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Car miss firing

3K views 23 replies 9 participants last post by  Srtnuf 
#1 ·
K guys, been dealing with this for weeks now and starting to really bug me. Just bought the car about a month ago. Has 3076r, 38mm external wastegate,walbro 255 fp,750cc injectors, 80 psi fpr set to 50 psi. Bfmi,tuned by agp, aem tru boost, and a few more things. The problem I'm having is a missfire. With the car in neutral I can rev it at 1000,1500, 2,3 don't matter it has a miss. Holding the rpm anywhere on the tach the car will make some loud "pops" quite frequently. And driving around if I'm taking it easy and slowly accelerating the let off and then lightly press the pedal again the car will sputter and feel like it's either getting choked of air or fludded with fuel. Did tps and rewire, never changed anything. No cel. No cel even on stock tune. Was running it in my shop and seems to be rich. Exhaust kinda burns the eyes a little bit. Previous owner never installed a/f gauge. I bought an aem wideband to install this weekend. I have no idea where to begin. Coil pack maybe? O2? Timing?
 
#4 ·
I guess a better way to put it is you know how these cars pop and bang when down shifting. Sometimes while downshifting there's always one or 2 louder pops in there. Well when I hold mine at let's say 3000 rpm in neutral it makes the same pop. Sounds like a gun fight in my shop lol
 
#6 ·
Absolutely, had a buddy with the issue and an occasionally blinking check engine light that would go away then come back.
 
#10 ·
Good deal, worst case scenario is one of them will need to be changed out. The best way I can describe it is a smooth idle with occasional misfires that won't set a code but will give a occasional blinking check engine light.
 
#11 ·
sounds like the common bad spark plug wires problem these cars have. the first time it happened to me, i wasted a lot of time and money replacing minor parts here and there, all to find out it was just bad wires. $35 fix. they tend to go bad frequently when WITH a heat shield, even faster without. Best thing i found was to get an aftermarket heat shield that installs right at the top of the valve train cover, and always keep an extra set of plug wires handy. solved it every time on my car and many others that i worked on.
 
#12 ·
New coil, new wires, new plugs,new tps,tps rewire,pulled Iac and it looked good. Car has a miss at idle,with solid motor mounts you can really feel the miss. Rev it up to 1500 it's really bad. 2-3000 it's like a gun fight. And now I notice if I put the lightest amount of pressure on the pedal the car will almost die before catching a few rpm. Also. If I rev it up a couple time then release the gas, the car sounds like it's got HUGE cams when it comes back to idle.( not all the time). No cel. I have no idea where to start besides getting injectors serviced and cleaned. It's running Rich to the point the exhaust fumes burns the eyes. And at idle my vac is at 16. On my tru boost.
 
#14 ·
My money is on injector(s). The reason being is that if the injector is leaking it is still flowing, so it's not enough to throw a code since technically it is working.
 
#15 ·
If you have a Diablosport that is capable of monitoring the misfires per cylinder, you can see which cylinder is throwing the misfire. Then swap injectors (I.E. Cyl 1 injector to 4, 2 to 3) and see if the misfire follows the injector. This is how I diagnosed my misfire (888cc Siemens). Needed to be rebuilt and the before/after flow test backed that up.
 
#16 ·
I may have missed it in earlier posts, but before you go drop a few hundred bucks on new injectors, check your plug gaps to make sure nothing got bumped out of whack when putting them in. Typical gaps for copper plugs is between .032-.035 (varies by tune, you are not stock so don't use stock specs assuming it will work) another thing I've noticed is that many of these cars (not all) do not like iridium plugs and have major issues with them. Again, it's not all of them, so this is not a one size fits all problem, but if you are running iridium plugs, try copper again gapped at .032 and see if it helps. If it does, then your car may just be one of the wierd ones. Tune as necessary from there. It's a random thought, as I've seen more people have success with iridium plugs than I've seen failure, but I've seen it enough to be aware of it.
 
#17 ·
K guys so still no progress. Still waiting on the guy to get back to me for injector servicing. But I have figured one thing out. Being unaware of which tune was which on the sct that came with the car. I had programmed the richest tune. Instead of the midrange tune. This may be the answer to why plugs were so black. And car smelling so rich. Could I have carboned up everything on the inside. Like combustion chambers n shit from running it so rich for so long? If I burn a few tanks of fuel maybe things will clear up? Might sound dumb but thought I'd ask
 
#19 ·
Got my wideband a/f installed today. Driving the car around a bit. Seems to be a bit better with the midrange tune. But still has the same sluggish issue. Idle the a/f are between 14-15. But I notice when I'm driving around town and let off the gas the af goes lean to like 12-13. Then when I gently come back on the gas af goes Healy rich up to 16s. It's the moment i lef off the pedal and back on again when it chokes up. But quickly catches itself and back no normal again. Should I put my stock injectors in and try those? Or stock map? I'm ordering Iac and both o2 sensors for the hell if it cuz I'm done guessing. Af at wot are in the 11's car works very well at rpmz over 2000. It's just immediate bottom end is where it's choking really bad. And will only choke once I release the pedal and come back onto it again. Followed by sometimes almost dieing when I come to a stop
 
#21 ·
K guys making a little progress with the hesitation and missing issue. my tuner came with 3 tunes when I bought the car. 3,5,7. Not knowing what was what, when I was playing around with tunes I left it on #7. I found out from a friend of mine that #7 would be the richest tune. So after setting to midrange (#5) the car seemed to run a bit better. But would still sound cammed when coming to a stop and the hesitation and bogging was less but still noticeable. I set the car on #3 tune and it did not like it. Car barley ran well enough to get going. So I tore everything apart in my shop tonight and noticed a hose going from underneath the break booster, across the fire wall towards the passenger side then down and back underneath the car had a huge hole in it at the rubber elbow. I replaced the elbow and now it seems to be a lot better. A lot nicer to drive and the hesitation is way less frequent but still happens every once n a while. When this hesitation or "sounding cammed" happens I see my a/f going well into the lean side. Sometimes so far lean my AEM wideband just shows "---". If I have a right understanding, over in 16-17 is lean right?The hesitation only lasts for like a second then the car runs fine. I know I have a small leak at my pass side intercooler piping I just need a better clamp. This shouldn't cause any symptoms like this should it? And what are some symptoms if I jumped a tooth on timing ?
 
#22 ·
Lean readings depend on what the car is doing. 16+ is lean no matter what.

14-15 cruising? That's fine.
14-15 WOT? Get off the gas!

Post some logs up of the car idling, WOT, cruising, anything. Sometimes with big injectors you'll get a slight "stumble". My car has it with 1025cc injectors. My AFR will fluctuate a tad but not go to "---", so not sure why yours is. Could have a bad rescale?
 
#23 ·
Air/ Fuel is read as Air first then Fuel. So 15:1 means 15 parts Air to 1 part fuel. So the higher the number the LEANER the ratio.


Sounds to me like your lean when you get back in the gas, thus the stumble/ mis fire.
 
#24 ·
Exactly terse. Car runs amazing all day. Only time I ever have problems is if I let off the gas and then back onto it. When I let off then back on it goes lean, stumbles for a second then perfectly normal again. I'm also dealing with an overboost problem. I'm not sure if the 2 go together or not.all lines are brand new. Agp 38mm external wastegate I pulled off and inspected. Spring is fine, diaphragm is fine.
 
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