Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing 1993 Plus8 high idle problem

Apologies if this is an old subject but I’ve only just acquired a 1993 Plus8 3.9 with the Lucas Hot-wire EFI system, and I believe my problem is known as ‘idle hang up’ and it’s driving me nuts !
Symptoms :- Starts perfectly from cold, runs at about 1500 rpm for a few seconds then drops to a steady 800. Throttle response is a bit sensitive at first with careful application of throttle required to prevent the engine 'bogging' down. Having replaced a defective thermostat, the engine warms up quickly. Now having driven away, the engine idle will only drop to 1400 even when the engine is fully hot, UNLESS one stops, when the speed sensor switch kicks in to drop it to 800 rpm. The speed sensor switch always works hot or cold, but once driving, the idle is back to 1400. Eventually after as much as 20 or 30 minutes of driving, the idle will drop nicely to 1000, unless of course the cars stationary.
Interestingly, the other day I drove off very slowly not exceeding 40 mph using very low revs and the symptoms completely disappeared. Next day, driving at normal traffic speeds and rpm, the symptoms were back.
So far, I have carefully cleaned the idle by-pass air valve, (rear right of the Plenum), and fitted a new temperature sensor that feeds the ECU.
It’s really annoying and embarrassing trying to drive with such a high idle, so any solutions would be most welcome thank you.
With regards, John.

My T16 engine plus 4 did that & it needed a new ECU.
GoMog has lots of stuff on +8s here
http://www.gomog.com/allmorgan/plus8index.html

Jads, it sounds like the idle control valve, try blanking it off , If it is still ticking over quickly then you may have an air leak.
Lloyds Specialist Developments are near you in warminster they are experts with these engines.
David Byrne

John

1993 is on the cusp of the changeover from non-cat to cat systems. Do you have cats? If positive then I would suspect one (or both?) of the lamda sensors. Another possibility (as per David above) is an air leak in the exhausts which is causing the lamda sensors to request the eco to overfuel.

First the "silly" question, you are sure that you don't have a sticking throttle cable?

Another question, is the ECU "chipped" at all with new fuelling maps? The Tornado chip is the most common, and many +8s will be equipped with this modification as it helps

My 1999 car (with cats) has always had a slightly high idle for a couple of seconds after lift off , but it does settle down pretty quickly thereafter , although bringing the car to a halt will accelerate the rev decline.

Thank you all, and GoMog looks a great site. The car does have Cats and I'll research testing the Lamda sensors. New throttle cable recently fitted.
Dave, when you say blank off the idle valve, do you mean remove it and blank off the hole in the Plenum, and thanks for the Lloyds tip.
How can I check if it's chipped, there is nothing in the pretty extensive history file.
I should add that at whatever the idle speed settles at, it is always very stable.

Cats were introduced as mandatory at the end of 1991, so this car will have cats.

I had a TVR with a "Hanging" idle and it was always the stepper motor. Not saying its the problem in this case but its well known to cause these symptoms and its easily rectified by cleaning and sealing the threads on reassembly.

John, you have my sympathy.

I used to run a standard 1991 Plus 8 3.9 efi with no ecu upgrades and no cats. Sometimes the car would, for no reason run at 2000 to 2500 rpm either on start up or refuse to drop below these figures when fully warmed up.

Although this behaviour only lasted for a few minutes at most each time, it could be highly embarrassing.

Of course as it was an intermittent fault it never did this when with the dealer

Unfortunately I have no solutions for you but feel your frustration. Good luck with finding some solution.

Jads , the idle valve is different on various engine years, on mine i removed the pipe to the valve( it comes from the rocker cover via a "T" piece the other end going to the inlet on the plenum) just blank the pipe and the idle valve and if it ticks over ok go and have a cup of tea.
P.S. the valve can get gunged up with vapour from the rocker cover.

David Byrne

I would just change out the bypass valve/ stepper motor, the symptoms you describe are classic symptoms. The last one I bought was only about £20 ,

Had the same issue with my old '93 +8, it turned out to be the stepper motor sticking.