Ficm Posted September 11, 2014 Share Posted September 11, 2014 I would like to make reference to pinpoint test AZ 11 when diagnosing DTC P0401 EGR low flow. I have difficulty wrapping my head around why MAF increases and how the PCM determines EGR low flow concern. I've read the EGR System monitor in the PC/ED, does anybody have a simpler explanation? Quote Link to comment Share on other sites More sharing options...
lmorris Posted September 11, 2014 Share Posted September 11, 2014 100% duty cycle has the EGR cooler bypassed,When you lower the duty cycle of the bypass valve you are forcing exhaust to flow through the cooler, if the cooler was restricted it would reduce EGR flow, thus increasing MAF. Exhaust flowing through a clean cooler will be slightly lower than when the cooler is bypassed, that difference is essentially 14 g/s. That difference is monitored using MAF. Quote Link to comment Share on other sites More sharing options...
Ficm Posted September 11, 2014 Author Share Posted September 11, 2014 Thank you for your input Imorris, please bare with me here. After the MAF reading, which other input does PCM use to determine EGR low flow? MAF measures air flow into engine, so in order increase MAF we give more throttle? Quote Link to comment Share on other sites More sharing options...
Brad Clayton Posted September 11, 2014 Share Posted September 11, 2014 Here are a few screen shots. 1 Quote Link to comment Share on other sites More sharing options...
Brad Clayton Posted September 12, 2014 Share Posted September 12, 2014 MAF measures air flow into engine, so in order increase MAF we give more throttle? The only reason this test has us raise the idle to 1500 rpm is so the engine won't stall when the egr is opened and spent gasses that cannot support another power cycle are introduced into the cylinders. There is a direct relationship between incoming air that is measured by the maf and egr gasses which are not measured by the maf. If you introduce exhaust gasses into the intake stream then the maf reading will go down similar to a decelerating engine (less air coming in thus lower maf reading). If the exhaust gasses start to make their way into the intake stream but get blocked (as in a plugged egr cooler) then the maf reading will start to rise again as if the egr valve were shut. The pcm interprets this as "no egr flow". 6.0's are notorious for egr flow codes even when the egr system is working correctly. This is usually attributed to skewed maf readings that are dirty as hell from shit air filters. 1 Quote Link to comment Share on other sites More sharing options...
8WA Sman Posted September 12, 2014 Share Posted September 12, 2014 So was the IAT2 only used as part of the EGR flow monitor/test on early 6.0 with no MAF? Quote Link to comment Share on other sites More sharing options...
tonybullitt Posted September 12, 2014 Share Posted September 12, 2014 That was my understanding that iat2 is used on them with out the maf Quote Link to comment Share on other sites More sharing options...
Keith Browning Posted September 12, 2014 Share Posted September 12, 2014 Jeez. I scrolled thorough this topic thinking Bruce was at it again with the screen shots. Nice job Brad! Quote Link to comment Share on other sites More sharing options...
Bruce Amacker Posted September 12, 2014 Share Posted September 12, 2014 My research shows MAF on both 6.4 and 6.7 to be in the low 30's at an idle with EGR closed and drop about 20 GPS (to about 12GPS) when the EGR is opened fully. I have had no issues with stalling on any trucks. Great quick way to test for EGR clogging. Nice job, Brad! Quote Link to comment Share on other sites More sharing options...
Ficm Posted September 12, 2014 Author Share Posted September 12, 2014 Clear as day now, thank you very much for the screen shots, much appreciated! Quote Link to comment Share on other sites More sharing options...
mchan68 Posted September 13, 2014 Share Posted September 13, 2014 I just repaired a '12 with a P0401 and P2457 a couple days ago. On mine though, the EGR cooler bypass valve broke off the shaft. That's the first one I ever had of one of these. Anyone else run into this? Quote Link to comment Share on other sites More sharing options...
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