Backfiring questions

So I just wanted to clarify,

Generally speaking, when the engine backfires out the intake, the timing is too early, and when the engine backfires out the exhaust, the timing is to late, correct?

And if so, how can the distributor be too early on one tooth and too late on the next? Would that suggest a faulty cam sensor or cps?

Best guess is your initial setup is wrong. TDC 1, is when the number one cylinder is at the *top* of the compression stroke and the timing marks are aligned on "0". If you want to go in a little deeper, the rockers for the number one cylinder have to be all the way up. It is possible to confuse the exhaust stroke with the compression stroke using the thumb method, less likely using a compression gauge.

More ways to get it wrong than you'd think. The pistons work in pairs, three pairs of pistons, with four strokes. The motor has to make two revolutions for all six to go through all four strokes. In other words the harmonic balancer has to make two full revolutions (pass "0" twice) for every one revolution of the rotor.

Second best guess is cross fire (spark taking an odd path), which is pretty common. Third best guess is firing order. Fourth best guess is the shear pin for the distributor shaft is sheared, but still catching some, in other words the rotor is not pointing where it is supposed to point. Fifth best guess is the teeth on your distributor drive are chipped. Sixth best guess is valve timing. Seventh best guess, bad valve or valve train parts.

Too much air or too much unburned fuel can also cause flashback (out the top) or backfire (out the back). Usually a faulty sensor CPS, TPS or MAP and/or a vacuum leak. Fuel builds up and ignites in the wrong place and/or at the wrong time.

Yeah, that's how I did it. First by thumb...then thinking I couldn't trust my senses, I used a compression guage.

Engine ran fine before, just had a vibe. I removed valve cover, oil pan, and timing cover. Verified timing was correct without changing anything. Changed the seal in the timing cover, changed all the gaskets, removed the tranny and replaced the flex plate and TC, reinstalled everything to spec, fired it up and now if I set it like it was originally, it sputters and dies without gas, if I give it gas it revs, but doesn't hold steady rpm and backfires out the exhaust. If I go one tooth forward, it runs really rough, won't go over about 2k rpm, and spits out the intake.

It's like I'm a half tooth off on my timing.

So I'm reading this installation guide for distributors in a Jeep 4.0 here:

http://www.justanswer.com/jeep/1mo0c-replace-distriburator-set-timimg-4-0-jeep.html

And it refers to a plastic alignment pin that is removed once the install is complete. Could I have knocked the distributor out of alignment while taking it in and out and laying it on it's side?

The only timing input you have is initial setup on the distributor and all this really does is (set, at least on the Renix) the minimum timing retard and maximum advance envelope *close* to number one (I imagine the OBD and Renix are pretty much the same in this instance). The CPS takes over from there and the PCM fine tunes the timing inside the envelope, with inputs from various sensors. The Cam or Sync sensor is all fuel and has nothing to do with the timing. But that backflash (or bucking) can be a symptom of an out of sync Sync sensor from what I've heard. Clocking the sync sensor isn't that involved.

Best guess and something you have to check is, that all the sensor connectors are clean, no oil, bent or loose pins to garble the sensor signals. Garbage in garbage out.

Worst case is you have the wrong flex plate, one that is clocked wrong and/or some kind of fabrication failure if it is new.

I've never changed out a flex plate on a 4.0. but from stories I've heard I'm going to change it at TDC one and make darned sure the windows in the new one line up as exactly as I can measure (compare) the windows on the old and new one before I ever remove the old one. More than a few guys have stuck the wrong flex plate in there.

Just for giggles take an old rotor, install it, then grab it and twist until it feels like it is likely to explode and see if the distributor shaft moves. Backflashes and backfires can shear the pin, but it kind of jams again and your rotor ends up in the wrong spot. That shear pim is in there to protect the cam/distributor shaft gears.

yeah, a bad flex or wrong flex plate has been my concern all along.

It's the only thing related to timing I can think of.

Don't suppose you know what the symptoms were after they put the wrong plate on, do you?

I hate to spend more money, pull it all back apart, just to find it's something simple.

When it starts up cold it doesn't sound bad...in fact I thought it was starting to fix itself, but as it warmed up and the idle dropped, it went back to it's previous state...but the whole time it has this fluctuation, it sounds like it's about to pick up and run smoothly for a couple seconds and then settles. and it has a couple mini-fluctuations within each big fluctuation like a hit-miss-hit-miss-hit-miss and for each group the engine rpm either increases or decreases. I hear no knocks, no pings, no bangs (except for the backfires).

The only time I can remember recently, who had that problem (backflash) is because he had the distributor 180 out. The only time I've had backflash is mixing up the firing order (hast makes waste ), when a CPS was seriously failing. And when the motor was running way lean (after reaching operating temp.) because of a flaky O2 sensor and it would backflash (buck or detonate) around 2200 RPM. I have had some crossfire flashbacks when I got my motor seriously wet and have seen some arching inside the distributor cap from dirt or moisture.

The guys I remember who put in the wrong flex plate had a no start or an occasional pop. I can't remember any who actually got it running, a search might be helpful, I vaguely remember some flex plate threads.

I've put the distributor in one tooth off on purpose, just out of curiosity, to see what would happen, it started fine. One tooth retard and it would idle fine but kind of crap out when I added gas, One tooth advance and it would buck and miss some, but would run fine at certain RPM's. I may have the advance and retard symptoms backwards, it has been awhile.

Backflash can damage stuff, I've seen it shear the shear pin in a distributor shaft, blow off vacuum lines and once explode the brake booster check valve.

Well, I'm getting more concerned it might be the flexplate. Found another thread about a guy replacing the flexplate and had similar issues. One of the responses was "I think we heard here not too long ago that all the new flex plates at the stores were factory defective?" Should have bought my stuff from moparoverstock.com and called it a day.

SanDiegoXJ said:

......
When it starts up cold it doesn't sound bad...in fact I thought it was starting to fix itself, but as it warmed up and the idle dropped, it went back to it's previous state...but the whole time it has this fluctuation, it sounds like it's about to pick up and run smoothly for a couple seconds and then settles. and it has a couple mini-fluctuations within each big fluctuation like a hit-miss-hit-miss-hit-miss and for each group the engine rpm either increases or decreases. I hear no knocks, no pings, no bangs (except for the backfires).

Click to expand...

Have you changed the O2 sensor? The fact it runs good
when cold and then runs worse when warm, points to
the sensor...?

Is there a raw fuel smell at the exhaust pipe?

yeah, swapped the O2 sensor too.

I'm seeing a lot of threads on people getting mis-matched plates.

I made sure diameter/circumference, bolt pattern, number of teeth were all the same...but I didn't think to match up the holes on the band to make sure they were the same.

I've seen a couple other threads where people changed flexplates, the jeep would start but not run, a new cps fixed it.

I have a new cps on the way, I'll try that...if it fixes it, great, if not it goes back in the box and onto the shelf for a spare/test unit.

I'm pretty sure I'm either going to have to put my old flexplate back on and see if that fixes it though...or at least pull this one off and match it up with the old one...but I don't see any other option that could set my timing off by half a tooth.

I hate to send you down the wrong path, so I'll just throw this out there. First off, which year motor are we talking about, I'm sure you posted that up someplace, but I haven't read all your posts.

When the guy did your motor did he swap the cam out? It could be any number of things wrong, but it sure sounds like your initial setup is wrong.

Having the sync sensor ( or cam position senor) clocked is important in OBD models. This is where it gets involved. It's possible the cam is out of time. Many timing chain crankshaft gears have two choices, some have a slot and you can adjust.

I'm beginning to think the flex plate or the cam is clocked wrong. If the cam is clocked (way) wrong it will often show up as low compression.

Pull the distributor and look down the hole with a strong flashlight, check out the gear teeth on the cam. Figure out someway to check that the pin (shear pin) going through the distributor shaft isn't broken.

Like I said, I hate to send you down the wrong path, but if I'd spent that much time with a motor and never gotten it right, I'd start over from the beginning. TDC 1, either verify it with a degree wheel and a piston stop or do it the old fashioned way with a curved piece of wire stuck down the spark plug hole and feel when the piston is all the way up, timing marks at zero. Now here is where you'll either hate me or love me, take the valve cover off, use either a dial gauge or another good way of measuring and check to see that push rod side of the cylinder one rockers are all the way down on the pushrod side and all the way up on the valve side and match in height, at true TDC (hopefully at zero on the timing marks).

I vaguely know how to clock the sync sensor, just what I've read, never had to do it myself. It seems to be critical on OBD motors. Check with somebody with the same year as your motor and see exactly where the rotor is pointing at TDC one. If the cam is timed wrong (wrong cam or setup wrong) the rotor and the sync sensor will never be right.

Like you said it may be your flex plate (sync ring) it may be something entirely different. Experience tells me it is likely to be the initial setup or something really simple that you are overlooking. At least it seems to be constant and not something intermittent.

If I took the valve cover off I'd also use a dial gauge and compare the height of all the cylinder rocker arms at the top and the bottom and compare. Especially if I had a new cam or lifters, Cams usually fail quick if they are going to fail.

You mentioned it seems worse when the motor was at operating temp. this usually indicates a sensor issue. Some sensors don't really come on line until the motor heats up. One sensor fails (shorts or partially shorts) and it can suck down the supply voltage to a number of sensors. The first sensor out of the envelope pops a code, but the sensor causing the grief may not show up as a code.

If this were a Renix I could probably be more helpful, I know them a whole lot better. I wish I was nearby, I'd stop in and try to help some.

I did live in Escondido for awhile, way back when. Most of the town was relatives, three Cops and twenty some bothers and cousins all in the same age group, we owned that town . They are still probably telling the story of how we filled some asshat Cops car full of gravel (he was parked in a gravel parking lot). Only took a dozen of us five minutes to do, while he was inside the bar making "bull of the woods" noises. LOL I bet the next time he rolled up his windows and locked the doors.

OK, quick recap...

94 Cherokee Country, 6 cyl, A/T
Engine was rebuilt 30 over with a hotter cam (I believe)
Engine ran well except for a harmonic vibration at 1750 rpm.


Before I disassembled, it started and idled beautifully.

One thing I knew was that the TC wasn't indexed and that I had a leak in the transmission oil pump. So after no changes in the engine, I removed the tranny to change the pump seals. The TC was scored and the pump bushing was trashed. I figured that meant the TC was wobbling, the holes in the flexplate looked a little wallowed out, so I figured new TC and flexplate was the best bet.

The new one showed up and I placed it on top of the other to verify it looked the same, but as I said, I didn't check the band holes to make sure they were in the same spot.

I figure the only thing that could throw off the timing this much is the cps or what it's reading from.

Fortunately, I cleaned the tranny while it was out so it should be a little nicer to take out this time....not that it was all that bad last time actually.

Just a thought before you start pulling all that
stuff off again:
How about getting an old-style timing light and
checking the timing? You can mark the harmonic
balancer at TDC with chalk, and put another mark
at 20 degrees BTDC or so, and see what the timing
actually is at different RPMs.

This would confirm if it is or isn't a timing issue,
and might save some work and point toward
a solution...

Back fire can also be caused by cam timing or a burnt/sticking valve.

Pull one spark plug at a time and see if any one effects more than the others.