Engine Misfiring

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MOPARJ

MOPARJ

What can I upgrade now?
Joined
Nov 3, 2006
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Thousand Oaks, Ca
While driving home yesterday evening in my Dart, I encountered some issues. I exited the freeway and noticed the exhaust had more of a pop to it as I was decelerating. When I came to a stop at the light, the idle was lower and the motor was not running smoothly. It did not have just one cylinder down, it sounded like all where firing weakly. When I put the car into neutral and rev the motor easily, it sounded really weak, like it was running out of fuel, not sputtering though. After a about 1000 rpm, the motor sounded like it abrubtly began running a little stronger, but even while holding the rpms at 1,500, the revs where still vaining up and down, plus there is some popping from the exhaust. I had a spare ECU, so I swapped it, to no help. I limped the car home. When at home, I noticed that everytime I shut the car off and restarted it, it would run fine, until I revved the motor to around 2,000 rpm or so and held it for a second. As soon as I let off the gas, the motor began running like crap again. Its hard to describe what it sounds like, but I know its not just one cylinder. It sounds like a 4 cylinder until you bring the revs up, then it sounds like the cylinders come back on line somewhat. It will continue running like this until you shut the car off, then it will be fine when restarting until the revs are brought up again. I repeated this process ten times with the same result.

At first, judging by hoe the engine was running and sounded, I thought it may be fuel related, but once I did the shut down and restart process, I kinda moved away from that. It seems like something electrical that resets itself or frees up when shut down, but craps out again after holding at a certain rpm after restart. I checked the vaccuum advance by plugging it and going through the running process, to no avail.

I am beginning to lean towards something in the distributor, but the only thing I can think of is the magnetic pickup, but usually when it goes, it just goes. Could be the coil, but seems like weird circumstances for it to fail.

If there is one thing that I would rule out, I would say mechanical failure within the engine, as something like that would be more constant IMHO.
Help me out if you can with opinions.
 
Weak charging system or bad battery can cause similar condition. Bad coil also. Listing the thigs it could be dont help much huh ? Good luck
 
RedFish said:
Weak charging system or bad battery can cause similar condition. Bad coil also. Listing the thigs it could be dont help much huh ? Good luck
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I dont think that its a weak charging system, as the voltmeter I installed a few weeks ago still shows 14 volts and the battery cranks fine.

Coil, very well could be, but I just need to make a correlation as to how it would act up after climbing to a certain rpm, then after shut down and restarted, run excellent and cleanly until I bring the revs up again.

Its a weird problem.
 
sounds like your carbs flooding, is it black smoking smell fuel ? pull A plug , just something to check
 
mr.318 said:
sounds like your carbs flooding, is it black smoking smell fuel ? pull A plug , just something to check
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I thought about that, but would it go back to normal when shut down, then run ok until revved to a certain rpm after startup? Unless its a float or something, I dont know. I did not see black smoke, as I have turn downs. I can smell unburned fuel from the exhaust though, as you would when a cylinder was misfiring. Its not totally unburned, but not fully nurned.

I cannot smell fuel while its running this way, so I am not sure.

The carb is an Edelbrock Performer 600 cfm manual choke.
 
the cyntrifugal advance working properly ? You can see it work with a timming light. Rotor button loose ? Distributer shaft loose in housing ? Water or cabon tracing in the cap ? Just stuff to unoverlook :)
 
ramcharger said:
Sticky advance plate in the distributor?
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I am thinking something like this, as the timing does sound way off lower in the rpms, until revs are brought up, then it sounds a little better, or more like an 8 cylinder motor, but still has some popping to it, or misfiring. The way the revs vain while holding rpms steady, say between 1,000-1,500 rpm seems weird to me, like timing. But as said, the fact that after shut off, it restarts easily and runs fine and idles fine until you bring the revs up and hold them for a few seconds. When you let off the gas, its running crappy again.
 
You are not going to able to guess which it is. If you have a spare good carb, I swap it. Best way to know if its the carb.

When I was reading your post, the first thing I thought of was that magnetic pickup in the dist. They can act up when hot. If its more than a couple years old, I would just change it. Cost $20 from even Napa.

No guarantees but they can go bad not just quit.
 
I may just go down to Carquest or Napa and pick up an entire distributor, since they are only another $30 over the pickup. I would usually spring for the quicker advance mopar unit, but since its a driver, I cannot justify the price. An orange box should create hotter spark anyways.

I am leaning towards the dist, mainly because of this weird intermittent behavior. If it was a fuel pump or filter, or even carb, I would expect it to continue consistantly without any breaks from shutting the motor down.
 
Well, I swapped distributors last night, reset timing and went for a drive.

All was well and the car ran and idled great...until.........

it warmed up some more and I took it out on the freeway(3,000 rpm@ 65 mph) for a few miles. As soon as I got off the freeway, it was acting up again. The idle was way down around 450-500 rpm, even misfiring amongst all cylinders. as if it had weak spark, but once rpms come up to about 1,100, the cylinders start firing better(all), but the engine still surges and is not a steady rpm.

It really seems like weak spark to me, but its just weird how it resets itself when you shut down the motor, then restart, although it is progressively becoming more constant when the motor is warm.

I would imagine there could be a power drain somewhere between the battery and the ignition components, but only when the motor is warm and is more aggitated after turning higher rpms? I kind of doubt it. My only other guess is that it could be the coil, even though it is not overly hot to the touch. It looks like the factory original piece.

Any other suggestions now that the distributor has been eliminated?
 
When it starts doing it again, take a jumper wire from the + of the battery to the coil feed side of the resistor. That will jump the resistor out of the system. If it cleans up you found your problem. If that doesn't work, go straight to the + of the coil. Do this only for a momentary test, otherwise, you'll overheat the coil. It will tell you if it's the resistor or wiring to the coil.

I'd put a timing light on it to see if it's jumping spark.

Could be getting a heat induced vacuum leak.

J, try to diagnose the problem first before tossing $$$ at the potential problem, your wallet will thank you. :-D
 
Sounds like the timing chain to me. Any idea how many miles are on the engine? Small blocks wear out the chain around 60K miles or less. The firing problem is caused by the slack in the chain moving around and screwing things up. When you start cold the chain is tight and when the engine warms up it is getting loose enough to throw the timing off. Turn it off, let it cool and the timing is back until it warms up. One of these days you will drive it home, park it, and it won't start the next morning. Had a '74 Charger that did this to me and it drove me nuts until I found an old Mopar mechanic. He changed the chain and got it running but it still wasn't quite right. Took a week off work and tore it down. Found 16 bent pushrods and 12 bent valves. Hook a timing light to it and have a look. If the timing mark is moving any at all you have a bad chain.
 
crackedback said:
When it starts doing it again, take a jumper wire from the + of the battery to the coil feed side of the resistor. That will jump the resistor out of the system. If it cleans up you found your problem. If that doesn't work, go straight to the + of the coil. Do this only for a momentary test, otherwise, you'll overheat the coil. It will tell you if it's the resistor or wiring to the coil.

I'd put a timing light on it to see if it's jumping spark.

Could be getting a heat induced vacuum leak.

J, try to diagnose the problem first before tossing $$$ at the potential problem, your wallet will thank you. :-D
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I will give this a shot, Rob. I wouldnt think vaccuum leak, though, because of the break in the poor running after you shut it down. I usually restart it about 1-2 minutes after shutting it down, and its fine, even idling for 15 minutes, until I bring the rpms up to 2,000 or and hold for a second, then bring them down. At that point, its running bad again. Doesnt seem like a miss, as whatever it is, it is affecting all cylinders.
 
340mopar said:
Sounds like it is lean to me.



Chuck
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I thought about this too, but it is heat related and is tricky on how it stops for a time after shut off and restart. But how so? Carb (Edelbrock #1405 about a year old) malfunctioning, or the fuel pump weakening when warm?
 
grumpuscreature said:
Sounds like the timing chain to me. Any idea how many miles are on the engine? Small blocks wear out the chain around 60K miles or less. The firing problem is caused by the slack in the chain moving around and screwing things up. When you start cold the chain is tight and when the engine warms up it is getting loose enough to throw the timing off. Turn it off, let it cool and the timing is back until it warms up. One of these days you will drive it home, park it, and it won't start the next morning. Had a '74 Charger that did this to me and it drove me nuts until I found an old Mopar mechanic. He changed the chain and got it running but it still wasn't quite right. Took a week off work and tore it down. Found 16 bent pushrods and 12 bent valves. Hook a timing light to it and have a look. If the timing mark is moving any at all you have a bad chain.
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I originally thought of this,but the shut down to restart time is only about 1-2 minutes, then its fine for a few moments until I bring rpms up and drop down, plus I do not hear any backfiring through the intake. There is no real misfiring,such as a dead cylinder/s, just sounds as if its starving for fuel (sounds like a 4 cyl is my best description, until rpms come up, then its very ragged; it sounds better at idle, but still ragged and there is popping through the exhaust)

The motor supposedly has 30,000 miles since the last rebuild.
 
i don't think it's the chain they'll give you a miss but not a come and go miss my opion, i always take the cap off turn motor by hand see how much play you have before the rotor moves, did you pull a plug? if so what's it look like? to see if it's firing on all cyls. i use the timinglite put it on one at atime if it flashes it's firing, and it could be a resitior they'll do weird stuff,
 
mr.318 said:
i don't think it's the chain they'll give you a miss but not a come and go miss my opion, i always take the cap off turn motor by hand see how much play you have before the rotor moves, did you pull a plug? if so what's it look like? to see if it's firing on all cyls. i use the timinglite put it on one at atime if it flashes it's firing, and it could be a resitior they'll do weird stuff,
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I now starting to wonder about the resistor. I have always thought that when they fail, they fail completely and will not restart. I could be wrong. This is cheap enough to give a shot, and I may have an extra on hand. It doesnt seem like wiring would just quit all of the sudden one day while driving down the freeway, so it may very well be this.
 
Carb?, Low gas with fuel guage sticking so it looks full? (I actually recently went through that). It sounds like fuel problems to me. Let it run and then cup your hand under the exhaust pipe for a second or two and then smell it. if it smells like fresh gas it's definitely to rich(possibly firing wrong), if it smells sorta like dirtbike fuel (wierd smell/ more air than gas) it's running too lean and that's where the popping is coming from(fuel pump is bad/really low fuel and guage is stuck). Maybe you got some bad gas? I hope you figure it out. I just drove mine up to the gas station, it was popping like crazy and running very rough with white smoke coming out of it, I thought I had blown a head gasket or something, I filled it up and then it ran perfect again. It really does sound like fuel. Trial and error is all I can say. Good Luck!

P.S. maybe somebody swapped a Chevy engine into it in the middle of the night, they sound a lot like what your describing.
 
DartSportGreg said:
Carb?, Low gas with fuel guage sticking so it looks full? (I actually recently went through that). It sounds like fuel problems to me. Let it run and then cup your hand under the exhaust pipe for a second or two and then smell it. if it smells like fresh gas it's definitely to rich(possibly firing wrong), if it smells sorta like dirtbike fuel (wierd smell/ more air than gas) it's running too lean and that's where the popping is coming from(fuel pump is bad/really low fuel and guage is stuck). Maybe you got some bad gas? I hope you figure it out. I just drove mine up to the gas station, it was popping like crazy and running very rough with white smoke coming out of it, I thought I had blown a head gasket or something, I filled it up and then it ran perfect again. It really does sound like fuel. Trial and error is all I can say. Good Luck!

P.S. maybe somebody swapped a Chevy engine into it in the middle of the night, they sound a lot like what your describing.
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I would think if it was rich, or flooding, it would be widely present sooner and at all times and I would be able to smell furl from under the hood. The lean condition would seem more consistant as well, with the fuel pump being the only culprit in my mind.
 
does it hesitateor stumble when you open the trottle rapidy ?
I've seen venturi bleed tubes just up a drop out of the casting (where they should be staked in place). About to give up here. Sorry
 
RedFish said:
does it hesitateor stumble when you open the trottle rapidy ?
I've seen venturi bleed tubes just up a drop out of the casting (where they should be staked in place). About to give up here. Sorry
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LOL...I hear you, sir...

Troubleshooting intermittent weird problems like this are hair puller sometimes. Seems to always happen to my mopars, until I fix it that is.:toothy10:
 
Troubleshooting over the internet is incredibly difficult. Kinda like trying to fix someones car over the phone. Having been a mechanic I relied on all my senses to diagnose a problem, that's something thats lacking on a internet forum.

That being said, I hope you find the problem and let us know when you get it fixed.
 
Well, I had some extra parts to try so I swapped the coil and ballast resistor.

No dice......

I didnt drive it on the freeway tonight, so it took longer to begin acting up. It gradually started, but with the same end result. I tried jumping on the gas hard at a couple of stop signs. No stumbles, it gets up in rpm quickly. If you just go easy on the gas, it hesitates, sounds weak, like it would if low on fuel, then gradually gets stronger. There is plenty of popping in the exhaust though and I did notice the exhaust smells different, definately not rich. No intake backfires. If it is fuel pump or carb related, its weird enough to run fine mometarally after a quick shut down and restart, but acts up again as soon as you gets he revs up high enough again for a few moments.

Wires look ok, with no visual cracking or damage. If anything, the alternator and voltage regulator are pumping out too many volts under throttle, up to 15 volts, 14 at idle, 10-12 in drive with all accessories on.

I guess my next process is a fuel pressure gauge north of the pump to see what readings I get. Should I be looking for lower than 6 psi?

Again, thanks for all advice. I post a lot of these questions, as not only is it building my wide mopar knowledge, but others can learn from it too.:-D
 
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