Troubleshooting stumble at idle in gear

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rich006

rich006

Learning as I go
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It "feels" like an ignition problem, but I'm not sure how to troubleshoot it. I may be missing something easy.

Here's the problem. After installing my new (reman) Super Six BBD carburetor, I have an intermittent stumble at idle, only when in gear (never in Park or Neutral). Some days it's there, some days it isn't there.

When it's there it happens at irregular intervals averaging about once per second. It often causes the engine to stall completely (embarrassing when you are showing your ID at the gate and the guard has to watch you turn the key again). With the old carb, instead of a stumble I had more of a hiccup, just a "putt" out the tailpipe with no stalling. Idle mixture was about the same, around 13 AFR on the dash wideband gauge.

Vacuum is 18.5" in Park, drops to 16 in gear and drops further when this stumble is happening. Ignition is stock Chrysler electronic ignition. Plugs, wires, ballast resistor and coil are all pretty new. I have raised the idle to 900 rpm in Park, but that doesn't really help with the stumble. It drops about 200 rpm in gear.
 
The ignition control unit is fairly new as well, installed about 12,000 miles ago. I've noticed that the problem gets worse the longer I sit in traffic or drive at low speed. It's as if the underhood temperature is affecting it.

If it was a vacuum leak, wouldn't the idle be rough all the time, in gear or not? Also, there's no stumble on acceleration at all.
 
How long have you been running the carb? can you pull plugs see if the carb is running rich? When plugs foul they start out acting up and it gets progressively worse untill you think the timing chain jumped time or something...:)
 
I just installed the carb last weekend. I haven't looked at the plugs, but I do have an AFR gauge in the dash. It shows around 13 at speed and around 12 at idle. When I first installed the new carb I set it to 14 at idle. When I noticed this intermittent stumbling problem I set it richer but that didn't help. Speaking of plugs, I'm running cooler plugs (NGK heat range 7) that I installed when I was trying to solve a long-standing ping. Now that the ping is gone I could try hotter plugs, but I don't know if there's any reason to think that would help.
 
i would pull them and read them anyway just to see if they are black and sooty but your AFR readings wouldn't indicate so...I am thinking it could be a carb stumble ' There is the idle circuit ,then the off idle circuit, the accelerator pump gives "pump shot" to richen it up between idle and part throttle and avoids the off idle stumble. You could check that quick look down the carb and throttle it not running and see if the acellerator pump is squirting nice and strong. I know the carb is rebuilt but it doesnt hurt to check. Youd notice that coming off ide when you just lay into the gas pedal.
 
The accelerator circuit is definitely working. The problem is only at idle, and only in gear.
 
Ok it sounds like a "misfire " at idle then? Can you get someone to sit in the car with it in gear and you hook up a inductive timing light to all the plug wires one at a time ? point the timing light at the valve cover or inner fender, you'll see the light being erratic on the suspect cylinder. Then you'll have the misfire pinpointed, if it is across all cylinders, then it is in the primary side, the points (or Ignition box if electronic) or in the distributor cap or coil. it could be a bad wire, I know they may be new doesn't mean they aren't bad. Parts these day are hit or miss. also sometimes the boot end isnt clicked all they way onto the plug or all they way down in the dist cap etc.
 
Thanks for the troubleshooting advice, that is just what I was looking for. I did check the wires were snug at both ends, but the timing light would be a good way to check firing.
 
It's possible you have a "drip".
When the stumble is occurring, look down the carb throats looking for an occasional drop of fuel to fall .
When it does, the engine will "stumble"..
If so, clear the air bleeds, and possible float adjustment .
 
rich006 said:
Thanks for the troubleshooting advice, that is just what I was looking for. I did check the wires were snug at both ends, but the timing light would be a good way to check firing.
Click to expand...
Yes it has helped me more than once, newer cars will set a misfire code the older ones we have top come up with our own tricks. It will isolate the problem cylinder and then you can eliminate from there, it may be a bad wire. I've seen new Belden wires last a year . They are on the inexpensive side and what most parts counter people sell because of low price but they can go bad quickly. Basically don't assume its not the wires, if the timing light test indicates something, throw a spare wire on and see if it changes
 
NGK 7 spark plug is too cold, should be a 5.
This doesn't sound like a carb/fuel problem...if you are just getting a momentary misfire.
Sounds like ign.
- plugs
- coil
- wires
- leaking spark plug boots
- dist p/up air gap, too big or too small
- worn dist shaft causing above
- if vac adv has been hooked up & dist has a lot of mileage, p/up wires can break from flexing causing no spark
- electrical connections in the ign system &/or low voltage to the ign. Try a jumper direct
 
Bewy said:
NGK 7 spark plug is too cold, should be a 5.
This doesn't sound like a carb/fuel problem...if you are just getting a momentary misfire.
Sounds like ign.
- plugs
- coil
- wires
- leaking spark plug boots
- dist p/up air gap, too big or too small
- worn dist shaft causing above
- if vac adv has been hooked up & dist has a lot of mileage, p/up wires can break from flexing causing no spark
- electrical connections in the ign system &/or low voltage to the ign. Try a jumper direct
Click to expand...
I put my ZFR5N plugs back in. See pic below of the 7's after being removed. I test drove with the 5's and there's no ping (the correct carburetor solved that).

The distributor shaft is in good shape. I didn't know there's supposed to be a gap between the distributor reluctor and its pickup. I had set zero gap last time I changed springs. I have now set the gap to 0.008" but I don't think the idle is much better if at all.

I'm not sure what you mean by leaking spark plug boots. The contacts where the wires clip onto the plugs are clean. I do seem to have a bit of oil seeping out past the plug tube seals, but it might just be running down from the valve cover which was leaking in the recent past.

When you say try a jumper direct, you mean from the battery + to the coil +?

Dartswinger70 said:
hook up a inductive timing light to all the plug wires one at a time
Click to expand...

My timing light worked last weekend but yesterday there was no light. I emailed Actron to see if I can get a replacement strobe. This is the $90 dial-back light with tach, which has been working great for several years.

Snake said:
change your box........................
Click to expand...

OK, I will. Easy and cheap enough to try.

These are the heat range 7 plugs I removed.
plugs.jpg
 
I think it could be your problem, i had stumbling at idle and my 360 stall out twice, so i change my icu with only 2000 miles and the new icu fix it.
 
rich006 said:
I put my ZFR5N plugs back in. See pic below of the 7's after being removed. I test drove with the 5's and there's no ping (the correct carburetor solved that).

The distributor shaft is in good shape. I didn't know there's supposed to be a gap between the distributor reluctor and its pickup. I had set zero gap last time I changed springs. I have now set the gap to 0.008" but I don't think the idle is much better if at all.

I'm not sure what you mean by leaking spark plug boots. The contacts where the wires clip onto the plugs are clean. I do seem to have a bit of oil seeping out past the plug tube seals, but it might just be running down from the valve cover which was leaking in the recent past.

When you say try a jumper direct, you mean from the battery + to the coil +?



My timing light worked last weekend but yesterday there was no light. I emailed Actron to see if I can get a replacement strobe. This is the $90 dial-back light with tach, which has been working great for several years.



OK, I will. Easy and cheap enough to try.

These are the heat range 7 plugs I removed.
View attachment 1715792670
Click to expand...
The color looks ok, I would say it isn't running rich. You can eliminate fouling...as in the idle mixture looks like its set right, you said you have good accelerator pumpshot for the off idle transfer. the carb looks like it may be ok.
 
I got out my multimeter and did the tests in the service manual. You're supposed to use a special test box, but it tells you what to do if all you have is a multimeter.

Disconnect the Electronic Control Unit (ECU) 5-pin connector.
With key on, engine off:
- Cavity 1 in the connector should be within 1 volt of the battery voltage; reads 11.3 while battery is 12.3 (within spec)
- Cavity 2 in the connector should also be within 1 volt of battery; it's 11.2 volts, just 0.1 volt lower than spec
With key off:
- Resistance between Cavity 4 and 5 (this is the distributor pickup coil) should be between 350-550 ohms; it reads 443 ohms
- At distributor connector (where it connects to the coil), either terminal should show an open circuit to ground; they both do (over 20000 ohms)
- ECU connector pin 5 (the pin, not the cavity) should show continuity to a good ground; it does (0 ohms to battery negative terminal).
- distributor reluctor air gap 0.008" (checked yesterday)

The last check is to pull the cable from the distributor cap center and hold it 3/16" from the engine. If you don't see a spark while cranking, replace the ECU. If still no spark, replace the coil. I didn't do this check because I do have a spark while starting the engine.

The last paragraph in the test procedure says if all of these tests do not reveal the problem, then the problem is probably in the control unit or coil.

I also measured the following just for kicks.

Voltages (with key on, engine off)
ignition coil: 5.3V at positive terminal, 2.1V at negative (I read somewhere that it should be more like 8 at the positive terminal)

Resistances (with key off):
Ballast resistor: 1.4 ohms across bottom and about 5 ohms across top (normal I think)
From ECU pin 2 to coil negative: 1.0 ohms (in other words good continuity)
ECU housing to battery negative: 0 ohms (ECU is well grounded)
 
Rich,
Post #15. Yes, a jumper from coil [+] to battery [+].
Plug 'boots'. These are the rubber pieces that hold the wire & slide down over the spark plug porcelain. They occasionally fail, go hard or crack, & the spark can leak away to the metal shell of the plug.
 
This weekend I replaced the NGK 7-series spark plugs with hotter 5-series plugs. I also reset the distributor reluctor gap to 0.008". Of course while doing that I also disconnected and reconnected a lot of things. And while reading out voltages last night, I swapped out my ballast resistor for a new one I had in the glove box (the measured resistance was not much different between the two). After all that, the idle was much better this morning, really not bad at all. Having done so many things at once, I'm not sure what did the trick. We'll see if it lasts. Some day I will switch to HEI.
 
Good that you found it. I remember testing an ECU on a 77 dodge truck, it just wouldnt start. The tests seemed to check out, and I got fed up and threw an ignition box on it anyway and the thing started right up. Sometimes you have to just guess. Its good to troubleshoot though and it saves spending alot of $$$ . I hope it works out.

i wanted to throw this out there, where did you hear to use NGK plugs? I recall they were a import plug I have seen/ran them in Mitsubishi etc. Sometimes plugs have an effect. I run Autolites on my Small Blcok Mopars, and the factory ran champions. I see those plugs have a "split" in the center conductor...I know some will say it doesn't matter but some will say it does. The split fire plugs they had out 25 years ago for example didn't work very well in real applications.
 
The shop manual actually does say if none of the troubleshooting reveals the problem, then change the ECU.

NGK plugs are popular on slantsix.org. Some like the ZFR5N in particular because of the longer extension into the cylinder. But others don't like that. I don't think I noticed any difference from the Champions I had before.
 
On the subject of plugs without fail every time I tried ACs in a Mopar I got a whine in the speakers that wouldn't go away til I changed them again for champions. So ac is a no go
I've heard good things about autolite plugs, tried em for the 1st time in my dakota only because the place was out of stock for champions.
I have used autolites before just never in a Mopar.
 
I know this is really simple but what are your idle RPMs in and out of gear?
 
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