Heres a little more detailed story line. Car starts to misfire and surge before engine harness replacement. I bought a new engine harness from Dave Barton. When it cam in and to make it easy, I removed the intake manifold. Injectors, rail, and FPR. A couple weeks later after all the wiring was buttoned back up and cleaned up I reinstalled what was removed and replaced a few sensors. Replaced 2 heater hoses and the 2 temp sensors. Fast forward to car is all put back together and running the first things that happen is the surging is still there and the misfire is gone since I swapped spark plugs. I know surging can be caused by air leaks, so I replaced the air bypass gasket "the one under the throttle body with the knob on it", intake manifold is of course new because it had been removed and checked the intake hose and replaced that since it had a few hole/breaks in it. So far at this point its running a little bit better. Between the time everything is put back together and the present, I have consulted a former instructor and looked information up online. I trust this instructor a lot as he has helped me in the past and has been dead on and not to mention can teach any automotive class without any books. He made the class and learning fun. Anyways he said pressure for FI systems are normally 35-45 PSI. I looked on O' reillys site and it said system pressure should be 40 PSI, Autozones reference said 43.5 PSI, and the factory repair manual "The Bently book" States depending on which LH system you have can be either 39.5 PSI or 43.5 PSI. Does anyone recommend me buying a second test gauge to confirm? Shall I remove the rail and try and see if anything is really stuck inside since I'm leaning towards an obstruction in the fuel rail. Please help me out and reply. This problem is driving me crazy. The car runs very well besides this problem and is in the best condition ever. Not gonna get rid of it.

Car Information:
1984 Volvo 240 DL
2.3L 4 Cyl. B23f Non Turbo Gas Engine
LH 2.1 System

Stuff I have done and tried:
Swapped between 4 FPR's - 1 original part when car was running fine, 2 new FPR's of the wrong part number "Bosch 0280160213" I later looked at the original and found part number "Bosch 0280160293" and had a pressure reading on it in "BARS". I converted Bars to PSI and it did confirm I should be running at 39.5 PSI. 4th FPR, I bought from a local parts store that I've been going to for nearly 20 years "Standard - Intermotor P/N: PR45" confirmed through the store that this one is the right part.

The stuff I have really tried are as follows: As you know I went through 4 FPR's and on the 2 Bosch wrong part number FPR's I had a substitute return line that went into a bucket outside the engine compartment. So I put a new hose on the return port of the FPR and the other end went into a large bucket on the floor. Considering it was running between 60-80 PSI at that time, it didn't squirt out forcefully but poured out with little force. I still at this point have not done a volume test or ran the pump with a line open say at the pump to check for proper pressure and function which I'll be doing in the next few days. This car isn't the only car I'm working on at the moment. Got a 92 Honda going back together after head job. Anyways between the 2 Bosch wrong parts and the Standard FPR the fuel coming out the new substitute return line has been about the same. I also have blown air through the fuel rail in hopes maybe it was clogged and have done so but maybe there's something stuck inside still and causing my problem. As of yesterday, I noticed that the fuel was low so I poured more in but still no luck. 2nd I also noticed that the fuel is orange, is this a problem? I don't know if this has anything to do with anything but when I pinch off the air motor hose, the surging goes away,runs smooth but backfires. I also didn't see a thermostatic door so I blocked the hole off completely. As to answer someone's question, te fuel pump was replaced nearly 5 years ago, so that probably isn't the problem since it was running fine this whole time and passed smog this past December. When you says faulty gauge, this is an OTC - Stinger line fuel pressure gauge Teed into the incoming fuel supply hose and fuel rail. As I stated earlier the fuel return or fuel coming out of the FPR seems a tad on the low side just based on the observation that I've been doing. Does anyone think I should run the fuel pump with no FPR on the rail to see if its obstructed?