Where is that clear section? Can't tell but looks like it is in a low section of the return hose? If so, there may be bubbles exiting the IP but you won't see them at a lower hose section because they "pool" and coalesce in a higher (highest?) portion of the return fuel line.
We use evidence of air in the IP return to infer air intrusion into the IP, where there should be no air. Air in the IP most often points to two failure modes:
1) Failing IP mainshaft seal allows air intrusion while engine is not running, which subsequently causes hard start condition. Not usually a problem once running, as that section sees (minor) fuel pressure when running, not vacuum. It may leak fuel when running.
2) Loose connection or leaky hose in fuel supply run to IP. Or the presence of an extra small fuel filter sealing o-ring. Often the smaller of the two sealing filter o-rings is not seen and therefore not removed during a filter replacement. Since a new o-ring is usually supplied on the new filter, the filter gets installed with two o-rings sandwiched together, causing air intrusion into the IP. This failure mode can cause hard starting, and almost certainly poor performance, especially under WOT (high fuel demand) conditions.
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1985 744 gle d24t
1985 745 gle d24t
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