Bizarre Engine Breathing Problem

Andrew Technician Commack, New York Posted   Latest  
Case Study
2012 Mercedes-Benz ML350 Bluetec 4Matic 3.0L (642.826) 7-spd (722.9)—4JGDA2EB1CA051778
Cuts Out
Crank / No Start
Low Idle

The customer was driving this vehicle and the engine suddenly cut out, they had it towed to the shop. At the shop it would crank and start, but would idle at only 300 rpm. After a short time, the rpm would drop and then the engine would stall. One code for the DPF and one code "idle too low". No other fault codes. One time the engine started just fine and the vehicle ran with full power and was taken out on a road test, where it cut out again.

The air filters were clean and clear. Fuel quality, pressure, volume, and rail pressure were all correct. The exhaust backpressure, DPF differential pressure, and DPF soot loading were all normal. I advised to check the rail pressure and exhaust backpressure when stalling, and to look for a problem with the air intake system, inspect the turbo, make sure the throttle is working, etc.

The shop found the problem soon after and they were nice enough to email me a photo. An intercooler hose had an internal liner that separated and blew up like a balloon, choking the intake airflow off. Sometimes the liner flattened out again and the engine could run at full load/speed, but most likely when it developed boost, it blew the liner wide open again, causing the stall and low idle on restart.

Martin Instructor
Burnaby, British Columbia

I use a simple "trick" in class Andrew, to demonstrate that the engine is an air pump first and foremost. If you cannot get air in or out, it's not going anywhere fast!

I insert a shop towel taped into a ball, at the throttle body, effectively blocking ~95% of the air flow after the MAF sensor. The engine starts and idles, but with no throttle response and stalls when attempting to use the accelerator pedal normally.

However, when pushing the throttle pedal very slowly, 100% throttle can be attained, but there is no RPM increase because there is no more air entering the engine. The clue is an extremely low MAF value. Once the issue has been identified, all should be well, save for an exhaust tailpipe that is similarly restricted. It's all about getting air into the cylinders.

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