# '79 Jeep CJ7 rough idle



## 47_47

Check for correct PCV, restricted, collapsed PCV lines.. Also check for a partially stuck open EGR (exhaust gas recirculation). These both will cause idle problems.

How many miles on the engine? Check the timing chain. Remove the distributor cap. Turn the crank shaft, by hand in the direction or engine rotation. The turn the crank against rotation, observing the rotor. Notice how far you had to turn the crank. It shouldn't be more than 5-10°.

If ok, do a compression test, then a cylinder leakage test.


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## Jeeper1970

Thanks. PCV is fine, EGR was plugged off when I got it, and didn't hook it back up when I replaced the carb. Can't find any vacuum leaks, either.

Unsure about mileage, previous owner didn't have it very long himself, and wasn't sure about mileage. Odometer reads 50,xxx but could be 150,xxx.

I had thought about the timing chain.


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## gotboost

Maybe take a look around the intake if you haven't already,pretty common with 4.2 and early 4.0's for the intake bolts to back out,
A can of carb cleaner for finding vaccum leaks,a compression and vaccum gauge will be a big help
Those engines also tend to carbon/sludge up so even if she fails a cyl leak down test of couse after the above had been done worth a try to run some combustion camber cleaner though her,not in the fuel tank stuff but poured though the carb,heck the more I think about it I just de-carb it now,then see where you are,Good luck and keep us posted....


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## 47_47

gotboost said:


> Maybe take a look around the intake if you haven't already,pretty common with 4.2 and early 4.0's for the intake bolts to back out,
> A can of carb cleaner for finding vaccum leaks,a compression and vaccum gauge will be a big help
> Those engines also tend to carbon/sludge up so even if she fails a cyl leak down test of couse after the above had been done worth a try to run some combustion camber cleaner though her,not in the fuel tank stuff but poured though the carb,heck the more I think about it I just de-carb it now,then see where you are,Good luck and keep us posted....


Good advice here, especially about using the carb cleaner and a vacuum gauge to find an intake leak.

Even though the EGR was plugged off, I'd still make sure it's not stuck open on some carbon or if the previous owner removed the valve, he may have damaged the gasket allowing a cross leak.

How were the old spark plugs? Did their condition tell you anything?

I reread your first post, have you checked the EVAP? Disconnect and plug your EVAP port where it enters the engine. You could also have excessive fuel stored in the canister giving you these gases.

Your on the right track, by starting with the basics. Good luck.


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## sssalas

*Poss Recalls*

Back in 1999 there may have been a recall on the Catalytic Converter, and likely an 02 sensor recall in 1994. Id write down the VIN number and call your Jeep dealer 'Service Manager' to find out if there were any recalls on your vehicle, just to rule out those drivability problems.


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## DIYtestdummy

Auto trans + hard to star cold = crank position sensor and/or throttle position sensor. How are the fuel pump and fuel filter?

Go here - lots of really smart people: http://www.jeepsunlimited.com/forums/index.php

:thumbup:

I've been in and out of the 4.0l and 4.2l engines and what I learned ws no match for all the info that this website has.


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## slickshift

A loose timing chain or timing out of spec would do it
It could be off 180*
As could mis-routed plug wires...
In addition to missing, cracked, clogged, or incorrectly routed vac lines and emissions lines

Any exhaust gasses shouldn't be in there till after warm-up

IIRC this vintage is still the AMC Jeeps
Not only can they have Chrysler carbs and Ford starters, the parts listed (at parts stores) can be downright wrong and specs off by a mile
The new-fangled (at the time) truck emissions laws could mean running changes on the assembly lines, never mind the AMC GiterDone w/WhatYaGot line practices of the times

I had to purchase two new oil pumps, and use parts from each to make one pump that fit/worked on my slightly earlier Jeep
I also ended up making most of the gaskets I needed

Lots of fun working on it, but most certainly because it wasn't my daily driver
lol


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## slickshift

...forgot about the choke...
Non-functioning or incorrectly adjusted

The new carb, if it had the mech, may still not be the right adjustment
And if you had to transfer it over, it may have been inoperative or out of wack from the get go


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## Jeeper1970

No cat exists on the Jeep, nor any of the sensors mentioned above. Those sensors didn't exist in '79. Fuel pump's putting plenty of fuel to the carb, and I replaced the fuel filter when I swapped carbs.

Timing's not off 180 degrees, I've been driving it, I did check the routing of the plug wires, too.

DIYtestdummy - I've been to JeepsUnlimited.com a couple times, but I do hang out at a couple other Jeep sites, I like 4wd.com.

Slickshift - Not only Chrysler and Ford parts, my factory tranny is a good ol' Turbo 400. Can't complain about that!

Me thinks the timing chain is way too sloppy. I'm gonna replace the PCV valve, too, that's simple and cheap. Thanks for the replies.


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## DIYtestdummy

AH - 1979, duh! Where's my head at? Hopefully it is a pesky PCV, but with the age a new timing chain may be the cure. Might wanna seafoam the junk out of it too - smooths out my idle with almost 200K.

4wd.com now owns JU since all the idiocracy happened.


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## Jeeper1970

Yeah, I've heard a lot of good stuff about the sea foam, haven't tried it yet, but that is on the list.

Also heard you can replace one quart of ATF during oil change, but run it no more than 1000 miles and change oil again. Apparently an old mechanics trick, it breaks down clumps of oil that might be blocking passages, supposed to clean everything out good, but still provides enough lubrication so it doesn't cause more harm than good.


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## chickentheef

*rough idle jeep*

check cam,lifters & push rods:thumbsup:


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## MgMopar

I just read threw the posts. A compression gauge was mentioned. Did you do a basic compression check?? It might lead you into the right direction. Easy enoght to get at the plugs on one of these! Go ahead and do a compression check on all cylinders and post the results. We can get a much better idea about the condition of the motor and what might be contributing to your problem.


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## Jperson80

Jeeper1970 said:


> I got a 1979 Jeep CJ7 with the 258 cu. in. inline 6 cylinder engine and automatic tranny and have only put around 50 miles on it. When I got it, it had a rough idle, no problem I thought, tune up, maybe a new carb and I'd be good to go. It's never that simple, is it? 🇳🇴
> 
> I replaced the carb and fuel lines, new plugs (gapped properly), new wires, dist cap and rotor, reset timing. I also checked electrical output of the ignition with the key turned to the running position, but with the engine off, and it was within specs. Got it idling a lot smoother, but still not where I want it. It's also still hard to get started, getting plenty of gas, no problem there. Once it starts and is somewhat warmed up, it's easier to start.
> 
> I was trying to start it today, and when it wouldn't, I pulled the air cleaner off to see the assembly and the carb full of exhaust gases. This is the first time I noticed it.
> 
> Possibly a cracked head between an intake and exhaust valve?



Did you ever figure out what was causing the rough idle? I have a 90 YJ that has a rebuilt motor from an 82 CJ7. 4.2L S6 and I'm having similar issues. The thing was purring like a kitten and all of the sudden I couldn't start it one morning. I ended up taking all the usual suspects apart and put it everything back together. It starts just fine but now my idle is jumping and it feels like it's missing a cylinder every so often. Any ideas would be much appreciated.


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## Bob Sanders

Jperson80 said:


> Did you ever figure out what was causing the rough idle? I have a 90 YJ that has a rebuilt motor from an 82 CJ7. 4.2L S6 and I'm having similar issues. The thing was purring like a kitten and all of the sudden I couldn't start it one morning. I ended up taking all the usual suspects apart and put it everything back together. It starts just fine but now my idle is jumping and it feels like it's missing a cylinder every so often. Any ideas would be much appreciated.


I don't know the yj at all. I have a jk which is a world different, but an intermittent miss is usually a fuel or ignition issue. Weak spark plug wires, or intermittent fuel injector (if the yj even has them). If it got better when you took it apart and put it back together though... I would look more at the ignition side of it all.


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## Jperson80

So yesterday, I replaced the distributor cap, rotor, plugs and wires. It starts just fine but still running rough (misfiring). I metered voltage with key on across coil and had 8VDC. After reading a bunch last night I went ahead and ordered a new ignition coil for $10 on Amazon. The symptoms seem to fit improper voltage coming from coil. Most likely it's something before the coil not supplying 12VDC but it's worth a shot. I figure the no start prior to this issue could have been a result of the coil failing anyhow. Let me know if anybody has any ideas and I will get that new coil in after work today.


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## raylo32

Not likely the coil. They generally either work... or not. but it's only $10.



Jperson80 said:


> So yesterday, I replaced the distributor cap, rotor, plugs and wires. It starts just fine but still running rough (misfiring). I metered voltage with key on across coil and had 8VDC. After reading a bunch last night I went ahead and ordered a new ignition coil for $10 on Amazon. The symptoms seem to fit improper voltage coming from coil. Most likely it's something before the coil not supplying 12VDC but it's worth a shot. I figure the no start prior to this issue could have been a result of the coil failing anyhow. Let me know if anybody has any ideas and I will get that new coil in after work today.


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