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MG MGF Technical - Erratic Idle on 1.8i

Dear collective,

My apologies for slightly highjacking you BBS; I am NOT the proud owner of an MGF but of a quite nice Rover 45 with a very nice MGF'ish 1.8i under the bonnet. (you can flame me now....)

I'm posing this question as my Rover dealer is currently a bit stumped as to what the problem with my car is. Since it's a MGF style lump I am hoping that you can help. The symptoms are as follows

At start up AND only for the first 30-60 seconds, the engine idles fine at about 800rpm, as I give the accelerator a gentle prod (to approx 1500rpm for negotiating the way out of the garage) the revs die, just before its about to stall the engine management picks it back up again (to 1500rpm) then it drops again, this repeats slowly homing in on approx 1100rpm. I should point out that all this hunting to find an idle speed occurs with my foot off the accelerator, very quickly i.e. 2-3 oscillations per second, much faster than you can push the peddle manually.

I should say that all of this has occurred AFTER the car had been repaired for "dying" while idling on the drive the previous week (i.e. start, idle for 30 secs, stall - turns over freely but never to restart). For the original repair, the dealer identified a faulty crank speed sensor and replaced it - it then ran for a minute or two until the ECU destroyed the sensor - they then replaced the sensor AND bunged in a new ECU.

With the latest problem, they have tried clearing the fault codes - there were nine random ones which they think were the result of a faulty download to the ECU or something and resetting the stepper motor. Since then it has logged a fault code for a failed injector but apparently there should have been another code to identify which one and this wasn't present so they don't trust this message either....

As they are having problems reproducing the problem, I now have the car back and have been asked to keep a diary of the exact circumstances under which the problem occurs. They are also hoping that the ECU will acquire a few more fault codes and have contacted Rover Technical support for help.

You help is politely requested (please nothing about selling it and getting an F)

Steve
Steve Buggs

I know with most cars the ECUs are self learning and take a few hundred miles to smooth out - my ford did this when brand new and stalled a few times - also hunted alot

After 500 miles it was fine but had good days and bad until this mileage had been reached
tony

Although I'm an F owner, I don't know the K engine; but it sounds like the IAC is malfuntioning (Idle Air Control), or you have a vaccuum leak in the manifold plenum. The IAC should set a fault code, but these computer setups are not all that reliable for troubleshooting, and either don't log the faulty sensor, or see another fault caused by the problem first.
I normally replace the fuel pump, and fuel filter in fuel injected vehicles before running around the computer sensors, also check the fuel pressure, the fuel pressure regulator could be a problem.
george

George,

Many thanks for your response. I agree that a vaccuum leak is a good possibility but I couldn't work out how it would only have an effect during the first 30-60 seconds.... As I said in my original post, it has had to have a new cylinder head (tappet noise due to badly aligned oilway drillings between block and head), so the possibility that this disturbance is the root of the problem is likely.

Tony,

The car did seem a little down on power so perhaps it is still "learning" but how come they don't have these problems straight from the factory?

Many thanks for your help. I knew that I'd have to find an "enthusiat" site to get any real info. If anyone knows where I can find a list of MEMS fault codes I'd be most interested.

Steve Buggs
steve buggs

Steve,

Perhaps a faulty/loose connection in the stepper motor circuit?

Bruce
Bruce Caldwell


Temp. sensor faulty ?
It could be faulty, but not showing up as faulty as it's not totally broken.
Have had this affect idle speed - and power.

Throttle position switch... will only affect engine braking and (surprisingly) I found not to cause the idle seeking that you are experiencing.

Hope this helps.

P.
Paul Nothard

Paul,

Have discovered that the throttle pedal "switch" was deleted around 1993 and a voltage of around 0.6V at the throttle potentiometer used instead (saved a switch).

For further information, the car was fine this morning and I'd started to believe tony's response that the ECU learning curve was causing the problem. Then this afternoon, when leaving work after the car had been parked on a wet and windy airfield for a day, the idle hunting problem briefly returned. I decided to press on anyway and the engine misfired several times while accelerating hard in 2nd. It then seems jerky for the next 2 minutes.

Perhaps there is a problem with one of the injectors after all. Hopefully when my Rover dealer get a response from Rover Technical support I can take the car back with a couple of dozen fault codes for them to chew on....

btw, does anyone know if the injection system on the 1.8 is simple multipoint or sequential multipoint?
Steve Buggs

If its missing under acceleration its looking more like injector/ignition problems causing that

I would also check Fuel Injection pressure - I know FI bikes suffer this sort of problem as they all have a return fuel hose which returns unwanted fuel to the tank - sometimes the return hose gets kinked causing "overpressure in the FI system which then causing hunting and missing as there is way too much fuel being dumped in the cylinders

Dont know if the F has a return on the fuel lines though ???

It may be worth having the spark plugs out as there colur may give some clues to what is hapopening in each cylinder !
tony

Tony,

A distinct misfire has now happened on two occasions while under acceleration in 2nd and from the stuttering (power/short pulse of no power/power again....) it feels as if it's only 1 cylinder misfiring 4 or 5 times in a row. I also get the feeling that might be what is causing the initial drop in idle speed, except that at idle it's much harder to "feel" the misfire. Given this and the fact that the one fault code my dealer managed to extract since clearing the 9 "dodgy" ones was an injector failure warning it does look more and more likely that it's an injector problem.

AFAIK, all Rover fuel injection systems use a Fuel Pressure Regulator and return unwanted fuel to the tank. The Haynes Engine Management Manual says that on MPi systems the pressure is set at 2.5bar referenced to the inlet manifold pressure.

btw, don't forget that I've only got an R45, not an F :-(

Also, all this is covered under warranty (the car is only 13 months old), so although I'm interested in the solution, I'm not really in a position to test/fix it.
Steve Buggs

No probs lol = i forgot and was all ready to strip the sucker into a thousand bits hehe !

Could be the injector or something as simple as a broken down spark plug but with electronic injection i would keep the diary and let the warranty sort it

I am sure it will be found and cured
tony

This thread was discussed between 06/02/2002 and 08/02/2002

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