Kangaroo just off tickover

[C Richards]

Odd problem, I have a 1987 911 completely standard.
When I start it from cold it will drive ok but with a fraction of throttle applied it will kangaroo (drive jerky) a bit, once the throttle is applied a bit more it is fine, this will happen at any speed, even if driving say at 50mph, take your foot off the throttle and just feather it and the car will jerk back and forth. this still happens when the car has warmed up. BUT park up and stop the engine for 10 mins restart and the car drives as smooth as a Swiss watch...Any ideas?

[C Richards]

OK, I've actually found the problem. I initially thought ICV, I had previously replaced this and it did help a little but the problem was still there. It actually turned out to be a problem of my own making, if I'm not going to use the car for a few days I disconnect the battery from my reading this looses some learnt settings from the ECU and reverts it to factory settings. I drove it yesterday and did my usual drive until it's warm then switch off and restart, I left the car overnight (with battery connected) and used it again today and it's perfect.

[Rtrorkt]

I chased a hunting idle, jerky throttle, engine shut off, etc. Replaced ICV, O2 sensor, head temp sensor. Was the idle screw turned out too much. Adjusted that and problem solved

[wazzz]

Originally Posted by C Richards

OK, I've actually found the problem. I initially thought ICV, I had previously replaced this and it did help a little but the problem was still there. It actually turned out to be a problem of my own making, if I'm not going to use the car for a few days I disconnect the battery from my reading this looses some learnt settings from the ECU and reverts it to factory settings. I drove it yesterday and did my usual drive until it's warm then switch off and restart, I left the car overnight (with battery connected) and used it again today and it's perfect.

Don't know where you read this, but this is not correct. The ECU on '80s Carrera is completely switched off from power supply (battery) every time ignition is switched off. No memory is maintained and no backup power supply is used by this generation of Motronic ECU. Hence it makes no difference whether you keep the battery connected or disconnect it.

If your problem has disappeared, it must be a coincidence.

[ant7]

Originally Posted by wazzz

Don't know where you read this, but this is not correct. The ECU on '80s Carrera is completely switched off from power supply (battery) every time ignition is switched off. No memory is maintained and no backup power supply is used by this generation of Motronic ECU. Hence it makes no difference whether you keep the battery connected or disconnect it.

If your problem has disappeared, it must be a coincidence.

[917_Langheck]

Originally Posted by Rtrorkt

I chased a hunting idle, jerky throttle, engine shut off, etc. Replaced ICV, O2 sensor, head temp sensor. Was the idle screw turned out too much. Adjusted that and problem solved

Can confirm this with same results.

Hunting idle at start up was due to idle bypass screw open too far. Jerky off-idle fully solved by relocating AFM potentiometer tracks, and installing a new O2 sensor, combined with properly adjusted bypass screw.

[GH85Carrera]

Originally Posted by wazzz

Don't know where you read this, but this is not correct. The ECU on '80s Carrera is completely switched off from power supply (battery) every time ignition is switched off. No memory is maintained and no backup power supply is used by this generation of Motronic ECU. Hence it makes no difference whether you keep the battery connected or disconnect it.

If your problem has disappeared, it must be a coincidence.

Yea, the fuel map is loaded from the massive 8 kilobyte CPU and you are off. Earlier 3.2 DME computers only have a 4K chip. No room for storing or learning anything.

[ccallana]

Originally Posted by GH85Carrera

Yea, the fuel map is loaded from the massive 8 kilobyte CPU and you are off. Earlier 3.2 DME computers only have a 4K chip. No room for storing or learning anything.

You young whipper-snappers - back in my day we wrote whole operating systems in 4k

[C Richards]

I've had this problem since I rebuilt the car over 5 years ago and I've never fully solved the problem, I did have the ECU rebuilt, I've replaced the ICV, fuel pump, fuel pressure regulator, and this was the only problem that remained, it was worse when driving in a traffic queue just around tick over. I've been using the car more and more lately and just noticed that when I park-up and go back to it it then runs perfectly and now when I leave the car for long periods with the battery connected (a number of days or even a week) the car starts and runs perfectly. Like you I'm surprised that an ECU of this age had a "learning memory" (or whatever it is called) so it could well be a coincidence BUT is is definitely something to do with the battery being disconnected.

[ant7]

From memory, The DME relay completely disconnects the positive power to the ECU when ignition is off, so not sure how leaving the battery connected solves your problem, more likely a slight air leak somewhere, or AFM needs the tracks inspected as they tend to cause drop outs in the resistor track over the most used part of the track, had the same issue with mine, moving the wiper in the AFM to an unworn part of the track sorted the issue for me, also; inlet manifold bolts can creep loose over time, allowing unmetered air to enter the cylinders, so another possibility ?
Ant.