slight surging problem

[jstaton]

Okay- I am traveling at a steady speed between 45-70 mph, on a flat highway and I get this very slight surge or miss on my 1991 IDI N/A. It doesn't change speeds in anyway but I can feel it. Is it the injector pump? or injectors? or maybe air in the lines? The car has 156K, rebuilt head and original injectors and pump. It was very hard to start this winter (I didnt drive it from January to March- I could not get it to start) When I did get it to start it took 30 sec to 1 min of continus cranking. Once it started I could jump right onto the highway. It starts perfectly fine in the warmer weather, no smoke. Doesn't use oil. Any ideas?

[jtanguay]

probably sucking air... how low was your tank???

[55mpg06]

As jtanguay said it probably was sucking air, you might want to check the clamps. Does this happen often, or just that once? If power is low aswell, when was the last time the fuel filter was changed?

[Fisher]

Check the Pump mounting Bolt on the back of the Pump.  If this is loose the engine will surge and act like you describe.

[jstaton]

I checked the lines (I had put all new hose clamps on when I put on my new fuel filter about 4 months ago) I thought that the fuel filter had something to do with it not wanting to start.
I do see a small bubble in the clear return line (the return line is the skinny clear that goes into the filter from the ps fender, onto the rear fitting, right?) The bubble is 1/8th of an inch (2-3mm).
One interesting thing did happen as I was trying to find the problem the other day, I turned the plastic knob at the bottom of the fuel filter and instead of an liquid (water or Diesel) coming out, it made a sucking noise as if there is a vacuum in the filter.
It seems to me that the car is drawing more fuel than it can get, it is inconsistent in it’s  “missing" or "surging" but I can feel it on the highway and it's slightly worst when the tank is lower and definitely worst once I am down to 1/8 of a tank (I usually get fuel at that point because it does feel like it is running out of diesel)
Also, I checked the rear bolts on the IP, are you talking about the allen head bolts? If so they look tight, but I didn’t check them with a wrench.
Any ideas?

[Fisher]

No, not the Allen Head bolts.  There is one IP mounting bolt on the lower right corner of the Pump.  13mm wrench, if it is not tight the engine will surge sometimes.

[jstaton]

I couldn't find anything wrong with the mounts or the lines, so i thought I would try to clean the injectors with some diesel injector cleaner. I disconnected the inlet and return line from the IP and put them into the diesel injector cleaner bottle and I let it run until it had consumed 8 oz or so. It seemed to cure the problem and the car seems to accelerate nicely compared to before. I never believe in fixes in a bottle, but this did the trick. Thanks again for the help.