car breaks up around 3500rpm

so last wednesday when I went to my dentist appt I had to stop and get gas. barely made it to the gas station since I was so low on gas and when I got there they were outta 93. since I was already late and wasn't going to be able to make it to another gas station (closest one was 10 miles in the opposite direction of my dentist) I put 2 gallons of 89 in the car. babied it all the way to my dentist appt keeping it out of boost cause I was worried about detonation with the lower octane. after my appt I filled it up with 93 and babied it for another 60 miles or so to make sure it had a chance to burn up all the 89 that was in the fuel lines still. since then its been stumbling and breaking up real bad at about 3500rpm. I'm not sure if it just happens to be a coincidence that it happened right after having to put the 89 in it or if that was the cause. The only other weird thing is it only seems to do it when the car has heated up. When I got up this morning and drove it, it was fine. I could do WOT pulls all the way to redline without and sign of hesitation. then about 5-10 miles down the road as I was pulling away from a stop sign it started stumbling again. I filled the tank again with 93 and put a bottle of valvoline complete fuel system cleaner in it. Once it cools down I'm going to go pull the spark plugs and check them all. any other suggestions as to what it could be? I'm also going to get in touch with my tuner and speak with him about it possibly being a tuning issue.

NEVER put lower octane gas in your tank than you were tuned for. Were you tuned on 91 or 93? I hope 91 because 93 is hella rare & expensive, at least where I am at.

It's ok if you don't boost, and baby it, but you have to realize that the 89 gas isn't just in the lines even if it got low.. It mixes with the 93 you put in and creates a mid-to-low-grade combination of fuels, depending on the proportion of each. If you put in a full tank of 93 to let's say 1 gallon of 89, you should be ok.. But if you didn't increase the proportion enough, this could be the issue. If you got new injectors etc, you know your own fuel system - it probably didn't need that cleaner stuff, as it works well to deteriorate seals.

I wouldn't push the car all the way (even though it seems to be doing ok) until you add more 93 octane fuel into it to make SURE all the 89 is out. I don't think it's a tuning issue, unless it has happened consistently since you left the dyno.

The low octane fuel, mixed with hot weather is a great combination for detonation. Realize, the extreme heat makes the fuel mixture lean out.. In addition to the already lower octane, this can be disastrous!

I'm tuned for 93. its readily available where I am so that's not really too much of an issue....except for when the piece of **** gas station owner down the street doesn't order his new shipment on time. The injectors are used DSM injectors and I have no idea how many miles are on them which is why I decided to get the fuel cleaner ****. plus it was 20% off so I figured it couldn't hurt. I'm gonna check the plugs in the morning and baby it till i get through this tank and fill it up again. hopefully thats all it is.

I hope so too! Keep us posted.

will do man.

91-93 i'm pretty sure is available based on state regulations. i know virginia's "premium" must be at 93 and it is expensive.

definitely sounds like a fuel issue. keep in mind open and closed loop operation and theory. i bet thats why why it doesn't start doing it until it warms up to temp and enters closed loop. i know i don't need to explain all that to you. just something to think about.

good point I didn't even take into consideration the whole open loop closed loop thing. we'll see how it is after I burn up this tank.

well I'm through a full tank and onto another half a tank of 93 now and its still having issues. Do you think maybe it could be a fuel filter issue? I've never replaced my fuel filter. so unless the previous owner did it then that filter has like 270k miles on it..... I also noticed that sometimes if I go around a sharp turn and give it some gas it'll stumble even at rpm's as low as 2k.

also waiting to hear back from my tuner about what he thinks the issue could be.

what fuel pump are you using? did you replace that in the build with a higher flow? (i'd check your build thread but i'm lazy) however i'm assuming since our cars fuel filter is part of the pump/sending unit then it would have been replaced if you upgraded to a walbro or other higher flow pump. if it is in fact the stock one trying to dump the tuned amount of fuel in then maybe it just can't handle as well as you'd hoped or expected and is having problems supplying the fuel... maybe it's wrong. but it sounds good in theory. i've also been drinking. but not too much. take it as you will. this is part of what i'm still researching in my slow build.

its a walbro but I only replaced the pump and not the whole sending unit/filter assembly. so it still has the original filter in it.