i ran into an interesting/challenging problem a week or so ago...

background:

brand new engine wiring harness and a complete rewire of all my elec modules by Beyond Redline.

i received the car july 8 and have been finishing the details while doing some moderate on the road testing. actually everything is looking good but...

i noticed a 2 to 4 psi blip up in my digitally logged fuel pressure. say, from 44 to 48

my fuel pressure wasn't properly tracking either. after a call to aeromotive "it is either a broken spring or a ripped diaphragm or it is some other component such as the pump" well it was neither. Luke at Beyond Redline suggested i check the ball and seat and as usual he nailed it. i had some foreign material on the seat. once that got removed fuel pressure tracked boost.

but i still had the spike.

so i bought a new FPR. and of course that did nothing.

i then took a closer look and found my AFRs were also spiking on the strip chart.

they were doing it at the same time. as was the raw voltage for my datalogged exhaust back pressure.

however... just to add to the confusion my two EGT logs had no spike.

so on my (new) Datalogit...

AN1 is AFR it spikes
AN2 is blank and will eventually be the O2 ground
AN3 is Fuel Pressure it spikes
AN4 is EGT Front no spike
AN5 is EGT Rear no spike
AN6 is Exhaust Backpressure it spikes.

battery voltage is consistant at 13.5
PIM has no spikes

i am now finally logging on a fast computer... 14 lines per second logging everything.

just for laughs i hooked up my old computer (4 lines per) and it also showed a very spiking reading... so it has nothing to do w the new computer.

i also have a brand new Power FC.

the spikes come approx every 5 lines.

anyone have an answer?

 

is the fuel pump switching from low to high over and over?

 

Could be a bad ground somewhere. Might be getting noise into your signals.

Dale

 

Everything spiking in unison like that? Has to be some electrical or software issue. Do the spikes change in frequency depending on load or RPM?

 

I've had my fuel pressure change in a similar fashion but it was more consistent and not just a blip. When my 20b was on the test stand, it would stay consistent with my set pressure of 44psi. It wasn't until I put the engine back in to the car that it started changing. It would go up to 48psi. The only difference with my engine running on the test stand vs the car was that I didn't have a gas tank that was pressurizing. I had to re-adjust my pressure gauge while in the car.

 

Check to see if the AFR, Exhaust Pressure, and Fuel Pressure sensors are all sharing the same ground at some point.

I'm not familiar with a Datalogit is to be wired, but I've seen this on another setup that used multiple sensor ground wires in the vehicle's harness... the TPS signal and Coolant signal were fluctuating when the ECU harness was shaken, but MAP was not. It turns out the TPS and CLT sensors were sharing a ground wire (which was failing intermittently), and the MAP had a separate wire which hadn't failed.

 

A diagram of your wiring, especially grounding, would be helpful. Have you thought about using shielded wire? Your factory harness has shielded wire and multiple ground points, so that is going to be less prone to electrical noise.

 

as mentioned, please describe your grounding setup, as that is a likely cause. Also if you have a current clamp you could check to see if the fuel pump is getting loaded up or not. Big fuel pumps draw enough current to float your system voltage a little bit, especially items that are on the same circuit branch, so if it is pulling a vacuum in the tank and then relieving or something like that, that could be the cause.

 

thanks for all the replies and help.

i did check and find the spikes average 2.5 per second without regard to RPM or load.

i will be examining the relevant grounds this a m.

 

you could try disabling the alternator (pull the grey plug)

even try removing the inputs to the datalogit one at a time, more than likely it is a poor ground

 

thanks to all who provided help.

i relocated 3 ground wires and have no more blips

howard

 

Howard,

Just for reference, where were your grounds and where are they now?

 

the 3 grounds were attached to a bolt that holds my exhaust back pressure sensor which is approx where the ABS module used to reside.

i moved the ground point about 6 inches over to a spot where it was easier to get it down to bare metal which probably was the problem.

my problem was not the location, but the preparation of the location.

again, thanks to all for your help.

 

Hey Howard - Is this the problem with the 200 mph car? let us know how that is going.