L series revving-mind of its own???

hi all, a little help here would be greatfully accepted

i have a '99 diesel with 140k on it. for a while now past 8 mths the car has intermittent revving pulsing upto 1200ish. then 1 day waiting to pull away the revs increase upto 3000-3500 ! foot on the brake it calmed down to 1300 ish foot off back up to 3k+... then while driving back home the engine starts lagging and management light flashes, clears for a bit, flashes, laggs, finally get home kill a revving 3k+ engine.
hunts for an answer on the net as garages are 'mystified' ' aint heard of that one mate!" try disconnecting the air flow meter and instant cure like it never happened.
over the past 8 months this has been an infrequent problem but sorted each time by the a-f-m.... until today.
a-f-m doesn't sort it, i leave the plug off,the car is at rest(engine off) for 45ish mins, try to start, the glow plug light doesn't go out but the radiator fan starts up!!!
reconnect plug, glow light goes out, cars revving again but i get it home.

anyone got any ideas or has heard of this before...?

thanks
russ

Now I was going to say throttle position sensor until I read through what you have to tell us.

I'd say from a gut feeling you have a bad earth somewhere. When seemingly unconnected systems start to interact then its usually because a shared earth is floating free of the frame potential. Its like you sometimes see cars that when they indicate the brake lights flash or other weird stuff like that, it all because a shared earth has come free of the bodywork.

Right, I reacon your problem is in the engine bay. Engine management, glow plugs and rad fans are all controlled from there. Have a good look round your wire looms in the engine bay and locate any connections that bolt to the bodywork. Specifically check under and behind the battery tray as the main connectors are there where the engine wire loom connects to the body wire loom. Also check for damage to the looms. We had one guy whose L series cut out after going over a sleeping policeman. Turned out the jult had damaged the ECU loom and caused the main ECU fuse to blow.

Also, take the cover off the high tension fuse box that sits behind the battery. Check all fuses and strap links are intack. While you're at it, check all fuses in the cabin fuse board below the steering wheel (Assuming your is RHD).

Report what you find.

Cheers,

Dave

Hi thanks for your reply, i have checked all fuses but will have to check earth points and loom tomorrow rain has not stopped play!. will report back tomorrow! Many thanks

Russ

No problem and good luck.

Dave

hi Dave

found 3 earth points around battery area, cleaned with emery- rebolted. found the main battery earth lead to body (approx 4 inch from '-ve' battery clamp) was loose!
just finished putting the car back together and all looks good. steady idling-no lagging. will give it a few days and see how it goes.
also found the metal coolant pipes under the air box severly corroded and a slight damp patch, so looks like i'll be back under there soon to replace them!

thanks again Dave for your help, i'll report back if anything more develops.
btw... if you disconnect the maf on this age of car should the management light glow?

regards
Russ

Now that is a result!! I hope it proves to be the fix.

Incidentally, if the battery comes off when the engine is running the the alternaters voltage regulation can spike something cronic, leading to damage to connected equipment. Most on-board electronics have a disconnecter that protects the unit when the voltage is damagingly high and restores power when the voltage recovers. This might have been your issue with the reving as the ECU was cutting its own power intermittently.

However, things like phone chargers are not protected in this way, so if you have any plug in chargers then check them carefully before you fry whatever they connect to!!

Best of luck,

Dave