Fuel Pressure Questions

[thorvald_skwimby]

Hi all, I'm hunting down some rough running issues in my '74 tii, I'm currently following the HackMechanics guide for sorting out fuel delivery issues. I cleaned out and coated the gas tank, replaced the fuel pump with a newer Bosch pump, blew out lines, changed leaky lines, changed or cleaned all filters, even had my injectors cleaned by the excellent PaulTWinterton. When testing my fuel pressure I'm getting readings north of 90psi on my gauge before ignition and while the engine is running. The gauge is placed right before the KFish pump after the fuel filter attached to the radiator and I'm not seeing any leaks. The guide states 29psi is the reading I should see and well 90 is way above 29. I'm assuming 90 is bad and way too much pressure in this case, let me know if that assumption is wrong. Where should I be looking for solutions?

[Son of Marty]

Your fuel pressure regulator located inside the barbed fitting where the fuel return hose hooks up is plugged up or broken, remove the fitting and under the retaining clip is a spring and piston take them out being careful not to loose the spring or piston they are under spring pressure so beware don't let them shoot out as parts are not available, clean it up and it should get back to working, it runs a bit rich at 90psi doesn't it?

 

 

 

Edited April 14, 2021 by Son of Marty

[Hans]

I'd be cleaning  up that green wire on the terminal.

[thorvald_skwimby]

Thanks Marty! I pulled the PRV and it was completely seized. I cleaned it up and threw it back in and initially it was reading around 35psi, but after a bit jumped back up to 40-45psi. I'm gonna take it apart again and try and give it a really strong clean and maybe an ultrasonic bath too. Seems like the spring might be a bit too stretched for running within spec now. Anyone know of a suitable replacement since these valves are no longer available. Could I just cut the spring down by a bit?

 

Hans thanks for the tip on the terminal, it's definitely on my list just focused on my fuel delivery problems now.

[xavier296]

I found that the spring actually loses pressure over time, leading to lower fuel pressure, not higher. I had to stretch my spring out a but to get 29 psi, but my in-tank doesn't put out as much dead-head pressure as yours does. Maybe 65 or so. 

 

Did your injectors have to be rebuilt or just cleaned?

[thorvald_skwimby]

The injectors were cleaned and reconditioned. "They are atomizing at 500-490-500-510 PSI.  Well within spec and balanced to within 3%." I'm going to check the lines again and look at the fuel side of the KF pump, searching around the forum there might be a few more places in there I need to clean. I will follow up here when I get that done and hopefully that can get things within spec. If not I might try and find some other PRVs to see if that really is the issue or change it out for a newer adjustable pressure regulator.

[PaulTWinterton]

On 4/14/2021 at 5:09 PM, thorvald_skwimby said:

after a bit jumped back up to 40-45psi. 

 

45psi will not likely damage anything or cause poor running.   90psi could cause pump seals to leak.  Low pump pressure on the other hand could limit fuel flow to injectors. 

 

I would revisit all other adjustments,  throttle, air, and ignition.  Make sure all air hoses and seals are intact.

 

Follow the pump PDF to adjust linkages from pedal to throttle body.

 

Adjust your timing to be all-in around 3k rpms.

 

Intermediate shaft, air/fuel screw and idle screw are the final adjustments which should dial-out rough running.   An AFR also makes this step more scientific than adjusting by "feel".

Edited April 16, 2021 by PaulTWinterton

[thorvald_skwimby]

Well I've narrowed it down further. I found out cylinders 2 & 3 aren't firing at all! Was playing around and pulled the plug wires while it was running and there was no change in the engine. I did a compression test a while ago and they were all within spec and I did a valve adjustment last weekend so I know that's all good. I have all new plug wires and plugs as well and I've verified spark. So I started looking at the fuel delivery from the KF pump. I pulled the hoses to the injectors on the KFish and cranked the engine and nothing came out of the pressure valves for those 2 cylinders. I pulled the pressure valves out and ran the fuel pump and fuel comes out of those ports fine so I think that those valves are just shot. I tried to clean them up a bit and put them back in but there was no improvement. Didn't have any more of the crush washers needed after that so I'm going to order some of those up. It looks like those valves are still available new but quite pricey so looks like I'm in for another expensive adventure!

[PaulTWinterton]

Call Robert at Jerry Fairchild Enterprises.   Ask his advice on pressure valve restoration or replacement.   They are only check valves after all.

 

Are you sure it's not stuck delivery valves? 

 

He can rebuild and calibrate your pump ( to 500psi injectors ) as well.  Last I paid was $800. and experienced quick turnaround.  He does not supply re-plated hardware,  although.  Only a thorough and professional rebuild. 

 

 

[Son of Marty]

It sounds like you have two different problems going on. the fuel pressure regulator should set the pressure at 29 psi regaurdless of the engine is running or not, and 2 cylinders not getting fuel which is a different problem altogether. 

[thorvald_skwimby]

38 minutes ago, PaulTWinterton said:

Are you sure it's not stuck delivery valves?

Paul I think we might be talking about the same thing. Maybe I wasn't using the right term. I'm talking about the valves that lines to the injectors screw onto. I unscrewed the lines and cranked the engine and no fuel came out of for the 2-3 hoses. I'll definitely give Robert a call and see what his thoughts are, I think this pump is long overdue for a professional rebuild. You saw how nasty the injectors were that I sent you, so I'm imagining the pump isn't in much of a better state.

 

37 minutes ago, Son of Marty said:

It sounds like you have two different problems going on

HAHA, if only it would stay at just two different problems!

[PaulTWinterton]

37 minutes ago, thorvald_skwimby said:

I unscrewed the lines and cranked the engine and no fuel came out of for the 2-3 hoses.

 

So it might be the delivery valves.  I think they can be resurrected. 

[SydneyTii]

Swap them around see if the problem follows the swapped valves, if not in may be the springs/plungers in the head of the pump.

I did one suction at a time and then one delivery at a time and it ended up being a stuck plunger/spring.

I ended up getting a rebuilt pump, kept mine to play with.

[xavier296]

Look at TheHackMechanic's post about taking the KF top off for reference, then rotate your KF pulley with the suction valves removed (don't remove them with a magnet), and you can see the plungers move. Sounds almost guaranteed that those two won't have any movement. If they don't move, soak in marvel mystery oil for a day, then tap them with a wooden dowel, then try your test again. The actual fuel delivery is crazy simple, its the metering below it that gets more complicated. 

[thorvald_skwimby]

Did a little more diagnosis after work today and swapped the valves around and 2 & 3 never pushed out any fuel. The plungers definitely aren't moving or are gummed up so much that they are moving quite slowly. When I cracked open the bolt for number 2 I heard a very slow and squishy sucking sound for about 45 seconds.

 

10 hours ago, xavier296 said:

If they don't move, soak in marvel mystery oil for a day, then tap them with a wooden dowel, then try your test again.

 

Definitely going to give this a try this weekend and see if I can get them moving again. Thanks for all the tips! I'll post back with results when I have them!