misfire after engine running fine for 20 min
 

Okay guys, ive done a good search of the site for a similar issue and couldn't come up with anything.

A little about my car
04 rx8 gt at

I run premix

new engine with less than 3k on it new coils, wires, and plugs

i gutted the cat, (mistake) the check engine light comes on every 50 miles or so for oxygen sensor slow response and lean condition. tried the anti fouler, no luck

So my main issue: I can crank the car let it warm up and it will run fine and redline well for a few miles, after that it starts to hesitate and random rpm ranges and I get a backfire sound coming from the exhaust. I checked my battery last night and it's leaking acid and reads bad on my load tester so im headed to get one now. I'm also going to get a spark tester to check the coils.

One last thing, all this happened one day when i accidently filled the car with 87 octane fuel, i was in a hurry to get to the heliport to fly out... since then i have pumped all the fuel out, put that fuel in my boat, and refilled the car with 93 octane. I always use 93 octane fuel.

My course of action is as follows>

Replace battery

check coils and wires for fire

Change plugs.

Fuel pump?


If there is anything anyone can come up with that might be a possible cause, your help will be much appreciated.

It isn't the 87 octane gas ;) I ran 87 in mine whe it was NA and had no problems at all. A tank of bad gas could do it though....water or dirt...

could be the fuel pump....could be a lot of things

Do you have a code reader, CHECKING THE CODES SHOULD BE THE FIRST PLACE TO START

Problem with no CAT is you always have a CEL so you won`t know if you have something new going on

the code reader said random misfire. The thing that gets me is it sounds like it backfires or afterfires from the tail pipe, not every time but a few. i have a code reader but it does it without throwing a cel.

There will be lots of backfires and pops with no CAT. That is normal.

But it does not explain the misfiring and running really bad? Does anyone know where i can connect a fuel pressure tester?

Fuel pressure adapter goes at the connector near the dipstick. Need a special adapter tbough

Put a new battery in today, Ran fine for about 20 miles or so, started to hesitate and wouldnt go beyond 5k rpm. I had my scanner plugged in, it had codes p0171 bank 1 system to lean, and p0139 oxy sensor slow response. I'm beginning to lean more towards a fuel delivery issue. I'm going to test the pump in the morning, otherwise if thats not it im having it towed to the dealer.

Any suggestions?

Nope ..
New O2 sensor ... it's my bet

Maybe exhaust leak or vacuum leak

Well i knocked the guts out of my cat.... i seen where others have done it and get the slow response code as well with no issues.... should i remove the plug anti fouler from the o2 sensor?

How do i test for a vacuum leak?

Do you have any Mods?

None, only a gutted cat and a k&n air filter. bone stock. The car runs fine for a few miles, then it starts sputtering between 1-2k rpm, if i push the gas peddle it just gets worse, if i dont let off the accelerator before it gets below 1k rpm, it dies. When it does that i can clear the codes and it will run okay but will hesitate around 5-6k rpm. im stumped. i been getting the p0139 o2 sensor slow response since i gutted the cat, someone recommended replacing the o2 sensor but i dont think it is that....

Can anyone help?

You will have to start somewhere and check everything. No specific codes means you dont have as much as a starting point.

sounds like the fuel pump to me, typical failure mode is for it to start out OK cold then go off as it gets hot

BUT you need an inline pressure gauge to verify this so don't just swap it out blindly

same thing can happen to fuel injectors

I cant get the proper connections to test the pump so i just used a hose clamp to the fuel line and the pressure guage and put constant power to the pump. it read close to 60psi, i let it run for about ten minutes and bleed it off a few times into a bottle and it dropped but cam right back to 60psi.... i did notice an inconsistant sound coming from the pump, kinda like a bad bearing or something, my mechanic told me it was the maf sensor today, replaced it and still no luck.... could i have a fouled plug? will a fouled pluf run good until it heats up and then not fire?

Checked for exuast leaks today. the o2 sensor was loose and so were the anti foulers. Removed the antifoulers put anti seize on the o2 and reinstalled. still no luck. going to order the bhr fuel pump upgrade tonight.

Just writing these little tidbits down in my handy dandy diagnostic book

<scratchy scratchy scribble>

I have had similar with questionnable plugs - in my case I remove them periodocally to check (and clean/regap where necessary) to max out their usable life. When they are getting really used up I will start getting small misfires at moderate load and backfiring will increase. That is my signal that it is time for new plugs.

Okay, with a closer look, i have noticed hissing or leaking air type sound coming from under the intake, while the engine is running. i guess its the vacuum chamber. I didnt have a vacuum tester at the time so i pulled the line off the air solenoid valve and blew on it and heard a leak under the intake. any thoughts?

I noticed today when reving the engine the vdi opens but the ssv does not. at what rpm will the ssv open?

SSV = ~3250rpm

APV = ~6250rpm

Someone please correct me if I'm wrong...............but don't think I am.

Damn, my ssv is def. not opening. I did check it and it moves both ways freely. Will the ssv only open when the engine is under a load? Will it open while in neutral just reving the engine? Thanks for your reply.

Just gonna send you here........... https://www.rx8club.com/showthread.p...&highlight=ssv

This guy knows way more that I.

Turns out the ssv only opens when the engine is under a load so its not that. I took it to the local rotary mechanic, he says its the temperature sensor, as his scan tool says the engine is only 70 degrees when he knows its fully warm. We can also hear the fuel pump running at full blast, i assume open loop. Why would the temp guage in the car read correctly and the scan tool read incorrectly? What would cause the fuel pump to be running wide open? I'm going to bring it home, do some more testing, and change out the temperature sensor (ECT) and reset the pcm and see what happens.... No luck, going to the dealer. Im sick with it and seem to get little to no response on here.... cant anyone help?