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backfire through intake,help

4351 Views 16 Replies 6 Participants Last post by  t-bird302
I have a stock 302 ho 89 mass air motor and its backfiring at low load operation,example at 50mph in 5th gear if i leave it in 5th and slowly excelerate it will pop and stumble but if I down shift and hit it hard its fine and doesnt act up at all,its also idleing a little rough. any help would be very appreciated. thankyou
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checked it allready along with the fireing order. also the cars running alot cooler then usual,hardly warms up.
this may be a long shot but it did happen to me.

Check your balancer for any type of movement. I spent days trying to figure it out. The outer part of the balancer and the inner part actually seperated but not enough to come apart. I couldnt see it until it was about ready to fall off.
Also check the operation of the EGR valve, a premature opening of the valve causes a similar problem. How to check it?....simply disconnect the valve and test drive the car, if the problem goes away, the EGR portion needs troubleshooting (or elimination, as some do). LUK/GL
thats already been bypassed, thanks anyway Joel5.0
Have you done the usual maintenance stuff? MAF cleaning, TB cleaning, timing check/cal, FP check. If so....a scan for codes would help. GL
I had the same problem...

I have a 95 mustang gt the last year the 5.0 was put in the mustang. My car would backfire through the intake and act like once it warmed up it would run good. CLEAN YOUR MAF SENSOR... AND Check to make sure your O2 Sensor's are functioning properly. I cleaned my MAF Sensor and changed my O2 Sensors and it is now running like a new car..
ok thanks i will try that ,i think thats the only thing i havent done yet, as for the codes that joel suggested i ran them and the only thing that came up was the egr but its not hooked up and thats the reason it blew code, i'll clean the m.a.f tommorow, and again thankyou for your suggestions
still no good,this is starting to get very frustating.:happyhapp
How many miles are on the motor? Also, does it do this when you are lugging (trying to pull from high gear at low RPM) the motor in 3rd or 4th. IE in 4th gear at 20mph?
t-bird302 said:
still no good,this is starting to get very frustating.:happyhapp
Hang in there......I know a lot of people that would give anything to have your problem than the one(s) they currently have with their try to solve yours.....lets define the problem itself.

When you say backfiring at low load you mean an intermitent miss, hesitation, stumble that is only noticeable at small load requirements (ie. cruising @35-45mph in 4th/5th, very small hill or incline, you keep it 4th/5th, press the gas 1/4-3/8 of its travel to keep same speed on the incline....right?).....If this is your case, we started looking at the possible causes from the very technical FI/EEC-IV engine control and forgot the most basic elements of engine performance.....well....enough of this but, we forgot to check the obvious....

I would start with the dist. cap, rotor, spark plug wires, spark plugs. You may have a plug wire boot that has sucumbed to heat and allowing spark to jump to its surroundings instead of the plug. You may have a cracked plug porcelain causing a spark problem. You may have a cracked dist cap or a coil that needs replacement. The EGR in your case, as you said, is not a problem. Just my other 2¢. LUK/GL
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no whan i said back fire i meant it unfortunatly its making a loud poping noise out of the intake,as for the plugs ,wires,cap and rotor ive already gone through all that,its got good spark also so i dont think its the coil,and it acts up on any kind of acceleration unless im wompin on the thing for some reason if i really jump on it and run it through the gears its fine, thats what i dont understand,ive gone through everything ive posted so far,and more and the problem seems to be getting worse but it might just be me.
Fuel pressure shouldn't be a problem since you don't see the problem at WOT........try running it with the FP regulator vacuum line off anyway, just to check.

You would probably have to check the PIP stator sensor and vanes reluctor of the dizzy. Check the shaft for lateral excessive play and the vanes for signs of them rubbing with the stator sensor or pick-up. I had a '94 with a similar problem and had to replace the distributor with a remfg. unit. LUK
Maybe a stretched out timing chain or a stuck valve?
Dude, how many miles on the motor?, also do you get any detonation?
thanks Joel i will try that, never checked the fuel pressure cause , i assumed the same thing that since it didnt do it at wide open throttle that it wouldnt be that,that would be when it needed the fuel most, as 4 monkey boys and mustangsaretuff suggestions , it doesnt get any detonation, only about 110k on the motor and besides 4 this problem it runs super strong and i think the problems to irregular to be a valve or timing chain, i know if it was the chain at least it would be all the time. thank you guys for the help its most appreciated.
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