Fuel vapourisation/ rough gear selection

eadmr04

eadmr04

Member
I have a 1968 3500 P6B and generally it runs like a dream. I've had a few small problems before mainly that when it's the car is hot and you stop, if you try to start it again, sometimes you have to crank it and eventually it will start but you have to try to keep the revs up. When you put your foot on the brake to then select drive, the revs drop and it dies. In the past I've normally stuck with it and eventually I can get the revs up long enough to get it into drive and pull away and then alll is generally ok.

I've been out today and the weather was hot and after I'd sat at a roundabout for a couple of minutes, I went to pull away and there seemed to be a loss of power but it picked up again ok and I thought I was imagining it but after the next set of lights it did it again. When we arrived at the car park I had slowed down then went to move along a bit more and the gearchange seemed very rough with quite a clonk! I then went to manouver and it died on me. I managed after cranking it to get it started and parked but going from drive to reverse it seemed very rough again. When we came home and it had stood for a couple of hours it all seemed fine again?

Would the syptoms described be the dreaded vapourisation and would the fact that the revs were really low make the gearchange rough, do the revs need to be at a certain level to make the drive selection smooth?

Any ideas would be gratefully received.
 
The lower the engine speed, the smoother the engagement of drive. The problem you have at the moment is the poor running which means you more than likely have the throttle open more for any given engine speed than you would if all was running correctly, which in turn will extend the kickdown cable thus raising the gearbox pressures causing the harsh engagement of drive and reverse. Sort the running problems and see if the gearbox problem disappears.
 
Do you reckon the symtems I've been getting are caused by vapourisation and if so what would be the best way to try to cure it?
 
It does sound like vapourisation given what you describe, but before jumping to conclusions I would thoroughly check the state of tune of the engine. Start with the points and plugs, then dwell and timing, before moving on to the carbs. Personally I'd do a compression check while the plugs were out just for the sake of it. Change the fuel filter as well, and make sure the bleed pipe between the rad. and the top of the inlet manifold isn't blocked. Blow through the radiator matrix with an airline to clear any road debris fom that as well. Faulty fuel pumps can also give the problem.
Don't worry, any hot weather won't last for long! :D
 
I've checked a few things over but couldn't find much wrong but did make an observation and wondered if it was normal. When I started it from cold I looked at the fuel filter and noticed you could see the fuel about 3/4 the way up the filter but once the engine was hot you could only see the fuel right at the bottom. I turned the car off and returned about an hour later and started it again, it just about ticked over and when I looked at the filter the fuel was about half way but I could see air bubbles bubbling through the filter. Is there supposed to be any air in the fuel supply and would it be normal to see the air bubbles rushing through the filter with the fuel? If this should not happen has anyone got any ideas what could be makeing this happen, would it just be vapourisation?
 
It could be that the pump is drawing air into the system somewhere between the pump and the tank, and it gets worse as everything heats up. With vapourisation it tends to happen in the carbs rather than in the filter.
 
It this first statement is true then read on. If it isn't then sorry for wasting you time.
Does the P5 have a reserve cable that is pulled out? Like the P6. (Sorry just read the first post again and noticed it says P6B and not P5B - Must get some glasses)
On my P6 i thought i'd got a fuel starvation problem. The problem was tracked to the fact that pushing in the reserve didn't move the valve back enough to allow the fuel to come from the normal exit in the tank. It restricted the fuel flow, made the engine run for a while and then stop, ran very hot.
Just a thought.
 
I've spent the last few nights trying to investigate the problem and am still noticing that the level of the fuel in the filter drops to almost nothing the longer the engine is running. I thought I'd try and check for leaks and swapping from main line supply to reserve but it made no difference.

Now the reserve pipe is see through and whilst I was looking at the tank end I noticed that it was dragging loads of air bubbles through it with the fuel, is this normal? The main fuel line is black so unable to see if it's happening on that one but would imagine it is. I take it that this should not be happening and wondered if anyone had any ideas what could be causing this? There are no signs at ll of any leaks etc. at the tank connections. Hopefully if these bubbles should not be happening and I can cure this, I should sort my problems out. Any suggestions???
 
As I said before it could be drawing air into the system through such a small leak that it's not yet big enough for the fuel to escape through it.(In the same way that diesels will refuse to start because there is air in the system, but no signs of any leaks) And as its a recirculating system although you see the bubbles in the fuel at the rear, the problem could be anywhere in the system, including the pump. All I can suggest is to check the whole system, and if you don't find anything, take a guess and overhaul the pump. (If it's the standard mechanical one)
 
Just to say. Exactly the same symptons as Alexis. Bubbles in fuel line, runs lean, runs hot and then runs out of petrol. Renewed the diaphram in the manual fuel pump. The original theory was the join where the electric fuel pump was taken out of the system was allowing air into the fuel line. This was repaired but the problem remained. The next theory, yet to be proven, was that value at the end of the reserve cable, when the cable is pushed in, is drawing air into the fuel line from somewhere. I'm guessing that there will be an o ring of some sort that is letting air in when the reseve cable is pushed in.
Currently as long as i leave the reserve cable pulled out all is well.
 
Any ideas where I can buy a complete set of fuel pipes, thinking of replacing them to try to eliminate possible leak?
 
I bought a length of flexible fuel line from my local car spares shop on the high street to replace a perished section. If I remember rightly it was only about 20p per inch.
Did the job perfectly.
 
Took the v8 to the East Kirkby rally yesterday which has been its longest run so far(35miles), Ran fine for the first 20 miles doing 50-60 MPH. Pulled out to overtake a tractor and just as I got past it the engine started missfiring and spluttering, pulled over in the next layby and had a look under bonnet to see a steady stream of air bubbles in the (new) fuel filter. As it was a warm day I assumed it was fuel vapourisation, let it idle for 5 minutes with the bonnet up then off I went. Ran fine for a few miles, overtook the damn tractor again I had passed earlier and it starts spluttering again.... This happened twice more but we got their Ok. I thought it will be fine when its cold when we go home, not to be though. Even though it was cold and raining hard we hadnt got 2 miles from the airfield before it started hesitating again. We got home 2 hours later after riding round all the country lanes (to keep off the A roads)at 30Mph and stopping every couple of miles but the problem got steadily worse to the point where it wouldnt rev over 1500RPM when in gear (revs fine in neutral though). I had my father with us in the car who is a semi retired mechanic who did his apprenticeship with BMC and even he said he had never come across a similar problem before. We got home and I switched off the engine and let it cool down with the bonnet up, even with the engine turned off there were still bubbles floating to the top of the fuel filter and the fuel level dropped until it was almost empty, We couldnt see or smell any petrol leaks anywhere though.

The fuel system is completely standard and origional (no electric pump).

Any suggestions most welcome as its got us baffled!
 
Hello Tricky,

The installation of an electric fuel pump either bypassing or in series with the mechanical unit will correct the problem of fuel vapurisation.

Detaching the metal fuel line from its fixings along the side of the block a fitting a heat proof sheath over the line will provide some benefit. The OEM sheath for this purpose was made from asbestos.

I am of the opinion that the problem stems from the fuel itself,..the vaporisation temperature being too low .

viewtopic.php?f=5&t=5095

Ron.
 
My own experiences of this situation were mainly from driving in the Alps a couple of weeks back and were proceded each time by extended periods of hard use then slowing down to walking speed or stopping completely such as driving 80mph on an Autobahn followed by joining a traffic jam to enter a tunnel - cue loss of power, coughing, spluttering and 15 minutes with the bonnet up. The other time was near the top of th Furka Pass where we'd been climbing for half an hour and then had to stop for some cows to cross. The cows moved and I drove round the corner where it cough and stopped. After an hour it was back to normal.

I put it down to the mechanical fuel pump not moving the fuel from the (cool) tank into the carbs quickly enough at idle speed to avoid being cooked in the fuel lines as it travels along side the engine. I have a feeling that an electric fuel pump will keep the petrol circulating quicker at idle speed so the float bowls are constatly being fed with fresh, cool fuel from the tank.

I assume that NADA P6's and SD1's don't suffer the same vapourisation problems?
 
I'll have to go and have another look but I dont think there is any insulation on (or near) the fuel pipe on the engine, pretty sure its fastened to the block with metal brackets.

Found this on Ebay
http://cgi.ebay.co.uk/Facet-replacement ... tsupported

Good price but will it work? also do I put it before or after the mechanical pump and where can I get an inertia switch from, dont fancy running it direct off the ignition switch??

I am inclined to think its more of a problem with modern fuel than the car, Dad had a V8 auto from 1984 to 1989 and never had any problems with it. "Formula Shell" did kill the engine on his 2000TC though (and a lot of Vauxhalls at the time).
 
Hi all,

A recurring topic is mechanical fuel pumps failing at around 100k miles, which happened to my first (RSA) series 1 V8 on a long roadtrip in the middle of nowhere. The one on my previous car worked fine but not when pointing up very sharp inclines (including ramps). If your fuel filter seems to bubble and froth but has little fuel in it, regardless of running temperatures, to my mind it sounds like a rebuild of the stock fuel pump could be tried, which is simple enough to do and inexpensive before considering more costly options.
 
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