backfire intake

well, I ask this in another thread and have searched and couldn't come up with another situation like mine. here goes it, 87 4.0 5spd. 220kmiles.
1.all new plugs (champion and autolite) put both sets in. makes no difference.
2.new plug wires
3.checked the cps with ohm meter. about 230 ohms...BTW freakin haynes manual has a misprint on that page. 6-20, says it shoud read open (infinite)
couple trips to parts store checking some new CPS revealed this
4.engine has a continuous backfire thru the intake from the #4 cyl.
5.when I take the wire off #4 it stops the backfire and runs good(less the #4)
6. compression is 92 on #4, didn't leak down after about 3-4 minutes.

it is like that cylinder is firing when the valve is open. any ideas?

valve sticking, or possible broken valve spring. do you have a vacuum gauge?

have no vac. gauge but can prolly borrow one. what should I check with that?
I was trying to reason out a valve sticking with the compression check I did. seemed like that would rule out a bad valve situation? im all ears

Put the compression gauge on num 4 when compression starts to build, put a curved piece of coat hanger on the piston top and turn the motor by hand until the piston is all the way up. Buy a leak down tester or make one, something with a tire valve so you can put some air into the cylinder. It doesn't take a lot of pressure, open the throttle plate and stick your ear close. You can hear if the valve is passing air. You don't really need a compressor, I've used a bicycle pump.
You can compare how fast the cylinder leaks down, with another cylinder, if you have a leak down tester with a gauge. A compression tester doesn't work well for this, because it has a one way valve built in. So the needle holds steady long enough for you to read peak compression.
You can also make a leak down tester, from a cheap compression gauge if the check valve is removable.
Clean the inside of your distributor cap out good. Cross fire inside the cap will also do this. Double check your firing order and make sure number 1 on the cap, is the number 1 cylinder.

grundan said:

have no vac. gauge but can prolly borrow one. what should I check with that?
I was trying to reason out a valve sticking with the compression check I did. seemed like that would rule out a bad valve situation? im all ears

Click to expand...

The needle on a vacuum gauge will wag pretty violently if a intake valve is hanging open.

compression checked good, but you turned it over a few times to get the pressure to that reading right...... it may be possible that the pressure held the valve shut, or the lack of .

with a vac gauge you're looking for a wildly fluctuating reading, which would be typical of a broken spring.....any way to pull the valve cover off for a "quick " look at whats going on there. You'd be able to see either problem from there.

it is also possible that 2 plug wires are crossed, which could easily cause that. double check firing order first, then approach a valvetrain issue if firing order is good....

triple checked the firing order, wires are on correct.
will check compression on all cylinders tonight and see if i can find a vacuum check deal and give that a go. sure hope it is nothing major

grundan said:

triple checked the firing order, wires are on correct.
will check compression on all cylinders tonight and see if i can find a vacuum check deal and give that a go. sure hope it is nothing major

Click to expand...

If it's backfiring through the intake, the vacuum gauge is almost certainly going to gyrate pretty radically. A leak down test, will probably tell you more.
Taking the valve cover off and shaking the rocker with number four at TDC will likely tell you if a valve stem is bent. You can compare the feel between the rockers, they will all wiggle a little when each cylinder is at TDC, but you can feel if a valve is a little lower than a companion, the rocker will often feel really loose. You can also see a rocker that is a little higher than all the rest, sometimes a valve will deform a little and look like this )- instead of like this T . And actually hold the valve end of the rocker up a little higher than the exhaust rocker.
It may be a damaged valve, it may also be a chunk of trash stuck between the valve and the seat. You haven't dropped anything down the intake or dislodged a large amount of goop in your intake lately?
It sounds like you have it isolated pretty much to number four, but some other things that will cause it to pop back through the intake are a bad TPS and a stuck open EGR. A really lean injector can also cause this and it would also likely be at a specific cylinder.

welp, got back at it last nite and found the problem....
pulled the valve cover and it was obvious as soon as I looked at the #4 valves/rocker arms.
I had a pushrod sticking thru the rocker arm about a 1/4", its just like the rod wore a hole thru the rocker. soooo... with that said, what do you think?
1.replace the rocker and rod and fire it back up?

on another note. there is lots of sludge in there arond the valves and junk. 2.should I be concerned with cleaning that out/off. if so how would you do it?

almost all the rocker arms look wet with oil except #2, does the oil come up thru the pushrods and lube like that?
3. curious now if any oil is getting on that pushrod and rocker, dont want it to wear out also.
4.I am gonna get some diesel fuel or something and clean that valve cover also, it looks gunked up with sludge. should I take those little screens out and clean them also? seems like it might fix some of the blow by into the airbox.
if I get her running right I will be moving on to figger out that oil getting into that distributor.
thanks for your knowledge and input

just found this thread about flushing...
http://www.naxja.org/forum/showthread.php?t=70540&highlight=flush+engine

IMO oppinion, moving the junk around in the motor is kind of asking for it. I cleaned the top of my head. I plugged off all of the oil drains with wads of paper towel and scraped all the junk off. Using the vacuum cleaner, very small screw drivers and whatever I could find to get in all the little corners. Some of the junk under the valve springs I sprayed out with gasket remover, but caught most of it with paper towels before it ran off far. The gist of it is, anything you can keep out of the bottom of the motor, has less of a chance of getting sucked up by the pump and being deposited someplace bad.
A few weeks after cleaning the head, I pulled the pan and cleaned that out. You start moving chunks of stuff around and there are many small oil passages that can get plugged up.
I've seen guys use a motor flush additive in there oil and completely plug the oil pump screen up.

update..and more ???

new pushrod and rocker on. turned the motor over with the coil wire off to make sure everthing was moving/nothing stuck. looked to be working fine. fired it up and let it run for a minute. oil psi is 40+ at cold idle. (temp in my garage was about 60 deg.
no oil coming thru the pushrods? dang it.
took all the rockers and pushrods out, soaked them and cleaned them to make sure the holes were clear. I guess I need to do either the ATF treatment or the kerosene? theres gotta be something in there keeping the oil from flowing up?
I guess it wont hurt anything to keep my valve cover off while trying to get the oil to flowing on top side.

I found an oiling diagram for a 1974 AMC 258, don't know how much help it is. The detail of where the lifter gets there oil from isn't there. I read somehwere that the XJ FSM had an oiling diagram, mine doesn't.
One odd thing I noticed, was that the cam gets oil first and then the crank. My diagram shows the main oil passage, going from the filter center, straght up an oil passage to just above the cam (between 3 and 4), and branches from there to cam bearings and down to the main bearings.
But the detail for the lifters (in my diagram) just isn't there. The detail does show the oil being pumped up to the rockers, through the push rods.
If someone has a loose I-6 lifter laying around, a slot and hole in the side, would be an indication of pressure oiling.
If anybody has an oiling diagram for the I-6, I'd really like to have one.
Some people have said the lifters are splash oiled. If they aren't and pressure oiled, no oil to the lifters, pretty much means no oil to anything.

Most likely a clogged lifter. The hassle with a 4.0L is that, unlike the 2.5L, you can't pull the lifter without pulling the head. So pull the head, pull the lifter, disassemble the lifter and clean it out. While you have the lifter out, pull the distributor and put an oil pump driver on a drill and run the oil pump to get the oil pressure up. If you don't see oil squirting into the bore, your block is plugged, otherwise, rehab the lifter.

Just for the record, you shouldn't put a new lifter on an old cam.