I have a '78 Country Squire (well, it's as _big_ and as _heavy_ as a truck -- close enough?) with a 400 that has about 130k miles on it. It has an intermittent, but frequent, misfire on #4 (rear pass. side) -- not missing continuously, but enough so that I don't see the typical 50 RPM drop when I pull the plug wire. It continues to misfire intermittently several times per second when I rev it up to 2~2.5k in neutral (have not tried higher). Seems to be better under load, though.



Over the past four years, for various reasons, I've replaced the following with new or remanufactured parts:
- distributor
- timing gears/chain
- vacuum hoses (Three cheers for Mitchell's Manuals!)
- carburetor
- ignition coil

Everything is stock:
- DS-II ignition
- 2150 carb remanufactured by Holley
(which is already starting to leak at the throttle shaft bushings after only three years...)
- original stock manifold, cam etc.

Dry compression is very good on all eight -- 153 ~ 165psi with #4 right in the middle of the range.

PCV vent and valve clear and free.

Wet float level set to spec.

EGR is enabled, and I monitored it with a vacuum gauge teed-in right at the valve to make sure it is allowed to close at idle. I had the valve and spacer off about a year ago to clean and check it - it closes completely.


Strong spark at the plug -- I can hear the spark loud and clear when I pull the wire 1/2 inch off the plug.

Spark plug is clean and dry -- slight tan deposits no different than the other seven.

Idle mixture is out three turns (spec is 1.5) with no improvement beyond that -- and I'm reluctant to run it even that high because I don't want to cook the cats.

Vacuum port (for body/brakes) looks like it comes right out of the #4 passage at the back of the intake manifold, so I disconnected and blocked it in case of a leak at one of the loads-- no effect.



So I guess the question is: what else can I check? Is there something inherent in the design of the stock carburetor/manifold that results in a lean mixture to #4? Is there some place where a vacuum leak commonly occurs that will affect only this one cylinder? Do I just have to live with the lope?

Like indyjames500, I am (or rather, my wife is) rather attached to my 2.5 tons of good old fashioned Detroit steel, so pushing it off the levee into the river is probably not going to be an acceptable solution.


Any ideas, insight, suggestions, questions?

Thanks for all your help.
Best Regards,

TrebleMaker

 

welcome treblemaker,
I don't think you'll make any enemies with a LTD wagon, I've never seen a guest list. besides what better place would you get the info you need. good info BTW on you dilemma. your car is the first thing I look for in the bone yards since they came stock with the 400 and 460 as option.

I have had a similar prob with my 400 since bought in 92 but in both banks of the engine when at idle or steady rpm. never pin pointed the cylinders, not real noticable its just anoying that its there. you wouldn't believe how much time I have spent with my ear to the tail pipes. it almost never does it in a pattern, the engine may cycle 3 or 4 times before it does it.
I have changed everthing external but the distributor housing. the miss was there when it was stock and it continues with all the aftermarket parts. did it with the 2150 carb and now the edlebrock. with or without the smog equipment. its a mystery.

the engine has also been rebuilt and the only thing reused is the heads. is this my ghost? I'll find out in a couple of weeks when I take the heads off during some more upgrades. maybe a cracked valve or worn seat? my compressing is good but lower around 145psi to your 153-165 which is real good. I have tried rerouting plug wires. tuned the engine to 19-20" of vacuum. still never fixed it. I just live with it.

hope that at least rules out a few things for ya.

 

Have you checked or replaced the plug wires? A strong spark with the wire off doesn't mean you are getting a strong spark in the cylinder. It takes more voltage to push a spark under high cylinder pressures.

 

Thanks for your replles. I've had the car for four years and it's done this no matter what I change. I forgot to mention that plug wires were changed about a year ago, and I've checked the resistance.

Could a weak valve spring or cracked/bent rocker or pushrod cause this without affecting cranking compression? Whats' the best way to check these?

I've thought about checking for vacuum leaks with propane or starter fluid, but don't like the idea of spraying that stuff around a hot engine, and I'm not sure how much of an RPM kick to expect if there is a leak (so would I know it if I found one).

All theory and no practice makes Jack's car a dull performer. So I appreciate any practical advice for how to find and exorcise this daemon from the family battle barge.

Thanks and Best Regards,

Treblemaker,
Who takes his mixed metaphors stirred, not shaken.

 

Sounds to me like you need shorter push rod. Check pushrod lenth. You may be able to shim it some ,around .02-.035 at the most .Drill a 5/16 hole in apiece of body metal and cut it to fit under the rocker stand. torqe the stand down. This should solve your problem. If it hasnt burnd up the valve and seat.

 

treblemaker,

I had the same problem a few years back with a similar motor. I turned out to be a bad valve. I had the head rebuilt and the problem was solved.

Sanders

 

mark a
you saying its from too much lifter pre-load. that plus a weak sping may do it, but the effected cylinder would have a lower compression reading

treblemaker, if you do decide to check you valve train for proper lifter preload you want to use a dial indicator w/ a magnetic base to get the best results. I think you want about .006 to 008" of preload. your chiltons or hayes will have the specs. and this chart will save you some time.
http://stangstable.com/Vlash.htm#Valve%20Lash%20Chart
you may also want to pressure test your springs being that they do have 130k miles on them.

Sanders, how did you discover your bad valve?

 

Vacuum leaks at the intake are fairly common on 335 series engines. Since you have good compression it's doubtful you have a valve problem. Spark plug wire routing is also critical on these engines but it's normally the #7 & #8 wires where you get "crossfire".

If you're nervous about using propane or ether then carb cleaner is a somewhat less flammable solution. Spray it around the runner going to #4 and listen for a change in the idle quality.

 

The motor I just rebuilt had the same problem. The compression was good but I did a leak down test and found a cracked valve.
I had the heads redone it was ok. after that.

 

Thank you all for the excellent responses.

Bill, am I being too paranoid about using ether or propane to find a vacuum leak? I guess the fan does a pretty good job of blowing it away.

I've not been able to find a lot of details on how to do a leak-down test. I can remove the check valve from my compression tester and inject 100psi air via the release valve on the side of the gauge. Is 100psi enough for a meaningful test, and how long to wait before measuring how much it's leaked down? How close should the leakdown measurements be among all the cylinders?

Thanks and Best Regards,

Treblemaker

 

Those symptoms are typical of a bad valve. The valve will seal sometimes, then as it rotates it will leak. The cylinder may still fire, but the power will be uneven.
A compression gauge records the highest compression pulses, the lower ones get masked.
A leak down test is a better tool, but you have to repeat it on the same cylinder multiple times, cranking the motor between tests.

 

I agree with danlee, sure sounds like a bad valve. It may pass enough air to hear it when you connect it up to 100psi.


Here is a pic of what one might look like.



We found this by pulling the rockers on that cylinder and then connecting the 100psi from the garage air to the sparkplug hole. You can hear the air rushing out pretty easily.

 

Bill,

I pulled the head off and the valve wasn't sitting in it's seat right.

 

My two cents. If you have good compression numbers it's probably OK internally. It could be a sticking valve = a candidate for a good dose of Sea Foam or MM oil.

You might also try advancing you intial timing a little bit. Do you know where it is set?