I have a 98' Suburban with 232K on the engine and 5000 miles on a new DS4 injection pump. I put in new injectors the same time as the pump. Recently I noticed when the engine is cold and it is cold outside (32 degrees F to 45 degrees F) the engine feels like it will occasionally stumble or miss when driving at cruising speed 45MPH or 55 MPH. It is not severe but it is only something I can feel. I see no drop in the engine RPM when this happens. What I mean by cruising speed is that I am not under acceleration or deceleration but just trying to maintain a constant speed on level ground. This symptom goes away once the engine is warmed up (and I assume the injection pump is warmed up.) When cold, there is no stumbling under acceleration and it is not down on horsepower. There is no white smoke, there are no engine codes. I monitor the fuel pressure on the output of the filter and with the Carter pump it is about 8 psi. I have changed the filter (of course) and I don't see where fuel pressure is an issue. I have also changed to a different PMD. The new pump came with a Stanadyne module on the side of the pump and I switched to the remote PMD and saw no difference. If I was to drive to the coffee shop 15 miles away and park the truck for 10 minutes, the stumble goes away and I will not notice it the rest of the day while driving.

Any thoughts? I have fresh fuel filter, good fuel pressure, no smoke, swapped PMDs. Is there something in the injection pump that could act up when cold? It is odd that this does not get worse under acceleration.

TimK