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Hello 5.0 Tech. So i've been experiencing a popping noise for quite a while now but havnt had the time to take care of it. It happens everytime i start my car up. After it cranks over and fires up it sounds very similar to backfire and will go on for around 30 seconds. I would compare it to popcorn but its not quite that rapid of popping. Ive heard it could be something wrong with the valves, but i dont think thats the issue. If i start the car up with the door open and lean over to the side of my car it sounds like its coming from the exhaust more so than the engine bay. Ive also heard that it could be an exhaust leak which i think would be more likely but it almost sounds like its coming from the muffler or catalytic converter. I've owned the car for a little over a year now and the noise started happening about 4 months ago. any ideas would be appreciated. Car is all stock with 76k miles. Has a leaking distributor o-ring if thats of any use.
 

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1991 Ford Mustang LX
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One thing I would check is the ect (engine coolant temp) sensor.Fuel,timing and iac valve operation are all determined by coolant temp,which is monitored by the ect sensor.Fuel is richened at startup to help keep the engine running.Its the same principle as an electric choke on a carb.Timing is also determined by the ect sensor at startup and the iac valve is opened/closed depending on coolant temp.Its opened 100% at startup and will begin closing as the engine warms up.If the ect sensor is going out,signaling a different temp than actual temp,it can cause your issues.So if the ect sensor shows 120°,but the coolant is actually 60° instead,the engine will receive less fuel at startup,timing will be lower and the iac valve wont be opened all the way.This will cause popping since the fuel mixture is lean.Remember,fuel is supposed to be rich at startup,not lean.Once the engine warms up,fuel will be leaned out.
You can test the ect sensor by disconnecting its harness,touching multimeter leads to the sensor terminals and measure ohms.Test the sensor cold then warm the engine up to operating temp and test it again.If your numbers dont match those in the chart below,the ect is bad.

Check the egr valve to make sure its not opening at startup and idle.Just remove the vacuum hose from the egr valve,plug it then see if the backfiring goes away.If it backfires while driving it too,take the car for a test drive to see if the backfiring stopped.If it did stop,the egr valve is sticking open or the evr solenoid is bad.The computer leans the fuel mixture and advances timing anytime the egr valve is open,so if the valve is stuck open (during startup or warmup mode) due to carbon buildup or a defective evr solenoid,backfiring can occur.The valve is only supposed to be open during part throttle/cruise conditions.Make sure the vacuum hoses arent reversed between the egr valve and diverter valve.

A dirty maf sensor can cause backfiring too,but it should do it at all times,not just for the first minute.Disconnect the maf harness and see how it idles.If the issue is gone with it disconnected,the maf sensor is dirty or defective.

The following link explains what happens during all running conditions.


Fuel Injection Technical Library » Strategies
 

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If it was an exhaust leak,it wouldn't stop after 30 seconds,it would continue the entire time the car is running.

As far as a valve issue goes,there are a few checks you can make.When the engine is cold and first started,the valve stem and valve guide have less clearance between them.These clearances will grow as the engine heats up,so if a valve is sticking,its most likely gonna do it while the engine is cold vs hot.

The first thing you should do is pull the spark plugs and check their condition.They should all be light tan.If one spark plug is wet,black,crusty and dark brown,but the remaining plugs are normal color,that indicates a fuel mixture issue within that cylinder,possibly indicating a sticking valve.

The next thing to do is a compression test,so leave all the plugs out when youre done checking them.Lay something against the gas pedal to hold the throttle blade open fully for the test.Remove the coil harness so the ignition will be disabled.Place the shifter in park(auto) or neutral(5speed) and if its a 5 speed,youll have to push the clutch pedal down before cranking the engine.Screw the compression gauge hose into the spark plug hole.Crank the engine over 6-7 times then get out and check the gauge.
***** If you dont wanna get in & out of the car constantly to do this,just leave the key in the run position,remove the small gauge red/blue wire from the small starter solenoid post,then jump a screwdriver across the small post and the large post with the positive battery cable attached.This will turn the starter over from outside the car,so you wont have to fool with jumping in and out of it constantly.If you use this method,the clutch pedal doesnt need to be pushed down because youve effectively bypassed the clutch safety switch,once you removed the red/blue wire from the solenoid post *****
Now that youve cranked the engine,write down the gauge reading for the first cylinder then move to the next one and so on.All cylinders should have a high reading and be very close to each other.Any cylinder that shows a 30+psi loss,from the remaining cylinders,indicates a valve issue is present.
Example: cylinders 1-6=150psi 7=115psi 8=150psi
Cylinder #7 has a sticking valve.

Another check you can do requires removal of both valve covers.Once theyre removed,jump the solenoid again as you watch the valvespring area.Make sure you see every valvespring move up and down in a rhythmic pattern,as the starter is cranking the engine.If a valvespring doesnt move at all,moves intermittently or erratically,the valvespring is sticking.Its also important that each valvestem is receiving lubrication from the oil.A valvestem seal is designed to leak just enough oil to lubricate the valvestem,but no more or no less.If this tiny amount of oil doesnt reach the valvestem,friction will begin between the valvestem & guide,causing the valve to stick and possibly seize down the road from lack of lubrication.On the opposite end of this equation,too much oil will cause a hard residue to build up on the valvestem,valve face and seat,which will make the valve stick or not close fully.If you loosen all of the rockers and swing them out of the way,then squat down and view the valvesprings from the side,the valvestem tip (of any valve thats stuck open) will be sitting much lower than the remaining ones.

One thing I failed to mention in my previous post is related to the smog system.This is probably a long shot,but the metal crossover pipe bolted to the rear of both heads should be checked also,to make sure the bolts arent loose.A hose is attached to the pipe on the passenger side which comes off the diverter valve for the smog pump.When the engine is cold,the smog pump sends fresh air to the cylinder heads,through the diverter valve then the crossover pipe.Once the engine starts to warm up,the air is cutoff to the crossover pipe and gets diverted to the hpipe instead.If that crossover pipe is loose at the head,its possible you could be hearing the noise coming from that pipe.I only bring this up because you stated the noise doesnt last very long.You would only hear smog pump air coming out of that pipe for the short amount of time the engine is cold.Once it warms up,all noise from the pipe will disappear,since its sent down a different pipe to the converters.

It wouldn't hurt to run a code test too_O2 sensor codes should pop up,if an exhaust leak is present.The computer will toggle the smog solenoids on & off then look at the o2 readings,in order to test the smog components. Codes should pop up,if any are present.
 
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