Talk Morgan Forums Morgan Sports Cars Morgan Aero 8, 2012< Plus 8, SuperSports, Coupe & Aeromax Hot start .... vapour lock

Appolgies if i seem to be carpet bombing the forum with posts - I'm (I hope) at the steepest part of the learning curve with my 'Mog at the moment, 3 weeks into ownership. Rapidly knocking things down as sorted, but still some niggles to get to grips with.

The car starts on the nail from cold and runs perfectly .... However. If its left to stand for 20 mins once warmed up it takes a very long crank to get it to fire up.

It always goes, but it can be 10 seconds or more cranking before it fires.

I'm pretty sure this is down to vapour lock - is it a known "thing" ?. Does anyone have a fix ?.

I've got a 2015 Aero Coupe, which I guess is going to be the same engine and fueling set up and it always starts immediately hot or cold.

I just googled it which made the following suggestions...

Crankshaft or Camshaft Position Sensors: As these sensors age, internal thermal expansion breaks down their circuitry when hot. They fail to send precise rotational signals to the ECU until cooled or subjected to extended cranking.

Fuel Pressure Leakdown / Leaking Injectors: If an injector drips or the fuel pressure regulator bleeds off pressure into a hot intake tract, a temporary rich or vapor-locked condition occurs, delaying immediate ignition firing.

Quick question as I am not that experienced on S5 and I know it was quite different due to the boot arrangement.

I have an S2 and a Coupe (yes I know, greedy). And the S2 has no issues. The Coupe can be fussy. The breather has been adjusted a few times and had several recalls. Is you starting issue perhaps related to the tank not breathing and so pulling back once switched off. So the restart is a bit starved? Not claiming any skills in this area but I did have that hot start issue after one of the “improvements”.

If it was a vapour lock I would be looking at the insulation around the pressure fuel feed from tank but nearest the engine.

Just before you try the hot restart next time, open the petrol filler cap (there's sometimes a rush of pressure in/out equalisation) and then try the hot start test

I have been messing around with the tank / breather to deal with fuel filling problems (see my other thread). Will definately try the "cap off" test next time it's warm.

I'm pretty confident about the one way breather valve (it's new a week ago) and changing it did not make any difference.

I have had issues with exhaust cam sensors (they put the EMS light on) but ive replaced both of them and the EMS light cleared itself after 5 drive cycles.

I'm planning to add an inline fuel pressure guage and if all else fails a "cooldown" fan, Regardless of the starting issues the engine bay gets incredibly heat soaked - I've even added some insulation on the bonnet stand rod as its too hot to touch comfortably. I suppose it recent UK temps have been a bit extreme, but it's done it again today @ 21C ambient.

Accepting your point concerning heat this is common. Several people have added additional bonnet louvres to reduce heat when standing. Several have also added them because they are pretty. Whoops did I say that out loud. I have never seen evidence of the impact when moving but after you have parked you can feel it through the vents.

The S2 and Coupe do both get a load of heat soak and high under bonnet temperatures at rest. As the car heats up the gearstick can become pretty hot to handle as I had a metal ball knob fitted, I disliked the factory one. Additionally the footwell surrounds and the chassis frame below the door can get pretty warm as well. Great in winter!

If it is really hot I tend to pop the bonnet knobs at the windscreen end which lets the heat out faster and does not really show. Never had anyone look or touch as they get nothing but love.

On the Coupe the breather valve for the tank goes to a pipe which exits down past the tank to the road. On mine it was kinked at one point and so the pressure could not release. Just as a check did you test that one? The cut off triggers constantly when filling following my last service and adjustment at Williams. Uttlerly pisses me off, not to mention I can no longer go more than 90 miles before risk of running out of petrol. It seems to be a magic combo of breather, angle of the neck mounting on body.

I've tried the cap removal when hot now - no change. Next up I've got a capillary adjustable thermo-switch and 2 slimline 6" rad fans.

The plan is to have a activation switch inside the car to enable the aux fans, then the thermo-switch to actually turn the fans on / off.

Position of the fans is planned to be underneath the 2 triangular mesh covered vents to the side of the headlamp cowls. When on they will blow hot air out. The only other slight concern I have is that the fans don't "windmill" excessively in normal driving. We will see ..........

I suspect that if fuel tank vacuum build up relative to vent restrictions were the case, then I might expect the engine to falter under acceleration, given the high fuel delivery demands...?
Using no more than logic, the idea of making use of a fuel pressure gauge to monitor the situation seems a worthwhile next step...?

Another thing I'd not mentioned.

My car has a Global Telematics Thatcham immobilizer fitted.

It's a "key present" system, so you basically disarm it and start engine immediately.

I also wonder if this were not there the OEM systems would run the fuel pump for a second or two to pressurize the rail before cranking ?. I don't seem to be hearing others saying they have the same problem so maybe the OEM BMW / MMC system is sound after all ?.

The digitally controlled world is full of "wonder," and is one of the reasons I am so fond of my Trad.... Though admit that it too has a rudimentary digitally controlled anti theft device... Sigh..! Though at least there is a little led that extinguishes to let me know the "key" has unlocked the device, Jeez it even seems to send some beeps through the Mog`s speaker system when initially deactivated.....Sigh.

I remember a friend who was an assistant service manager at a Nissan dealership telling me in the late 90`s that activating the door handle to initially access the car caused an inordinate amount of electrical processes to begin, only one of which was to prime the fuel system....

Perhaps taking some degree of the wonder out of the process by checking the fuel pressure... Given after 20 mins one might expect that the system might be likely to loose much if any pressure...and if it did, I might then be thinking the fuel pressure release valve set point....? But then I have zero hands on knowledge of working on modern EFI systems, and am using no more than logic, flawed or otherwise...

At least your battery, starter motor and alternator are getting a good work out till you hopefully manage to resolve the issue...?
Fingers crossed.

I would be investigating further into the fuel supply line, it sounds to me as if a non return check valve has failed and is letting the fuel bleed back or evaporate before the high pressure system or even in the high pressure side - relay or valve failure is where I would be looking.

My Plus 8 will take an extra turnover if conditions are extremely hot such as we experienced recently and evaporation has taken over under the bonnet and this normally ties in with me needing 1 Ltr of water to drink..

Latest - I've fitted the aux cooldown fan with a capillary thermostat strapped to the fuel rail.

If I set it to 55C it stays off whilst driving the car .... and kicks in when the car has been shut down after about 3 minutes.

Slight problem, It stays on for about 40 mins whilst the engine cools and I think its drawing about 15 amps - so possible flat battery issues on really hot days.

I've adjusted to 70C and it still kicks in on shutdown .... waiting to see how long it stays in for at this setting.

Next up, Ive ordered a fuel pressure measurement kit (surprisingly cheap @ £25). There's a schrader valve test point on the fuel rail so I'll be looking for about 70psi engine running and paying particular attention to pressure decay on shutdown.

What I can't explain is that it starts on the nail when cold but cranks for ages when hot.

If it were a faulty check valve (apparently there's one as part of the in-tank pump unit) then why only a problem when hot ?.

Hols now for a week - I'll be back on the case on my return.

BTW, have you done a diagnostic scan? But nothing you have mentioned is too serious, IMHO. Nothing to fret about..though fretting about one's Morgan is a hobby.

I am not as sure as you are as to the cause. Yes, vapour lock is a common reason for hesitation in starting a hot EFI engine. But I have found it is more often a leaking injector. They both create a temporary situation where insufficient fuel and fuel pressure slows a start as the fuel pump brings the fuel rail back up. Also, a hot engine is harder to start than a cold one as the internals have expanded, tightened and the starter must work harder because of it. As Luddite suggests, modern engines are made to run ever hotter.

There is a cure for leaky injectors. Clean, refurbish or replace the leaking guilty injector (s) which implies you find which it is. Cleaning is often effective and can be accomplished in situ by professionals. They detach the fuel feed and replace it with a cleaning solution that flushes and unclogs. Low mileage car injectors are especially vulnerable. And storage gums them all up. The cost for professional cleaning in situ in the UK, according to search engines, is 150£. I clean mine yearly at the start of my mogging season, no point in doing at other times and notice a difference. I learned to do it at home as that is more convenient. One merely needs the right interface to the fuel rail and a can of injector cleaning fluid made for the purpose.

Of course, a true injector specialist can do the best job with the injectors out of the car and create a report on each injector flow and its spray pattern. If you are looking for perfection, buy 16 injectors from a break-up yard, send them to an expert to refurbished them and test Then chose from them a set of 8 with the closest matching flow rates and patterns. If you choose, you can then sell the rest for more money than the entire process cost.

BTW, a rad fan should run for no longer than 5 minutes after the hot engine is turned off More runs the battery down and definitely pre-age it. In many cases, I have told moggers, to power the fan from the ignition switch. The reason for running a few minutes after the engine is merely to protect other components (plastic!!) from a too hot engine bay. Did you have melting components?

Lorne

Hi Lorne,

Thanks for the reply. The car has had a fairly odd life so far. Bought new by a collector and registered in 2019, it had only covered 140 miles by 2024 (I dont think he ever drove it). It went to the next owner in 2024 and he drove it to Devon and France (once) before hurting his back dinghy sailing and then not driving it again for some time. Fast forward to a month ago (Early Aug (2026) and it arrived to me with 2040 miles on ..... it's like brand new, but spent plenty of time idle for fuel to congeal etc.

It put the EMS light on on the way home, after much faff I realised generic OBD scanners etc dont work - so it went to Ledgerwood for a proper scan. The EMS reported to a failed exhaust cam sensor, so I put 2 new ones on and it reset itself after 5 drive cycles. There were a few other codes showing - tank fill level and crankshaft sensor signal, all cleared and the EMS light has stayed off for 800 miles.

I always like to get my vehicles "just so" when I first get them, so there's been a very intensive flurry of activity - dealing with various niggles, upgrades and adjustments and at the same time getting up the 1st few rungs of the Morgan learning curve (This is my 1st Morgan). Fuel injectors are one of the things I sell for a living, I've got a flow test rig for producing matched sets and an ASNU cleaner. I've also been running on Vpower since I got the car with the theory it helps remove deposits etc.

I'm planning to work through the problem logically, next up will be fuel pressure tests - Injector cleans / replacements & check valves if I find leakage. There's definately something not quite right with the way it starts at the moment even if it does run perfectly once going.

Regards, Richard