Everything I attempt to do besides gauge installation is based on the info I’ve gained from a Bentley Manual and a Haynes Manual, as I am still a rookie in repairing.


As you can see, I installed this inline fuel pressure gauge on my 1990 Volvo 240 LH 2.4. I thought it would be helpful in troubleshooting my current lean-running engine or any time there’s issues in the future since there’s no OBD II scanning capabilities. Before doing this, I wanted to remove fuel pressure so I ran the car with fuse 6 (main fuel pump) removed. After a minute, I could feel the engine to stall very soon from lack of pressure, so I just tuned off the car, removed negative battery terminal, and then installed the in-line fuel pressure gauge. The battery was disconnected for more than an hour so the ECU must have reset too.

Now, I wanted to use the gauge to test my fuel system since I have a lean running condition, so I put fuse 6 back in, tightened negative battery terminal back in, and then I made this jumper with an on/off switch and a 20 amp fuse, which I used to short out the fuel pump relay harness to test the fuel pump.


I jumped red/yellow wire and red wire. I turned on ignition II and turned jumper switch to on. I could hear the pump working from my seat and I saw the installed gauge building up pressure till I reached steady at 38 PSI. Put fuel pump relay back in.

After all of this, I relaxed and then checked on the fuel gauge an hour later and saw that the gauge was still holding 38 PSI. I tried starting the car about 3 times and it would stall out in a second or two. The PSI went down to 20 PSI after I unscrewed the gauge from the T fitting a little and some pressure was relieved so I screwed back to what it was. I tested the fuel pump again with my jumper in the relay harness, and the fuel pump was working still and I let it build up to 24 PSI as shown in the first picture. I also put the relay back in and could hear the click from it when I activate ignition II (ignition parts powered). When I crank the engine to start, I could also feel the second click from the relay (fuel pumps activated). Also the car battery is at 12.6V now. Fuses are good too.

So, do you think the resetting of the ECU must have caused this issue and forgot how to run the engine that was previously running lean with idle and running issues? Appreciate anyone’s help

 

Well, simply disconnect the battery, reset the ECU shouldn't do any damage otherwise my 89 will be long gone. I do remeber read somewhere, some later model ECU's have little problem, for instance, my fuel pump never comes on when I put my keys in II position, it will take about 5 cranks for first start (which is about 1~2sec), that might explain why you dont get pressure on position II. If you build up pressure fine it might be a fuel flow issue, lots of things can make it run lean, but I guess you just want to get it start again right now.

A little dum question tho, you sure the fitting you installed and with the gauge fully in, won't have any restriction on fuel flow? I seen some fitting/gauges can be screwed very deep inside, which will give you a reading, but will block regular flow. And you sure the fuel line is not accidently kinked? I would start by remove the gauge and see what it does and go from there.

 

Before I installed the fuel press. gauge, I did check to see how deep the gauge thread goes to make sure i won’t block the flow in any way, and it was clear.

AND I actually got the car to start running. I shorted the relay again to turn on the fuel pump until it went to 38 PSI, and cranked the engine and it still stalled. Then I also suspected that my MAF sensor could also be the culprit since it has never gotten replaced, so I unplugged it so that it won’t rely on it’s possible bad readings and go into Limp Mode, and it started! This test may not be reliable in figuring out that the MAF is bad, but I’m still going to replace it, and I’ve already cleaned it two weeks ago.

It idled a little rough and ran rough with random engine shakes just as it did before so it never improved, which was expected. I made the risk of driving it to college and I made it, but on the way back home, there was a point where engine shook consistently and the shaking kept increasing until it finally stalled out and the engine would keep stalling out when trying to start it again.

I then popped the hood and looked over at the fuel pressure gauge and noticed that it was on 10PSI, so it stalled due to lack of fuel pressure. You could also say that maybe there could be air intake issues that cause this stall, but I was monitoring my air vacuum gauge for the entire drive of the day and I was always at 15 in Hg at idle, and it lowered gradually the more I accelerated and greatly increased briefly when I decelerated. Normal airflow right?

Looks like my fuel system is not properly building up pressure, and I wish I could have monitored the fuel gauge while driving to see how it performed the whole time. I got it towed to my house and tried starting it again without luck. I tested the fuel pump by using a jumper on the pump relay harness and it worked and I let it go up to 38 PSI. And then IT STARTED.

These symptoms have me believing that there’s a fault in the fuel system and possibly the MAF too. Either way, the MAF, fuel pump, and fuel filter have never been replaced, so I just want to replace these 3 parts anyway. Fuel pump sounds loud and buzzy when testing and other times quiet and smoother. Everything on in-tank fuel sender unit has been replaced not too long ago so that’s the last thing to check if problems persist. And thank you for reaching out!

 

Well, glad to hear you figured something out, I would start from the fuel filter, because seems to me your pump can eventually buid up enough pressure, but can't keep it while you are driving, this sounds a lot like a somewhat blocked fuel filter where when you accelerate, the fuel pressure slowly drops because not enough fuel is going through, but after you shut the engine, it will pump the pressure back up again. A dirty fuel filter can make the pump go loud as well, I think these pumps are not quite to begin with but can last a very long time.

Keep monitoring the fuel pressure with the engine running is a good idea, you can rev it while watching the fuel pressure gauge.

Cheers!

 

depending on where you hooked up your vacuum gauge, the is more negative behind the throttle when low load, and increases while high load, the pressure infront of the throttle is opposite, but generally very low vacumm over there, the OE volvo fuel economy gauge is actually a vacuum gauge hook to the intake. When you release pedal while the car coasting down the road, the vacuum should be most negative behind the throttle.

 

A challenging problem(s). Since there is a lean running condition that is constant(?) I would attack that first. The fuel pressure from the main pump under the car is, I believe, constant under all conditions. The pump has a built in pressure regulating valve that keeps its' output pressure constant. The amount of fuel delivered to the cylinders, under all conditions, is controlled by the computer varying the pulse width to the injectors. So, I would monitor that pump output pressure under varying, temperature, engine speed, and load to eliminate any problem in the fuel delivery system.

There is I believe a second pump and filter in the fuel tank also that could affect delivery to the main pump.

 

So I’ve installed the new main fuel pump, fuel filter, o2 sensor, and MAF sensor like I’ve wanted to since they have never been replaced in its 34 years. I ran the fuel pumps individually for testing and they worked fine and both spewed out fuel (I didn’t measure the volume they let out in a certain timeframe though; I should have done that). The ECU has reset since the battery was unplugged and so after I started the car and same results like last time.

The car would start and stall in a second or more.I disconnected the MAF to hopefully start it from limp mode like the last time when this worked. Attempt failed. I shorted the fuel pump relay harness to run the fuel pumps until it reached 38 psi and it held the pressure for the 30 minutes I waited. Still failed after more attempts.

But finally it started after one last random attempt of turning the key. Fuel pressure at idle stayed at 35 PSI. Still idled rough and didn’t improve after 15 minutes of waiting for ECU to learn. I drove for about 1 minute and car felt like it was gonna stall soon so I went back in my driveway and opened the hood to see that my fuel pressure went down to 29 PSI. Fuel pressure issue is evident again and it only seems to drop when I actually drive. By the way, fuel pressure regulator is new and tested good too.

Next plan:

• Test volume of expelled fuel from in tank fuel pump. If test fails, I’ll check the hose on the in tank fuel sender since only the in-tank pump and the filter sock have been replaced not too long ago by previous owner’s mechanic.

• Test ECU (because I found out it’s a pink label 561 and they are supposedly notorious for causing issues with the fuel pump system or other problems, so I’m getting a used 951 instead. Even if ECU is good, I still want one of these as backup as failure is more likely bound to happen with the pink label. My ECU also makes a weird whining sound sometimes that gets louder the more I accelerate)
• Im not even going to test crankshaft position sensor or ignition control module, but simply replace it since replacing all super old original sensors is another side mission of mine. Will do this next paycheck.

 

While your in the tank peer in and see if the bottom looks shiny. If there's dirt it could repeatably clog the filter. Be careful not to make a spark with your tools or light. Also, if you use a vacuum to clean the opening it may suck up fumes that could be ignited by the motor.

 

I replaced the Crankshaft Position Sensor and Ignition Control Module. There was still no improvement, so then I moved on to inspecting the fuel sender unit from the fuel tank and I found the problem (the hose was cut in half right next to my thumb)



I replaced the hose, reset the ECU, let it sit idle for about 15 minutes, then did a test drive through the streets and freeway. All issues are gone and drives so much better. Thank you for all the help everyone.