MOD500

On the way home from the airfield yesterday the engine missed a couple of times.

Happened mostly between 2 and 3, 000 revs on light throttle, the engine would miss once or twice then be fine for another 20 miles or so ... happened quite randomly. When the engine missed the OBC display would dim momentarily and the speedo needle would drop 10 mph or so, and then both would return to normal after a split second. The final time it missed the CEL and another warning light (didn't catch which one as happened so quickly) flashed very briefly.

Half way home I stopped at a Service station and changed the DME thinking it might be at fault, but the problem reoccurred after 30 miles or so. I think the issue might be a relay given the light show on the dash? I noticed when changing the DME all the relays in there where hot to the touch, is this normal?

Can anyone help please?

Thank you in advance.



Martyn.

eclou

I don't know exactly how similar it may be in the 993's but in the 944 turbos when maxing out the fuel injectors on their duty cycles the DME will get hotter - especially if running aftermarket injectors with a higher than stock impedance. Is it still doing it after the car got a chance to cool off?

MOD500

I changed the DME after a 60 mins driving at a steady 80 - 90 mph on the way home from the track, so no sustained WOT.

Cheers



Martyn.

Felix

This strikes me as an overall electrical supply issue; check your battery leads and the voltage at the battery with the engine on and off. Alternator belt tight and in good condition? Might be the alternator or voltage regulator itself.

A DME issue might cause rough running but should not cause any OBC display issues and only illuminate the CEL.

ACEparts_com

Has it had the wiring loom upgrade?

MOD500

Thank you Phelix, will check the leads and voltage on/off and report back. Belt is tight and sound, and had a new battery just before Christmas. Could be the voltage regulator / alternator, they are original AFAIK so may be knackered.

Ace, had a new wiring loom about 18 months back, so all should be sound there.

Will report back with the voltages

MOD500

Engine off, voltage across battery = 12.5 V

With engine running = 14.1 V

Think a trip to the shop is in order to get it all checked out. Is the voltage reg part of the alternator or a separate unit?

Thanks for the help.



Martyn.

Felix

The regulator and brushes are in one unit located at the back of the alternator.

Felix

Also be curious to see what the voltage is with the electrical load cranked up - rear window demister, HVAC blower on max, main beams on, etc.

MOD500

Tinkered further with the multimeter ......

Again fired the car up and saw 14 V across the battery.

I then turned the heater blower to full, put the little snow flake A/C on, rear de-mist on, full (HID) beam, and turned radio up loud .... the reading was 12.5 V and fell to 12.3 V within a minute or so. Upon turning all the above off the reading increased to 13.6 V and was climbing quickly.

So I guess the alternator is on the way out, thinking back driving home from the track the weather was all over the shop meaning was turning lights on and off, wipers, and heaters, which might explain the erratic nature of the misfire. I assume when the voltage drops below a critical voltage it sends the ecu, etc bonkers.

Also, whilst writing .... what lifetime do people reckon the HT leads, dist cap, and rotor arm have? Mine were last changed in Dec 2005 when the mod work was carried out (12k miles ago), was thinking whilst the car is in the shop to have them changed. Would this be prudent or a waste of dough?!

Thanks again




Martyn.

911/Q45

I'd say those items are good for 60K miles. Your troubleshooting sounds like it's in the right direction, but I'm not convinced it's the alternator. I'd guess a loose connection somewhere that faults over bumps. I had a battery once that had an internal break that did odd things.

Felix

12.3 at the battery with the engine running is definately too low - could be a bad connection or worn brushes or perhaps a failed diode in the alternator.

2 years/12K miles out of leads, dizzy cap and rotor isn't much provided no cracks/splits/heat damage.

Q

MOD500
my first 993 turbo did exactly that
eractic OBC and accessories
light throttle, hiccup
light throttle, hiccup, stall
light throttle, hiccup, eratic

later that day, it stopped - DEAD.

Alternator.

[It had travelled 60 000miles at that stage]

MOD500

Update .....

Turns out the alternator is FUBAR; was only fit enough to charge the battery when little or no ancillaries were operating (like the rear de-mist, heaters, etc). So a new one is to be fitted early next week.

Also, whilst (another set of) new discs and pads are going in am having the brake fluid changed to Castrol SRF, will very interesting to see how it performs compared to the R600 in there at the moment, and the Superblue that preceded it.

Might be a silly Q, but whilst the car is in the shop should I have the N75 frequency valve thing changed out of precaution? What does this device actually do? Is it hard to swap out?

Thinking of some neons under the front too, would look real fly when cruzin'?

Thank you for all the help

Kind Regards



Martyn.

Felix

As I understand it the N75 valve is an electrically controlled valve that supplies boost pressure to the wastegate actuators at the direction of the ECU. It opens and closes multiple times per second as required to gently bleed off boost pressure when the ECU decides the air mass going through the MAF is too high for the current conditions (rpms, load, knock, IAT and of course the maps). It seems they get lazy as they get older and don't open and close as quickly as they should which sometimes results in an over boost condition. They're about £40 or so and it should be easy to replace if the alternators's already coming out. I have a new spare if you can't get one quickly enough.

I would recommend blue neons; that's what I use. Bought mine from http://www.chav_your_ride.co.uk