My GMC, 5000, 396cu/in Isuzu will lose power down to 40 MPH while driving. I changed all filters, replaced fuel lines, checked for dirt in all filters. It starts good any time in cold weather and does not miss while driving. I got in it the other day and it ran great. Now this morning it is starting this all over again. I have no bubbles in radiator. I don't add water. The engine has about 300,000 on it. I need some help guys. This has been a great truck in the past.

 

/me moved here for better exposure
Are you saying that it has no power above 40 MPH?

 

Electric or mechanical lift pump? Any smoke, exhaust have an unusual smell?

 

Hello?

 

zukes3140,

you can re-open the thread if the problem hasn't been resolved.