Engine cuts in and out

[chuckfryer]

'87 GN, lots of mods but none recently except for a switch to vacuum brakes.

Problem appeared out of nowhere a week ago: at TPS of about .70 to .85 the engine cuts in and out. Lower than .70 and higher than .85 it runs fine. RPMs are in the 1800-2000 range when it happens.

Powerlogger doesn't reveal any problems that I can see. I attached the data file, but had to change the extension to .txt for it to upload. Change it back to .dat before viewing in Powerlogger. It was cutting in and out from the 7 second mark to 13 seconds.

To add to the mystery, the problem clears up after driving for about 20 minutes. I've confirmed it's not directly related to coolant temperature, although I suppose some other component might be slowly warming during that time.

Any ideas? TIA

 

[tcgn]

engine cut out part throttle

You may have a bad TPS sensor. The voltage breaks off at a certain throttle point. I usually use a lab scope to monitor the voltage, key on engine off.
Apply throttle slowly and watch the voltage rise smoothly without any interuption from 0.42 to 4.70. If you see a glitch, then the TPS is bad. Its hard to see the break looking at the numbers using a volt meter
Good luck