Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing 3.9 v8 cuts out at bottom of a hill

Has anyone got any experience, or advice, as to why a 1995 3.9 v8 would cut out When stopping going down a steep hill?

Absolutely no issues on the flat or lesser hills, just on the steep ones.... though I did find, second drive, bit early to say it was definitive, that if I pushed the clutch a few times just before the junction it wouldn’t stop.

After it did it to me first drive internet wisdom suggested a sooted up idle control valve, so I checked, it was a bit dirty, I cleaned, and took another drive but still the same.

Fuel pickup issue ?

is there a possibility of clutch drag? Had it once with an MGB that hadn't been used for quite some time and I assume the flywheel pressure surface had become corroded. The car would creep on a level surface with the clutch pedal fully depressed. Took about 200 miles for the phenomenon to disappear. It might be that freewheeling down hill to a standstill causes the engine to stall due the the drag on the clutch. Why it wouldn't also do it on the flat though I'm not sure.

I’m vaguely dismissing fuel because it starts perfectly after, and it’s not an issue on the flat, slight hill or even after and emergency stop.

I will though change the fuel filter just in case.

Does it do it with a full tank of fuel?

Just under a full tank.

Couple of things to check.

There is a box halfway up (or down) the speedometer drive, this feeds a signal to the ecu to tell it the car is moving. under normal circumstances, as you come to a halt, you take your foot off the accelerator, the ecu knows the car is still moving, and keeps the fueling up a bit, and then when you come to a halt the ecu gracefully drops the revs allowing the engine to come to idle. If the ecu loses the knowledge that the car is moving it doesn't drop to idle in a graceful manner and could stall. Going downhill is an issue because your foot is off the gas pedal and the ecu will have reduced fueling, and the engine is still turning (engine braking) so the ecu may drop fueling even more to get down to idle, but the engine continues to turn at speed, then when you do stop and disengage the clutch suddenly the engine dies because the ecu cannot introduce increased fueling quick enough to stop the engine stalling. Whereas with the signal from the speedometer cable pickup the ecu knows the car is still moving and even though your foot is off the gas pedal it knows to keep a bit of fueling until the car comes to a halt, and then it gracefully drops the engine speed to idle. In control terms its what I call integral windup, but ecu manufacturers make up other names for it. So if you lose that signal you will probably find that the ecu doesn't like periods with your foot off the gas and then going to a halt, it will be unable to react quick enough to give enough fueling to get to an idle.

Second possibility is loss of an O2 sensor, or sensor heater, the engine tends to run OK, fuel consumption will go up a bit, but the ecu doesn't handle the drop down to idle very well. However in this case its a general issue and not just to do with hills (or long periods with your foot off the gas pedal)

That’s great info, thanks, but is there a way to check those myself or does the car need a mechanic with diagnostic software?

The speed transducer box is under the hood(s) and attached to the bulkhead, you should be able to locate it as the speedo cable is in two sections one feeding into the box and the other feeding out of the box. Maybe a first step is to check that the signal cable is still attached. I will need to refresh my grey cell with regard to the expected output voltage. Essentially there should be an electrical pulse coming out of which the frequency will be related to road speed.

Reference is made here https://www.gomog.com/allmorgan/StepperMotor.htm#TRANSDUCER - see 3. THE ROAD SPEED SENSOR (aka transducer)

One way to check is to put an oscilloscope on the output of the speed transducer, or a DVM may suffice at a low speed

0-2.0V and 10.5-13.0V. Signals when road
speed >3mph.The signal should go up and
down between 0-2.0 Volts and 10.5-13.0
Volts, changing every 4-5 metres. The
signal is 8000 pulses per mile.

See https://www.gomog.com/allmorgan/HOTWIRE/RV8diagnostics2.pdf

Another is to put a code reader onto the ecu, Rovergauge will display the road speed signal - Rovergauge is free software, you just purchase a cable from your PC to plug into the connector in the EFI loom, located somewhere close to the rear of the underside of the glove box.
http://www.remap-14cux.uk/gadgets.html
https://github.com/colinbourassa/rovergauge/releases/tag/0.10.7

If you were nearby I could connect one of my code readers to your +8, but I see you are in Cornwall. Just in case you may wonder why you don't get a MIL light on to alert of an ecu error, it's because there is no MIL light. There is a light in the cluster but it lights when the inertia switch triggers and cuts out the fuel pump. I have wired in a MIL light and modified the ecu program to enable the light, but in addition to aĺerting of an error it also comes on and off when it feels like it, which is probably why they disabled the function.