Fuel Slobber happens when diesel engines are started cold and even worse when the ambient temperature is cold. This was a common occurrence back in the 1970’s and 1980’s when there was no variable timing on most of the Cummins Engines. The 425-B Caterpillar did have advanced timing for cold idling engines, so the fuel slobber wasn’t nearly as bad. The STC-444 Cummins Engines also had advanced timing at idle so they were much cleaner. Many owner-operators would see the fuel leaking out of the flex pipe right behind the turbocharger and it looked like engine oil, when it was fuel and water mixing with the soot in the exhaust system and it looked like oil. This problem would cure itself once the engine temperature came up to the correct operational temperature, which usually didn’t occur until the truck was driven. With the large cooling systems and the amount of air that passes through the engine during idling, diesel engines tend to run cool. The typical result of burning fuel in a modern diesel engine are about 67% nitrogen (N2), 11% Carbon dioxide (CO2), 11% water vapor (H2O), and 9% oxygen (O2). Less than 1% makes up all the bad pollutants that diesel exhaust has recently become famous for. On a cold start of a diesel engine the water vapor may be cold enough that it turns into liquid water in the exhaust system. Especially on trucks with vertical exhaust stacks, this liquid water can run back down the stack and leak at various clamped joints making a mess on the chrome elbows and even leak out of the flex pipe. This problem normally goes away in a very few minutes if the truck is put to work, but if it remains idling on a cold day the liquid water can be quite a nuisance. The feeling is the liquid water is not good for modern diesel particulate filters, especially if the engine is shut down before it gets hot enough to turn the liquid back into a vapor and purge if from the particulate filter.
This slobber can also run out of the exhaust manifold at the connecting joints and run down the side of the engine. If this happens to your engine, just spray the slobber with a spray soap or penetrating oil and wipe it off with a rag. Many penetrating oils are also a very good cleaning solvent and will keep the engine from rusting.
Blow-by; another mess on the engines and the chassis of the truck. The higher the horsepower and the harder the engine is run, the greater the amount of blow-by the engine develops. Blow-by is compression getting past the piston rings and all engines have it. Our Oil Trap, which mounts at the bottom of the blow-by tube, is a wonderful item to catch the oil residue, which is present in the blow-by gasses. The Oil Trap will catch about ½ cup of oil every 10,000 miles. It’s better to be held in a container than spread underneath your truck. If you want to see the effects of blow-by on the streets, drive a motorcycle down the strip in Las Vegas while it’s raining. When you put your foot on the roadway it will want to slide out from under you. I can tell you that from personal experience of trying to hold up an Ultra Classic with a passenger in the back seat! Many mechanics will tell you the engine is wore out if they see blow-by coming out the tube, this is NOT true, oil consumption is how to measure if the engine is wearing out. One gallon of oil consumed every 2,500 miles is considered worn out and needs rebuilt. Many times, an owner-operator will call us and say they had their truck on the dyno and the blow-by is high and the shop recommended an in-chassis rebuild. We ask what the oil consumption is after an oil and filter change, many times they say the engine will run 8,000 or more miles before burning the first gallon, this engine is not wore out. Don’t worry about the blow-by; go by oil consumption to determine when it’s time to rebuild the engine.
I haven’t talked about this for many years, however now that the Big Cam Cummins Engines are becoming popular once again, let’s talk about them and the small 3/8 ID blow-by tube mounted on top of 1 of the 3 valve covers. This may have been sufficient when the NTC-350 and 400 horsepower engine was popular. Then we came along and started increasing the power up to 700 and 800 horsepower. Along with the horsepower increase, the blow-by also increased so we would install a breather tube on each of the 3 valve covers. Problem solved, blow-by must escape out of the engine, if the blow-by tube is not large enough, the blow- by gasses will try to escape up the turbocharger drain tube. The oil coming out of the bearing housing of the turbocharger is in a whipped foamy state and drains via gravity. All turbo drain tubes must be no more that 30 degrees from vertical or the oil can be forced past the seals of the compressor wheel and the turbine wheel. Now think about this, if blow-by is trying to escape up the turbo drain tube, the turbo oil will not drain down the tube, but be forced out of the turbo seals. Many turbochargers are replaced because of a restriction in the blow-by tube. The newer ISX Cummins have a filter prior to the blow-by tube and it must be changed. NEVER put a piece of heater hose on the bottom of the blow-by tube, the hot gasses will soften the rubber and the wind under the truck will push the hose horizontal and choke off the blow-by gasses. Now a turbocharger is blamed for leaking oil when it’s that piece of hose you put on the blow-by tube.
I write these articles to make you think, I want you to be aware of what is going on under the hood of your truck while you are cruising down the highway and especially when you’re climbing a mountain.
We are getting ready to do a bi-monthly video, which will appear on U-Tube and our web site, more on this next month.
If you want to e-mail me, you will have to call the shop and Cathy will give you my address, if I put it in this article the spammers will bombard me with junk e-mails. Always put a subject with your e-mail. I do prefer phone calls to e-mails; it’s just old school thinking!
Written by: Bruce Mallinson, Pittsburgh Power, Inc.
3600 South Noah Dr., Saxonburg, PA 16056