Avoid Engine Failure in Isuzu Diesel Engine 4BD2

Para español: Evite el Fallo del Motor en Isuzu Diesel Engine 4BD2

4BD2 Isuzu Diesel Engines 

A problem we’ve been seeing lately is the development of cracks in the 4BD2 heads. This can be a problem on trucks in the 1992 – 1998 model range with 4BD2engines – both the Isuzu NPR trucks, as well as the Chev W3 and W4 and the GMC W3500 and W4500 trucks. This does not apply to the GMC W5500.


One of the main causes of engine failure in this range is from the truck running hot. If you notice where your truck is starting to run a little bit warm, it is very important that you check your coolant levels & keep your coolant level in the radiator full.  If your radiator and reservoir is full and you still notice that it is running warm, have your radiator checked by a certified radiator tech to have it rodded out or replaced with a new radiator.  Some possible problems are the flue inside the radiator gets stopped up causing it to fail.  Or trash, debris or dirt gets the outside of the radiator clogged and you can pressure wash or steam clean the outside of the radiator to fix this problem.  You should check your coolant levels at least every time you fill up with diesel. Do NOT trust that the reservoir is full. Remember you must never open the radiator cap when the engine is warm. The pressure and hot water will injure you severely. Another problem causing the engine to overheat is a bad fan clutch. The clutch should never spin freely or not move at all, but should have some resistance when you turn it by hand. These engines do NOT come with a shutoff switch when coolant gets too low. So by the time you realize that the light comes on or you hear a knocking, it is too late. You have already damaged the head. When the engine runs hot, it results in the cracking of the head. The head will crack in between the valves resulting in major coolant loss. You can tell that the head is cracked if when you remove the radiator cap, you have excessive pressure in the cooling system.   This engine first 1992 – 1998 Isuzu NPR as well as GMC W3500, W4500 and Chev W3 and W4. 

Busbee’s Truck Parts offers new replacement heads complete with valves and springs. We recommend that you use an Isuzu OEM gasket head set for this replacement, available at your local Isuzu dealer. Do not use aftermarket gaskets. We have new heads at $1850. each for Isuzu 4bd2 diesel engines 92-98 models.

Fan Clutch Failure

Another thing that causes these engines to run hot is fan clutch failure. One way to determine the problem is to test your radiator cooling fan . While the engine is shut off, spin your fan. It should not spin freely at all, it should have a consistent amount of drag. It should have some resistance when you spin it while the engine is shut off.

Another test you can do is when the engine running, observe the fan. If it’s not turning at all or is turning slowly in relationship to how fast your engine is turning, then it, too, may need replacing.

Watch this video if you’d like to see this explained.  
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