Results 1 to 20 of 20

Thread: Why Cheap Pressure Gauges Fail

  1. #1
    Registered User rossiz's Avatar
    Member ID
    CZCC-30163
    Join Date
    Feb 2014
    Location
    Seattle, WA
    Age
    53
    Posts
    323

    Default Why Cheap Pressure Gauges Fail

    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
    Click image for larger version. 

Name:	image.jpg 
Views:	30 
Size:	104.1 KB 
ID:	74298

    Gear that drives the needle - all teeth worn out in the center
    Click image for larger version. 

Name:	image.jpg 
Views:	34 
Size:	113.4 KB 
ID:	74299

    Gear that pushes needle gear - look closely for missing teeth in the center
    Click image for larger version. 

Name:	image.jpg 
Views:	34 
Size:	118.6 KB 
ID:	74300
    '78 280z - Daily driver/work in progress...

  2. #2
    Supporting Member 240dkw's Avatar
    Member ID
    CZCC-15138
    Join Date
    Sep 2007
    Location
    Powell River,BC, Canada
    Posts
    353

    Default

    Me thinks it is due to vibration

  3. #3
    Registered User rossiz's Avatar
    Member ID
    CZCC-30163
    Join Date
    Feb 2014
    Location
    Seattle, WA
    Age
    53
    Posts
    323

    Default

    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.
    '78 280z - Daily driver/work in progress...

  4. #4
    Boat Anchor Repairman Captain Obvious's Avatar
    Member ID
    CZCC-24608
    Join Date
    Jul 2011
    Location
    Philadelphia, PA
    Posts
    1,845

    Default

    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.

  5. #5
    Registered User S30Driver's Avatar
    Member ID
    CZCC-21075
    Join Date
    Mar 2010
    Location
    Seattle Washington
    Posts
    163

    Default

    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.
    Attached Images Attached Images  
    1977 280z 06/77

  6. #6
    Low Budget/High Value
    Member ID
    CZCC-20342
    Join Date
    Oct 2009
    Location
    Washington County, OR
    Posts
    3,629

    Default

    Quote Originally Posted by Captain Obvious View Post
    I could have sworn I posted about it a while ago,
    Post #6 here - http://www.classiczcars.com/forums/f...l-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.
    Last edited by Zed Head; 09-28-2014 at 10:11 AM.
    1976 280Z, with some minor modifications

  7. #7
    Registered User rossiz's Avatar
    Member ID
    CZCC-30163
    Join Date
    Feb 2014
    Location
    Seattle, WA
    Age
    53
    Posts
    323

    Default

    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!!
    '78 280z - Daily driver/work in progress...

  8. #8
    Registered User siteunseen's Avatar
    Member ID
    CZCC-24724
    Join Date
    Jul 2011
    Location
    Alabama
    Posts
    1,967

    Default

    I took mine out too after fixing my issue, CTS potentiometer tweaker. More cracks in my boat that could leak.
    1972 240Z #918 New Sight Orange
    1977 280Z #305 Light Blue Metallic
    1972 240Z #110 Persimmons Red

  9. #9
    Z geek at large FastWoman's Avatar
    Member ID
    CZCC-19635
    Join Date
    Jul 2009
    Location
    Hampton Roads, VA
    Posts
    2,787

    Default

    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.
    My last three sports cars while I still owned all three:

    2001 BMW Z3 Roadster 3.0i soft/hard top (sold)
    1966 Ford Mustang Coupe (sold)
    1978 Datsun 280Z (enjoying very much )

  10. #10
    Registered User Stanley's Avatar
    Member ID
    CZCC-24191
    Join Date
    Apr 2011
    Location
    Redondo beach
    Posts
    641

    Default

    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

  11. #11
    Boat Anchor Repairman Captain Obvious's Avatar
    Member ID
    CZCC-24608
    Join Date
    Jul 2011
    Location
    Philadelphia, PA
    Posts
    1,845

    Default

    Quote Originally Posted by rossiz View Post
    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.

  12. #12
    Boat Anchor Repairman Captain Obvious's Avatar
    Member ID
    CZCC-24608
    Join Date
    Jul 2011
    Location
    Philadelphia, PA
    Posts
    1,845

    Default

    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!:

  13. #13
    Registered User rossiz's Avatar
    Member ID
    CZCC-30163
    Join Date
    Feb 2014
    Location
    Seattle, WA
    Age
    53
    Posts
    323

    Default

    ok, wow...

    that's really cool - where did you pull the vacuum? would this be a good tool to cross-check for vacuum leaks?
    what if i have crappy vacuum in my system (even w/no leaks) would that screw things up and give false fuel pressure readings?

    and, of course - cost and source must be shared, please...
    '78 280z - Daily driver/work in progress...

  14. #14
    Boat Anchor Repairman Captain Obvious's Avatar
    Member ID
    CZCC-24608
    Join Date
    Jul 2011
    Location
    Philadelphia, PA
    Posts
    1,845

    Default

    The gauge is from Orange Research and I got it off ebay for $40. So it's not disposable and lives in the toolbox as a diagnostic aid. It's a little too valuable and bulky to be a permanent installation. I got Orange Research 1203-PGS series.

    I pulled the vacuum off the intake manifold off the carbon canister purge source which is the nipple near the cold start injector. I don't see any reason why you couldn't use that gauge as a standard vacuum gauge by hooking up the LO side and leaving the HI side open to atmosphere. Here's how that gauge works...

    Inside there is a floating piston and cup seal that is spring loaded to one direction. The pressure ports are connected to a cavity on either side of that piston and push/pull it to one side or the other depending on the magnitudes of the pressures. The spring pushes toward the HI side, so pressure on the HI side compresses the spring and pushes the piston towards the LO side. Vacuum on the LO side will also compress the spring and pull the piston towards the LO side.

    One of the neatest parts to the whole thing is that there is no mechanical connection between the piston and the needle movement... It's MAGNETICALLY COUPLED!! There are no springs to wear out, no gears to strip, nothing like that. The only link between the wetware and the needle is the magnet.

    Here's a sketch of the guts design:


    They also make a diaphragm version for gaseous applications where there is a diaphragm between HI and LO instead or a cup seal. The diaphragm does a better job of sealing between the two sides, but for liquids you don't need that.

    So it's true real time differential.
    Don't have to do the math in your head at real time. It does it for you.
    No springs, no gears, no linkages.
    Cup seal between HI and LO sides. (I'm not positive what seal composition I got, but it's either Buna-N or Teflon, either of which is suitable.)

    It's also got a reed switch built into it that you can adjust to close at a desired pressure if you want to set off an indication or something. Red warning lamp if the differential gets too high or too low. Of course, I'm not connecting to that feature, but it's there.

  15. #15
    Boat Anchor Repairman Captain Obvious's Avatar
    Member ID
    CZCC-24608
    Join Date
    Jul 2011
    Location
    Philadelphia, PA
    Posts
    1,845

    Default

    Quote Originally Posted by rossiz View Post
    what if i have crappy vacuum in my system (even w/no leaks) would that screw things up and give false fuel pressure readings?
    Forgot about this part...

    No. It will read true under all conditions and that's part of the beauty of it. The trick is that if you've got vacuum problems, your fuel pressure regulator will be adjusting the pressure WRT to the same leaky system.

    In other words, even if you've got a vacuum leak, your fuel pressure should be OK because the regulator is referenced to the same leaky source. And this kind of gauge, being also referenced to the same leaky source should show proper fuel pressure even if there is a vacuum leak.

  16. #16
    Registered User rossiz's Avatar
    Member ID
    CZCC-30163
    Join Date
    Feb 2014
    Location
    Seattle, WA
    Age
    53
    Posts
    323

    Default

    way cool - i like the way that works.
    i can see a setup with the ability to shut off the pressure and/or vacuum independently via a couple of simple valves it would be a nice diagnostic tool - especially with a dual-scale dial that would also read in-hg.


    love it!
    '78 280z - Daily driver/work in progress...

  17. #17
    Boat Anchor Repairman Captain Obvious's Avatar
    Member ID
    CZCC-24608
    Join Date
    Jul 2011
    Location
    Philadelphia, PA
    Posts
    1,845

    Default

    I'd be happy to send mine on loan if you cover shipping. It's a big brass chunk so it's not the lightest device in the world, but I bet it could make it round trip for less than $20. I might be able to take the reed switch housing off the back, and if so, it might even fit into the smallest flat rate priority box which is cheaper.

    Shoot me a PM if you want.

  18. #18
    Z geek at large FastWoman's Avatar
    Member ID
    CZCC-19635
    Join Date
    Jul 2009
    Location
    Hampton Roads, VA
    Posts
    2,787

    Default

    Very cool! I checked ebay, and there was a nice 50 psi one for about $20 used. Unfortunately you may have temporarily set that market on fire. The gauge sold in the few minutes between when I did the search and when I looked at that listing! The rest are considerably more expensive or have inappropriate scales. Grrrrrr.... But I'll definitely keep my eye out for one. Thanks for passing along the tip to those of your Neanderthals friends who aspire to be Cro-Magnon!
    Last edited by FastWoman; 09-29-2014 at 02:37 PM.
    My last three sports cars while I still owned all three:

    2001 BMW Z3 Roadster 3.0i soft/hard top (sold)
    1966 Ford Mustang Coupe (sold)
    1978 Datsun 280Z (enjoying very much )

  19. #19
    Boat Anchor Repairman Captain Obvious's Avatar
    Member ID
    CZCC-24608
    Join Date
    Jul 2011
    Location
    Philadelphia, PA
    Posts
    1,845

    Default

    Sorry if just created a run on those gauges. Keep your eyes out and I bet you can snag one. I didn't look into it, but I bet I don't even want to know what they go for new from an authorized distributor!!

    And here, I made this for you:

  20. #20
    Z geek at large FastWoman's Avatar
    Member ID
    CZCC-19635
    Join Date
    Jul 2009
    Location
    Hampton Roads, VA
    Posts
    2,787

    Default

    Mmmmm.... One horsepower! Me like...
    My last three sports cars while I still owned all three:

    2001 BMW Z3 Roadster 3.0i soft/hard top (sold)
    1966 Ford Mustang Coupe (sold)
    1978 Datsun 280Z (enjoying very much )

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Epic fail
    By IdahoKidd in forum Open S30 Z Discussions
    Replies: 14
    Last Post: 06-10-2014, 10:59 AM
  2. Replies: 14
    Last Post: 11-25-2012, 12:31 PM
  3. Difference in Oil Pressure Gauges
    By Marty Rogan in forum Electrical (S30)
    Replies: 2
    Last Post: 08-27-2011, 04:00 PM
  4. What can cause a condenser to fail?
    By 72' 240 in forum Interior (S30)
    Replies: 10
    Last Post: 04-18-2005, 10:06 PM

Bookmarks

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •