Sunday, December 14, 2014

Mercedes MBE 900 Diesel Engine Emissions and Engine Codes

EGR - Exhaust Gas Recirculation Maintenance

I want to share some observations and solutions with emissions and engine codes we have been experiencing with the Mercedes MBE 900 diesel engines in our bus fleet. We have a dozen of these engines 2007 - 2008 emission controlled and they are all off warranty so this has allowed us to learn by "doing" with the various emission and engine codes that normally occur. The time we have spent troubleshooting these various problems has taught us a lot and we have adapted to the typical problems that happen with this model engine.

The EGR Cooler above had to be replaced due to a leakage problem. With a coolant leak they will eventually plug up with a sludge build up if it does not get attended to. If the EGR cooler is not leaking and has a soot build up it will require removal and a hot tank cleaning at your local radiator repair shop.

They are not cheap to replace however if they leak you'll experience coolant loss and engine code problems because of the soot wet mess that will develop inside the cooler exhaust core. This will cause poor exhaust gas flow and reduce engine power. The EGR cooler exhaust gases flow to the EGR valve which mixes the cooled gases with the charge air intake reducing NOx (oxides of nitrogen) creating reduced emissions out of the tail pipe.

Charge Air Cooler Failure Mercedes Code 2631

The charge air cooler to the left split open right at the core on the engine side. This problem was hard to detect because of the noise going on when the engine is running.

The reason we found it was because of a pressure check on the cooler itself. There was a 2631 Mercedes engine code which is a "turbo boost performance" issue.

Mercedes has a procedure for troubleshooting this code and it's quite involved. You have to check the boost pressure sensor for soot build up then using the software check the EGR operation along with the intake throttle valve etc. Any part of the engine that isn't allowing full flow including the air intake and boost side will produce the 2631 code.

In this case we have blown charge air cooler causing a big drop in boost pressure. The good part is we found a problem the down side is it's located 10 days away in Memphis. This bus is down until then. NOTE: We used a regulator to pressure test this cooler at 30 psi and made plugs that can be clamped at both ends.

Mercedes Diesel Engine "low doser fuel pressure" Code

The doser code activated because the fuel pressure supply in the system was too low and this caused the regeneration process to fail.

The "doser" system shoots a mist of diesel fuel down stream to cause the regeneration operation to activate at the DPF (diesel particulate filter).

No regen operation meant the DPF was going to build up soot and ash to the point where the engine would eventually shut down. This is serious since the engine control unit is programmed to protect the engine and will not allow it to run when the emission parameters are out of the ball park.

The Mercedes power service literature is very good for troubleshooting information. This test shows how to measure the return fuel going back to the tank. At idle the lines are hooked directly to the secondary fuel filter return and a tee fitting is hooked into the overflow valve.

There are 2 fuel regulators one for the doser fuel side and one for the high pressure injection side (unit pumps). If there is too much fuel returning back to the tank the fuel pressure reading drops.

The fuel pump is a positive flow gear pump that is engine driven. Fuel pressure must get up to at least 65 psi to supply the doser system (does the exhaust regen operation) and the unit pumps which provides the high pressure fuel to the injectors.

Once we changed the 2 valves in the fuel return lines "overflow valve" and "doser fuel pressure regulator valve" the exhaust regen started working again and the engine power improved drastically. So it was a win - win in this case.

If you're interested in viewing more blog posts photos and videos check out and look for "blogs". I'm posting under the name "schoolbusmechanic"

Friday, December 05, 2014

Cummins Diesel Engine Training

Training is everything especially if you own the newest diesel engines in the market. I recently went down to Vancouver (BC) to get some training at the Cummins training centre and it was well worth it. The ISB 6.7 liter diesel engine is by far (in my opinion) the best medium duty engine available.

They have designed their own emission system including selective catalytic reduction and DPF (diesel particulate filter). Getting the emission levels down to EPA levels is a huge challenge. Cummins is reducing 90% of harmful exhaust out of the pipe. We own 9 of the ISBs and getting 10 more next year.

The support from our local dealer is very good which was one of the factors we looked at but I know even if the support wasn't there this would have been the engine of choice regardless. Having 'Insite Lite' software is a "must have" and without it you're sunk. Throwing parts at a problem and guessing what's wrong will cost you a ton of cash.

Another part of the ISB is the VGT (variable geometry turbocharger). Providing boost pressure for engine power is only one of the uses for this component. It also serves as the exhaust brake and  helps heat up the engine during a parked regeneration. The angle of the turbo changes to cause the restriction needed.

Not only did we have classroom training but also went out and did a lot of hands on training using the Insite Lite software. The tests you can run with this software is endless. Gone are the days when you had to crack an injection line to look for a miss. Now it's all on the software with the click of a button.

For technical information there is Quickserve Online where you can find parts and manuals to help out with repairs. A manual is essential and without it you are taking a chance of screwing up the job. Information is like gold to a mechanic having the proper schematics, torque specs and step by step instructions.

I recommend anyone working steady on the ISB 6.7 or any Cummins engine needs to go get a Cummins training refresher. There you can ask questions and get the right training so you can dive into any repair with confidence. These days without a laptop on your toolbox with the proper software is like living in the stone age.

Saturday, September 27, 2014

Diesel Engine Emissions A New Frontier In Preventive Maintenance

When 2007 rolled around and the EPA increased their emission standards everybody was waiting on what the original equipment manufacturers were going to do.

The diesel engine has always evolved through the decades but the challenge in the new millennium was to reduce soot and ash discharge levels out the tail pipe.

The catalytic muffler was the first on the scene along with ultra low sulfur diesel and low ash engine oil. The muffler transformed into the DPF diesel particulate filter which traps soot and then through an oxidation process remove harmful particulates from entering the atmosphere.

The very difficult problem for the engineers and the battle to stay within EPA specifications is costing Cummins, Mercedes and Navistar a lot of money in research and development. The progression to cooled exhaust gas recirculation worked for many years along with the DPF technology.

However the EGR by itself wasn't enough to keep up with environmental demands. Navistar this year will no longer supply diesel engines in the medium duty market. The EGR pictured above is out of a MaxxForce DT diesel engine. We have several of these engines in out fleet and this component needs regular maintenance or replacement at least once a year.

At the vary least removal and cleaning of the soot and ash buildup is necessary or the shaft that opens and closes the valve sticks and eventually causes the EGR valve to stay open which creates problems with power and take off ability. Every one of our fleet buses are checked for stuck EGR valves especially the older units.

Mercedes EGR valves have the same problem and eventually the electric motor that opens and closes the valve mechanism does not have enough strength to over come the soot covered shaft and valve assembly. At the end of this year Navistar medium duty diesel engines like the MaxxForce DT and MaxxForce 7 will not be manufactured any more.

The Cummins ISB will be taking over the field with it's own technology including the SCR (selective catalytic reduction) system that uses DEF diesel exhaust fluid to reduce emissions. They also have the VGT variable geometry turbocharger to increased heat to assist in the regen process that cooks out the soot build up in the emission system.

It's a heavy duty challenge to keep up with emission demands but when the going gets tough Cummins diesel engines get going. They have really excelled over other diesel engine manufacturers and went outside the box to over come the high demands from the EPA to reduce emissions levels to zero through out the transportation industry.  

Saturday, January 25, 2014

Mechanics Hub Membership Site Tour

I not only post here but also at Mechanics interactive site for mechanics and non mechanics to hang around and talk about machinery /equipment in all industries. I joined in 2009 and have enjoyed sharing information, videos and photos.

Mechanics Hub also has a forum, job board and industry news along with many other related links to some great information. If you click on the 'Blogs' link you will find my articles there on diesel engines, school buses and several other commercial transport related posts.

Mechanics Forum

I know from experience that finding good information online is extremely difficult. There are lots of forums and sites that want to sell auto parts etc. but not a lot of reliable information to answer mechanical questions. I see sites that offer "ask a mechanic" but that comes with a fee. The MH forum has categories for construction, highway, bus and coach, forestry and many more.

Great Information For Tech Schools

There are a lot of sites on the Internet and I haven't seen many better than MH for the simple reason that they deliver quality information. If you are looking for a Technical school in your area there is a link on the site for technical schools in Canada and the USA.

There are a lot of jobs available if you're willing to pack your bags. I would encourage anyone who wants to get into the commercial transport or heavy duty mechanic trade to have a look at the jobs board and see what's available. There you will see jobs all over the continent that pay very well. I an seeing a huge need for mechanics these days and there will be an even larger demand in the future so now is the time to get to school for your pre-apprenticeship course and start pulling wrenches!

See my Mechanics Hub Members Tour Video

Hey I appreciate your visit!

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Thursday, June 27, 2013

School Bus Garage Photos

I've been taking photos in our school bus garage since 2005 and didn't want them to go to waste so here is a short video with RANDOM pics. There's everything from instant snow chains to engine repair.

 2005...I remember it well, we still had mechanical diesel engines in the fleet. They were semi-retired as spare buses and just this year we are auctioning off our last mechanical engine equipped units (Cat 3116 models). The 3116 Cat gave us pretty good service but nothing stellar to speak of.

 I believe there were 4 engine rebuilds because of the exhaust valve dropping at a very bad moment. Usually the high heat pulling full load would cause the mishap. It was a lot of work since the block was solid without removable piston liners. The entire engine had to be removed from the chassis, stripped down and rebuilt from scratch. The rebuilt head ran about 1500 dollars Canadian, if the cylinder wall was scored then the block was sent to a machine shop which MUST know the repair steps when boring and re-sleeving these engines.

There is a special jig to be used to secure the block so it doesn't warp during the re-bore. It adds up to about 5K labor and parts. This of course is far less than buying a long block. The top end on the 3116 was the weakest part of the engine so valve adjustment is crucial. The engine operating temperature is a factor as well so they need TLC or they start to heat up over 200 degrees.

The engine fan in the pusher school buses were transversely mounted so it was hard to keep the temperature down especially in warmer months. We disabled the shutters (opened permanently) and installed an over riding dash switch to turn the engine fan on full which took some horsepower out of the engine. In the video you'll see the coolant sensor that runs the hydraulic fan solenoid.

When you unhook the sensor wire the fan runs full on. The dash switch we installed for the driver just caused a break in the circuit. It should also be mentioned that the cooling system also had to cool the transmission oil which could get up to 200 degrees. The radiator mounted sideways at the back of the bus had a hard time cooling especially when the bus was in 3rd gear with a full load climbing a hill in 80 degree weather. Thanks for viewing this post and I hope you liked the video.

Monday, May 20, 2013

The Mechanic Red Seal Journey

Mechanic Red Seal Certification
The coveted Mechanic Red Seal is the culmination of a four year apprenticeship and on-the-job experience. I never forgot about the people I worked with who took the time to explain how systems worked and the best tools to use.

The stepping stones to achieving your Mechanic Red Seal will vary depending on fate and possibly luck in some cases. Above all don't stop trying and show your true colors to perspective employers. Stay in contact and don't allow them to forget your face.

Attitude is everything. Every year our shop accepts a commercial transport pre-apprentice for a one month practicum. Every one of these guys are energetic, respectful and humble. The college instructor hand picks these 20 something future mechanics and it's very easy to work with them sharing over 100 years of combined experience in our shop.

There is a difference when working in a fleet shop compared to a customer shop. However there's no rushing around making quick decisions for the sake of saving time and money for a customer. A fleet shop has spare units to hand out and when it comes to our school bus fleet we can not cut corners for the sake of time. Transporting children means special attention on safety.

There are so many mechanical fields available. When I apprenticed the commercial transport and heavy duty trades was combined. We covered trucks and earth moving equipment so the curriculum was very broad. Now those two areas are separate trades and specialized. Just having a journeyman ticket doesn't mean you qualify for everything available. There's mining, construction, trucking, automotive, logging and so on. Pick one and go for it with the most enthusiasm you can muster.  

Monday, March 25, 2013

Mercedes 900 Diesel Engine Runs poorly after fuel filter change

The Mercedes MBE 900 Diesel Engine. What I have experienced since we acquired these engines in 2005 is they are built well and stand up to the rigorous start and stop routines involved in a school bus fleet. The pre 2007 engines are not emission controlled like they are today and have given us very little grief. However I would like to share a tip on a troublesome problem we had with a 2005 freightliner conventional school bus.

The Video I made above explains how to properly bleed these engines. Strangely enough we were told to crank over the engine continuously after a fuel filter service until the engine fuel pressure built up. At first we were scratching our heads thinking we're living in the new millenium and this diesel engine has no priming pump and only one starter...that was destined for failure eventually from the constant load it would have to endure turning the engine over and over just to prime the fuel filters.

We tried to innovate and pressurize the fuel tank and crack a line at the secondary fuel filter to help the system prime up faster. Unfortunately some of our new buses had fuel guage sender problems and there were buses running out of fuel out on the road. There was no choice but to load the hell out of the starter to get it going again.

By the way if you loosen the fuel line from the unit pump to the head the line must be replaced. Recommended by the dealer / engineers. We were forced to crack the lines only once and re-tightened them after start up and did not have any leaks. So I imagine the odd time the fuel line will not seal properly after it has been disturbed?  

Engine Runs Poorly After Changing Fuel Filters

We recently experienced a problem with one of our MBE 900 diesel engines after a fuel filter change. It ran poorly and was not picking up fuel. It eventually kicked in and ran fine for a few days. However it died on it's bus run and had to be towed. It ran like it was starving for fuel like air in the system. The engine would idle fine but crap out under a load.

One thing noticeable was when priming the fuel system with the pump kit the pump just kept filling the lines and filter without the typical solid feeling at the pump handle indicating the system was primed and ready to go. Eventually the Mercedes diagnostics instructed us to check the return fuel volume from the head and the unit pumps.

There should only be 1.7 Liters of fuel returning to the tank every minute at an idle. We had double it ended up being the overflow valve (similar to a check valve) which is in the return fuel plumbing to maintain around 60 psi. It was not doing this and caused a loss of back pressure in the return system which caused the hard starting, rough running condition. Now our shop will recognize the problem after these symptoms pop up again.

Sunday, February 24, 2013

Special Tool Saves Components When Welding

Protecting Vehicle Components While Welding

This tool (Ultra Pro from Napa) is what we use in our shop and has worked successfully for us for several years (click on photo to enlarge). I was asked what kind of coverage is available if it fails and a component burns out because of it from a high energy surge from welding.

Not sure about that question; I contacted the selling dealer and they have not come up with any answers. The other option is to disconnect the battery ground when welding. The only inconvenience from this is you have to reset your digital components like clocks and timers. Modern School Buses have several modules within the multiplex system and using either one of these precautions is mandatory to save an expensive component from failing.

I have not replaced any modules in our buses yet; the main problem being corrosion to wiring and connections. The connector plugs still get moisture inside in some cases and the use of dielectric grease helps a lot to keep moisture away from the terminals. Oxygen, moisture and electrical current can team up nicely and corrode connections quickly and cause trouble.

A lot of times I have found with corrosion on module connectors there are cross electrical signals that occur and some circuits get energized with constant battery power. We had an instance where 3 different circuits were operating on their own even with the ignition shut off.

With the wiring diagram we were able to determine where the circuits started from and it led us to a corroded connector at one of the control modules. As a preventive maintenance step we have applied dielectric grease to all of the module connectors.