Here's an odd thing.

Installed a fuel pressure gauge last night. Checked the pressure at idle and it was rock solid (about 3.4 Bar) on the gauge. I'm sure the needle didn't twitch.

Today been running leakdown tests. Warmed her up at idle (the WSM leakdown tests are based on warm engine) and again I'm certain pressure was rock steady. I've just done the leakdown tests (found leaky injectors) and replaced relay and fired up engine to check relay replaced OK.

Now the needle twitches very slightly but continuously around the idle figure of 3.4 Bar. At first it was twitching a lot - maybe +/- 0.1 Bar - but now it's just twitching slightly and continuously.

All I've done is clamp the delivery hose and return hose when I did the tests. I'm sure I'm not imagining things so curious as to why the pressure would now show very slight variation from previous rock-solid reading. The gauge is glycerine-filled so should not be susceptible to vibration.

Grateful if any LH injection gurus out there could explain what might be happening!

 

I have the same on my car when it gets warm.
Rock steady when cold and at the spec. when hot the needle fluctuates +-0,2 bar on my car.(not always though but most of the time)
I do not have any leakage and good pressure after 30 minutes on the meeter.
I have been thinking about the dampers being old and once gettong heat soaked stop to do their job properly?
The fuel pressure regulator has been changed recently so it should have nothing to do with this.

/Peter

 

Thanks for that reassuring info Peter. I was wondering whether it was a cold/hot thing and which fuel component could be affected by this.

Only other idea I had was whether running the engine off/fuel pump jumpered test last night had made one or more leaky injectors worse and that this led to the wobbly pressure. They don't normally see such high fuel pressure so maybe this made the leaks worse?

I may replace the dampers when I do the intake job if your theory is correct.

Thanks again

 

My injectors was cleaned two Years ago and tested to be ok both for spray pattern and leakage.
I have a brand new set of Ford Racing injectors that I will put in this winter.
Also plan to replace both dampers at the same time.
I have tested the dampers and they do not smell of gasolin when puling the vac lines from them and if I apply vacum they hold just fine.
Only reason for my theory is that I can not think of anything else causing this.
Not sure but the membrane inside the dampers should probably suffer from age and long term hot working condition I assume?

/Peter

 

Is this a standard guage or liquid filled to reduce or dampen the vibrations.

Ken

 

I use the liquid filled one that Carl at 928 motorsport sells.

/Peter

 

Ditto - mine is Carl's liquid-filled gauge

 

There's a little 'yellow' lever on top of the guage. If you open it with the engine running/guage pressurized it should stabilize, then close it again.

Worked on mine.

 

I'll try that now Malcolm. Before I was worried that idf I opened it under fuel pressure, all the liquid would be blown out the fill port followed by jet of fuel!

...That did the trick. Also now reads same pressures it read yesterday whereas today everything was a few 1/10s of a bar lower. Must just have been some kind of bubble vibrating in the gauge.

Thanks!

 

I have tried this several times and it doesn't make much difference on my car.

Thanks/Peter

 

When the engine is cold the the LH computer is in cold enrichment mode, making the mixture richer. It does that by increasing the injector pulse width, keeping the injectors open longer. All injectors fire every engine rotation and at the same time, so the period of open time is closer to being the same as the period of closed time. The guage has less time to switch from the open injector state to the closed injector state before it needs to go back to the open state again. As the engine warms, there's no longer a need for the cold enrichment, so the injector open time is reduced to make the mixture leaner. The injectors are then open for a shorter period of time in comparison to the time that they're closed. There's a longer closed time for the gauge to react to. With the engine cold, it may also be running at a higher idle RPM. That would reduce the total available time between cycles, also making the open time longer compared to the closed time as well. A slower reacting gauge would be smoother hot or cold, and a faster reacting gauge would twitch more hot or cold.

Sometimes you can freeze the gauge needle at the different points pretty well with an adjustable timing light.

 

Brilliant explanation - many thanks!