure in a cylinder is the product of the pressure at the beginning, times the pressure ratio factor of the dynamic compression ration. I have attached a picture of a compression ratio vs pressure ratio graph. The dynamic compression ratio may be lower than the spec’d compression ratio due to later intake valve closing. There is a you tube video “thoughts on compression from…
Hi Brian, - I have seen snap throttle in-cylinder pressure waveforms with 16psi on exhaust stroke @5000rpm, marked as "no issues". No idea what that would mean for gauge values at the O2 sensor, take with a grain of salt... - To compare compression of two cylinders you either need to catch them at the same (or very similar) RPM and the initial (corresponding to approx. 570-590 deg. mark…
Hello Dmitriy, I wish I had a mechanical gauge at the time to check and compare values at the 02, the reason being that I am wondering what we would "see" with the mechanical gauge. I have been messing around looking at the files for this vehicle, originally I thought I was seeing lower compression on the restricted exhaust and this was confusing me, turns out I was not comparing apples to…
Can't comment on the in cylinder. Things I would look at are IAT, MAF, O2,Load, Fuel Trims, misfire count or even a vacuum gauge. IIRC Cylinder Misfire count on the restricted bank will be higher on the front cylinders. Popping out the intake is another sign, I would expect on posted vehicle the IAT will be raising on heavy accel(should drop)
I wish I would have looked at IAT as a indicator! Yes misfire counters actively counting only Bank 1, surprising fuel trims were not affected as much as expected. Vehicle was not able to be driven when I was looking at it (blocked inside shop), and I did not look at Load.