TooTallMike Posted August 1, 2014 Author Share Posted August 1, 2014 Chaps, I spent a few more hours yesterday looking at the GMC again. I had already changed the LT wire from coil to distributor so it wasn't that. John Comber very kindly offered the use of his running GMC as a donor for parts so I swapped out the entire distributor including LT wire, cap and 7x HT leads. This made no difference and in fact the truck began to misfire sooner than before (I doubt this has anything to do with the change though). As soon as it misfired I pulled over and checked the temperature of the coil and points and neither were more than lukewarm. I wedged my multimeter into the glove box and used it to monitor the LT feed to the coil (max. observed 14.2V) - there was no observable electrical misbehaviour during the periods of misfiring. I also monitored the coil to dist LT voltage (max. observed 11.2V) and again no disturbance when misfiring. Someone else suggested disconnecting the alternator to eliminate any possibility of over-charging. I did this but it made no difference. I attempted to swap in the carb from the 'donor' truck but the tickover was very rough and it coughed and spluttered when attempting to pull away so I was forced to re-fit the old one. (Incidentally once home I refitted the carb to the donor truck and it immediately flooded so I need to take the top off and see if the needle valve has got stuck.) At this point I decided to go for broke and drove the truck hard until it actually wouldn't go any further. Sod's law meant it stopped in a rather dangerous location but c'est la vie. I then carried out all of the normal checks such as spark at the points, spark from the king lead when the points are flicked, no overheated components, fuel coming out of lift pump etc etc and was unable to find anything wrong but it would not start. I took the top off the carb and it was pretty clean. After a while it was able to re-start and I was again able to limp the truck all the way home driving gently. I then completely dismantled and blew out the carb. This morning I'm going to do the same for the lift pump and while that side of the engine is accessible I'll also check valve clearances. And so the saga drags on... - MG Quote Link to comment Share on other sites More sharing options...
HWade Posted August 1, 2014 Share Posted August 1, 2014 Hello Mike Just a thought a friend of mine had a jimmy with a similar problem, it turned out to be something in the the fuel tank sucking up on the pickup pipe. The coating on the cork of the sender unit had started to breakdown leaving pieces in the fuel tank. Once tank was cleaned out no more issues. Just A thought. Howard Quote Link to comment Share on other sites More sharing options...
Grasshopper Posted August 1, 2014 Share Posted August 1, 2014 Thinking about it I witnessed a similar thing with a Gardener engined Leyland Titan. It had new lift pumps, new injectors (sprayers in Gardener manuals), injection pumps, even a new fuel tank. Turned out one of the drivers would dip the fuel tank with a rolled up new newspaper and bits of it would fall off into the tank occasionally blocking the stack pipe....the saga went on for weeks until someone cut open the fuel tanks and discovered them full of newspaper! Quote Link to comment Share on other sites More sharing options...
gas 44 Posted August 1, 2014 Share Posted August 1, 2014 Thinking about it I witnessed a similar thing with a Gardener engined Leyland Titan. It had new lift pumps, new injectors (sprayers in Gardener manuals), injection pumps, even a new fuel tank. Turned out one of the drivers would dip the fuel tank with a rolled up new newspaper and bits of it would fall off into the tank occasionally blocking the stack pipe....the saga went on for weeks until someone cut open the fuel tanks and discovered them full of newspaper! I bet he was put on a charge...........:blush: Quote Link to comment Share on other sites More sharing options...
TooTallMike Posted August 2, 2014 Author Share Posted August 2, 2014 And the winner is... Well, we took the truck for yet another test drive yesterday having put back on all the original ignition components and cleaned carb, lift pump and all fuel lines back to the tank. It still misbehaved but after a longer period of time than before. We then connected a gravity feed jerrycan from the passenger's seat to the carb and it ran perfectly. Very annoyingly we had already tried this weeks ago but at that time it didn't make any difference. I can only presume that we had an airlock or similar which gave the erroneous result and sent us off on the wrong track. My conclusion is that the lift pump is at fault, even though there is nothing apparently wrong with it when it's in bits on the bench. It's the only item which is attached to the engine and therefore affected by the time/heat delay that we have seen. I believe any other faulty item in the fuel system would cause a problem on an erratic basis rather than consistently after a certain period from cold. Thoughts so far suggested are the pivot bush seizing or the spring losing its 'spring' when hot, also sticking non-return valves. It's an unusual model of lift pump for a GMC and doesn't have a lever which has made it all the more annoying to work with and diagnose. Anyway I now plan to fit a low-pressure electronic pusher pump at the tank and do away with the mechanical unit. This will give three advantages: 1) positive pressure throughout the fuel system simplifying leak-finding 2) minimising the possibility of vapour lock 3) ethanol-proofing Many many thanks to everyone who has given of their time and brains to solving this problem. It has been a very interesting intellectual exercise exploring the subject of ignition systems and I've learned a lot on the way, even if the final conclusion was in fact un-related. Thanks again - MG Quote Link to comment Share on other sites More sharing options...
Richard Farrant Posted August 2, 2014 Share Posted August 2, 2014 As they say,...."99% of ignition problems are fuel problems" :-) I say that as the engine ran for about 10 miles or so then stopped and restarted after a while, that something might be being sucked up against the pick up pipe and after vacuum in the fuel line drops it falls away and you get going again. Had this years ago when working for the army. It was a Bedford which had broken down several times before, it got recovered and I found a repair invoice in the tank that would occasionally get sucked against the pipe. Quote Link to comment Share on other sites More sharing options...
N.O.S. Posted August 2, 2014 Share Posted August 2, 2014 And we had it with tractors being fuelled from water jerrycans - the continual slinging on top of lorry load, being tipped out and retrieved often by hydraulic grab, had caused the enamel lining of the cans to break away. A small flake of enamel spent weeks floating around and occasionally blocking the lift pipe. Whenever the tank was drained it sat discretely on the bottom and could not be seen. So what gives with the excessive arcing on points then? I also had similar not not identical problem last very long run out (very very hot day) - engine revs and power slowly died away but very smoothly unlike your truck, almost as if the governor butterfly was closing (first thing we checked). All electrics appeared fine. 2 valve pump with priming lever checked out ok as far as we could tell. Never did find out why but I need to ensure it doesn't happen again so am very tempted to fit a pump too! Quote Link to comment Share on other sites More sharing options...
N.O.S. Posted August 2, 2014 Share Posted August 2, 2014 (edited) P.S. Roy does not think too highly of my plan - reckons the 2 valve pump given the range of engines it is fitted to will deliver plenty enough fuel for a mere 270. I on the other hand am looking forward to not having to prime the pump every time I use the truck with any more than a fortnight's break in use (however do folk with no priming lever on the pump get on?).:-D Edited August 2, 2014 by N.O.S. Quote Link to comment Share on other sites More sharing options...
Richard Farrant Posted August 2, 2014 Share Posted August 2, 2014 P.S. Roy does not think too highly of my plan - reckons the 2 valve pump given the range of engines it is fitted to will deliver plenty enough fuel for a mere 270. I on the other hand am looking forward to not having to prime the pump every time I use the truck with any more than a fortnight's break in use (however do folk with no priming lever on the pump get on?).:-D Hi Tony, My Bedford is also blessed with no priming lever, a cost saving by GM no doubt. It is never any problem, just a turn over for about 10 secs after it has been standing for weeks / months and it will fire up. I also had problems some years back with the dreaded red coating from a jerrican, it would roll up and get sucked up the pick up pipe, my fault for removing the strainer from the neck on one occasion to save splashing. Caused me no end of bothers until last bits were removed from the tank. Quote Link to comment Share on other sites More sharing options...
John Comber Posted August 2, 2014 Share Posted August 2, 2014 Mike that's all better news I hope, still in my mind wondering why the points arced so hard on the way back from W&P , unsure if that is fuel related :??? Perhaps on the fuel side to be doubly safe the tank wants draining and a Hoover out? Quote Link to comment Share on other sites More sharing options...
HWade Posted August 3, 2014 Share Posted August 3, 2014 Tony, My GMC tractor unit had no primer, yet every time I went to start her she seem ok, on the other hand I had another truck with a primer, and that was a sod you had to prime her. Mike regards fitting the electric pump, I would stillbe inclined to flush out the fuel tank, even if you go electric if theres crap in the tank it will suck up, and cause a problem. Howard Quote Link to comment Share on other sites More sharing options...
gas 44 Posted August 3, 2014 Share Posted August 3, 2014 Our Gmc has a primer but to be honest half a dozen turns and it fires up. Never use choke.Luck of the draw I suppose. Our Dodge needs almost a check list to start, half choke,half hand throttle,6 pumps on pedal and whoooosh, miss one of those and its takes a few mins to start. Quote Link to comment Share on other sites More sharing options...
TooTallMike Posted August 5, 2014 Author Share Posted August 5, 2014 An extra non-return valve was fitted in the fuel system because this truck was suffering from fuel run-back and the lack of a priming lever meant it took forever to lift (this was when it was still 6V). I'm now adding two and two together and realising that was another symptom of the pump beginning to go bad (still drove ok at that point though). I agree about thoroughly cleaning the tank out and giving it a good hoover. I'm still keen on the electronic pump as I've seen several 6-valve pump diaphragms reduced to canvas in parts over the last year, presumably due to the corrosive effects of modern fuel. Points arcing may have been down to poor quality repro points? Certainly the 'original' Delco Remy set it now has fitted are considerably more substantial than the copper-coloured set that had got hot and been arcing. Could it even be that the Delco set being made of thicker material were simply able to absorb and then dissipate heat more easily? Of course until the fuel pump has been changed and a successful road test completed it could still turn out that the problem remains and we were just lucky to get home ok. - MG Quote Link to comment Share on other sites More sharing options...
TooTallMike Posted December 30, 2014 Author Share Posted December 30, 2014 Hi folks, For the sake of tying up this thread, I am pleased to report that the GMC has now completed a couple of journeys with none of the aforementioned problems. It now has an electric fuel pump and starts very quickly whether hot or cold. The pump is mounted to the X-member inboard of the fuel tank and there is a large in-line filter to deal with any junk. It was an easy conversion costing around £50 in parts and a couple of hours work. I personally think this is the way forwards to avoid a number of the issues relating to modern fuels in old vehicles. Thanks once again for everyone's help and advice! Regards - MG Quote Link to comment Share on other sites More sharing options...
fv1609 Posted December 30, 2014 Share Posted December 30, 2014 Well done Mike that is a relief. I'm sure there were many of us watching on the side lines wondering how it was going but didn't want to keep making posts badgering you. It is always good to hear of a diagnosis & a successful outcome, so we can all learn & store the info away in case a similar problem strikes on our vehicles! Quote Link to comment Share on other sites More sharing options...
gas 44 Posted December 30, 2014 Share Posted December 30, 2014 Thankyou for the update, its always good to know the outcome of problems. Quote Link to comment Share on other sites More sharing options...
R Cubed Posted January 5, 2015 Share Posted January 5, 2015 Would be nice to see some pics of what parts you have used. Glad to hear you have got to the bottom of it. Quote Link to comment Share on other sites More sharing options...
Don Robert Posted January 6, 2015 Share Posted January 6, 2015 Would be nice to see some pics of what parts you have used. Glad to hear you have got to the bottom of it. I am glad you have got to the bottom of it...I posted way back on post no.45 that I had similar problems on my GMC that was fuel related and a strong contender for your fault. Cleaning 70 years of silt from the system has been totally successful. and needed a variety of wires to push down the pipes to dislodge all imbedded debris..on the tank side of the filter... The GMC comes with a fairly sophisticated fuel filter as standard...easily serviced with new filters readily available.... I don think "Modern Fuels " should be confused with poor quality fuel/ old pump diaphrams that will fail irrespective of which fuel is used. Ethanol/alchohol was widely used in Petrol in the Uk from the late 1940,s to the late 1960,s when most cars had mechanical fuel pumps. Quote Link to comment Share on other sites More sharing options...
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