All:

I’ve finally bought an OTC fuel pressure gauge set to check fuel pressure on my ‘88 L98 IROC. The engine is completely stock.

Key on no start pressure hits about 44 PSI

Fuel pressure with the vacuum line disconnected is approximately 44

Fuel pressure with the vacuum line disconnected remains at 44 PSI at any RPM I was brave enough to run up to.

Fuel pressure with the vacuum line connected is around 38 at idle, and drops steadily as RPM’s increase.

This fits with my observation before that BLM consistently drops as RPM increases.

30 seconds after shutoff, fuel pressure was still at 42

What I don’t completely understand is WHY...because I would assume that fuel rail pressure should, if anything increase slightly as vacuum drops, and the fuel pressure regulator does it’s thing.

 

Your understanding of the regulator is correct. As vacuum increases, fuel pressure will be regulated lower, to its minimum point.

What may be confusing the issue is that you also need to monitor vacuum to compare the regulated pressure at a specific vacuum. My suspicion is that the regulator is working properly.

 

I suspect the regulator is probably working properly as well, but I’m being somewhat cautious.

I bought a set of the Delphi Multec replacement fuel injectors, but decided it would be smart to make sure everything else is 100% before installing them.

 

Fuel pressure with the vacuum line connected is around 38 at idle, and drops steadily as RPM’s increase.

Isn't that backwards? Shouldn't pressure INCREASE as vacuum drops. (RPM increase...)

 

I’ve done a little additional research, and suspect the issue is that vacuum is only going to drop significantly under load.

I need to rig up the fuel pressure gauge so it can be seen while the car is being driven, or perhaps use a vacuum pump so I can artificially raise and lower the vacuum the regulator is seeing with the car in the driveway.

The fact that the fuel pressure is at 42 with the vacuum line completely disconnected leads me to think the regulator is doing it’s job...it’s the way I’m testing it that’s the problem.

...or at least I THINK that might be what’s going on.

Thanks for the input! I’ll keep checking. I’d rather, if there is an actual problem deal with it while the plenum and runners are off to replace the injectors, than have to pull it apart a second time later, because I missed something.