popping and backfiring in 4th gear

jpal

Member
Joined
Dec 27, 2018
so I was able to go for a ride for the first time this year and the car ran good except that it pops and backfires when in fourth gear just off idle.so I can cruise at say 55 mph and let off the gas,but when I just push the peddle off idle,if I keep it there it will start to pop and backfire,if I push the peddle further,or let the peddle back to idle,it will clear up and run fine.it does not do this in any other gear.any ideas or recommendations are greatly appreciated.
 
funny thing, 5yrs ago i chased a similar issue on a 87 Turbo T (gone, but not forgotten) and it would not show up on scan tools and was intermittent. Wound up being a faulty ignition module. got to the point i (reluctantly) was doing part replacement until i found it.
 
so I went through my notes of adjustments I made over the winter to get the car to idle nice and I had to turn the iac screw in quite a ways,where it was actually opening the butterfly quite a bit,so I adjusted the tps accordingly,so my iac number when the car was warm was around 20-30 and the tps was .44,so this morning I turned the iac screw back out to where it was just starting to open the butterfly and adjusted the tps to .44,car went back to idleling a little rough,not as smooth as it was,but acceptable.went for a ride and the car still has a hesitation and the motor seems to surge a little in 4th gear when just picking up the throttle like yesterday,but it did not pop or backfire once,so so the adjustments seemed to help.these are the numbers when I got back
at idle
rpm 750
iac 43
tps 44 at idle/4.90 wot
maf 5
lv8 35
int 130-140
blm 150
02 cc increase about 1 to 2 numbers each frame
mv bouncing between 170 and 730 each frame
at 1400 rpms in park
iac 61
tps 65
02 cc stop counting
mv bouncing between 50-160
maf 9
lv8 35
int 128
blm 139
 
BLM's are through the roof a nd the 02's are locked up. Toss a new Denso 02 sensor in it and try it again.
 
so I was able to do a little more driving today,popping and back firing is still there just as you pick up the throttle in 4th gear.another issue I found today and not sure if it is related,but It will only build about 8 lbs of boost.i hooked a vacuum gauge to the line that goes to the fpr and had 17 in hg.i then put it on the vacuum line that goes to the egr solenoid and only had 12 in hg there?some scan numbers I took while driving at 1500 rpms
int 128
blm 134
lv8 65
 
Had an issue like what you are describing. in my case it would only build about eight pounds of boost then backfire and stumble until I let off the gas pedal.

Got to watching the AF gauge and saw that it was going lean when going into boost.

Turned out that the cam sensor needed adjusting and the MAP sensor was bad.

There are many reasons for the engine to go lean and it can be found by logging.

You have a power logger ?
 
I don't have a logger,just scanmaster,its stumbling with an occasional backfire just as I touch the throttle and only in fourth gear and if I push the throttle further it clears right up and is fine or if I let off the throttle it is fine,its just in that one small spot,just past idle,that it acts up.it does not pop or backfire at all during boost and there is no knock or knock retard.im thinking maybe a bad tps?????
 
Key on engine off, scanmaster on tps. Very slowly push the throttle watching the scanmaster carefully. If the volts don't rise smoothly or it hangs then the TPS has a dead spot.
 
replaced the o2 sensor,it does not lock up now and instead of going up 1 or 2 numbers each frame,it now jumps up between 8 and 15 numbers each frame,but it did not fix the popping or backfiring.the tps seems to go up smoothly from idle to wot,but I did notice that at idle,it does not stay at the same number.so I set it at .42 and push the pedal down and when it returns to idle it will be either .44 or .40.is this an indication of a bad tps?as far as only building 8lbs of boost,i pinched off the pulse line that goes to the waste gate actuator and went for a ride and it was still slow to build boost,but it went right past 8 lbs and kept climbing till I let off.what does that mean?
 
TPS .4-.44 at idle should be no problem.

Careful with playing with the waste gate. That is not recommended. Slow to build boost can be mis aligned waste gate puck, waste gate not all the way closed to build boost or pre turbo exhaust leak. With the line pinched or disconnected you could easily over boost the engine and blow some head gaskets. Have you checked for pre turbo exhaust leaks? Cracked headers ?


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Pull off the actuator rod and see if the flapper valve on the exhaust elbow or downpipe swings freely.
 
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