Uncomanded Reving.

[tommytwo]

I sent a message to Peter C after reading that he had got lots of advice from guys on the Mercedes forum regarding  uncommanded reving.  Sorry for my slow response to the replies I recieved, I dont always log in unless something grabs my attention. A big thanks to those of you who tried to help me with this problem.

My car is a 1992 Rover 820, the first of the Mk2 820,s with multi point fiuel injection. I think I have found the reason for the problem, I had set the plug gaps 6 thou too small.  I cant believe I did this but  I did and I am a retired pecision enginneer.

I used a set of Imperial feeler gauges and converted the figure in the Haynes book, given in Metric, to Imperial. My feeler guages are from my apprebticeship days back in the sixties, the markings on the blades are now very hard to read. Of couse, I could have double checked that I had the right gap using a micrometer but for some reason, I did not do this.

My standard NGK  BK6RE plugs, are now gapped correctly and I hope ths has now cured my problem. The problem only occured after driving 20-25 miles, then the engine would rev up to 3000 rpm before falling back and then repeating the performace.

So there you have it, it was the nut behind the wheel that caused the problem, not the car!

[tul66]

Can't help thinking it would be far easier buying a set of metric gauges for £3 though!

[mercedade]

820s experience issues with stepper motors and air intake leaks, both of which can play havoc with reliable revs.

Check around the rubber throttle body 'sandwich' mount as they degrade and leak.

Stepper motors can be reset with ease: http://pixels1.com/cars/cars/rover-800-820-throttle-body-housing-motor.html?i=1

[RayMK]

With luck you may have fixed it.  However, in my experience plug gaps being incorrect will not cause uncommanded revving.  I've experienced it with a 1985 BMW 520i (E28) - that was caused by a failing ECU and randomly revved the engine on light load increasing revs by about 500rpm , and also with a Ginetta G26 fitted with a Ford Cologne Injected V6 and autobox which randomly revved at tickover to 2000rpm, giving interesting brake test results when in Drive.  The Ginetta's ECU imported from the donor engine was fighting a constant battle with the Cortina wiring loom.  I scrapped the BMW (it was rusting as well) and sold the Ginetta,  as the Beemer was beyond economic repair and the G26 proved the be an incurable schizophrenic. 

[The_Equalizer]

I had similar on my Merc CLK and it finally ended up with EML on with a fault code of P0420. As a completely desperate measure, before starting to swap out sensors or bankrupt myself on a new cat at £700 from Mercedes, I went for the snake oil salesman answer which, amazingly, did the trick. EML, when reset, was out for good and the engine wasn't fluttering on start up. I presume it must have been a crapped up O2 sensor.

[willswitchengage]

Providing there are no leaks I would have thought the only thing that could cause such significant random revving (with no change in load) is the throttle. In an electronic throttle, of course they could in theory go haywire and start opening at random. But with a manual throttle surely the only thing that could open it is the cold-start assist or whatever it's called. I can't see fuel metering making much of a difference, if anything I'd expect it to cut out the engine than increase power output and thus rpm.

[mercedade]

15 hours ago, tommytwo said:

 

 The problem only occured after driving 20-25 miles, then the engine would rev up to 3000 rpm before falling back and then repeating the performace.

 

 

3 hours ago, willswitchengage said:

 But with a manual throttle surely the only thing that could open it is the cold-start assist or whatever it's called.

I think even the early T16s had electronic stepper motors controlling the idle stop. My K plate 220 Gti did, which is what's made me think it could play a part here.