# Heavy intermittent missing - 68 GTO



## NZ-GTO (Jun 7, 2015)

I have an engine miss problem that I can't get to the bottom of and wondered if anyone had experienced (and solved) the same problem.
My 68 GTO auto has a standard 400 engine and trans. I bought the vehicle recently on the basis that the engine had been recently rebuilt (<100 mi on it) and nothing I have seen leads me to think that is not the case. It starts and runs perfectly however from time to time mainly when driving at light throttle opening I get a very heavy hesitation. If this occurs at idle the engine dies as if the ignition was switched off. 
The car has new:
1) plugs (checked - all look good)
2) Distributor cap
3) Points and condenser
4) Coil - ballasted
5) Ignition switch
6) Fuel pump and fuel tank
I have replaced the wire between coil and distributor as the old one was hard and cracked. I can't see any earthing of the wire from coil to solenoid and tugging on that lead doesn't induce the miss when it is running.
It has the correct Quadrajet which I have reset the float level.
I have reset the timing (35 degrees total advance at about 3000rpm) and reset dwell to 30 degrees.
It starts and runs well but every second or 3rd time it will start missing and hesitating as above - the condition always clears but might persist for 5 mins or so.
I am running out of ideas - the most likely suspect seems to me to be the 12v ignition system. Any assistance would be most appreciated down here in New Zealand!
Cheers - Chris


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## PontiacJim (Dec 29, 2012)

It sounds more like a fuel problem rather than an electrical, but it may be electrical. 

The fact that it clears up is why I say it may be fuel related. Electrical typically in my experience either runs good generally or runs poorly/quits generally, not so much running good then poorly, then clears up, and repeats.

1.) I don't know what type gasoline you have available, but the ethanol (alcohol) blend gas we have can create problems similar to what you describe IF there is water in it. The ethanol draws water to it and if you hit some of it that has settled out of mixture, it will stumble & run rough until you burn through it and clears up again.

2.) Even though you have changed out your tank/fuel pump, sometimes new parts can be defective. I might install a fuel pressure gauge on the carb inlet line and see what my reading is and if any change when it begins to stumble/run rough. Your symptom has the ring of a carb on the verge of running out of fuel as well. Your needle & seat inside the carb could be sticking shut. There is a little snap/wire that clips around the top of the needle and then attaches to the float. This helps to lift the needle up out of the seat when the float drops. If this is missing, the needle could be momentarily stuck, causing a no fuel situation, and then pops up under pressure so gas can enter the bowl and refill it. Do you recall seeing this when you reset your float level?

3.) Make sure you choke is not fluttering or momentarily closing on you. I might try locking/wiring the choke open just to check if you are not sure.

4.) Your carb could be actually flooding out the engine. A gas leak internally, gas leaking past your needle & seat, or the float sticking, could allow this to happen and be the cause of the stumbling and/or shut down of the engine at idle because the engine is loading up with gas.

Not to scare you, and this is grasping at straws, do you have a mechanical oil pressure gauge? I would want to observe the oil pressure when the engine stumbled or almost quits. You said the engine was recently rebuilt (and I don't know if you consider the rebuilder of good reputation or not), but I would want to make sure that something internally (a bearing) is not seizing up and causing the problem. If the piston-to-wall tolerances are too tight, the piston will expands when it gets hot and it can seize, but there would be no way to know this. Again, probably not happening, but I would want to have a mechanical oil pressure gauge to watch oil pressure.

That's my 2 cents worth of opinion on it. Again, it could indeed be electrical, but I would not rule out a gas/carb problem either.


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## Lemans guy (Oct 14, 2014)

SoLid advice from Pontiac Jim. I would just add to check the coil with an ohmmeter and or swap out a different known good coil. Make sure that your ground near the coil firewall is clean and solid as well.

For it to stall or hesitate it loses the proper mixture as Jim described or the strong spark....the coil delivers that strong spark...

So you carb has to be set up right including the choke system fast idle cam etc...

A spark plug wire shorting to ground can make that happen too....look at it run in low light in the evening....see any stray spark? Or use a spark tester to see if spark is shorting .....watch your fingers

Alternators can cause weak electrical systems to perform poorly, is the belt tight? Alt good? How is the fuel filter?

You can richer up ( or lean up) the carb idle screws a quarter turn or so and see if it goes away.....since you said under idle or light throttle,.....those are times when the idle passages in the carb are at work or just transitioning.......

So you might want to explore that just a little


Good luck...you will get itatriot: 

didn't we invade there once? I forget....

Probably one of the guys left that GTO....

No that was another Pacific spot......


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## NZ-GTO (Jun 7, 2015)

PontiacJim said:


> It sounds more like a fuel problem rather than an electrical, but it may be electrical.
> 
> The fact that it clears up is why I say it may be fuel related. Electrical typically in my experience either runs good generally or runs poorly/quits generally, not so much running good then poorly, then clears up, and repeats.
> 
> ...



Thanks very much for the advice above PontiacJim

1) The problem has occurred with three different fuel suppliers one of which contained Ethanol - I have never heard of issues with NZ fuel containing water. Have pulled the carb and emptied the bowl and filter - no sign of water so seems unlikely although might explain the intermittent nature of the problem and the heavy hesitation
2)The spring lifting the fuel valve is in place - I reinstalled it per the manual. The float valve sticking was my main theory until I tried crimping the fuel line to the carb with pliers to see the effect - when the engine finally died it did so very gradually and "softly" - not at all like the "dead stop" that I get when the miss occurs. That's when I shifted to the ignition line of thinking. I would be interested in your thoughts on this?
3) Ok will restrain choke and see what happens.
4)No evidence of fuel leaking or flooding on or around the carb - I do have a rebuild kit for the QJ so will go ahead and install that.

I have also installed a new radiator and water pump and the engine is definitely not overheating. Have a mechanical Oil Press gauge and pressure is good at all times.

Thanks again.


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## NZ-GTO (Jun 7, 2015)

Lemans guy said:


> SoLid advice from Pontiac Jim. I would just add to check the coil with an ohmmeter and or swap out a different known good coil. Make sure that your ground near the coil firewall is clean and solid as well.
> 
> For it to stall or hesitate it loses the proper mixture as Jim described or the strong spark....the coil delivers that strong spark...
> 
> ...



Thanks for the response LemansGuy
The coil is brand new as are the leads, cap, etc and there is a strong spark. What I thought I could do to bottom out the ignition 12v theory was to attach leads so I can watch a voltmeter attached to the distributor side of the coil from in the car - if the voltage collapses when the miss occurs it would confirm the theory or if not I can look elsewhere.
Will also look into the other points you raise.

BTW New Zealand was invaded during WW2 - troops from the Pacific came down here for RnR - as the saying goes - over-paid, over-sexed, and over here. :thumbsup:


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## NZ-GTO (Jun 7, 2015)

Update - I have run the vehicle while monitoring voltage at the coil and while the engine is missing the coil supply remains at approx 12v so that isn't the issue. 
So now looks like fuel supply / carburation as PontiacJim and LemansGuy suggest. No visible external issues with the carb when missing is occurring - ie: choke flutter, or external signs of flooding.
Any further ideas would be welcome.


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## chuckha62 (Apr 5, 2010)

I hate to sound like a broken record, but you may have too much fuel pressure which will unseat the needle and seat in the float assembly and cause a severe over-rich condition.


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## 68GTO4004Spd (Jun 19, 2008)

Stupid question, but do you have the grounding straps, one on the frame to fender, one on the engine block to firewall?


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## Lemans guy (Oct 14, 2014)

Lots of stuff to check....vacumn leaks around the intake runners can cause stalling and rough idle. Listen with hose, check with smoke if you can.

But if we talk the fuel side a minute.....Is it a rich stall or lean stall?

When it is idling if you put your cupped hand over the carb horn, does it run better?....you have acted like a choke then and richened the mixture, by blocking off air.....so you may have a lean stall.....

Need more fuel then, mixture screws, float, fuel pressure etc....

Too rich? You will have to leaning it out....very slight changes on carb mixture screws can cause this. But you have to have your choke and fast idle screws set up right to factory specs before you can do anything correctly.

You will get it .....and PS as a lover of history Great respect for those New Zealand and Australian Troopers in all of our past conflicts.....

Solid as they come, now we did have some disagreements with George III, but we ironed that all out as well......we just don't do Royalty well....

But we get along better now......atriot:


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## BearGFR (Aug 25, 2008)

Is it possible for you to record video of the behavior and post it somewhere that we can see/listen? That might help.

Bear


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## chuckha62 (Apr 5, 2010)

68GTO4004Spd said:


> Stupid question, but do you have the grounding straps, one on the frame to fender, one on the engine block to firewall?


You certainly should. I believe the factory had three of them although I can only think of the placement for two at the moment. Since your motor and transmission mounts are all rubber, you need a reliable path to ground to complete all circuits.


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## NZ-GTO (Jun 7, 2015)

I have confirmed that the issue is the ignition - I had pulled the distributor to recheck it and was resetting the timing - the missing started while I had the timing light connected and each time the engine was missing the timing light went very dark. I have rechecked the two power supply wires (ignition switch and solenoid) to the coil +ve and can't detect any interruption in 12v supply so my attention is now very much on the distributor. I have replace the (almost new) AC Delco points/condenser set - I noted some arcing marks on the think metal strips that connect the points arm to the condenser - one of the two strips was broken where it attaches to the condenser frame. I decided to replace the whole thing (points and condenser set), and at the same time ordered a HEI distributor as the vacuum advance is not operating on mine which looks to be original.
So far so good - while running in my driveway and on one run on the motorway it is all good.


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## NZ-GTO (Jun 7, 2015)

NZ-GTO said:


> Update - I have run the vehicle while monitoring voltage at the coil and while the engine is missing the coil supply remains at approx 12v so that isn't the issue.
> So now looks like fuel supply / carburation as PontiacJim and LemansGuy suggest. No visible external issues with the carb when missing is occurring - ie: choke flutter, or external signs of flooding.
> Any further ideas would be welcome.


I have confirmed that the issue is the ignition - I had pulled the distributor to recheck it and was resetting the timing - the missing started while I had the timing light connected and each time the engine was missing the timing light went very dark. I have rechecked the two power supply wires (ignition switch and solenoid) to the coil +ve and can't detect any interruption in 12v supply so my attention is now very much on the distributor. I have replace the (almost new) AC Delco points/condenser set - I noted some arcing marks on the think metal strips that connect the points arm to the condenser - one of the two strips was broken where it attaches to the condenser frame. I decided to replace the whole thing (points and condenser set), and at the same time ordered a HEI distributor as the vacuum advance is not operating on mine which looks to be original.
So far so good - while running in my driveway and on one run on the motorway it is all good.


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## NZ-GTO (Jun 7, 2015)

NZ-GTO said:


> Update - I have run the vehicle while monitoring voltage at the coil and while the engine is missing the coil supply remains at approx 12v so that isn't the issue.
> So now looks like fuel supply / carburation as PontiacJim and LemansGuy suggest. No visible external issues with the carb when missing is occurring - ie: choke flutter, or external signs of flooding.
> Any further ideas would be welcome.


Problem solved!!
I had the miss show up when I was running it in the driveway with a timing light connected. Whenever the miss started the timing light went dark - confirming the ignition as the source of the issue.
I was also able to confirm that there was no issue with power supply to the coil leaving the distributor as the likely culprit. (New coil and -ve wire from coil to distributor)
In the end I replaced the points - although the set installed looked quite new with no significant pitting there was some arc marking on the points arm.
Anyway the short version of the story is that the problem is gone and all running well.
This emphasises to me that the ignition and specifically the 12v part of it should be the first port of call in an intermittent miss situation. 
THANK YOU to everyone who helped with comments above.


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## PontiacJim (Dec 29, 2012)

Excellent. Glad we could all help in trying to solve or point you in the right direction. The beauty about "old cars" is that there are basically three things to check when having such problems: fuel, ignition, air. No computer crap or sensors to complicate things. Just do a simple step by step process of elimination and you can usually track it down.


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