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Another GMC ignition system conundrum


TooTallMike

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Hi Folks,

I have a perplexing problem with a GMC which I would like to throw open to the masses. I hope I've covered all the tests we've carried out as I don't believe we've missed anything but here's hoping...

 

Symptoms: The problem began a couple of weeks ago after the truck sat unused for a couple of days. The engine will start first turn from cold and tick over fine but after about 5 minutes will splutter and cough, and then recover itself. It will then tick over and after a shorter time will cough and splutter, then recover. As it warms up the periods of good running get shorter and shorter until it runs rough and eventually conks out. Once allowed to cool it will re-start and the process repeats. Initially I looked at fuelling as we've done some work to the fuel system (electric fuel pump, carb rebuild), but after swapping to a known good carb which was just as bad I put on a spark tester and discovered the truck is losing spark during the misfiring moments. All cylinders do the same, and with two spark testers the loss of spark can be seen to be occurring in two locations at the same time. To me this rules out plugs and plug leads. The sequence of ever decreasing periods between failure seems to me suggestive of something electrical breaking down under heat/load.

 

What we have tried (in order of trying):

replaced condensor

voltage check at coil including during misfire periods (13.5V - 14.8V)

replaced coil (yes it is a 12V coil!)

replaced rotor arm

replaced distr. cap

replaced king lead

replaced points

checked LT wire from coil to distr.

wired direct from battery '+' to coil

wired direct from battery '-' to screw on distr. body

checked insulation of LT post through distr. body

trimmed tags on condensor terminal as we thought they might be earthing to the base

continuity and resistance checks between points and distr. body etc.

compression test showing all 6 to have good and similar compressions.

 

These changes were performed one at a time and then tested, and the symptoms did not change throughout.

 

All ign. parts fitted were new from Rex Ward.

 

What am I missing...?

 

The next move is to fit a known good coil, distributor and leads from another truck but I'll admit this is clutching at straws since we cannot find anything wrong with what is already fitted.

 

Thanks in advance for any suggestions - Mike

Edited by TooTallMike
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Hi Mike. Since you have changed pretty well everything I'd say could be plug leads or possibly plugs. Check the distributor shaft for wear, I had one that was giving all sorts of odd symptoms eventually traced to play in the shaft.

Hope this might give you a clue.

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One thing I should add is that it has a hideous blow from the exhaust manifold joint. The owner is reluctant to let me touch it this side of the show season in case of breaking studs. It doesn't make all this any easier but it's probably wise as they will break. We disconnected the fan belt and sprayed all around the inlet joints and couldn't detect a change in engine note so I don't believe it's got a leak 'in' anywhere. I know exhaust blow can cause issues with back pressure but I think the missing spark means this is not relevant.

 

- MG

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Hi Mike. Since you have changed pretty well everything I'd say could be plug leads or possibly plugs. Check the distributor shaft for wear, I had one that was giving all sorts of odd symptoms eventually traced to play in the shaft.

Hope this might give you a clue.

 

I had similar problems with spitting back through the carb and backfiring from the exhaust when warm and under load with a GMC.

Like you I went round and round :nut: until I finally pulled the distributor and found it was a French replacement unit and the base plate had become loose on the shaft. It was held on by a peened down flange that had worn allowing the base plate to move about and therefore the points gap to wander all over the shop, also when the shaft warmed up the base plate would stick in the advanced position. A quick squirt with the Mig on low power solved all the issues in this particular case.

 

I'm not suggesting this is your answer Mike but it might be worth checking

 

Pete

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I had similar problems with spitting back through the carb and backfiring from the exhaust when warm and under load with a GMC.

Like you I went round and round :nut: until I finally pulled the distributor and found it was a French replacement unit and the base plate had become loose on the shaft. It was held on by a peened down flange that had worn allowing the base plate to move about and therefore the points gap to wander all over the shop, also when the shaft warmed up the base plate would stick in the advanced position. A quick squirt with the Mig on low power solved all the issues in this particular case.

 

I'm not suggesting this is your answer Mike but it might be worth checking

 

Pete

Thanks Pete, I'll swap the distributor for the spare and if it cures the problem then that's something I'll bear in mind.

- MG

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Hi Mike. Since you have changed pretty well everything I'd say could be plug leads or possibly plugs. Check the distributor shaft for wear, I had one that was giving all sorts of odd symptoms eventually traced to play in the shaft.

Hope this might give you a clue.

Thanks Degsy, I'll have a look. Sorry you couldn't make it to A&E, I enjoyed meeting you last time and hoped you'd be there again.

- MG

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Mike, I've also had oddities trace to distributor faults, not on GMC but on related engine families. As you say, though, the failure after increasingly shorter amounts of time does sound rather like a component breaking down. Your idea of a third set of (preferably known good) parts may be a good one - as you know, it's by no means unknown for new or NOS parts to be faulty out of the box.

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Thanks Degsy, I'll have a look. Sorry you couldn't make it to A&E, I enjoyed meeting you last time and hoped you'd be there again.

- MG

 

 

Hi Mike, Likewise, although it could have been under better circumstances for you. I would love to have been there but I've got so many different health problems if I was a horse I'd have been shot years ago:D I would definitely look at changing the plug leads especially if they are suppressed type. After I retired I used to help a mate out in his village garage and on average we changed at least three sets a week. For reliability I've always gone back to the old wire type on my own vehicles.

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Mike I can test your coils for insulation breakdown. This ideally needs to be done when the coil is hot as the insulation reduces to about a tenth of what it should be when cold. When cold at 5 kV the insulation should be many gigohms otherwise it will cause trouble when hot. I can do these hot & cold tests at WPR & test rotor arms etc.

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Hi Mike, glad you got home ok from A and E.

 

Are you sure its an ignition problem ? Did it do this before you fitted the electric fuel pump ? might be too powerfull and is over coming the float valve in the carb and flooding the engine, if so changing the carb would not help. Try using a gravity feed to the carb to rule this out. other than that its down to the electrical bits, need to be changed for known working items. One last thing make sure you have not got NGK plugs in the engine they can cause all kinds of problems, as I know.......

 

Good luck

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One last thing make sure you have not got NGK plugs in the engine they can cause all kinds of problems, as I know.......

 

Interesting comment what was the problem with the NGK plugs ?

 

I've got a bit of a mission at the moment about some manufactures plugs and their unsuitability for older engines so I'd be keen to know your findings.

 

Pete

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Hi Mike, glad you got home ok from A and E.

 

Are you sure its an ignition problem ? Did it do this before you fitted the electric fuel pump ? might be too powerfull and is over coming the float valve in the carb and flooding the engine, if so changing the carb would not help. Try using a gravity feed to the carb to rule this out. other than that its down to the electrical bits, need to be changed for known working items. One last thing make sure you have not got NGK plugs in the engine they can cause all kinds of problems, as I know.......

 

Good luck

 

Hi Mike, I'm with Richard to a degree. You did not say why you change the fuel pump but did the problems start after that ?

Gary

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Hi

 

We have had the same issue as your self, problem was found to be inside the dizzy is a big piece of Bakelite that holds a bolt that feeds the condenser and points. we found it was juicy inside and the Bakelite had split on the back edge creating a short. Upon this we also found burnt points and rotor arm so also replaced the condenser. All is fine now and have done some miles without a cough!

Worth a look at and hope it helps?

 

Regards Michael

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Interesting comment what was the problem with the NGK plugs ?

 

I've got a bit of a mission at the moment about some manufactures plugs and their unsuitability for older engines so I'd be keen to know your findings.

 

Pete

 

Well, it was not on anything military, but an old E reg sierra 2Ltr pinto, our daily driver, about march time decided to carry out some maintenance and changed the plugs (NGK), leads, dist cap and fuel filter as I had not done any of this for some years, no reason to change any of it, did not have any running issues at all just though it was about time to do it.

 

Come november time it started getting colder like it does and the wife started saying it was getting hard to start in the mornings, (never ever had anything like this with it before) it did however eventually start and ran fine even when hot also would restart when hot too.

 

Steadily got worse and worse till it would not start. Discounting the plugs as they were "new NGK's" tried everything else ran out of ideas so got a friend round to help and we started working through it in a methodical manor. First thing he said was lets see the spark, I took a plug out and gapped it and he saw it was an NGK plug, he said well thats your problem, pointing to the plug, ? I said, ngk throw them away and get some proper ones. We did test the spark and to say it was weak and tiny and only a yellow glimmer was an understatement. I had kept the old motorcraft plugs which we tried and lo and behold got a masive fat blue spark. !!!!!! fitted the old ones back in and had no more problems.

 

Another friend who has a Triumph 5SW and after an engine rebuild he could not get more than about 10 miles out of a plug, he use to swop them round, but it got worse and worse down to only a couple of miles !!!! they were NGK's too changed to Champion's and I think he is still on the first plug he put in !!

 

Draw your own conclusions to this but I will not ever touch another NGK plug.

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I would not knock NGK spark plugs until you are sure the ones you are using are not fakes. It has been widely known for several years that there are a lot of counterfeit NGK plugs in circulation. I generally use Champion but also NGK and never had a problem. Probably due to the fact I but them from a reputable supplier. If you buy these plugs from places such as Ebay and they are cheap then a good chance they are not genuine.

Check out the makers warning, http://www.ngk-sparkplugs.jp/english/techinfo/fake/

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In desperation I fitted the distributor from another working truck and now it seems to be ok. The points in the 'new' distributor were a bit burnt so I fitted the old ones out of the 'old' unit. This means that the following have been changed:

condensor, bakelite insulator through body, rotor arm, cap, leads. We are leaving it for now as the owner is taking his daughter to her prom in it later this week. After that I want to swap the 'old' parts back in one by one until the fault returns.

 

To answer some of the questions above, the electric pump has been on for over 150 miles now. More importantly though, the spark could be seen to be disappearing when the engine coughed, whereas now it can be seen to be consistent.

 

Thanks to all who have contributed and i will update in a week or so when I've looked into it further.

 

- MG

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I wrote too soon!

 

The 5-mile trip to the prom went fine, however on the way back it began doing the same coughing and spluttering thing. The owner pulled over and as he was trying to call me it cured itself and he drove home with no further problems. On Saturday he drove it 20-odd miles to the Capel show with no trouble. I've not yet heard whether he got back ok.

 

Someone suggested excessive alternator output as I measured 14.5V at the battery at high revs. This does seem a bit in excess of the 13.8V I've always understood to be the normal maximum. However I have no idea if or how this could be the cause of the problem.

 

I plan to fit a spark tester in line so that next time it misbehaves he can pop the bonnet to see if it is losing spark.

 

- MG

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Odds on are it's still ignition related have you checked security of LT wires on ammeter, ignition switch and the condition of the switch itself.

I had ignition switch on a Dodge that worked fine until the truck jarred on a bump and the engine would stutter and then pick up, turned out the internal contacts were burned and arching. That problem took a long time to find and only by accident when I knocked the switch getting in with the engine idling and got the same result.

 

Have you run a vacuum gauge test yet ? it may help to eliminate valve, seat, and valve train issues although given that it has run fine under load for 5 miles I would be surprised if this is where your problem is.

 

Failing that something floating about in the fuel tank causing a temporary partial blockage or an accelerator pump piston that sticks again not very likely I'm afraid.

When does it happen.... under load?....on over run?...,at idle?.....when cold? when hot? .....

all of the above ?

 

Pete

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If I read the symptoms right all the plugs lose spark causing an engine cough then it runs normally?

 

I'd bypass any switch in the cab and run the ignition right off the battery.

 

Looking at the ignition circuit you have the ammeter, the IGN switch, IGN primary filter (depending on your year) then coil, Distributor and plugs.

 

I'd run the Black wire from the coil (again assuming you have the correct color coded wires) to the battery (+) and then hit the starter. That would rule at any electrical issue aft of the firewall.

 

 

If you need a wiring diagram: https://www.tm9-801.com/searchTM9/tm9_pages/316.jpg

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I would not knock NGK spark plugs until you are sure the ones you are using are not fakes. It has been widely known for several years that there are a lot of counterfeit NGK plugs in circulation. I generally use Champion but also NGK and never had a problem. Probably due to the fact I but them from a reputable supplier. If you buy these plugs from places such as Ebay and they are cheap then a good chance they are not genuine.

Check out the makers warning, http://www.ngk-sparkplugs.jp/english/techinfo/fake/

 

Quite agree, I have always bought most stuff from my local motor factors which have been going for more years than I can remember, did discuss with them at the time and they were going to look in to it but I was assured by them they source all their parts from reputable suppliers.

 

Still wont touch NGK any more.

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Have to say I'm really surprised at the comments about NGK plugs. You have to go with your personal experience, of course, but coming from a motorsport background we always swore by (rather than at!) NGK; I've seen NGK plugs and leads producing increased power on a dynamometer vs. Champion.

 

Since then I've used NGK where possible, Bosch when I can't get NGK, and never had a problem.

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If I read the symptoms right all the plugs lose spark causing an engine cough then it runs normally?

 

I'd bypass any switch in the cab and run the ignition right off the battery.

 

Looking at the ignition circuit you have the ammeter, the IGN switch, IGN primary filter (depending on your year) then coil, Distributor and plugs.

 

I'd run the Black wire from the coil (again assuming you have the correct color coded wires) to the battery (+) and then hit the starter. That would rule at any electrical issue aft of the firewall.

 

 

If you need a wiring diagram: https://www.tm9-801.com/searchTM9/tm9_pages/316.jpg

 

Sadly wiring the coil direct to the battery to bypass the entire electrical system was one of the first things I tried but it made no difference.

 

Since its last brief hiccup, the truck has now done around 50 miles without trouble. I've left an in-line spark tester on it so if it plays up the owner will immediately look so we know if it's electrical or 'other'.

 

One suggestion is that the alternator may be putting out too much voltage. I'd normally expect to see around 13.8V but it was 14.5V when I tested it a while ago. This is at the high end of normal output range. I need to test it again under more controlled conditions to confirm but could it be that it's cooking something?

 

- MG

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Hi MG

 

Once had a car that had the same intermittent spark problem, and in the end we put one of those ignition monitors on it to see if it would help diagnose the problem. Car worked perfectly so we took the unit off car started acting up again put the ignition monitor back on problem went away.

 

Traded the car in five years later with the monitor unit still on and running good.

 

Cheers Phil

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