1989 Jeep 2.5l backfiring/popping through the intake

[jay_powers]

Hi, I have a '89 Wrangler with the 2.5l. It pops/backfires back up through the intake whenever I give it throttle. There are no vacuum leaks. I have replaced every sensor. I have swapped out the PCM, no change. I have checked fuel pressure, 15 psi. At this point I found excessive play in the distributor when rotating the crank, so I pulled the timing cover and found the chain to be stretched, and the cam sprocket and crank sprocket timing marks were mis-aligned. Aha! I said, and replaced the timing set- I set the crank to #1 TDC on compression, rotated the cam forward so that the marks would line up properly, and installed the new set. After reassembly, I checked the distributor and made sure that it was pointing to 6 o'clock when the timing mark on the cover was at 0 BTDC. I then fired it up, and the issue remains. Backfiring through the intake. Could this be a camshaft or valve issue? What would be the next move?

[Moses Ludel]

Hi, jay_powers...You've really done some work here.  I would suggest running a basic compression check or, better yet, a cylinder leakdown test like I describe at http://forums.4wdmechanix.com/topic/75-engine-diagnostics-compression-gauge-versus-a-cylinder-leak-down-tester/.  

Considering all of the improvements you've made, you may have a bent pushrod(s), bent valve(s) or valves sticking in their guides.  In any case, I'd like to rule out a compression loss or valve clearances that are too tight.  The valve clearance issue is discussed thoroughly at my vlog:  http://www.4wdmechanix.com/jeep-232-258-and-4-0l-inline-six-and-2-5l-straight-four-valve-clearances-and-adjustment/.  The 2.5L four valvetrain is similar to the 4.0L and 4.2L sixes.

If your 2.5L was rebuilt with the block decked and head surfaced, it's possible that the valve clearance is too tight or on the margins.  Camshaft lobe wear can sometimes contribute to backfire, though usually the lobe and lifter wear simply drops compression and performance.  Also, that degree of lobe wear is accompanied by audible lifter issues.

Go back to basics that any engine needs:

1) Normal compression plus cylinder seal

2) Proper valve lift

3) Proper valve timing

4) Normal oil pressure

5) The remaining issues are tune and vacuum seal (which includes the intake manifold-to-head and the EGR valve function)

Note: Make sure the EGR valve is not sticking open.  It should be closed at idle and open at tip-in of the throttle—then tapering off as engine speed increases.

Rule out abnormal cylinder seal, low compression or improper valve clearance...A long shot would be spark crossfire due to carbon tracking inside the distributor cap or spark wires that are so worn they jump spark between each other.  A poor signal at or from the crankshaft position sensor can also cause backfiring.  I've seen CPS wires melt on exhaust headers and cause a misfire/backfire from the grounding effect.  

Moses

Edited January 5, 2016 by Moses Ludel

[jay_powers]

Hi Moses, thanks for responding. I did a basic compression test and found all 4 cylinders to be right around 120 psi. One thing I noticed is that when I first start the jeep up with a cold motor, I can give it plenty of throttle and it won't start backfiring for approximately 45-60 seconds. After that, it starts to pop under throttle and gets progressively worse as it warms. There is also no lifter noise when first starting it up,  but after a couple of minutes when given throttle there is definitely some kind of rattle. I can manually compress the EGR valve while its running and the idle drops, that tells me the valve is functioning properly. The exhaust is very black and sooty, with strong fuel odor. I wish I had pulled the camshaft while I had the timing set off! If I remove the valve cover and measure the upward travel of the valves What type of # should I be looking for?

[Moses Ludel]

jay_powers...Valve lift is not provided for stock camshafts; however, mild aftermarket RV camshafts are close to stock spec, usually with slightly more lift than stock for better low end torque.  You might compare your findings to a very mild RV camshaft.  Take into account the rocker ratio if measuring lift at the valve stem dip or measure lobe lift on the lifter side of the rocker arms. 

For a quick check, simply do a comparison of valve lift.  The four intakes should have uniform lift.  The four exhaust valves should have uniform lift.  Seldom does a camshaft lose all of its lobes at the same time.  At least some of the lobes should be on spec, and these would obviously be the ones with the most lift.  Use this as a baseline.

If lifter/valve lift is normal and valve timing correct, your sooty exhaust spells incomplete combustion or over-rich fuel mixture.  These issues could be TBI/pressure regulation, too much fuel pump pressure (wrong pump like an MPI type?), a plugged or restricted fuel return flow to the tank from the TBI, a defective EVAP canister or vacuum routing issues, or ignition, spark timing and distributor malfunctioning.  You share that you've replaced "every sensor".  I'm assuming you mean the O2, CPS, TPS, MAT, MAP and so forth?  Test the ignition coil when hot, a bad coil often functions okay until warm.

Moses

[GoatBoy4570]

I hate to say it but think this is a classic lobe has gone flat on the cam symptoms....well in my experience anyway. When they start to pop out of the carb like that its ``Ùgh time to change the cam``

If you fashion a piece cardboard to act as a push-rod oil deflector you can briefly run the engine with without the valve cover and not make too big a mess.

Look for the rocker that's not moving or hardly moving, that's your bad lobe.  Did you notice the problem after going for a long drive, you got home and it wasn't running quite right, an then it got progressively worse?

My guess is its an exhaust lobe, the popping back is non exhausted air blowing back past the intake valve at the start of the valve opening cycle, its worse when you give it throttle as you're feeding it more air to back pump.