Why Cheap Pressure Gauges Fail

[rossiz]

So I was having some hesitation at WOT and some funny starting issues which felt like fuel delivery problems (they were, due to a clogged pre-pump fuel filter) and when I went to check my fuel pressure, the gauge had crapped out - again. This is the second one I've had, and both lasted a couple of months before dying. My gauge is installed with a brass T fitting just after the fuel filter in the engine bay, and I've been using generic hardware-store variety pressure gauges ($10) so no huge disappointment, but I wanted to find out what the problem was.

Probably because my pre-pump filter was so filthy, the pump was working hard to keep up pressure (which was causing the stumble at WOT) so every time the injectors fired the pressure would drop instantaneously, then pop back up, causing the needle to constantly flutter by about 1-2 psi. This constant fluttering actually caused the little brass gears inside the gauge to wear out, till there were no teeth left at the mesh point.

With a new pre-pump filter in place, the new gauge is much more steady, so I'm hoping it will last a little longer this time. I am, however, thinking of just getting an electronic sender unit and wiring a gauge inside the cabin to more easily monitor pressure.

Gauge inner guts

Gear that drives the needle - all teeth worn out in the center

Gear that pushes needle gear - look closely for missing teeth in the center

[240dkw]

Me thinks it is due to vibration

[rossiz]

i thought so too, but their really isn't much vibration where it's mounted - right against the fuel filter, with rubber fuel line on either side of it. if you saw the needle flutter, you'd get it right away. the new gauge w/new filter is pretty rock solid now.

[Captain Obvious]

I experienced the exact same failure of a pressure gauge. I could have sworn I posted about it a while ago, but (even with the advanced search**) I honestly I can't find it. So if I didn't talk about that, I'm sorry because I could have saved you the trouble. My experience went like this...

I was hunting for hot restart issues and I installed a cheap pressure gauge just like you did between the filter and the fuel rail. I tracked the cause of my hot restart to a bad check valve at the pump, but after fixing that I figured I would just leave the gauge installed as a diagnostic tool just to keep an eye on things.

Over the next few weeks my fuel pressure seemed to be gradually dropping. But the interesting thing is there seemed to be no change in the way the engine was performing. Tracked the issue to a failed gauge with eaten up brass gears inside just like you had. The pressure shocks of the constant hammering from the injectors opening and closing ate up the brass and started skipping teeth.

I put a new gauge in the system and verified that it was a measurement problem only and my fuel pressure was fine, and then I took the gauge off the system so I didn't ruin another gauge.

**Mike, if you see this, I'm not sure the advanced search works as intended... What I was thinking would be an "AND" search on keywords seems to be an "OR" search instead? Send me a PM if you want to discuss? Maybe it's just me, but I couldn't get it to do what I wanted.

[S30Driver]

For about 30 bucks, why not put in a silicone liquid filled gauge for dampening.

This is the one I have, over 5 years, still accurate.

[Zed Head]

I could have sworn I posted about it a while ago,

Post #6 here - http://www.classiczcars.com/forums/fuel-injection-s30/52413-78-no-start-w-out-fuel-priming.html

I used the "site:classiczcars.com captain obvious pressure gauge gears" method in Google. But it works n the top search box on the forum also, since it's the same effect, through the Google engine.

Edited September 28, 2014 by Zed Head

[rossiz]

CO: now that you mention it, i do remember your post - guess that qualifies for "independent verification"

James: that looks like a good solution, i'm glad to hear yours has held up for so long. i thought the silicone was just to keep the needle steady against vibrations for more accurate reading (my needle flutter was pretty big so you always had to guess and average reading over a couple psi), and that the injector pulses would still have the same effect on the internal gearing, but without transmitting it to the needle. so i figured while it may look better, it would still wear out.

thanks for that suggestion, i'm gonna order me up a marshall gauge and treat my engine bay to a nice sst component!!

[siteunseen]

I took mine out too after fixing my issue, CTS potentiometer tweaker. More cracks in my boat that could leak.

[FastWoman]

Hmmmm.... I'm thinking a digital fuel pressure gauge, a digital vacuum gauge, and a wide band O2 sensor in a triple gauge pod on the driver A-pillar.

[Stanley]

I got at 0-10 psi liquid filled McDaniel gauge after reading some rave reviews of it on an RX-7 site. They also have gauges that would be right for F.I. setup. I've had it installed (under the glove box with a high pressure AN line tee'd from the mechanical fuel pump) for over a year, it's big, easy to read, steady. After removing a glass inline filter I could see the typical pressure go up from 3.9 to 4.0 psi. They're not retail but will email you information for local distributers. Mine cost $65. Good industrial stuff.

Pressure Gauges, McDaniel Controls, Inc. - After all it costs more to replace a defective gauge than the original purchase

[Captain Obvious]

i thought the silicone was just to keep the needle steady against vibrations for more accurate reading and that the injector pulses would still have the same effect on the internal gearing, but without transmitting it to the needle. so i figured while it may look better, it would still wear out.

Rossiz, I share your theories on the silicone fill and will be interested in any ensuing discussion. I've not done any investigation, but what you described above is exactly what I would expect as well.

And Zed Head, Thanks much for finding that. I searched and searched and couldn't find it. I searched so much that for the rest of today I had myself convince that I really DIDN'T post it, but only THOUGHT about posting that info. Thanks for helping me back (closer) to sanity. Now that I know it's really there, I'll dig a little into why I couldn't find it. That'll help me in the future.

And I've got the end all of fuel pressure gauges for all you Neanderthals to drool over. Give me a little bit to upload some pics and I promise you'll be jealous.

[Captain Obvious]

I've been meaning to post this thing, and this thread is a great segway... I bought a differential pressure reading meter. It's got two ports on it for the high and low pressure connections and it provides a true real time differential reading.

There are a couple different designs for differential pressure meters, but I really liked on this one. Here's the pressure reading with the engine running and just the high side connected. The vacuum side is open to atmosphere and it reads like any other single input pressure gauge that you would hook up to the fuel rail. The reading is lower because the vacuum is higher. Also you can't see it (because it's a still pic), but if you rev the engine, the reading is all over the place accounting for changes in manifold vacuum:

Then you hook up the vacuum side, and it's like magic... Rock steady under all engine conditions. Goose the throttle. Hold the RPM's high and then snap the throttle closed? No change. My FPR is doing exactly what it's supposed to be doing, and the pressure drop across my injectors is rock steady where it's supposed to be even in the face of rapidly changing manifold vacuum. It's awesome!: