Horrendous vapour lock!!

Post Reply
Posts: 11
Joined: Wed Jan 03, 2018 7:25 pm

Horrendous vapour lock!!

Post by BYY922 » Mon May 07, 2018 6:50 pm

Gentlemen, I’m suffering horrendous vapour lock now we have hot weather. I’m considering an electric pump to hot start prime when hot starting or sitting in traffic, anyone tried this?
She’s a 1935 14, mech pump was recently rebuilt and runs/starts fine in all weathers/ driving conditions except hot weather.

Posts: 192
Joined: Mon Sep 07, 2015 4:38 pm

Re: Horrendous vapour lock!!

Post by TonyG » Wed May 09, 2018 1:04 pm

I spoke to a number of people at events regarding fuel vaporisation while i was restoring my 12 Tourer. Some people seemed to get along fine with the original arrangements of fuel line and mechanical pump, while others experienced issues and had implemented a variety of solutions to alleviate the situation. The problem is well known in that modern unleaded fuel vaporises when things get hot, causing issues particularly with carburettor engined cars. With that basic fact in mind, I built my car up with the simplest and, hopefully, most robust of the solutions I have seen. Specifically, the fuel line from the tank to front of the car was routed along the chassis on the opposite side to the exhaust to reduce any warming of the fuel en-route. Also, the mechanical fuel pump located beneath the manifold was junked in favour of an SU electric pump, mounted on the bulkhead on the other side of the engine bay. My carbs, by necessity, are mounted 180 degrees to the norm so the float chambers do not sit directly above the manifold. This enabled me to route the fuel pipes from the pump to the carbs on the rocker box side, again keeping as far away as possible from the exhaust. Additionally, all the under bonnet fuel pipes are metal braided high pressure items, which may resist the heat a little better. So far, I have not had any vaporisation issues.

All the steps I have taken were easy enough to build in for me since the car was in pieces anyway and I have incorporated the same things into my 12 saloon, which is currently undergoing a similar restoration. However, I appreciate that it won't be as simple to do the same retrospectively, but not impossible. The hardest part would be re-routing the fuel pipe if it is on the same side as the exhaust on a 14? All the other things would be quite straight forward, although an SU pump will cost around £100 and the braided fuel pipes about £50. Most importantly, the end result would, in my experience, be robust and safe with regards to fuel pipes and connections.

Simply plumbing in an additional pump sounds easy but may throw up an almost equal number of challenges; Assuming you are planning to use the existing pipe to the tank and putting the electric pump in parallel to the mechanical one, that either places the pump close to original one with, one assumes, the inherent issues of vaporisation or positioning the pump elsewhere under the bonnet with pipes running about the place and teed into the carburettor feed. Alternatively it could be positioned closer to the tank and 'pushing' the fuel towards the mechanical one, which I think some people do if/when their original pump becomes weary. However, i suspect the pump would have to be running all the time for that to work, plus there is the need to wire the pump to a fuse on the ignition switch feed so cables etc to run.

I look forward to hearing how you get on but hope some of my suggestions are also of help.

Tony Gilbert

P1 12 Tourer
P2 12 6 Light Saloon
Discovery 3
Discovery Sport

Posts: 219
Joined: Tue Oct 22, 2013 8:04 pm

Re: Horrendous vapour lock!!

Post by RobHomewood » Thu May 10, 2018 6:51 pm

My '39 14 has had the fuel line moved to the nearside similarly to avoid the exhaust.
An electric pump was mounted on the nearside inner wing. Having changed this original 'pusher' electric pump for a 'puller' SU pump as advised I still was experiencing fuel starvation when going up hills. I then moved the SU pump to the offside, lower down in order to avoid the fuel having to go uphill to the pump and downhill afterwards. The line now runs around the front of the engine under the radiator, much nearer to level from the back axle. It is better but not perfect as the route from the tank is much longer and I get bubbles in the fuel anyway (cavitation or leak?) Today I had a little vapouristaion issue (it took 3 starts of the engine to get way after parking in the sunshine for 10 minutes or so) and on a long hill, on top of a lack of power, a faltering starts starts after a couple of minutes of climb.
I am indeed thinking af adding a pusher pump down by the tank to test whether this improves the hill climbing. I seem to hit a terminal velocity on hills which I suspect to be fuel limitation, having worked on all the other variables I can think of.
I am also preparing a pair of vented engine side panels to allow the engine to cool better (its a 16 engine anyway) as well as fitting a 'heater bypass' hose ( like the heater curcuit would be but without a heater) to improve coolant flow but I can live with a small incidence of vapourisation I think.
Nothing is straighforward is it?

Posts: 11
Joined: Wed Jan 03, 2018 7:25 pm

Re: Horrendous vapour lock!!

Post by BYY922 » Fri May 11, 2018 6:22 am

Thanks for your comprehensive replies. My fuel line is on the o/s chassis rail already so that’s taken care of. The pipe enters a pre- filter ( simple plastic see through for convenience and observation) then into the mech pump on the o/s of the block before crossing the block between the radiator and engine and travelling up to the carb. I’m thinking it may well be the hot air from the rad/block that’s affecting the metal pipe (uninsulated) or the lack of heat shielding from the exhaust manifold below the carb. It has louvred panels along both sides of the bonnet and holds full throttle with no problem until a few miles on a hot day. Then it won’t idle or accelerate heavily nor will it hot start after parking.
I changed the coil and gave the ignition system a good clean and checked all lt/ht connections and they seem fine. The pre-pump filter has sat nearly full of fuel since Monday and hasn’t drained back so Imthink that’s all air tight.
A pusher pump is a last resort but not much work to do and should allow draw through when running on the mech SU pump.
I’ve got a show on Sunday so it needs to be sorted by COB Saturday!

Posts: 11
Joined: Wed Jan 03, 2018 7:25 pm

Re: Horrendous vapour lock!!

Post by BYY922 » Sat May 12, 2018 5:08 pm

Just a quick update, fitted a Facet Posi-flo (that did sterling service in my Land Rover Lightweight) to the offside by the tank. Got it all nice and hot and it restarted several times without issue. All this was in the garage however due to another wet weekend, but I’m due to attend a classic car show tomorrow so that’ll be the acid test.
Electric pump is running in series with the mechanical quite happily with no overflowing so maybe that’s sorted the vapour lock.

Posts: 11
Joined: Wed Jan 03, 2018 7:25 pm

Re: Horrendous vapour lock!!

Post by BYY922 » Sun May 13, 2018 3:43 pm

Well, just attended a local car show, about 14 mile round trip, all good. Hot started fine, so I’ll call that a satisfactory result.

Posts: 11
Joined: Wed Jan 03, 2018 7:25 pm

Re: Horrendous vapour lock!!

Post by BYY922 » Mon Aug 13, 2018 8:09 am

Well, had to jinx it didn’t I!! The electronic pump is now sucking air intermittently ( couple of loud ticking bursts every 20-30 secs) and won’t pull full throttle so I’m guessing air leak suction side? Had pipe out of tank and seems ok on reserve and normal. The rubber hoses either side of pump are as was a couple of weeks back when it ran fine on a 30 mile round trip. Any ideas?

Posts: 68
Joined: Fri Aug 02, 2013 7:44 am

Re: Horrendous vapour lock!!

Post by jp928 » Sat Aug 18, 2018 7:22 am

In my youth I saw vapour lock problems on a P2 and P3 out here in Oz. One easy helpful job was to paint the bottom of the tank silver to reduce heat uptake from the roadway. Personally I favour a pusher pump mounted as close to the tank as possible.
jp 26 Rover 9, previously P2,P3,P4,P6,P6B,RR, Disco

Posts: 28
Joined: Fri Feb 15, 2019 1:19 pm
Location: Tunbridge Wells area

Re: Horrendous vapour lock!!

Post by Geoffrey » Sat May 23, 2020 3:18 pm

I made the more recent post called Carburation issues because my car was suffering from intermittent fuel starvation. I could not figure out whether it was a vapour lock, a sticky carb float needle, fuel pump issue or what. I had an electric pump fitted just under the bonnet on the off side. I made a shield to protect it from exhaust heat but it made no difference.

To cut a long story short I have now re-sited the pump. I have bolted it to the battery carrier under the rear seat so that it is close to and below the fuel tank. Initially it just sucked air. This was because the union of the fuel pipe to the main/reserve solenoid unit was not good and allowing the pump to draw air. Having fixed that issue the early signs were good. Much better pressure at the carb (I had trouble getting the pipe connection to the carb to seal properly) and the pump seemed to be working a lot less hard than previously. I then took the car for a good 25 mile town and country run. No starvation issues at all and that is a first. It will usually die at a junction or just after at least once, before picking up and carrying on.

I have read posts about push and pull pumps. I have no idea what mine is but all it has to do is push the fuel up the line as it is effectively fed by gravity to the pump.

I was always rather sceptical of the vapour lock theory. I think it needs a lot of heat to cause vapourisation. I also run a Sunbeam S8 motorcycle which has its fuel line virtually touching the exhaust manifold as it makes its way towards a carburettor squeezed between two exhaust outlets. Never get vapour lock on that. If the connectors between the pump and the tank have been disturbed then their seal may be poor allowing air to be sucked in.
1947 Rover 12
1952 Sunbeam S8
Norton International Special
1957 AJS Model 20
1952 Velocette LE
2018 Royal Enfield Himalayan
1999 P38 Range Rover

Posts: 487
Joined: Sun Oct 19, 2008 5:08 pm

Re: Horrendous vapour lock!!

Post by GOY189 » Sat May 23, 2020 10:12 pm

Push pump = suitable for P4's, rear mounted. Pull pump = front mounted as in Morris Minor/1000


Post Reply