Missfire!

[Tim Hill]

Hi all,

For the past three months my new 3.2 has behaved perfectly. However in the past two weeks it has developed a few problems.

The first I put down to a dead battery (lumpy running and having to jump start the car every morning were the reasonably obvious simptoms). Replacing the battery apeared to cure the problem and for another 2 weeks all was well in porsche land.

However, today on my drive to work (approx 23miles on duel carriageways) it began to run very rough indeed.

Any ideas? or am I going to have to run it upto Autofarm for a proper look see?




Migration info. Legacy thread was 18138

 

[Used Porsche]

This is from a email I sent to the PCGB but they didnt publish.



There is a new manual (which I purchased after fixing the problem!)

for the 911 Carrera printed by Bentley Publishers which contains this

paragraph regarding the electronic ignition on the 911.


Page 200-13 Engine management

"All the cars covered by this manual use engine management systems

that rely on precise electrical signals for proper operation. Some of

the DME circuits operate on very low current and are sensitive to

increased resistance due to faulty or corroded wiring or connectors.

If any of these signals are distorted, incorrect or missing the car

can develop major driveability problems."


Several months ago the car started to fire on 3 or 4 or 5 cylinders

when started from cold - then it would pick up on the offending

cylinders when the engine was warm.(see email below)


THE ONLY PROBLEM WAS CORRODED FUSES - an instant fix (with much

improved smoothness and performance) was to replace the fuses and to clean the terminal connections into which they fit!


There appears to be a steady trickle of problems on the problem pages

of the magazines and web chat room sites all of which are contributed

to by corroded fuses.


Ray Northway hit the nail on the head when he said he had seen this

with earlier cars but not the 3.2 - but the reason its happening now

is that the cars continue to age!

The car also reponded very well to a new distributer rotor arm and a clean up of the distributer cap terminals.





Migration info. Legacy thread was 18142

 

[adam s]

Tim,

Stevo's certainly right to suggest replacement fuses and a clean around the terminals. Rotor arm renewal and clean up of dist cap is only a few quid too.

I had suspiciously similar symptoms to yourself, which proved to be an aging alternator. This may explain why the battery discharged (it wasn't being refreshed during a run). If the alternator is not producing enough juice, or what it is producing is being lost by poor connections/earth straps/fuse holders etc, this will cause the ecu to malfunction and incorrect electrical signals being sent to injectors etc. This will make for a very lumpy ride indeed.

You need an electronic charge tester to compare current at back fuse box with battery at idle, and same again with engine under moderate load.

Good luck.


Migration info. Legacy thread was 18148

 

[Tim Hill]

Adam,

When the battery went I had the breakdown chap put a voltmeter on the battery terminals to check the charge rate at tickover. It was bang on spec at 14.5V. I guess I could get a meter from work and test at load but I suspect that the altenator is OK.

I'll try cleaning the fuses (I assume that you mean the ones in the engine bay Stevo?) and let you know what happens...

In the mean time I have tried cleaning all the HT leads and the distributor cap with a healthy coating of WD40 but this made no real difference (although it did start very easily).


Migration info. Legacy thread was 18149