Technical The fantastic mystery of the great engine gremlin(318 Magnum "misfire")

Hey all.

Got a weird issue with a fresh 318 for my 55 plymouth that defies all explanation. An engine "miss"...

The issue: Engine idles fine, revs fine in park, but it runs rough, REAL rough at any RPM below ~1800-1900rpm going down the road. Above 2,000RPM the engine will sort of smooth out, but the sounds of pops and burbles coming from the exhaust indicate that the problem is still there, just masked by the RPM. It feels like total ignition failure(on ALL cylinders) for a split second(the length of time varies with RPM, but it'll give that classic, interrupted 'RUP--RUP--RUP--RUP' sound taking off from idle. A misfire). Then out of nowhere, VERY rarely, for approximately 5 to 15 seconds at a time, the 'miss' will go away, the engine will run perfectly fine, it'll pull strong, and then it'll go back to the same shenanigans as before. I haven't been able to discern any pattern or cause for this.

Things I've done to troubleshoot:
-Checked for vacuum leaks at every possible point with a can of starting fluid: found none(I also plugged certain accessories like the brake booster)
-Swapped the carb for two other known good ones(two quadrajets and an edelbrock 1406): no change
-Swapped the distributor from HEI to a brand new points unit: no change
-Fiddled with every possible base timing: no change
-ran a hotwire directly from battery to the distributor(thinking it was bad/intermittent wiring): no change
-Checked, rechecked, and reset valve lash/preload(it's a hydraulic roller cam) to zero: no change
-Listened VERY carefully for any lifter clatter with all my ears: found none
I also checked the plugs, they're black, but not oily or sooted particularly bad. They're also all uniform to each other. Motor itself has less than 500 miles total on it.

Engine Specs:
Stock 318 magnum heads(that aren't cracked between the valves, either)
edelbrock performer intake
hughes whiplash cam
SS full roller rockers
Crank, rods and .020 over pistons out of a 318 LA.
Everything else is pretty much stock.
oh yeah, and a 904 torqueflite and a factory converter runs behind it.

So it's not the carb, it's not the distributor, it's not the lifter preload, nor a vacuum leak, and all of the cylinders' plugs look the same. This is the most illusive gremlin I have ever dealt with(and I've dealt with some doosies before, like the mystery of the disappearing brake fluid where there were no leaks, but a smoking engine). I'm going to need the magic of a graybeard wizard in order to exorcise this demon, for my engine must be possessed by satan himself.

So... before I make this post any longer... any ideas?

 

Does the transmission have a lock-up torque convertor? What kind of manifold vacuum do you have at idle and steady cruising speed?

 

No lockup torque converter. I'll give a reading on vacuum tomorrow.

 

Doing the math on this, sound to me you have ruled out above the short block stage in parts...define fresh. You might have to look there, you just never know.

 

Very fresh. Short block is from a 98 durango. Block was hot tanked, bored and honed at a machine shop. All new everything. As fresh as a used engine could possibly be. The rods and crank are used, the pistons had less than 500 miles on them in a LA block and still micced out fine. There is absolutely no blowby at idle nor WOT coming out of the breather. It's extremely fresh. The freshest.

If you can think up any plausible explanation for the short block being a problem here, I'll certainly listen and investigate it, as I'm completely stumped on getting rid of this gremlin.

 

sounds like a condenser

 

Fresh eh. Fresh as Florida squeezed? lol.

I don't want you going down a rabbit hole on my account, but who installed the cam and chain. There a fuel pressure regulator of any kind?

Check teeth on the gear that drives the distributer. Remember it a guess, that could almost make sense and if I keep typing it'll almost sound believable.

 

A vacuum gauge will help you figure this out .
This sounds like a text book example of what they are good for .
There are charts on here and other places that show how the needle will act and point you to the answer.
They are invaluable in diagnosing engine problems.

Tommy

 

I'll play.
Why are your spark plugs black? Have you considered using a wideband O2 sensor?
How about a compression test and leakdown?

 

Good ground on engine??? I remember my friend had a snap-on diagnostic reader that you could hook up , take with you on a test drive. It would show the ignition pattern like a allen scope That would be handy for this....

 

Checked the firing order , right?

 

I'd go for the vacuum gage also, how did the valve guides look when you had the 318 apart?

 

You said the plugs are all the same, sooty. They should not be sooty unless you have to much fuel or to little spark. You don't say if you have changed the coil or not.

 

It is possible that when you changed the distributor, you replaced it with one with a bad condenser, even if new, or oil or grease on the points. Just because you replaced a potential bad part with a new one doesn't mean the new one was good. That has happened to me before.

 

I had a very similar issue on an old 250 Chev 6 cylinder, and the issue was the plug wires failing. Couldn't take the load, but would fire good at idle. Check them out too.

 

Find someone who has an ignition scope and knows how to use it. Correct use of a scope will either pinpoint or eliminate your ignition system in a very small amount of time. Your symptoms certainly sound like they are ignition related even though you swapped ignition systems. Are you using a separate coil? Was it common to both systems?

 

Intermittent faults are the worst to troubleshoot. I’m guessing engine ground on this one. Everything else you described sounds good.

 

Very likely. A poor connection of some kind some where.

Ben

 

What kind of plug and number is in this thing? Lippy

 

This. Hook a scope to it and see what is happening, interpret it well and you know why it's doing it too.
Unless you like doing the endless trial & error, then keep doing that.

 

Your description sounds like a vacuum leak. But, internet long distance diagnosis...
Look at the intake again. See if you have an intake port leak under the intake into the lifter valley.
Is your intake compatible with the magnum heads/ports?

 

What does the Hughes cam company say about your issue?
Seems to me a call should help.

 

At 900RPM idle in park(fully warmed), the motor has 18" of vacuum. I don't have a long enough vacuum line just yet to run the gauge into the cab to check cruise vacuum.

I have the battery ground bolted directly into a clean spot on the driver's side head, with copper washers between the head, wire, and bolt. I'll take the ground off and clean everything off again just to be sure.

Now for further shenanigans this engine is pulling on me, After checking idle vacuum, I accidentally left the nipple off the smaller brake booster vacuum line(two vacuum lines coming off of the booster for other accessories/vacuum canister). The vacuum leak at the booster didn't entirely fix the issues, but the rough running "miss" is far less severe. What exactly are the chances all three carbs, two quadrajets and an Edelbrock, are running DEAD rich all in the same way? I pulled one of the quadrajets off of a running 305 chevy that I know does NOT run rich.

It's an LA intake, but magnum and LA heads share common ports, all that's required is a slotting of the bolt holes in the intake to accommodate bolting onto magnum heads with their straight up and down intake bolts. I did port match the heads to the intake, and I do own a bore scope so when I swap back to the original carb I'll have a look at a few of the ports to make absolutely sure the intake is indexed right.

What the book calls for on a 98 5.2L Durango. Copper core plugs.

As for swapping ignition systems:
I swapped everything, including plug wires. Originally had a 1 wire Chevy HEI style distributor on there and converted over to a stock point style distributor. As far as the HEI goes, I swapped coils, the little c shaped ignition module, and after that didn't do anything, I pulled the distributor completely apart and swapped the pickups. After that I was figuring it was a intermittent broken wire somewhere inside the dist, so that's when I swapped for a brand new points distributor(something I wanted to do anyway for simplicity).

I'm going to pick up a spare points and condenser in town today. After rechecking the ground connection on the head I'll try swapping those two things over to see if it changes anything.

 

Check your grounding on engine and distributor.

 

Still same spark plugs? If they are platinum, or same plugs you started out with, change to a new set of conventional plugs.

 

I offer 2 additional guesses.....check the plug wire ohms reading. New wires can be less-than-good.
Second item is the zero lash on the lifters. Not usually done in a Mopar.

 

Just ran a jumper wire from batt neg directly onto the dist body. No change in behaviour. I'll have to wait until Monday to swap the points and condenser, website said in stock and they didn't actually have any in stock in the store.

They are conventional copper core plugs.

 

A quick intermittent condition leads me to believe it's ignition.

Couple of things...

Your spark plugs should not be black. They might be somewhat fouled. Install a new set of plugs. Go from there.

Also...

You don't want to run a wire from the battery to the distributor. You might burn out your points. I hope you meant that you ran a wire directly from the battery to the coil.

Doing that but with no change, as you said, would indicate no bad connections or breaks in the wire.
But...
... it would not tell you if the hot wire is grounding out someplace. The intermittent ground would still be present unless you disconnected the original wire.
Inspect the hot wire for missing insulation. You might want to run the engine in a dark environment and watch for electrical arcing under the hood.

 

Might verify ignition voltage is correct and consistent. Possibly the feed wire from the ignition switch is low on voltage due to a bad connection or faulty switch contacts.

 

Check both the wire from the battery to the coil and the wire from the coil to the distributor. Especially the wire from the coil to the distributor where it goes through the body of the distributor. They often will intermittently ground out there if the rubber grommet is missing.