Late this spring got the 1999 Corvette out of storage as winter temps were gone, now around 65 degree days
Car had been sitting since late fall so time to get a testrun on it and check everything out
About 5 miles down the road I noticed the electronic Autometer fuel pressure gauge ( $288 cost) reporting fuel rail pressure going as low as 54 PSI
The GM specs for fuel system functions from 55 to 61 PSI so 54 PSI
bothered me as in hotter weather that would be lower and cause high lean conditions
Tried the easy swap first, bought and replaced the fuel filter / regulator, did a testrun and still Autometer reported as low as 54 PSI
Then yanked the fuel rail and checked all 16 O-rings for fuel injectors, torqued fuel rail mounting bolts, testrun - same results
Now stuck doing the bitch of buying new pump unit, replacing the left tank fuel pump/ float on jack stands
Which means draining both gas tanks, hassle getting pump unit out and in
OK, fuel system is replaced at lot of money and labor and damn enough do a testrun and the Autometer is still reporting lower PSI values, like WTF
OBD-II live data showed when Autometer gauge reported low PSI the fuel trim, O2 sensors, fuel injector pulse width, etc reported perfect and no signs of lower fuel pressure
Now I suspect the almost new Autometer fuel pressure and gauge, this is a electronic setup, only operates from 0.5 to 4.5 Volts
and being Autometer's specs is gauge functions fromzero to 100 PSI
that means small amount of mVolts per PSI
, fuel pump functions +/- 2 PSI of 58 PSI
means the gauge operates in less then about 1 volt, no margin of error for and degrading or drifting.
It dawned on me that the gauge reporting was higher
when engine was cold
and then values dropped as engine got to operating range where in this Corvette is 185 degrees coolant temp
as that is what the Tstat is
Autometer sender is made if thin metal and only 1" by 1" in size.
It mounts to a fitting on fuel rail which of course is at top of engine and where it would be the hottest
Ran some cold morning testruns and then later in 90 deg weather and was clear all along the sender was at fault
Contacted Autometer's support who told me that they only warranty this $288 product
for just one year
I then sent a full report with findings to Autometer and got an email from someone in their marketing group
He then sent me a new sending unit free of charge
Installed and tested it to get about the same results !
So now I start doing thermal testing using a J type probe to a Pyrometer, doing testruns and mapping thermal values that was recorded live by laptop software
Found is when sensor is mounted to fuel rail after testrun measured around 130-140 degrees
Then remotely mounted the sensor 16 inches away from engine, close to firewall and did testruns
As long as the sender measured less then 120 degrees,
the PSI output it reported was closer to correct
Then dawned on me of another testcase, the Corvette has a custom hood, with scoop and openings for bringing in colder airflow and extracting hot air out
When testing first started the opening were set to reduce airflow about 60%
and why testing above, sender was about 130ish deg rather then 185 deg of Tstat
So final testcase, scoop openings set to allow maximum flow, sender still 16" away
Sure enough in several daily testruns over 4 days of testing example below, Cold start gauge reported 59 PSI,
after few minutes at idle reported 57 PSI
During several 25 mile runs, in 70 and 90 deg weather
gauge now reported correctly staying at 57 PSI most of time, decel a bit and up to 58 PSI
Not once when in colder temp did the gauge report less then 57 PSI so without a doubt the sender degrades output (mVolts) around 120ish degrees and worse by 140 deg.
With the hood scoop fully functional and weather was 70 degrees, as long as car was doing over 20 MPH
the sender measured only about 10 degrees higher
and temps only spiked when at idle at around 130 degrees but as soon as car above that 20 MPH the sender temps would drop again.
Only slight increase in temps during WOT.
All this testing was repeated to both of the Autometer fuel pressure senders I had with repeated results
I had sent marketing of Autometer finding and results two weeks ago - ZERO response
So in the end customers spend almost $300
for a electronic fuel pressure gauge,
a simple mechanical one is like $25 so you'd expect for the big bucks
that you could rely what it reports and instead cost a bunch of money replacing the fuel system to find the gauge is prone to degrade as temps increase
and in most cases a stock car with no vented hood, 195 deg Tstat and coolant fans commanded by engine management not to come on till coolant temps
are over 220 degrees would be worse.
Click on any photo to see full size :