Backfire through intake manifold, rough running dohc.

[zleeper]

Has anyone heard of a backfire in the intake and misfire symtoms on a dohc? I have a 92 3.4 that backfires under light acceleration and seems to have an ignition miss at idle and throughtout all rpm. I have replaced the plugs, wires coils, icm, timing belt alternator and I am looking at the crank sensor or fuel pressure regulator or map sensor. Everything I replaced was not to fix the problem, but just as maintenance. This thing has had killer power until just recently. And yes this is my first try at the forum.

[your_I.D.]

I'd start with a compression test and check sparkplugs Also check that the plug wires are in the correct firing order. Who put t-belt on?

[SappySE107]

Yeah, I would start with compression cause it sounds like you have a bent valve or really bad cam timing.

[GTP091]

Back fire through intake manifold

The exact same thing happened to my car. I changed out every sensor I could get my hands on. The problem ended up being my Knock Sensor. After I changed it out and did an ECM relearn it idles smoothly and doesn't back fire out the Plenum, there's no hesitiation and it can rev over 4000RPM again. Unfortunately in my troubleshooting stretch my alternator gave up and that set me back a couple weeks. It was a good thing it did because its very difficult to remove the knock sensor without taking out the alternator. If you attempt to do this I'd replace the wiring for the Crank and Knock sensor as their known for wearing and causing problems. The Knock Sensor is responsible for advancing or retarding the timing depending on engine load that's why it'll idle rough but as soon as you touch the throttle the engine can't compensate for it.

[SappySE107]

RE: Back fire through intake manifold

The knock sensor is not responsible for advancing timing, only retarding on engine knock. I wouldn't go throwing parts at a motor, its very expensive and time consuming. Diagnose the problem and if you can, run it on a scan tool to see what the sensors are actually seeing. Unless you have a code or massive KR, the knock sensor probably isn't your problem here.

[RollinonZ34]

Mine did the Misfire thing ( especially when cranking and warming up)
until i changed the Bad leaky Lower Intake manifold gasket.

Do you have the high idle until warmed up? Telltale sign of gasket.

I do want to point out though that i also changed & reset Timing belt when
all of this was done. so chances are it was the Timing, but i always
suspected that it was the leaking gasket causing the problem.

Also, i ain't no mechanic so my opion don't mean as much as most
of the other members...but thats my story & im sticking with it