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Low voltage under full power

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Help. Freescan shows voltage drops under full power, dropping as low as 12.3 volts and returns to 13.6 under normal cruise. New Bosch alternator and new battery. Cables good and I even added an extra ground wire. Anyone got a clue as to where the voltage is going or if this is normal?

I'm looking for a reason that causes the car to go from perfect operating conditions at computer reset and progessively gets worse over the next 50 miles, like it learns backwards getting dumber the more you drive it.

Marc in CA

89 SE

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Hi Marc,

My voltage also follows the same changes. Its strange (you would expect it to be easier to hold 13.6v at full engine speed), but I have put it down to the point at which the voltage is measured in the ECU. It would be nice to see what the voltage is at the battery terminals, logged by an independant device (not the ECU).

I am not a freeescan or coding expert, but I do know that when the ECU is reset it will run the engine 'safe' (i.e. a little rich) for a while while learning the new parameters. Idle immidiately after reset tends to be better than idle after the ECU has learned. When I had a stumble problem, it was always better emmidiately after the reset and got progressively worse. My problem was an irritant rather than a major issue. I progressivley worked through all the componants that effect the running qulity, replacing as I went, and things got progresivley better.

It seems that all componants have to be spot on to get near perfect running and idle quality.

I changed (in order of largets effect to smallest)

O2 sensor (biggest improvment)

fuel pump


fuel presure reg



plug leads

coil pack


Even if freescan is saying the O2 sensor is working OK, it is worth changing since mine seemed to be working fine but the engine ran much better with a new one (use genuine Lotus part, people hace had problems with non OE O2 sensors in the Esprit).

Good luck

Malc (also 89 SE)

Edited by Malc Holmes

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maybe the belt to the alternator is slipping at higher engine speeds. Try some belt dressing to stick the belt a little bit more to the pulley.

89 White Esprit SE

...a few little upgrades....

93 RX7.....Silverstone

....slightly modded...Muahaha...

New Addition:

1990 300ZX TT......Hmmm

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Thanks guys. I have replaced the O2, TPS, plugs, wires, ECU, New belts, New Alt, checked vacuum leaks, IAC new. Maybe it is weak on the fuel delivery, I haven't check that because it pours the fuel in under full throttle and takes off. The secondarys work according to Freescan.

My experience tells me it is a lean condition, just feels like a weak burn under cruise. Another thing that happens is after it gets 50 miles on the reset, it stumbles a bit at the stop sign and then catches itself and burns the tires off at half throttle with no turbo spooling (a few pounds of boost really). That feels like a timing issue. Maybe I have a bad ECU again or the memcal is lost in space.

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