Engine behaves like Dr. Jekyl and Mr. Hyde

[JBCumminatya]

I've been noticing two distinct running modes of my engine. One "strong and smooth". The other "gutless and rough".

When I start the engine, it may idle rough with more than the usual amount of vibration being transferred to the steering wheel and accelerator. As I drive, the engine may become "strong and smooth" with less noticeable vibration.

Before I shut off the engine, I think to myself "will the engine stop smoothly" or will I have to "wait for the shake!" that causes anything loose in the truck to rattle.

A couple nights ago, I left the house for a new lift pump at CumminsNW. The truck ran beautifully, until my return trip home whereupon I could feel the engine vibration increase as I had to push a little harder on the go pedal to keep my speed up.



If this were a mechanically operated gasoline engine, I would suspect an intake or exhaust leak.



This past week I installed a BHAF and new lift pump. Changed the fuel filter last weekend.

Engine still exhibits Jekyl and Hyde syndrome.

Anyone have any ideas?

 

[SEmerson]

Do you have a fuel pressure gauge? I had the brushes go out in mine and when the pressure fell I would get a stumble, lose of power and a shutter.

 

[JBCumminatya]

I have a handheld gauge 0-30psi but I don't have the fittings to hook it up yet.

I replaced the lift pump on 2/9/02. I don't think lack of fuel is the problem. I have noticed the engine change while driving or idling.

 

[JBCumminatya]

Sorry, I forgot to mention that I had all of the intercooler rubber hoses reattached because they were all leaking.