Jeep Wrangler driving off-road with a 4BT Cummins engine
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Cummins 4BT and 6BT Problems

Common 4BT / 6BT Issues

The Cummins 4BT and 6BT engines are widely known to be bulletproof. If there is an engine out there that old school guys always go to is the older mechanical 4BT Cummins. The reason these engines are still used in industrial and jeep swap applications is due to their reliability. This engine is a pure mechanical engine with very few electronic components. However, no diesel engine is perfect which includes the 4BT and 6BT. This short article will explain the few issues with these engines and how to diagnose them.

Block Issues

Cummins 53 Style Block Issue

Cummins 53 Style Block

The first generation 6BT (5.9L) 12V engine was produced from 1988 – 1998 and had no casting issues with the block or head. From 1999 to 2002 some 100,000 of the 24v ISB engines stamped with the “53” on the crankcase have been known to be bad castings. Overtime these 53 Blocks are susceptible to cracking causing coolant leaks. The cracks start out very small but these 53 Blocks will eventually have to be J-B welded, hot welded or cold stitched back together. These repairs can extend the life of the engine but overall the engine will need to be replaced when the block goes south for good. There is some evidence to indicate the 53 style block was used in some 12v models but the cracking seems to be much smaller than the 24v engines.

The blocks were manufactured by Brazilian company TUPY. Other Cummins blocks are known to crack but the “53 Style” by in large has the most issues because the ratio of iron in the casting was significantly less than what was required by Cummins. Cast Iron blocks are a combination of Carbon, Silicon, Sulfur, Steel, Limestone, Manganese and Phosphorus with alloys containing Nickel, Copper, Chrome and Molybdenum. The Carbon content should be 1.8-4% of the mixture with Silicon content being 1-3%. In addition to the incorrect cast iron chemical composition, poor design led to thin walls in the water jacket that made the blocks prone to cracking. The cracks in the block are caused by overheating, towing heavy loads and over revving the engine.

Dowel Pin Issues

Killer Dowel Pin Issue

Location of the "Killer Dowel Pin"

The other big issue with the 4BT and 6BT (5.9L) engines is the weak dowel pins. The dowel pins are known to either crack or become loose. The pins were made from steel and located on the front timing gearcase. The problem with the steel pins is that the constant heat cycles expand and contract the steel causing it to unhinge from the casing. Once the pin falls from the casing either one of three things will happen. The best case scenario is it will fall into the gearcase but miss everything and safely land in the oil pan causing no major damage. If the dowel pin falls into the timing gearcase and hits one of the gears in mid rotation it can cause the pin to immediately bounce around and eject out the side of the crankcase. If this happens oil pressure will immediate drop and probably destroy the engine due to lack of lubrication. Oil will go everywhere but if you catch the drop in oil pressure in time you can save the engine. In the worst case scenario the dowel pin will get lodged in the timing gears throwing off the timing and causing the camshaft to break. Once the timing is off the valves will ram into the pistons causing catastrophic damage to the engine. This last scenario is called the “Killer Dowel Pin” and has been discussed by many in the community at great lengths over at 4BT Swaps.

The good news is that the Killer Dowel Pin issue can be prevented before anything horrible happens to the engine. The engine needs to be torn down (valve cover, cylinder head and camshaft) to gain access to the timing gear case which includes also removing the lower pulley, overflow containers, fan and shroud and lower pulley. Once you locate the dowel pin you can install a metal tab over the pin to keep it in place or JB-Weld it in place. Many aftermarket companies sell a killer dowel pin kit which includes a metal tab, new timing cover gasket and front crank seal to put everything back together in an upgraded fashion. While you are inspecting the timing gearcase tighten up the gearcase bolts and couplings. These bolts can also loosen and fall into the geartrain and cause damage.

Other Smaller 4BT / 6BT Issues:

Cummins 4BT Gear Issue

Cracked Cummins 4BT and 6BT Injection Timing Gear

Injection Pump Gear Cracking - The gears for the injection pumps sometimes develop hairline cracks from the keyway up to the teeth. Once a crack develops it can change the timing slightly. The fix is to replace the gear.

Injection Pump Fuel Line and Pin - 4BT engines sometimes develop low power, low boost bad fuel economy randomly. The cause of this can be a number of issues but most likely it is a loose fuel tube and pin. When the turbo builds boost the adjacent chamber it pushes pressure down on the fuel pin. When more power is requested the engine tells the injection pump to add more fuel. This fuel tube and adjacent pin break going into the injection pump register which causes low fuel pressure and fuel leaks. The solution is to J-B Weld the tube and pin back into place. This will fix the issue and restore fuel pressure 5-10 psi.

Fuel Filter Connection Issue – Make sure the fuel filters are properly tightened to the cylinder head. Also check the feed line from the lift pump to the cylinder head. There sometimes can be pin hole leaks in the line. Any leaks can cause fuel to flow back into the intake feed.

Drivetrain Issues – The added weight of a 4BT or 6BT into a swap can cause drivetrain stress down the line if not properly installed and modified.

JB Weld Image

J-B Weld is the recommended expoy for quick long-lasting bonds

Cracked Cylinder Heads – Some cylinder head castings are prone to cracking and burned pistons between the 3rd and 4th holes. This occurs is roughly 3% of 4BT and 6BT cylinder heads. This issue is not unique to these models, as this is also a known issue for models like the ISL. Mostly the heads are pretty good with these smaller Cummins engines.

Oil Cooler Issues – The oil coolers sometimes have issues. Common problems can be oil in the coolant or coolant in the oil. Coolant in the oil typically is caused by a head gasket issue.

Injector Problems – The injectors for the 4BT and 6BT are usually pretty good and don’t have the issues of the common rail injectors of the newer style electronic engines. If you are seeing white smoke it most likely means an injector is leaking, oil is burning or coolant is burning.

Intercooler Issues – The intercooler and the adjacent pipes have some issue during install processes in swap applications. Getting a 4BT into a Jeep is no easy task and during the install process the intercooler and pipes can be crushed or pinned against the frame causing damage.

Overall, the Cummins 4BT and 6BT (5.9L) are excellent engines. The reason they are so popular and still in use is that they are easy to maintain, are built well and will get the job done for 1,000,000 miles. If you do run into any issues these engines have a wonderful and expansive network of mechanics and enthusiasts to help you figure out the issue and have you back on the road in no time!

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