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I have a Proton Wira 2004, which has the same engine as a 1994 Mitsubishi Colt. The car is fuel injected.

Recently, the engine seems suppressed when starting the car. Plus, there are popping sounds coming from under the hood. What I mean by "suppressed" is that the engine is struggling to start or "rev" at first then it becomes normal after a couple of minutes.

I recorded the sound.

  1. What could be wrong?
  2. How do I diagnose this?
  • Who downvoted this? it's a pretty good question – Cc Dd Nov 20 '16 at 4:51
  • Question does the popping sound like it is coming from the intake manifold or the exhaust pipe? By the way your english is pretty good :) – Cc Dd Nov 20 '16 at 4:56
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    thanks, the sound is coming from the intake manifold probably, it is coming from under the hood. – shahawi273 Nov 20 '16 at 5:57
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    Until it gets warm – shahawi273 Nov 20 '16 at 8:11
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    There are ~7 engines that both of those cars share. Do you know what liters or ccs your engine is? – Zach Mierzejewski Nov 20 '16 at 16:48
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It sounds like a backfire.

here are a few things to check

  • grab a timing light and see if your spark timing is where it should be.
  • pull the spark plugs and look for damage corrosion or if they are black, white, shiny if you lay them all together in the order they were pulled from the engine and take a good close picture of them you can upload that here.
  • grab an obd2 reader and see what codes if any are listed
  • check the fuel filter closest to the injectors and see if it is full of crud
  • check your fuel pressure
  • check vacuum hoses for cracks or leaks
  • you need to be careful and have a fire extinguisher but you can find vacuum leaks and intake manifold leaks by using a butane torch. Don't light the torch but turn the gas on and slowly move it around the engine. !!! DO NOT !!! let it backfire while doing this it could cause problems. DO NOT open the torch all the way just enough so a leak will suck in more fuel and increase the RPMs. !!! NOTE !!! THIS CAN BE DANGEROUS IT CAN CAUSE FIRES, IT CAN EXPLODE, USE JUDICIOUS AMOUNTS OF COMMON SENSE, SOME PROTECTIVE GEAR, AND HAVE A FIRE EXTINGUISHER THAT CAN PUT OUT OIL AND FUEL FIRES READY AND CHARGED. If you wave the torch over a leak that leak will suck the extra fuel in and cause the RPMs to rise. This will show you a place to start looking for the leak.
  • if you have access to something that can give out engine readings then taking them down will be very useful. We mostly want to see readings from the mass airflow sensor, the oxygen sensor, throttle position, vacuum pressure, and fuel pressure.
  • get a vacuum gauge and see if your vacuum pressure is steady while idling and what it is at. If it is all over the place or even wobbling a little note the maximum and minimum pressures it wobbles to.

now for some questions

  • between seconds 7-9 do you remember how hard you were hitting the gas?
  • between seconds 6-7 were you hitting the brakes at all? or what was that whistle? if you were hitting the brakes did you suddenly take your foot off at just about 7 seconds or possibly pushed the brake down harder?
  • between seconds 9-10 you hear first a "kebut" sound then near the end a click. What is the "kebut" sound? And I am assuming the click is the gas pedal being floored. or is the kebut sound the gas pedal being floored and the click the backfire? If I am wrong on both of these then when did you floor it? before the Kebut after the kebut but before the click at the click but the click was a backfire or just after the click?
  • what was happening to the gas pedal between seconds 20-29? To me it sounds like it was idling either with the gas pedal slightly open or completely closed till a small burble at 24 seconds then fine idling till 26 seconds when I suspect something else happened with the gas pedal.
  • did these backfires suddenly happen or has it been gradual?
  • can you remember any repairs or modifications the car has had that may have lead to this issue?

most likely you have a lean condition or at least we are going to hope you do this can be caused by:

  • vacuum leak
  • improper mass airflow readings
  • weak fuel pressure
  • other sensors telling the computer to go lean
  • it can also be really bad timing or spark jumping to another plug. This is unlikely but not impossible. you have a coil pack and if it's damaged maybe you can get a plug firing when it shouldn't and maybe the computer is sending the wrong signals. These are pretty unlikely though. it would have a much better chance of happening in an old style distributor.
  • or it could be a combination of these that is causing the lean condition.

With the lean condition the whole system fires sooner and hotter which in some cases can cause intake explosions.

the thing we hope it is not is a valve not sealing in its seat. I would expect this to pop much more and not just a couple times while revving but it's possible this might be the cause. The reason it would stop after getting hot is metals expand and this could help keep the valve upright in the valve guide. The way to check this is to pull a spark plug run the engine and see if it pops with 3 cylinders. With the spark plug out of the engine this will effectively leave 0 compression and 0 spark which will mean 0 backfires once the right one is pulled.

So basically go through some of the tests and see what you can find and let me know.


I mentioned in a comment what work was done for the routine maintenance? replacing a timing belt? fuel filters? the air filter? was all the vacuum lines plugged back in?

Typically when you have something that changed or was done and something else starts happening there is a good chance they are related. For example: replacing a fuel filter may change the flow pressure of the fuel if the pump was already weak it can also knock crud into the fuel rails causing the injectors to drip rather than spray (not likely but possible). changing the timing belt and misaligning it (highly unlikely since it would have to be so turned around the car shouldn't run). replacing spark plug wires and switching them (common but usually causes very consistent issues so in your case this is ruled out). Changing an air filter or something else under the hood and forgetting or accidentally disconnecting (can't see it pull part out and unplugs it) a vacuum hose or an electronics connecter (very common and probably a likely cause).

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    First of all thanks for a quick and thorough responce Secondly I recorded this sound while the hood is open and i was revving the engine from the back of the engine using a dial so I wasnt using brakes at all and every "kebut" heard is when I rev the engine, actually these kebut sounds are the explosion sounds that I was reffering to and it is much louder in reality – shahawi273 Nov 20 '16 at 8:46
  • Also I was tevving the engine at its max – shahawi273 Nov 20 '16 at 9:05
  • These backfires suddenly happened after some routine maintenance to the engine itself – shahawi273 Nov 20 '16 at 9:07
  • I will go through these steps and keep you posted thanks for your interest – shahawi273 Nov 20 '16 at 9:09
  • @shahawi273 updated answer below the page break line ... what kind of routine maintenance? was a timing adjustment involved or possibly a timing belt was replaced? – Cc Dd Nov 20 '16 at 9:26

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