Ornery 318 - refuses to idle smoothly.....any insights?!?!

[dartswinger]

Hello there hot rod PhD's....

I've got a persistent surging idle in my 1972 Dodge Coronet Wagon.....which is threatening to drive me crazy....

I would characterize this as a lumpy, erratic idle that sounds as if it is about to stumble and die, if the RPM drops below 1000.  i've checked for vacuum leaks, plugged off all the vacuum ports on the carburator and still there is no improvement.....really curious what I might be missing here!

The car has about 64,000 original miles on it, a California Car from Ventura County with no rust and lots of time spent nidden in garages away from mother nature.

I have been patiently attempting to work my way through this for a while now. The lower half of the engine has not been rebuilt, but I have checked the compression. To date, I have done the following:

1) Checked compression in cylinders - checks out OK
2) Pulled heads and had valve job done
3) Replaced points ignition with chrysler electronic ignition
4) New Plugs, Wires
5) Replaced 2 barrel stock intake with Edelbrock Performer and topped off with new Edelbrock 1406 Carburator (600 cfm)
6) New Fuel Pump and Filter
7) New Alternator and Battery
8 ) Adjusted idle screws on carburator

Timing is advanced just a hair over TDC (which is stock setting)

Before I replaced the 2-barrel with the new 4-barrel, the engine ran exactly the same.....erratic and lots of misses.

Sincerely hoping that someone has a bit of encouraging advice!  Very much looking forward to someday driving this car without blipping the gas at stoplights to keep it running.....

Thanks so much guys!!!!!
-Mike T.
motorcity_69@yahoo.com

[Bruce Dorsi]

Welcome to the RRT!


You didn't state if the problem was present before all the items you mentioned were done.

Here's a few random thoughts:

1 - Weak or broken valve spring.

2 - Vacuum leak at under-side of manifold.

3 - Ignition misfire at low rpm.

4 - Worn cam lobe.

5 - Bent valve.

6 - PCV Valve.

7 - Worn timing chain.

8 - Vacuum leak at brake booster.

9 - Incorrect firing order.


I'm not a MOPAR guy, and I'm not familiar with the 318's, so my suggestions are generic.

[dartswinger]

Thanks for your thoughts - really do appreciate it!
And thanks for the welcome to RRT --- !

I used to post problems here once in a while under the "dartswinger" name a couple of years back, I think before you guys re-organized this website. I lived in Detroit and had multiple challenges with a 1971 Dodge Polara with a 383. You guys helped out every time!!!!! thanks so much. I also visited the famous vintage junkyard in Paw-Paw, Michigan that Fat Cat and crew had been raving about. Glad to have found you again, and to see you thriving. Without a doubt one of the most enthusiastic and positive gearhead support groups on the net! So happy you guys are here. So thank you. I remember a lot of help from Wayne, from Crosley, and Enjenjo back then.

The problem was there before I began all of the attempts to correct it....

When I first bought this Coronet Wagon, which was about 3,000 miles back, it ran smoothly. But over about 1,500 miles or so, it began to run rougher and rougher....so I assumed that the valves were burning, seeing as the car wasn't equipped with hardened exhaust seats (pre-1973 for Mopars, I believe).  And dumping ordinary unleaded into the gas tank was taking its toll, I figured.

So dutifully I yanked the heads and had those looked at - and the machine shop up in Santa Ana said they were the cleanest, most intact set of heads that they had looked at in a long time. Did not require much at all to finish up the job, they assured me that if something was making the engine run rough, it likely wasn't the valves in these heads.

So I have been chasing it since!

For the heck of it, I test-fit an MSD ignition in the car last night, to see if a hotter spark would correct a perhaps too-rich idle condition, but this had zero effect on the idle. (For that matter, the engine is starting to shudder at higher rpm's also.) So the intensity of the ignition spark is less and less suspect.

You mentioned a worn cam lobe.....yikes.  Curious how durable a cam made in 1972 would be.....the car does have 64,000 miles on it....hmmm, soft 1970 metal? It is also the original timing chain, which may be worn I suppose too, but so many people have gotten hundreds of thousands of miles out of 318 engines (bread and butter v-8 for mopars!) that I am startled to think of this possibility. I don't rule anything out though, thats for sure!

Still very puzzling.

thanks for your thoughts!!!!  I do hope someone has a revelation (myself included! lol)

thanks again -

Mike T.

[enjenjo]

Since the car sat for so long, valve springs are a very real possibility, they can look fine, but unless you check them you don't know. For the same reason, setting, the cam is a possibility too, usually shows up as a rough idle, and as it gets worse, poping out the exhaust, or back through the intake. The problem was worse with GM cars, but all of them had problems with cams in the late 60's early 70's.

bu the way, welcome back

[SKR8PN]

The timing chain is easy enough to check,just take the distributor cap off and rotate the crankshaft back and forth. While turning the crank one way,stop,then watch the rotor while you reverse the crank rotation. How far does the crank turn BEFORE the rotor begins to move? The farther you can turn the crank before you see movement in the rotor, tells you how much slack,or wear,there is in the chain and sprockets. I would be very suspect of the camshaft, springs,AND the timing chain...............

[DrJ]

The timing chain is easy enough to check,just take the distributor cap off and rotate the crankshaft back and forth. While turning the crank one way,stop,then watch the rotor while you reverse the crank rotation. How far does the crank turn BEFORE the rotor begins to move? The farther you can turn the crank before you see movement in the rotor, tells you how much slack,or wear,there is in the chain and sprockets. I would be very suspect of the camshaft, springs,AND the timing chain...............

After checking that, if it's not it I'd suspect your 1000 rpm idle.
It's not supposed to be that high because it's no longer using the idle circuit at that rpm.
It's in the "midrange" circuit and will typically surge the way you described.

If you have monkeyed wit the idle screws, reset them to where they were originally, or around 2-1/2 turns and back off on the idle speed to around 600-700 rpm and then adjust the idle mixture screws for your idle.
You said you plugged all the vacuum ports...If that distributor has a vacuum advance, hook it back up.

[Jokester]

This is a puzzle!  

At first I thought vacuum advance, or low fuel pressure, or major vacuum leak, but you've replaced the distributor, carb, and fuel pump with no improvement.  I agree that the 1000 rpm is too high an idle and the carb is no longer using the idle circuit.  I'd check for vac leaks by setting the rpm as low as you can without surging, then spray the carb, intake, etc with water or ether or WD40, practically anything.  If rpm changes at all you have found a leak.

But since you have already changed so much stuff, vac leak sounds unlikely.  I suspect it's in the valve train.  It's about the only thing thats not been touched.

Good luck.

.bjb

[ram-rod]

try replacing your coil they can be a pain people think they never give trouble. but i have seen cracked insulator towers  tracking on carbon  crook intrenal windings  coils dont like to get hot or being vibrated

an old electronic dizzy of mine had a worn top bush and
was increasing the gap to jump on only one cylinder the rest were fine
 
also a freind just replaced the  fire wall moduel on his car
with a bosh one  of a local product GMH commodore
recons its the best thing hes ever done

[Land Yacht]

Does it have any emission equipment, like egr etc?

[dartswinger]

Thanks for the continued advice....

The car is a 1972 california emissions vehicle, which meant that it came with a charcoal cannister. Apart from that, I don't see any EGR equipment of any kind, and with the newer intake manifold that I installed (the edelbrock performer 318/360) there is no provision for that either. To rule out any further vacuum leaks,  I unplugged the vacuum lines that had run to the charcoal cannister assembly also. (but the car ran rough before I did that - it doesn't seem to make any difference.)

I have set the idle down into the 750 rpm range also - it doesn't seem to make any difference. The car starts to stumble and surge in that range.

Ironically, my 1970 Dodge Dart, which just had its slant six 225 motor rebuilt, is acting EXACTLY the same! It is if the two cars caught something contagious between the two of them. Now both of them stumble and surge at idle. Putting the Dart into gear makes it just about die. And with that car I have rebuilt the simple 1-barrel carburator, installed a new alternator, battery, electronic distributor, an MSD ignition, and all new plugs/wires....timed it to just 5 degrees advance....it breaks my heart! Runs so badly. Incredible. And the slant sixes are usually bulletproof, smooth runners, even under the worst circumstances. Crazy.

I have tinkered with and tuned-up mopars for over 12 years now, and never had issues like this....so puzzling!

I admit it is possible that both cars have some amazing vacuum leaks somewhere....but they are eluding me. Have checked what I think to be the obvious places so far.

Soon to be taken to a more knowledgeable mechanic than myself, I fear!

I will keep you posted as this pair of mysteries unravels.

Thanks again guys.....I know this is a difficult one.

-Mike T. scratching head and pondering his pretty cars that don't run so pretty