VIOLENT bucking/hesitation at WOT
 

Well, ive got a new problem! yay!

the UIM was off about a month ago. i removed double throttle completly. as well as sanded/polished some inside of the UIM. dont know if it matters, but i believe my AWS hose, going to the elbow, is also removed.

now, i can not go WOT in 1st gear. it will accelearate briefly, and then VIOLENTLY start to buck. of course i get off of it at that point. If i switch quickly to 2nd and WOT, same results. unusable/violent hesitation. however, if go to second smoothly, and allow it to stay in second more a few seconds, i can go WOT and it pulls like crazy just fine.

last symptom, WOT in 4th at high RPM,say 5500 or more, i get similiar hesitation, but no where near as violent, but still unusable.

ive searched and i believe it may be my TPS. especially since it started after i messed with UIM.

what do you guys think? other possibilties?

sounds like fuel cut to me; what are your boost levels at? factory ecu?

You may have damaged some of the vacuum lines that go thru the intake manifold that control the turbo's if you still have the factory twins with the rat's nest.

please list all of your engine mods

Sounds like the TPS out of adjustment, or other electrical problem.

Violent bucking is usually a sign of an electrical/electronic control issue or in extreme cases, a fuel system problem. Fuel cut occurs at a set boost level/rpm with the stock ECU, so it would be even more likely to happen when he "stays in second for more than a few seconds, then goes WOT". Stock turbo system problems don't result in violent bucking.

apexi intakes,and downpipe, blitz sbc id boost controller. its a mazda reman with about 6k miles on it, turbos were replaced at the same time,running stock sequential. virtually all of my rats nest has been replaced with silicone hose also.


problem was happening before boost conrtoller. I dont think its fuel cut, because im not seeing quite 10 psi on my first turbo in any gear. i have not got the blitz completly dialed in yet.

does fuel cut happen at 10.6psi in high rpms? the blitz unit never gets that high i dont think. i will have to check into it some more.

the most violent and for sure occurance is in first gear.

I've seen this happen MANY times; overboosting causes fuel cut, and the stock twins will put out over twenty psi before the bearings explode. That's well over fuel cut parameters. TPS out of adjustment won't cause violent bucking, and idle would be slightly rougher than normal or fluxating depending on how off it it. Quite simply, if the TPS was removed to do some polishing or cleaning, it should have been reinstalled exactly where it was (the markings from the screws). At any rate, i'd be focusing on fuel cut rather than electrical.

^ my idle is a little high at about 1100rpms, sometimes it will settle a little lower than that. but its not hunting, or rough at all. so maybe not tps?

oh! i forgot to mention that accelerating in a normal drving fashion in 1st will often cause a bucking/hesitation as well, but no where near as violent. but i have experienced similiar feeling in all manual vehicles ive owned, in first gear using slow, almost consistent acceleration, they often jump/buck a little.

Fuel cut chart:

this might be an unrelated question, but, looking at that chart makes me wonder.

i was working on something unrelated to this problem and i got a 15 psi spike, and it woulvde keep right on going had i not got off of it as quick as i could. no fuel cut then?

but thats not important at the moment, im just concerned about this craziness. i cant beat my buddys evo from a dig without 1st gear! lol

Fuel Cut
 

There is a time-delay built into the stock ECU before fuel cut will occur. Spikes (and 1st-gear boost) usually are too quick to get a fuel-cut response from the ECU.

Dave

......soooo.... d=are you saying first gear is too fast for a fuel cu response? cause thats where im having my worst problem?

Well, I'm no expert on this, since I've never encountered fuel cut, but, basically, yes. I would suspect a different problem, like fuel starvation due to fuel sloshing away from the pickup in the tank, etc.

Otherwise, I don't have much in the way of intelligent input on this.

Dave

Two possibilities that I've personally experienced (and never enjoyed):
1) Drop in fuel pressure due to a fuel pump problem
2) Injectors not firing properly

Both will feel violent if you don't let off quick enough to prevent the drivetrain from slapping.

Dave

hmmm....well this sucks.


is it even a possiblility that installing a PFC would cure this?

Primary coil failure gave me the symptoms you describe. The car ran fine under light loads.

Yea, when I had some problems with my ignition I had some pretty viloent bucking, and misfiring. Check that you didnt leave a ground for the coils, or anything ignition related off. After that, I would spec out your coils and see if theyre still good.

well i started changing my plugs and wires yesterday.....to be honest, ive never changed a spark plug on any vehicle before.lol. but after crawling around down there, i see how simple it is....well....if i had the right tools!

i only had a crescent wrench at hand....that SUCKS. i gave up after getting 2 of them in, along with their respective wires replaced.

this whole engine only has around 6-7kmiles on it, so i got a feeling it wasnt these wires. if i were to clen them a little, i would not be able to tell the old from the new!

however, my plugs were pretty black, but , like i said, ive never done this before, and im not positive what they would should look like. ill get a picture up later today.

i dont think this will fix my problem, because it seems to me, if it were this, it would be affecting every gear similiarly, but its not.


oh, i also got a good deal on a PFC last night and plan to put it in today hopefully. its says fds4 on it,so its the new one. and he said jason from rx7store put the base map back on it, as it was used when he got it.


ill give an update later.

Check the TPS values. When these Pots wear off these can have dead spots. Maybe close to full throttle.

^ive got the how-to from the fd3snet website and im gonna look into it, as that was my first suspicion. but i dont fully understand what it does, or how it works.

like i said, the problem did start after i had, had the UIM off for a week or two. i assume i couldve knocked it out of place?(dont really know what im talking about) or something?

but im gonna check.


another tid-bit. i had the fuel filter changed as soon as i bought it. so its got maybe, less than 5k miles on it.

Ignition and TPS problems are generally load and RPM sensitive. So they can affect all gears similarly.

Dave

does this bucking happen EVERY time you drive or only once in a while? When your car bucks, is it when you have your radio, lights, heat on? You said your car idles fine. I would reccommend checking ground wires.

I had the same bucking problem at lower rpms with a perfect idle. First gear was always the worst but it did occur in other gears as well. Culprit was a loose ground wire. Fixed the ground wire, bucking stopped.

ok. changed plugs and wires, changed oil, cleaned up and tightened all engine bay compnents.

as to the post above. that was one of my first thoughts. i have not found any ground loose as of yet. i did however improve the dinky little ground wire from the UIM to the firewall. and it may have help smooth out the driving, barely, but it didnt cure it.



i put in my power fc last night. following dales suggestion, i went to close the idle bleed screw and then open it a half turn. to my suprise, i bet i turned it, easily 10 times, before it was closed!

i checked everything an fired it up. idle sounded great! it quickly settled to about 900rpm, and just sound really smooth. i let it learn the idle for quite a while as suggested.

driving impression: i like it! the PFC has made the car run considerably smoother while accelerating. I have not WOT in 1st yet, as i was still researching the safety factors concerning the "base mod" map that comes on the PFC.

it does however accelerate perfectly smooth in 1st gear, drivning slowly, where as before it tried to buck a lot. i have a slight hunch, it may be cured.

last thing, ........looking at my TPS through the commander, im seeing

VTA1 .34 and WOT 4.46

VTA2 .76 and WOT 4.97

are these off enough to cause any problems?

Just a simple question, have you done anything electrical to your vehicle prior to this?
Are you currently using the stock ECU? This sounds very similar to my problem I had before. What I did was when I was doing a fuel pressure testing, I had to have the car running and pull the fuel pump relay to have it die off residuel fuel... When everything tested and done, test drove the car, everything happened exactly what you've described above. I had stock ECU, later what i figured was, I need to reset the ECU (this is done by: taking off negative battery cable, have foot on brake for 10-30 seconds, reconnect negative cable, ECU is reset) Test drove the vehicle, no more violent bucking whatsoever.

In this incident, I figured that Mazda does not tell us tech/or owner that, when working with anything dealing with electrical, if they see the KL15 (key on signal) the ECU will set a fuel cut to prevent things from hurting themselves. Therefore, I think you might've done something while you had the key in the ignition on, and causes the ECU to react violently to that matter. Therefore give it a try and see what happens and let us know more from there on.

-AzEKnightz

update.

The car has performed wonderfully for about 2 weeks. Ever since i put in the PFC idle has been wonderful and no more hesitation or bucking........until tonight.


took it for a drive for the first time in a few days, and its doing it again:Wconfused nothing has changed...?

so the PFC rules out a bad ecu, or fuel cut.

my TPS settings are very close to what they should be?

i cant figure out why it was completly cured for 2 weeks, and now its back:Wconfused

yeah your TPS settings are fine they are within the correct ranges. Do you have a way to monitor the fuel pressure while it bucks/hesitates?
It really does sound like some sort of fuel cut problem. Not from the ecu cutting fuel, but maybe the fuel pump intermittently shutting off or failing? an injector sticking? How are the wires on your injectors? somtimes they can get baked and then the shielding will fall off and the two wires can rub.. I can't remember if the power FC commander shows injector duty cycle.. if it does could he monitor each injector individually? then watch to see if one of them shows the duty cycle drop off as it hesitates?
Just ideas

bad O2 sensor maybe? thats what caused the same symptoms in my FC (ls1, I know I know, blah blah blah) and it was a hard one to diagnose but a new O2 sensor fixed the problem.

good luck.

under full throttle the ecu doesnt use the o2 sensor for the fuel maps... most guys running the PFC turn o2 feedback off anyway..

This may be a ballpark suggestion, but I had a very similar thing happen on my 300zx when I replaced the radiator. I figured, why the hell am I getting violent bucking after changing a radiator? It turns out that I ever so gently knocked the CAS connector and caused it to get a poor enough connection that the ECU occasionally went into panic mode (I guess as the connector intermitantly made/lost connection) cutting my ignition and fuel. It was a horrible feeling. Pulled the connector, cleaned it, and reinstalled with great results.

I have this same problem! but only when my gas is almost at E

That is called fuel starvation. Do a search on it. Good way to blow a motor also.

To the OP, what boost are you seeing and at what RPM? Does it start bucking at a specific Boost level or RPM level?

Also, what mods was the PFC tuned for? Installing a used one can be a dangerous proposition (i.e. blown motor) if you don't know what the map was tuned for.

^ it is a used PFC, and im running the base mod map.(i have intakes,and DP) BUT this problem was happening before the PFC.


quick re-cap ,as it seems a lot of posts have not read the whole thread.

It violently bucks/hesitates in FIRST gear only. it was happening BEFORE the PFC. and as far as i know, nothing has changed to cause it.



i did some testing tonight.

it did it over and over, until i decided to smoothly accerlate past the transition to the second turbo in first gear, THEN i WOT around 4600rpm, and it was smooth, and pulled hard. I re-did this test a couple times, with same results.

then i tried it again from a slow roll, in first turbo range. and it was smooth! i tried many times again, including launches from a dead stop, and it was smooth.


so, confused:Wconfused as to what happened, i did some more pulls while watching my commander and my boost gauge more closely. I didnt notice anything that jumped off the screen of my commander,....grant-it, i may not notice anything even if it was off, as i am just learning all this.

My boost gauge in 1st never exceeds about 8 psi. it will run to about 8,drop to 5(a problem im having in all gears) and by the time it has a chance to get boost back up to around 8, its time to switch gears.


so as of this second, the car works again. and i still have no idea whats really going on. when it works....it works GREAT! when it doesnt...well...



is there a way to see fuel pump voltage from the commander?

install a wideband and see what the AFR's are when the hesitation occurs.

well your boost pattern is off then... you should have a 10-8-10 boost pattern with the stock turbos. Probably a vacuum hose somewhere. Or a bad boost leak. After numerous problems with my whole vacuum nest crap i went non sequential and now single turbo. it's so much simpler.

It's hard to say what your original problem was. Especially without having more way to monitor vital parameters.

^ yes, boost is off. and its been off since i bought the car. Its a mazda reman with new turbos, and they hooked all kinds of things up wrong.

ive just recently got it to 10-5-10, after 6 months of owning it. im pretty sure the boost pattern has little to nothing to do with the bucking. it didnt do it when i bought it....it just developed it out of no where? and then it dissapears just as quickly. ...."there have always been ghosts in the machines"

my boost pattern is pretty good in all other gears, except for the drop to near 5psi, instead of 8.

No but you can check sensors in the etc menu iirc.

Next time it starts happening, repeat the test procedure and isolate the RPM, boost etc where this happens.

The FAQ thread has a good link to turbo troubleshooting if you want to start researching your boost pattern. Judging by the boost levels, I'd probably suggest first checking if the restrictor pill in the wastegate line was replaced by a silicon hose that doesn't have a restrictor. That limits boost to 7 PSI but sometimes shows up as 8 PSI on diff boost gauges.

ok, thanks. ill try to isolate everything as best as i can.

and as fa as troubleshooting the turbo system......im unfortunatly, reasonable versed in the subject:icon_tdow lol. Im currently running a blitz sbc id EBC, using version 2 of the wastegate diagrams. i suspect i may have to try version one instead and install a pill in the prespool line. see, my car, since ive owned it, has never had the pills. Mazda never reinstalled them when they replaced everything, and they also hooked up a lot of vacuum lines wrong. and when i bought it, it was making 4-0-0 :lol:

slowly but surely ive been correcting all the issues, and making minor upgrades along the way. ill update if i isolate it or if it returns and i have more info.

thanks

update please...what become of the issue?

its still doing it.:icon_tdow and it appears now to definitly only be related to the first turbo, in FIRST gear. Same as before, if i accelerate gently through the 1st turbo range, and THEN WOT, everythings fine.

also, i was adjusting my blitz EBC last night. i turned down my gain settings and the car is decidedly quicker. but as i was testing, i developed a NEW problem! alright! ........i guess it could be related.

after some hard runs, i would slow down to acceptable speeds, even have the car in neutral for a while. then, when i would go to boost again, theres no first turbo! it happened many times but after playing with the throttle and changing gears a few times, it would come back online.

im aware of the turbos being parrallel after 4500rpm and back down to low(is it 3000rpm), but this was not that. i was at idle for a while! somehtings sticking. sounds like to me, they could be related.

whatcha got for me degeeseman!:)