So, with perfect timing for the snow and ice the idle speed decides to intermittently rise to around the 1800 mark . This can last for the whole of my journey to and from work ~ 17 miles and is fine on the dual carriageway but unnecessarily exciting in crawling traffic or ice covered lanes. I'm presume it's not going to do the auto box a lot of good either
Any thoughts? Air leaks have been suggested but I know not how or where to look for them. An engine re-start _seems_ to reset the idle speed but not always for very long.
I assume the RR has an IAC to control the idle? The one on my car keeps doing similar things leaving me with an idle of 2000-3000. If i then manage to stop and open the bonnet and hit it it goes back to normal, or it does it on its own - sometimes quicker if I turn the ignition on and off a few times.
Second hand ones are cheap on ebay if you want to try a swap.
I had the problem on an '83 V8 RRC Manual. Im my case the Advance retard unit had a perforation in the er..erm.. diaphram in the unit allowing air to be sucked through and screwing up the timing. The engine pulsed but revs seemed to sit lowest around 1100 rpm and upwards.. Equally as fun at junctions and the lights as everyon assumed I was up for a race.
May be completely unrelated but similar so though it worth throwing in the the idea pot.
And if you get to the bottom of it, please tell us! I have a regular customer who has had over 15 hours of free labour on his disco 3.9 now, because I won't bill him if I can't effect a cure. His does something similar, occasionally racing up the revs but it will come back to normal idle as soon as you come to a halt and then select N or P on the autobox, then it is fine until it next goes over 20mph then the cycle starts again. If you stop then restart within say 2 minutes it does what it should - fires straight up and runs to 1400rpm then settles almost immediately back to idle, yet if you let it sit for more than 5 minutes it will not fire without giving it a little throttle and holding the revs up manually for a few seconds until it clears its throat! So far, it has had:-
2 x idle speed stepper motors, both new. a 3rd stepper motor for diagnostic purposes, taken from a good running vehicle. Ecu replaced, no different so original refitted. Base idle settings checked & adjusted. AFM replaced with known good unit. Throttle position tx replaced. Temp sensors (water and fuel) replaced. Wiring inputs (and voltages) from autobox neutral switch and road speed sensor all checked ok. Harness con-checks carried out ok. Fuel pressure checked ok. New plugs, leads, arm, cap and vaccum advance capsule. New ignition amp and coil.
This is the first time I've ever been beaten by a rover V8.......... and it's pissing me off severely! Badger.
Lambda probes - they tend to either work ok or wander off one way or the other (rich or lean), but they don't give erratic indications as a rule, and they have no control over rpm. Badger.
In my case on an 89 Vogue SE it was a faulty plug on the sensor that was about 7 years ago so I can't honestly remember which of the temp senders was giving an erratic reading and causing the ECU to read the engine as cold so go into the fast idle mode your choice but one of the pair on the top of the engine. They are cheap as chips from your local Prince of the Sudden Lack of Light dealer Derek
Lots of interesting info, in the "Real-Life Diagnostics" section.
And as to Lambda sensors... Scroll down to... "Best of the Rest".
You don't need a specific make or model of automotive osciloscope to make measurements, just one that will do the job (without itself suffering) and the right info so as to diagnose the problems.
As I said above, lots of good info on that site. Hope something helps.
The easy way to spot air leaks is with an automotive smoke machine. It mildly pressurises the induction system with smoke, and you look for where the smoke comes out. The harder way is to have the engine running and use water spray to try and find an area where the engine stumbles or surges when the spray is there. A lot of people use a hand spray bottle for this.
However, noting that I am not that familiar with the model in question, I'd suggest that it might be intentional. The fast idle could be because the engine is running too cold and going into fast idle to warm up.
So perhaps monitor your temperature? If it is on the cold side, you might want to look at your thermostat, it might not be operating quickly enough (to close). If it was my bus, I'd be sticking a bit of card board over the radiator, sized to reduce the available cooling by some fudge factor that you would need to work out by trail and error.
I'd suggest the first thing to try is using the card board, sized to block from 3/4 to all the radiator, leave for work earlier than usual, monitor your temperatures and adjust your cardboard as necessary if it looks to be on the hot side.
Thanks for many interesting (and thoughtful) suggestions.
B********d thing has stopped doing it (for the present) which is just as good now that the roads resemble ice rinks again.
As far as I can tell the IAC (Ignition Advance Control?) is fine. When I aquired this RR the advance was faulty - pierced diaphragm - but this had no noticeable effect on the running of the engine! I will have another look at it though.
Possibly a bug in the ECU which may explain why power cycling sometimes cures the fault.
No lamba probes in my CAT-free heap.
Temp probes on the top are a good route to try (nice and cheap)
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