94 gmc 3500 6.5 turbo. after replacing cylinder heads because of leaks. . and not removeing injector pump,or wireing to pump, just the injector lines . it is hard to start. engine idles well for few minutes the starts to miss and run rough . after 10 minutes aprox. the service engine light comes on . when service light is on it runs great and revs up perfectly . with these codes, we check for a bad ground or brake in harness . but not found. not believing injector can be bad. it ran fine before we tore down , other than it blowing coolant in overflow.

I was told by a pro that code 18 almost always means the IP needs replacing. I had those exact same codes when my IP went. I replaced the IP and all was good.

Well, this is not an IP problem per say. More likely than not you missed a ground that came off with the head. Check to make sure there are two on the passenger side head near the back on an intake bolt and the water blockoff plate. And there should be one on the back by firewall too near the filter manager.

Otherwise, make sure the filter harness is not still on the Optical Sensor harness going into the IP. The engine harness should reach to the OS on top of the IP without the filter in line.

The reason it runs better with the SES light is because you are going into backup fuel mode aka limp mode, and it sets a contact pulse width and its probably ignoring the OS as a timing source.

Otherwise you have, or caused and air leak in the fuel lines. Is the Lift Pump working?

we did check all ground . they where fine ,also changed fuel filer and bled system of air . lift pump was filling filter and lines.so we where (assuming) it was working well . we where using a scanner that did not read any sensors on ij.p.,that made it more harder to diagnose. so the fellow that owns the truck ,which is an employee at our auto shop decided to take it someone with deisel exp.after a week of trying to fix the problem in our spare time.he had it towed to a deisel repair shop, to diagnose the problem. they said it was in oil pressure switch. which was shorting internaly, causing it not get full 12 volts. they repaired the switch. it runs perfect. this has been learning exp. for me. i have been the auto repair for 25 years.

Yes, well the Oil Pressure Switch runs the lift pump, never good to assume it working correctly. thats why you ask questions here, the OPS is the most common failure item on this engine and your friend should modify the system by installing a relay to power the lift pump with the OPS power output to the coil of the relay to trigger it. Then it will only need to pass the current to excite the relay, instead of all the current to run the LP.

You had classic symptoms of fuel starvation/air leak, which is why asked if the lift pump was running. the 6.5 diesel electornics are different that any other vehicle and must throw out all assumptions. However they are not difficult to understand and diagnose with the proper technical manuals and experience.