I want to check my fuel pressure and the condition of the Fuel System return.

What should I look for in a fuel pressure tester or are they all about the same?

How do I go about testing the Fuel System Return? I'm guessing it's related to fuel pressure somehow...

Thanks!

 

I'm going to try for partial credit on this one.

The tested variable is normally the fuel pressure, but I don't know the procedures or target pressure(s) for our cars.

Once you have those, then if the pressures check out OK I think we can infer that the return system is functioning normally. This would have to be so, seems to me, since the pressure is regulated by varying the quantity of fuel returned to the tank. If the result is good, the return function must be good. (The pressure regulator is a kind of relief valve.)

FWIW ...

 

I'm trying to dog down codes P0172 and P0175 - Rich Banks A and B.

Following the JTIS:

I've been through the air intake system and it is tight and sealed.
Checked the MAFS, IATS and ECTS and they appear within spec and normal in operation.

Next up is the Fuel side - JTIS says to look for too much fuel pressure or leaking injectors.

How much is too much fuel pressure I wonder?

 

scott, I can only tell you what my AE records for fuel pressure when the car is running.

At 1440 rpm, I recorded 55.2 psi. I don't have an idle measurement at the moment, but can if you'd like. I also can do 'key on' as well as startup/idle.

 

Thank you very much! An idle measure and a Key ON measure would be most helpful.

I have just a pound or so of pressure at Key On and it snaps immediately to 33 at Startup/Idle. (I'll get a high RPM reading this afternoon).

At shutdown, within a few seconds the pressure drops to about 25, holds for a fraction, then to 10, hold, then to near Zero. I assume this is the proper functioning of the Fuel System Return process.

I would have expected more pressure at Key On - I thought the check valve kept some available. Oh well, my problem is TOO MUCH pressure anyway and it starts like a champ.

 

Check the vacuum line to the fuel pressure regulator. Make sure it has not fallen off, or deteriorated and make sure you have 15-20 inches of vacuum at idle.

I believe the way the regulator works is when the vacuum drops, it restricts the return line, therefore raising the system pressure, so you want to make sure the vacuum is OK at idle where there is full vacuum.

Also using a hand vacuum pump, test the fuel pressure regulator and make sure it can hold vacuum from the vacuum port and no gas is leaking from the vacuum port.

 

The pressures you report are certainly not normal. The pressure should jump to 41 plus or minus at key on. Thirty-three is way low.

The pressure should hold after key off far better than what you report also. Reverend Sam went through much the same problem in this thread:

https://www.jaguarforums.com/forum/x...-please-44285/

 

I had these same symptoms with my last car (not a Jaguar). It took a couple of tries to start the car but, once started, it ran perfectly. It turned out that the culprit was the fuel pump and the symptoms would probably have gotten much worse over time. Once I had the pump replaced pressure was normal with the key on (engine not yet running) and pressure would hold when the engine was turned off.


Doug

 

Thanks.

I replaced the FPR about a year ago when I had a hard start issue but I'll test it anyway. I replaced it's hose just the other day.

Somewhere I've got unmetered fuel leaking into the system...

 

Bad gauge! I'm usually pertty selective as to what I get from Harbor Freight, but I thought a pressure gauge might be OK...It wasn't.

My new Actron gauge reads 43 at start and maintains that level at idle. At shut off, it holds steady at just over 40 for as long as I could wait, over 1/2 hour...

I'll cross excessive fuel pressure off my list of causes.

Thanks!

 

Did you actually replace the MAFS mentioned in your other thread?

 

You must be reading my mind; No. Here's why:

My new air filter arrived at just the time we were getting all excited about fuel trims and measuring MAFS output against Steve's standard. I took the new air filter and the new MAFS and old air filter and old MAFS and made a series of measurements in all four combinations.

It appeared that the new air filter and old MAFS gave me good readings, so I sent the new MAFS back figuring I'd quickly find the source of the problem elsewhere.

Since then I've checked/replaced everything the JTIS suggests except the TPS and looking for leaking injectors.

The problem persists. This afternoon I'm thinking that it might be best just to replace the MAFS.

 

Yeah, I was thinking your MAFS is either just a little out of spec, or else has an intermittent.

Guess you could try to clean it with CRC MAF cleaner and see if that helps first.

 

Yes.

Check my next post though, I might be on to something...

 

bad/ dirty connection at MAF harness can cause that. make sure the connector pins are clean and tight. I've seen this many times.

 

Thanks, I did that earlier but I still think it's the sensor right now