Hi Folks, Thank goodness for the internet. Folks from around the world gathering to share knowledge and information.

I have a 96 with a 5 speed in a 3/4 Ton 4x4, the Cummins has begun a scary habit of having the oil pressure fluctuate dramatically and drop off very low under load above 2250 rpm. If I let off of the throttle it will recover. It won't do it when cold, but when warm, especially when hauling a load or cruising at highway speeds (sometimes). Otherwise, she has good oil pressure all the time and doesn't seem to have any rod or crank noises that are unusual. I've changed the oil and it appeared this remedied the situation. But after 1,000 miles on this oil it has returned. I have also noticed a small but definite reduction in power. The truck does seem to smoke more when first started. Which color of smoke is okay and which is bad, I can't remember, blue or white? The truck has 182,000 miles on her, it is my second Dodge Cummins and I love them. Please let me know what I should look for or if this has been seen before.

Thanks,
Bryan

 

Are you using diesel rated motor oil?

 

The oil pressure sensor on these trucks is well known for frequent failures. The first place to check is the connector where the wire clips to the sender itself. They get greasy and loose. If that connection is good, check the sender.

If the oil sender is good, then you need to drain the radiator, pull the oil filter and the oil cooler plate under the filter. There is an overflow valve located in the housing on the oil cooler that will also give symptoms very much like you describe. That valve gets stuck open, and will let the pressure drop dramatically.

As far as smoke is concerned, white when cold is OK, white or blue when hot is bad and indicative of low compression. Black smoke is unburned fuel and will occur under temporary high loading conditions, or when an air pressure coupling between the turbo and the engine intake has worked loose (or if a turbo fails.)

 

Thanks Gents, for your replies. Yes Bill, I have used either Rotella or Pennzoil 15-40 Diesel rated oil since new every 3,500 to 4,500 miles. A very good point Bill.

Thanks RCW, no blue when warm, mostly white and black when cold, occasionally a trace of blue when cold at initial starup the first few seconds. I will pursue both the sender as well as the relief valve within the next few days. I will look to see if there is an available port to attach a mechanical pressure sender to the engine to find out if indeed I am losing oil pressure or if it just appears that I am.

Regards,
Bryan

 

Bryan, there is a 1/8'' pipe thread port with a plug in it on top of the oil filter housing that works excellent for a temporary gauge. It will read a bit higher than the stock gauge, best to use at least a 100 psi gauge.

 

Yes indeed there is. Thanks Bill

Regards,
Bryan