I need some help diagnosing a fuel pressure issue.

I just rebuilt my 3.8 in 87 GN. Got things back together and fired it up. Things are looking really good. Even scanmaster numbers are looking good after some driving and testing and adjustments. I did notice that it was burning an incredible amount of fuel even though scanmaster numbers didn't indicate a fuel problem, and driving seemed nearly perfect, lots of good power, no stumbles or misses, turbo and boost worked fine.....even better than before I rebuilt everything. I decided to check fuel pressure and do some adjusting if necessary to possibly address fuel consumption issue. I connected fuel pressure gauge and turned on the ignition without starting the engine. I checked the gauge and the first thing I notice is a after the fuel pump shuts off the pressure starts to drop, and pretty fast too. I would say it takes about 30 seconds for the pressure to drop from 40 to 20 psi. And after some checking and re-checking with no change, i notice that as the pressure drops, the slower it drops as the pressure goes down. I have a racetronix 255 fuel pump with hotwire kit installed, Adjustable fuel pressure regulator, turbo tweak 5.7 chip with 42lb injectors to match chip. I've had that combo for about 4 years and never had an issue, always good fuel economy. I tried adjusting fuel pressure, with the regulator backed all the way off, I can't get it to go below 40 psi with vacuum line off, (I know it should be set to about 43psi with vacuum line off, but I was playing with the regulator to make sure it's working properly), I can raise up to 60 psi plus no problem (could go more but I didn't try going any high than that). I put new o-rings on the injectors when I installed after the rebuild. So I'm thinking that the fuel pressure drop with engine off could just be a leaky check valve on the pump, but I'm also thinking that the pressure regulator doesn't seem to be acting right, and I'm also thinking about the incredibly poor fuel economy. There is no external fuel leaks anywhere. So do I have leaking fuel injectors......bad pressure regulator......combination of both? And with the excess fuel consumption, why doesn't the numbers on the scanmaster indicate it? I need some advice.

Chuck Leeper

8/10's of a century!
Staff member
What are the SM numbers?
Any data logging?
How long did the injs sit?
Either the return line is kinked/plugged, or the reg is toast.
What are the SM numbers?
Any data logging?
How long did the injs sit?
Either the return line is kinked/plugged, or the reg is toast.
Here are the scan master numbers after going for a short drive and engine is warm and at idle.
O2- 070
AF- 05
L8 - 35
BAT - 13.5
INT - 128
BML - 134
CLT - 164
ATS - 64
RPM - 800
TPS - .42
IAC - 38
CC - 70
No malfunction codes
Injectors sat for a little over 3 months before reinstalling. I checked and the return line is not plugged. I know have to adjust to get the IAC number down a bit. O2 number doesn't seem right though. When I was playing around a bit a couple days ago the O2 number was hanging around 830 with engine warm and at idle. Do I have a bad O2 sensor? I have a new pressure regulator and a fuel rail mounted pressure gauge coming in a couple weeks.


Chaos is a ladder.
TT chips are open loop at idle.
O2 sensor:
cc = Cross Counts

Cross Counts are the number of times that the factory O2 sensor has crossed from lean to rich across the stochiometric point (0.441v) during the sample rate of the scan tool. The more active the sensor is, the better job the ECM will do in keeping the A/F at the desired stochiometric ratio for idle and part throttle driving. At wide open throttle, it is not used. Many scan tools may show this number as a cumulative addition so that the first frame may be 10 and the second may be 30 which means the second frame was actually 20. When the total reaches 255, it resets to zero and starts over.

Given that a tool like the ScanMaster samples at approximately 1.4 second intervals and more sophisticated tools like PowerLogger may provide 20+ fps, one must take this into consideration when looking at the results.

Something like 10-40 cross counts per 1.5 seconds is probably normal. An O2 sensor that is sluggish and consistently is on the low end (count-wise) should be replaced. O2 sensors are cheap and it does not cost much to replace one periodically anyway. If you don't, gas mileage and drivability will soon go down hill.

As stated prior, leaded racing fuel will often drastically shorten the life of the O2 sensor. Symptoms are reduced output, sluggish cross counts, and/or a check engine light. Many believe the Denso sensors resist degradation much longer than do the factory AC sensors. When leaded gas has been used, even the Densos will show a sluggish cross count, but, will sometimes clean back up with use. Watch the counts, if they don't recover to a normal level, put another one in if you want optimum mileage and part throttle performance.

If you have a chip that provides an Open Loop idle, it will probably show zero cross counts at idle and this is normal. Check the cross counts at cruise. Also, do not expect cross counts at wide open throttle.

michael evans

Well-Known Member
I had a similar issue when I got my car. At idle no fuel leaks but when the pressure would rise the hand-tight connections at the frame would leak.