just want to start off by saying thanks for all those who post great and informative information on here. I dont post much on here but i do come on here quite a bit to research any issues im having. I usually find my answer most of time. I did find quite a bit of posts with similar issues but it seemed that none of them lead to a cause or fix.

anyhow heres what im working with 87 4runner 2.4 non turbo automatic located in socal.

PROBLEM: intake backfire/popping

had a bad head gasket with about 225k on the odometer, so i got a new longblock and swapped over all my bolt on stuff...heres a quick list of whats been done to the truck so far. Oh and please note the truck would run good for the most part when i put the motor in with exception to some slight idle issues and adjustments etc. Couple weeks ago the motor developed some backfire/popping through the intake under load (at about 3k rpm's). Since then i've been testing and throwing parts at this thing and am now getting desperate as i need to smog the truck in a couple weeks.

new O2 sensor (got a code, code is now gone) - (UPDATE another denso 02 installed i replaced under warranty just in case it was faulty)
new long block (has about 3k miles on it now, includes new timing chain with metal guides, new head, new waterpump etc)
adjusted TPS to spec (UPDATE new TPS now installed (carquest brand)
cleaned TB and part of upper plenum
replaced TB to Plenum gasket
new NGK plugs gapped to .031 and new NGK wires with new longblock
replaced Distributor (with known working distributor)
new distributor cap and rotor
new fuel filter
fuel pump appears to be functioning properly (hums nicely when the key is in the on position) - (UPDATE new fuel pump (Denso brand)
new heater hoses
new radiator hoses
new coolant temp sender (for gauge)
timing set to 5 BTDC (with jumper in place)
reset valve lash to .008 intake .012 exhaust
replaced igniter and coil (with known working igniter)
replaced wiring connector to igniter
new to me diehard battery
FPR and EGR tested good
adjusted idle screw
new o-ring in idle adjust screw on top of the TB (it would back itself out)
checked everywhere for vacuum leaks, used a full can of carb cleaner (found one between the lower plenum and upper plenum, replaced gasket fixed)
new radiator
new thermostat
new PCV
fuel damper on fuel rail is still intact and has screw
compression test (#1 160psi, #2 159psi, #3 160psi, #4 151psi)
swapped in a good used AFM
removed the exhaust pipe from the manifold to check for clogged cat (to be honest the truck was so loud after removing that pipe i couldnt even hear if the intake was popping)
steady 18hg from the brake booster hose at idle

im running out of ideas and time.....any help from the experts?

truck starts up every time, idles up nicely on cold start as it should then settles at 800rpm

i can ease the throttle up slowly but then starts to backfire from the intake at about 3k rpms and if i rev it up hard it starts to pop as im revving.

only thing else i can think of is....
PCM/ECU is faulty
clogged cat
AACV faulty

HELP its driving me crazy!

 

Is your timing chain off a tooth or two? That's most likely cause of a backfire through the intake.

Possibly distributor is off a tooth, also.....

 

My guess is firing order

 

interesting ..... as mentioned truck was running fine and all of sudden developed the backfire. When the timing chain or distributor jump a tooth is there a noise at the time it happens, jolt etc?

also updated the first post....forgot to mention new fuel filter

 

If it was running fine, then started backfiring....hmmmm....

Did someone play a trick on you & swap plug wires? That's what it sounds like~

 

stuck exaust valve??

 

i sure hope not, that would be a smooth trick if someone was able to do that while i was driving the truck.

i was thinking a possible burnt valve, but with the compression numbers coming back good it kinda eliminated that possibility. Will a stuck exhaust valve still produce good compression #'s



im gonna try revving the throttle up to where it pops and pull out the spark plug wires to see if i can identify which cylinder is causing the backfire.

 

I had one of the valve adjustment screws strip out of the rocker arm. That would stop the valve from opening. Your compression numbers would prolly look fine (you're sucking in from the intake and compressing it, just not blowing it out the exhaust) IMO. Not saying that's what's causing it, but it could happen.

 

i will recheck the rocker screws and recheck valve lash. Did the valve lash at operating temp, burned the heck out of fingers and knuckles doing it!

if i back track ....didnt start backfiring until after i adjusted the valve lash. come to think of it the valves were pretty quiet when it was running good, i decided to pull the valve cover and check the valve lash since the motor was new and i had already put couple thousand miles on it. I adjusted them to .008 intake and .012 exhaust. The valves were alot noiser after the adjusment and then it developed the backfire a few hundred miles later. Went back in there and adjusted them to .007 and .011. Made no difference.

Also if it helps it was a gradual popping...as in it started off very faint then got louder in louder as i was driving home from work. This leads me to believe it was something that gradually failed or that gradually came out of adjusment.

 

my truck would backfire upon deceleration. it was some bad vacuum lines in the egr system

 

found this thread seems to be the closest i've found to my symptoms http://bb.bc4x4.com/archive/index.php/t-125401.html

im gonna also pull out and TPS and replace with a spare that i have.

knock sensor? can that cause a backfire?

UPDATE: hmm looks like it can...i never even thought of that

How will I know if I have a faulty knock sensor?

We're not being funny here, but you would experience engine knocking and vibration. The knocking can become increasingly loud where you can hear it over your engine noise. If that doesn't happen, you will experience rough acceleration with a jerking sensation like you are hitting a speed limiter. You may also experience misfires, backfires, or smell a strong engine exhaust or burning smell from the pre-ignition happening in the cylinders.


Read more: http://www.redlinemotive.com/replace...#ixzz1tBXipksb

 

Hey check your camshaft. There have been a string of bad cheap cams wearing out.

 

just spent another hour on it. Put the exhaust pipe back on the manifold and now i have an exhaust leak. Looks like i need some new "O" gaskets.

anyhow was getting an intermittent code"7" even though the TPS was tested within spec. I just cleared the codes and got code "7" again so i swapped out the TPS with a spare. Code cleared....getting no CODES now but still getting BACKFIRE under load.


Pulled each spark plug wire separately as i revved the motor and it still popped on each cylinder. Also wiggled the knock sensor wire will revving ..same result. Wiggled ignition and ignition coil wires ...same result.


gonna try and get a spare knock sensor...

 

You mentioned the issue happened after adjusting valve clearance. I would re-trace your steps at that point.

"Back firing through the intake represents Power and or exhaust stroke with one or more intake valves still open."

Incorrect Igntion Timing?

Incorrect Distibutor to Cam Gear Timing?

Incorrect Engine Timing?

Incorrect Valve Clearnace(s)?

Valve(Lash) Clearance too tight?

Sticking or Stuck Intake Valve(s)?

 

what is your base timing set at now? sorry gotta ask. i dunno if the dist will rotate far enough to mess timing up that bad but hey easy check. hold down bolt could have been loose and dist rotated while you were driving.

 

timing still at 5 BTDC

 

im gonna pull the valve cover off again and set back to .008 & .012. Is there anything besides checking the cam timing while i have it off ?

 

Check and make sure all valves are opening and closing properly at the respective times.

If #1 cylinder is at Top Dead center:

#1 TDC(Top Dead Center) Read to fire Both valves should be closed.(Rocker arms loose)(Distributor Pointing at #1 cylinder)

#4 TDC(Top Dead Center) Finished Exhaust Stroke Both valves closed about to start intake stroke.

Ect.

 

i must ask....will the truck idle fine at 800rpm if the valve lash is off or if i have sticky valves?

 

If it were me.... And since it started after a valve adjust... I would rotate the engine with valve cover off. Watch each cam lobe until it was completely off the rocker arm. Then I would verify clearance. At least verify some slack in the rocker arm. Yes there are faster ways to adjust the valves, but I would start very simple and break it down one step at a time. But that's me...