Hi all,

I have some time over Christmas and want to try to either sort out the issue or identify the cause. I have got some advice sometime ago but wanted to start fresh.

I've owned my FD for 11 years and this issue has been from the very beginning since owning it. Given it's a weekend car, it doesn't bother me much but its the only issue so I want to fix it. When I purchased the FD, it was relatively standard. It has a PFC and exhaust with a manual conversion using an auto harness. It was a known issue even to the mechanic (rotary specialist who knew the vehicle) and was recommended to do a full re-wire ECU to fix it. I just want to eliminate other possibilities before doing this.

What the issue is:
I get quite a bit of jerking back and forth most noticeable at low RPM and more so now with a twin plate clutch. It happens at certain times. First, when I let off the throttle completely, after a second, there is a hiccup (rocks forward). Secondly, if I hold the throttle at a particular point (1/4 throttle ish) it will constantly rock back and forth. And lastly, if I let the car decelerate , when the rpms get to 2500rpm or lower it jerks sporadically. It makes getting back on the throttle jerky as well.

I've got 2 videos to help understand the issue though may be difficult.

Things i've done so far...
1. All mounts have been replaced and confirmed it isn't drive train slack

2. Double checked it isn't tuning related (been on the dyno numerous times and checked everything)

3. Replaced TPS and re calibrated it. TPS reading via PFC
VTA1
Fully Closed - 0.85
Flly Open - 4.56
VTA2
Fully Closed - 1.73
Fully Open - 4.98

5. I did get my rotary mechanic to look into the issue a bit while doing a single turbo conversion. They checked over everything and their only conclusion was wiring and again to do a full re-wire ecu. The wiring does appear to be a bit butchered.

If the problem is in fact wiring, I am happy to eventually do a new harness or wired in ECU but wanted some guidance from you all to cover all basis rather than just believing a mechanic who wants money.

Any help is appreciated.

 

I'm in a very similar boat with my car and I believe it's a mechanical issue but I can't quite figure out what causes it. I have run my stock ecu and it does the same thing even after I tuned my car.
Personally, I believe it's caused by:
1. Bad FPR
2. Fuel Damper issues
3. Ignition coils
or 4. One of the many sensors

I believe that it's a cycle of slight lean events that becomes constructive enough to rock the whole motor but I have no idea if that is the problem or not.

I wish you luck in the hunt for a fix!

 

Even with my almost totally OE driveline, I get jerking, mostly in 1st gear (rarely in 2nd) during low or no acceleration. It is caused by play/springiness in the driveline components causing a resonance between the rotating and non-rotating (the weight of the car) masses. It's been there since new.

When I owned my 1st-gen RX7 (SA), it had exactly the same behavior. I've also had similar behavior on several other cars. I have just learned to drive around it. So I never drive at a constant low speed in 1st, because 80% of the time I'll get the resonant/jerking behavior. Once you learn how to avoid it, the technique becomes the way you always drive the car.

 

If you have an adjustable FPR make sure your base fuel pressure is at the correct level. I recently had to reset mine and at the proper 43.5 psi that I was tuned at pretty much all of the jerkiness went away..... and I'm running a Japanese market Exedy twin disc with their lightest flywheel.

 

Do you have a lightened flywheel?

 

Thank you for all the responses.

Fuel pressure is at the correct level.

Sure am. The jerking is more noticeable since the twin plate and lightened flywheel

I've driven my friends FD and it's so much smoother than mine. Jerking in first is expected, but I get jerking in every gear and low RPM and when letting off the throttle.

 

Have you checked the throttle dashpot? If that is not working it will allow the throttle to close too suddenly, possibly causing "jerking."

 

I have replaced it and adjusted it in the past. Whats the best way to check if it is actually working correctly?

 

To be honest, I've never checked mine, since it has always functioned properly. Sorry to be of no use here.

 

If your throttle slams all the way closed without being slowed by the dashpot, then it needs to be replaced or adjusted.

 

What RPM are you idling at? You have the TPS set too high, the range is .1-.7v and .75-1.25v when warmed up and fully closed. With the PFC I shoot for the high end of the spec.

It is hard to tune deep vacuum, very light throttle areas on the dyno. Can't tune full brake stalling issues on a dyno at all. If you have a wideband installed it is much easier to see why the jerking/bucking is happening it can be from overly rich or overly lean.

Lower the TPS setting first, so that the PFC can see that the car is actually at idle.

 

Thank you. I must of adjusted the settings when it was cold rather than hot. I will adjust it to the higher setting when I can and report back. It idles at around 1000rpm. It does have a street port with 2000cc injectors so it is lumpy.

 

I adjusted the TPS today. It was a little out but now in spec

New figures
VTA1
Fully Closed - 0.1-0.7 = 0.55
Fully Open - 4.2-4.6 = 4.4
VTA2
Fully Closed - 0.75 - 1.25 = 1.25
Fully Open - 4.8 - 5.0 - 4.98

I've noticed a change in throttle when I get back on the throttle after decelerating. It previously felt more jerky as though there was a flat spot but seems to be much better.

I don't have a wideband but when this issue was being looked at on the dyno, apparently there was no change in AFR's as originally it was thought to be a tuning issue but they then deemed it mechanical/electrical. I took another video today to show you some more behavior.

Driving in second... let the car decelerate (you can notice the hiccup when I let off) it decelerates fine but when it goes down to 1500rpm, it jerks quite a lot. I don't let it decelerate that much but just wanted to show you what it does.

Any other suggestions?

I'm not sure what is normal behavior but I tested it today. It looks like only the bottom butterfly (out of the three) slowed down but only at the very last couple of mm's. The dashpot is completely tightened. I can only loosen it if I had to adjust it.

 

I let the car cool down a bit today and took it out again. The car was 70 degrees Celsius (150 degrees fahrenheit) and cruising out of my street at 2500rpm, it didn't hiccup when I let off the throttle. I never really drive it when its cold so never thought to see if the issue only occurred when it was at operating temps. I will take it out tomorrow when its cold to see but all the jerking certainly happens in the first 1/4 throttle.

 

What are your fuel cut setting on the rev/idle screen? If you could just post up your map it would be a whole lot easier.

 

Sorry it's been a while. I'm sorry I don't have the Map.

 

@rexhvn did you end up smoothing it out fully? I have a 55k original miles ‘95, pretty minty and I rarely find anything out of shape. I have an intake, DP, hi flow cat, catback, manual boost controller to keep boost under 9 psi, since I’m still on stock ecu. but I’ve been experience the jerky on/off throttle at lowers rpm’s (under 3500 or so) as well, even in 4th gear. No issues steady state acceleration or decel, only transient tip in / tip out.

I measured TPS, it’s in range. Definitely not driveline slack since I just replaced engine and diff mounts (FFE 68 durometer engine mounts, Mazda speed diff mounts). I also adjusted my dashpot per service manual, helped a little but not the smoking gun.

I’m going to test the other emissions valves, but ultimately I’m thinking the intake (which was my most recent engine mod) flows enough that it’s too far out of range for the stock tune. I read another thread where someone with low miles stock ecu like mine but stock air box found a screw missing on their air box - after clamping correctly their problem went away. Not sure that’s considered a “vacuum leak” that far up stream, but in any case is “tune related.”

im also going to adjust my throttle cable and pedal, gonna **** myself if that’s the solution. I’ll also check my wideband for lean condition.

 

First off, a missing airbox screw won't affect anything at all. Don't know where you saw that but that's a big red herring.

Is your air pump working? If it's removed or if the fuse is blown and it's not engaging the car will have rough on/off throttle and rough idle with a stock ECU.

The only vacuum leaks that can have any effect on drivability is post throttle body, between it and the engine.

Dale

 

Thanks Dale. additional air upstream of the throttle body/MAP sensor aligns with my conventional knowledge, I'm really just grasping at straws here lol my air pump is real noisy especially with the HKS RS intake, so it seems to be working and I can hear it coming on at part throttle and on decel -- but I will check it again for proper disengagement. My idle is great, which makes this more difficult because I don't have many of the other symptoms I've seen in other threads, when the car is not well damped on tip in/out. My idle may be a tad high (tach shows 800 - 900 rpm, will check it at diagnostic port), would throttle plates set too open at idle affect tip in/out?

other things I've checked:
MAP sensor vac tube - no leaks
MAP sensor voltage - good at idle and with vac tube removed and plugged, did not do the other pressure checks because I don't have a vacuum tester
adjusted my throttle cable - feels better, but didn't fix the issue
IAC valve - anti-afterburn check was good, will do other voltage checks
same issue with A/C on
Tried to read my Wideband O2 real time, but it moves quickly so didn't find anything conclusive

spark plugs are only 6k miles old, but will replace them along with O2 sensor (only 15k miles old)
My diff is not making any noises or acting any differently
My battery is 8 years old, but voltage somewhere around 13V from memory and has no issues starting
I will check my cat for temps before and after it, as well as do a visual inspection for clogging (Bonez hi-flow, 8k miles old)
I'll also check for vacuum leaks at throttle body or after, with starter fluid

- Chapin