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Cutting off at idle #3084102
10/07/22 03:47 PM
10/07/22 03:47 PM
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New Kent, Va.
bobbyb Offline OP
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I’m having a issue that I can’t diagnose…my 69 Roadrunner with a 400/451 keeps cutting off at idle and in gear. Motor has 870 Holley, elec. fuel pump w/6lb of pressure, Mopar electronic ignition. Can’t seem to figure out the problem, hoping someone might have an idea, thanks.




2004 Dodge Rumble Bee 5.7 Hemi

1969 Plymouth Roadrunner
Re: Cutting off at idle [Re: bobbyb] #3084105
10/07/22 04:18 PM
10/07/22 04:18 PM
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What's your RPM at idle?

Re: Cutting off at idle [Re: stumpy] #3084107
10/07/22 04:36 PM
10/07/22 04:36 PM
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bobbyb Offline OP
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1000 rpm, also it seems to occur after the car is hot and starts right up after it cuts off.




2004 Dodge Rumble Bee 5.7 Hemi

1969 Plymouth Roadrunner
Re: Cutting off at idle [Re: bobbyb] #3084156
10/07/22 09:00 PM
10/07/22 09:00 PM
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Originally Posted by bobbyb
1000 rpm, also it seems to occur after the car is hot and starts right up after it cuts off.

I would guess electrical / ignition related.
Start with the easy stuff, Bulkhead connector, possible ballast on the edge of failing, GROUNDS between battery, engine, and body.
What type of ignition system? beer

Re: Cutting off at idle [Re: TJP] #3084167
10/07/22 09:47 PM
10/07/22 09:47 PM
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bobbyb Offline OP
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Mopar electronic ignition conversion, and already have changed module and ballasts resistor and also the coil, actually moved the coil mount to the inner fender thinking maybe it was getting hot.




2004 Dodge Rumble Bee 5.7 Hemi

1969 Plymouth Roadrunner
Re: Cutting off at idle [Re: bobbyb] #3084197
10/08/22 12:54 AM
10/08/22 12:54 AM
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It sounds like you have a electrical "high joint" AKA high resistance open, when one part of the ignition feed circuit gets hot and expands and stops conducting at low RPM due to low voltage from the alternator. When you go to start it the 12V direct to the coil bypasses the ballast resistor and makes the motor start, when you rev it up the increased current jumps the high joint and allows th emotor to run until the RPM go down low enough to stop the current flow again.
1969 was a very bad year on Mopar cars electrical systems whiney shruggy
It will take some time and effort to find it and fix it but you can do it, don't quit.
I would start at the coil and work back through those two circuits, both the start and run feeds to the ballast resistor and then into the coil, remove all the slip fit connectors and clean and tighten them and then retests them, one at a time up wrench
The bulkhead connector can be a real challenge to clean ad tighten but it can be done by removing them one at a time from both sides of the bulkhead connector and look at them real hard and see if they slide off and on easy or hard. Clean and hard is better up work whistling grin
Are you still using the original mechanical voltage regulator and single field alternator?
If so check the voltage that it is running on when cold and again right before it shuts off when idling warm or hot..
I'm thinking it may be dying due to the volts dropping below the run voltage that ECU has to have too idle when warmed up scope up

Last edited by Cab_Burge; 10/08/22 01:55 AM.

Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: Cutting off at idle [Re: Cab_Burge] #3084203
10/08/22 04:55 AM
10/08/22 04:55 AM
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I’ve had that exact issue before and it’s the ecu not having a good ground. Like cab said, resistance is getting higher at low rpm.

Two ways to attack. One is cleaning and sanding the mounting points on the back of the ecu and also the firewall so it has no paint or rust preventing the ground. Other thing to do is to run a dedicated ground to it. I prefer that method because it eliminates the problem for good as it can always corrode again and leave you stranded. As tjp said, grounds are always a good place to look.

As cab said wiring can be an Achilles heel on these things. Make sure you’re factory ground wire is in good shape. Adding a ground never hurts on these cars. Overall things just work better. Last 69 rr I had had the best electrics of any of my cars by the time I sold it. Extra grounds, bigger factory alt with dual fields, electronic regulator, fresh under hood harness from year one that included the late model stuff added like the regulator and ecu, cleaned bulkhead, added volt gauge and bypassed the amp gauge. Flawless.


I want my fair share
Re: Cutting off at idle [Re: SomeCarGuy] #3084294
10/08/22 11:27 AM
10/08/22 11:27 AM
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both Cab and SCG have excellent suggestions.

I will mention a few others that were "FUN" to find.
1. Freshly restored 72 Cuda with all new harnesses. it would die every time one closed the R door but start right back up and run fine until the door was closed again. Problem : When the Connector for the control module was molded, one of the female terminals was to deep in the connector and BARELY touching the pin on the module. closing the door would open the connection and the motor would die. The reason i mention this is it was a potential HIGH RESISTANCE connection.

#2. Mis-wired ballast resistor when conversion was done. They had pulled the 12V signal for the Control module off the WRONG side of the ballast. Electronic components are finnicky. Sometime a 12v component will work at ~8 V and sometimes it won't. Why? numerous variables, temp, voltage, resistance and it's mood LOL

#3 Wire to pickup coil inside the distributor was broken internally. I assume it was from the vacuum advance moving and flexing the wire, BUT I have also found brand new wire with internal strands open inside the insulation. these can usually be detected by pulling opposite directions on the insulation. If a section seems weak or "stretchy" discard it.

#4 Bad crimps an on new harnesses. There are two parts to the crimp. The first crimps the copper wire. It should be tight with NO insulation inside the crimp. The second part is the strain relief which crimps on the insulation. it should be snug and prevent the insulation/wire from moving When inspecting flex the insulation where it enters the terminal. if it feels weak, it is likely breaking or broken internally.

#5 You've already replace d the ballast but i will mention it for others. The resistors will sometimes start to deteriorate internally causing intermittent issues like you experiencing

#6 Bad coil wire. They can be also broken internally or have had a bad connection on one end which has Burnt the conductor from arcing.

#7 as suggested make sure the male and female terminals fit snug for a good electrical connection.

#8 Any added terminals should be PULL tested to verify the integrity of the crimp.

#9 be aware that different terminals require different crimpers. Some are some what universal ( Home depot, harbor freight etc). Some should also be crimper a specific way. BUBBA doesn't adhere to theses ideas and believes a pair of pliers will work just fine or a hammer and punch on a battery lug tsk rolleyes musik


keep us posted beer

Re: Cutting off at idle [Re: SomeCarGuy] #3084345
10/08/22 02:22 PM
10/08/22 02:22 PM
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New Kent, Va.
bobbyb Offline OP
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Cab burge and some car guy, thanks for responding, what you’re both saying makes sense and I’ll pursue both. Just to let you know what I’ve got, I restored this car and just started driving it a year ago, a few hiccups along the way including this one, all the wiring was replaced, engine compartment and dash/instrument harnesses. Not saying they’re perfect, but letting you know. The alternator is high output dual field and voltage regulator is compatible with the conversion kit. With everything under the hood being new it’s just frustrating having this issue. By the time I figure this crap out the season will be over. Thanks




2004 Dodge Rumble Bee 5.7 Hemi

1969 Plymouth Roadrunner
Re: Cutting off at idle [Re: TJP] #3084352
10/08/22 02:31 PM
10/08/22 02:31 PM
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bobbyb Offline OP
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Thanks tjp, sounds like you have had not so good experiences with new wire harnesses, as I mentioned earlier mine are new also but doesn’t mean they’re perfect, I’ve had a few issues with new products on my car in the last year.




2004 Dodge Rumble Bee 5.7 Hemi

1969 Plymouth Roadrunner
Re: Cutting off at idle [Re: bobbyb] #3084356
10/08/22 02:40 PM
10/08/22 02:40 PM
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nothing there states you checked for spark or gas when it stalled..so when it stalled, what did you do? did it start back up, did it crank, did you wait for it to cool off, did you stick your head and look down the carb to see if gasoline is pumping in when you move the accelerator linkage, check for spark at the plug, at the coil? look for 12 volts on the positive side of the coil when it stalled? fuel pressure is good?

Re: Cutting off at idle [Re: bobbyb] #3084357
10/08/22 02:47 PM
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and stay FAR AWAY from those cheapo, bolt on, "temporary" [oh YEAH ?] battery terminals ! panic
i hate those things with a passion, and if they are on a nicely restored vehicle, those things immediately make me question the rest of the vehicle.
there have been more problems caused by those things than i can count.
if you have those, get new cables before you do anything else !
just an old dumazz's opinion and 60yrs of playing with these toys.
your mileage will vary.
beer

Re: Cutting off at idle [Re: AARCONV] #3084443
10/08/22 10:13 PM
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Originally Posted by AARCONV
nothing there states you checked for spark or gas when it stalled..so when it stalled, what did you do? did it start back up, did it crank, did you wait for it to cool off, did you stick your head and look down the carb to see if gasoline is pumping in when you move the accelerator linkage, check for spark at the plug, at the coil? look for 12 volts on the positive side of the coil when it stalled? fuel pressure is good?


in his second post he states
Quote
1000 rpm, also it seems to occur after the car is hot and starts right up after it cuts off.
wink

Re: Cutting off at idle [Re: bobbyb] #3084444
10/08/22 10:16 PM
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Originally Posted by bobbyb
Thanks tjp, sounds like you have had not so good experiences with new wire harnesses, as I mentioned earlier mine are new also but doesn’t mean they’re perfect, I’ve had a few issues with new products on my car in the last year.

We've had a few, especially with M and H harnesses. Some mis-wires, bad crimps etc. Do keep us posted beer

Re: Cutting off at idle [Re: bobbyb] #3084480
10/09/22 09:05 AM
10/09/22 09:05 AM
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Does it gradually slow down and quit or is it instant like turning off the key?


In theory, there is no difference between theory and practice. In practice, there is.
Re: Cutting off at idle [Re: GomangoCuda] #3084979
10/10/22 07:55 PM
10/10/22 07:55 PM
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bobbyb Offline OP
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I can sit in the car and watch the tach and it’s holding steady at 1000rpms in gear, then it will have sort or a blip and drop to 950/900rpm and then it just shuts off, and like I said earlier it will start right back up again.




2004 Dodge Rumble Bee 5.7 Hemi

1969 Plymouth Roadrunner
Re: Cutting off at idle [Re: moparx] #3084981
10/10/22 08:14 PM
10/10/22 08:14 PM
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Originally Posted by moparx
and stay FAR AWAY from those cheapo, bolt on, "temporary" [oh YEAH ?] battery terminals ! panic
i hate those things with a passion, and if they are on a nicely restored vehicle, those things immediately make me question the rest of the vehicle.
there have been more problems caused by those things than i can count.
if you have those, get new cables before you do anything else !
just an old dumazz's opinion and 60yrs of playing with these toys.
your mileage will vary.
beer


X 2 up

Re: Cutting off at idle [Re: bobbyb] #3084988
10/10/22 08:34 PM
10/10/22 08:34 PM
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Originally Posted by bobbyb
I can sit in the car and watch the tach and it’s holding steady at 1000rpms in gear, then it will have sort or a blip and drop to 950/900rpm and then it just shuts off, and like I said earlier it will start right back up again.
Do you know if the timing starts to retard when it slows down? IF not, check that out to see if the timing is not at full retard while idling and dies when it starts to retard more scope


Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
Re: Cutting off at idle [Re: Cab_Burge] #3085579
10/12/22 07:22 PM
10/12/22 07:22 PM
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I’m going to look into your suggestion this weekend cab burge, thanks




2004 Dodge Rumble Bee 5.7 Hemi

1969 Plymouth Roadrunner
Re: Cutting off at idle [Re: Cab_Burge] #3085596
10/12/22 08:55 PM
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Originally Posted by Cab_Burge
Originally Posted by bobbyb
I can sit in the car and watch the tach and it’s holding steady at 1000rpms in gear, then it will have sort or a blip and drop to 950/900rpm and then it just shuts off, and like I said earlier it will start right back up again.
Do you know if the timing starts to retard when it slows down? IF not, check that out to see if the timing is not at full retard while idling and dies when it starts to retard more scope


A good suggestion, I always try o make sure the advance is fully out at no more than 200 RPM above hot Idle. beer

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