Rough Idle and Jerking at Constant Speeds - Chevelle Tech
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post #1 of 33 (permalink) Old Jul 25th, 17, 4:33 PM Thread Starter
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Sean
 
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Rough Idle and Jerking at Constant Speeds

Hey Everybody,

I posted a similar thread like this in the wrong area I believe. But this time, I have more information and the right thread.

So, to the story! I did an engine swap in my 67 El Camino about 2-3 weeks ago. When I was driving it one time while the engine was still under 100 miles, I felt a light lunge when I was slowing to a stop and the car started to run weird. The idle became quite horrible and shaking the entire car quite badly, and the car would slightly jerk when at constant speeds. Now at first, it seemed like it only did this when in gear. So like in Neutral or Park it would idle normally and not feel like the hood was going to shake off. But more driving would have it happen only once the car fully warmed up and has been driven a good couple miles, and also is now starting to idle rough in Neutral or Park. NOT as bad as reverse and drive mind you, but still quite bad. Transmission is a 2-speed powerglide column shifter, with the GM Crate 250/350 as the new motor.

But I've been doing some troubleshooting and tweaking things here and there and testing out the symptoms. I found a small oil leak from the driver side valve cover that was causing small puddles of oil on the ground. So I fixed that with a new gasket. Problem still persists(I knew it wasn't going to fix it, but it had to be done anyway) I can't find anything disconnected like vacuum hoses, transmission is topped off/a little over full, and the timing is perfect at cold starts and when it's fully warmed up.

My father is convinced that it's the transmission/torque converter going up. Which makes sense, except for the fact that it would be running like crap during cold starts to, right? Or he says a vacuum line is coming loose. But I can't find one anywhere.

Any help is much appreciated everybody!

MORE INFORMATION if needed/wanted: The car runs great at cold starts. It starts right up, runs well, idles well, and the transmission shifts just fine. It's just after the engine has been really warmed up. Like, 5-10 miles warmed up and then it starts running real weird. I will say that it seems to happen faster with a little jerkier peddle usage and on hotter/humid ass days. There is no Transmission fluid leaking from what I can tell. There was some right after the engine swap, but it hasn't since then. Just small oil puddles from the ****ty gasket as mentioned before. But that problem is gone as far as I can tell.

67 El Camino (2sp Powerglide, new 350 crate engine)
13 Malibu(SO COOL! Naw, it's my daily....)
02 Avalanche(Family Truck, passed down to me as my first vehicle. Gone, but not forgotten)
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post #2 of 33 (permalink) Old Jul 25th, 17, 5:37 PM
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Re: Rough Idle and Jerking at Constant Speeds

Sounds like my recent issue: ran like crap, when warmed up, misfire, backfire. Changed the coil and seems that was the problem

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post #3 of 33 (permalink) Old Jul 25th, 17, 11:47 PM
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Re: Rough Idle and Jerking at Constant Speeds

Good call on the coil. good thing to check.

Maybe you picked up some debris in the fuel line.

I was on Vacation with MY Ford New build 300 miles on it and I took off from Oklahoma with the family to do a loop to the west coast and back.
I was in the desert and hardly any people on the road..There weres 3 cars all following each other I was #3.
I pulled over because of what I thought was vapor lock issue with the Motorcraft 2 barrel.

The guy in front of me turned around to see if I was ok.
I said I was fine.

Well the carb was full of fuel..HHHMMM
Sometimes it would choke and puke and just slowly die like it ran out of fuel.
Then it would perk back up.

That started happening every 10 minutes.
I pulled the carb right there in the desert. laid out paper towels in front of the Van and took the carb apart.

It all looked perfect..nothing in the boosters.. nothing in the jets.
Needle and float look perfect.. Inlet did not have a filter as I had inline filters elsewhere.

Last thing I pulled was the seat that the needle sits in.

There was a sliver of rubber up inside the seat body that must have came off the fuel line when it got pushed onto the steel tube.
That piece of rubber would flip sideways and block the flow of fuel.. Then all of the sudden it would play nice and allow fuel to flow.

Leave no stone un-turned. Or something like that.
Keep us posted on the results Sean.
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post #4 of 33 (permalink) Old Jul 26th, 17, 12:00 AM
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Re: Rough Idle and Jerking at Constant Speeds

Sure you don't have a vacuum leak causing your idle problem and part throttle surge?

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post #5 of 33 (permalink) Old Jul 26th, 17, 12:37 PM Thread Starter
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Re: Rough Idle and Jerking at Constant Speeds

Quote:
Originally Posted by zombie1969 View Post
Sure you don't have a vacuum leak causing your idle problem and part throttle surge?
Not from what I can tell. I'll list the lines that I can see. It could be it's not enough vacuum pressure? How would I check for pressure?

Here is the list of lines I have checked and I know what they're for:
  • Brake booster line that connects to the intake is good.
  • Line from the intake to the firewall seems good(I lose track though after it goes down)
  • The carb&distributor lines seem all fine and connected.

67 El Camino (2sp Powerglide, new 350 crate engine)
13 Malibu(SO COOL! Naw, it's my daily....)
02 Avalanche(Family Truck, passed down to me as my first vehicle. Gone, but not forgotten)
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post #6 of 33 (permalink) Old Jul 26th, 17, 12:40 PM Thread Starter
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Re: Rough Idle and Jerking at Constant Speeds

Quote:
Originally Posted by George Halpin View Post
Sounds like my recent issue: ran like crap, when warmed up, misfire, backfire. Changed the coil and seems that was the problem
Points or HEI? I have an HEI distributor. Could it be that the distributor isn't getting a full 12volts? I've heard that might be an issue.

67 El Camino (2sp Powerglide, new 350 crate engine)
13 Malibu(SO COOL! Naw, it's my daily....)
02 Avalanche(Family Truck, passed down to me as my first vehicle. Gone, but not forgotten)
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post #7 of 33 (permalink) Old Jul 26th, 17, 1:04 PM
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Re: Rough Idle and Jerking at Constant Speeds

The module in the HEI sometimes flake out when hot. Get a new one and report back.
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post #8 of 33 (permalink) Old Jul 26th, 17, 1:26 PM
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Re: Rough Idle and Jerking at Constant Speeds

How about a power valve or ? At a steady cruise and idle perhaps its "cycling" the PV, etc, as its transitioning from the idle circuit to the power/cruise...

Just something else to consider...


Pull the vacuum hose that goes to the transmission, and see it that helps. I have seen vacuum modulators of the transmission leak, causing a slight vacuum leak... The trans will shift harshly when vacuum is disconnected, but otherwise the car should run fine (make sure to cap off the vacuum port on the carb when doing this).

Once you go RAT, you never go back...
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post #9 of 33 (permalink) Old Jul 26th, 17, 3:05 PM
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Re: Rough Idle and Jerking at Constant Speeds

Quote:
Originally Posted by GMSean View Post
Points or HEI? I have an HEI distributor. Could it be that the distributor isn't getting a full 12volts? I've heard that might be an issue.
Yes ,full 12v needed at crank and run.
Check the voltage at the coil terminal with the key on -should be 12v.
If you didn't rewire for 12v ,that's likely some of the issue.
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post #10 of 33 (permalink) Old Jul 29th, 17, 1:38 PM
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Re: Rough Idle and Jerking at Constant Speeds

You said " The idle became quite horrible and shaking the entire car quite badly, and the car would slightly jerk when at constant speeds. "

Sounds like it may be misfiring.What do the spark plugs look like ? One thing that can contribute to spark crossfiring and misfiring is old plug wires and/or the wires not being clicked onto the plugs when installing them.Any cracks in the porceline of the plugs ? Also check the distributor cap for cracks or carbon tracking.
One way to tell how healthy an engine is running is to find a nearby road with a little bit of a hill.Any weakness an engine has is multiplied by the hill and if it's nearby to you its a useful place to check a car out without having to break the law doing it.

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post #11 of 33 (permalink) Old Jul 30th, 17, 6:26 AM
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Re: Rough Idle and Jerking at Constant Speeds

When checking for 12v with HEI, the engine needs to be running. Just turning the key to 'ign' & testing for 12v can give a false reading.
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post #12 of 33 (permalink) Old Aug 7th, 17, 9:37 AM Thread Starter
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Sean
 
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Re: Rough Idle and Jerking at Constant Speeds

Hey Everybody. So I took your advice and ordered a new coil awhile ago. It arrived on Friday so I installed it over the weekend. It seems to have fixed the issue of the shaking and constant speeds. And I took it for almost 100 miles yesterday just driving around to really just make sure that problem was gone. The engine still has a low idle when in gear but that is just due to the crappy Powerglide that's in it currently. I'll be switching to a TH350 soon enough and hopefully with it being new the engine won't have so much load on it.

Thank you guys very much for the help. My car is now drive-able and I'm extremely happy about it.
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67 El Camino (2sp Powerglide, new 350 crate engine)
13 Malibu(SO COOL! Naw, it's my daily....)
02 Avalanche(Family Truck, passed down to me as my first vehicle. Gone, but not forgotten)
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post #13 of 33 (permalink) Old Aug 7th, 17, 1:33 PM
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Re: Rough Idle and Jerking at Constant Speeds

Quote:
Originally Posted by GMSean View Post
The engine still has a low idle when in gear but that is just due to the crappy Powerglide that's in it currently.
Adjust the idle setting on your carb so that the idle speed isn't too low when it is in gear. There are loads of PG cars out there that idle just fine. I think it is more of a carb adjustment issue than a transmission issue.

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post #14 of 33 (permalink) Old Aug 28th, 17, 6:34 PM Thread Starter
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Re: Rough Idle and Jerking at Constant Speeds

Reviving dead thread. So, replacing the coil did not work in the end. The coil I had was going bad, so it wasn't a bad idea. But the Transmission I believe now has bit the dust. Well, atleast the torque converter has. The car runs like crap when in gear, and now, even outside of gear it idles a little rough(not happy since it doesn't even have 400 miles on it). The TQ is so bad that it kills the car immediately. The other day when I had it out, it was doing the jerking thing, but then it got real bad and died on me on the road. I pulled over, it started, then died immediately. I turned the idle screw up real high, just so that the engine would run, but barely. So then I had to crawl it home by putting it neutral when coming to a stop and keeping my foot on the gas peddle. Because even in neutral, the car was idling real low and felt like it was going to die. So, not good I would say.... I started it up the other day, it runs again, when cold... so when the idle is nice and high, as soon as it's been runing a couple minutes though. The idle slows down some and the engine jerks every-once and awhile, like it's missing randomly or something..... But I think that transmission/TQ might have messed up my new engine..... I don't really have the money for a new Transmission right now and I can't find anything used that's cheap. Plus, all that doesn't matter if the engine is messed up.

67 El Camino (2sp Powerglide, new 350 crate engine)
13 Malibu(SO COOL! Naw, it's my daily....)
02 Avalanche(Family Truck, passed down to me as my first vehicle. Gone, but not forgotten)
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post #15 of 33 (permalink) Old Aug 28th, 17, 10:01 PM
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Re: Rough Idle and Jerking at Constant Speeds

First thing I'd check is the fuel delivery. If it is a Holley, pull the needle valve assemblies out one at a time and flush them both directions with brake cleaner. Use a filter in-line between the pump and carb. Spark plug wires would be next - if they have much age - replace them.
Good Luck.

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