OK- it's been a while, so I think I'll post this again...
I was seriously thinking about selling my car at one point because of this one incredibly annoying problem.
This is no different when the engine is hot or cold...
I have a new TPS... it's good... it's adjusted. The S-AFC indicate about .7v (it's the full range plunger on the 90).
The car has ALWAYS had this problem as long as I've owned it (2 engines, 2 TPS's, 2 ECU's)
I am really at a loss here.. I cannot solve this problem.. and I'm not exactly clueless when it comes to 7's!

When driving the car, cruising at a steady speed, say at 10% throttle position- if I slowly let off the gas to slow down, the throttle touches a point where it goes into full decel (engine brake).. and it's kindof a bit of a jerk going into that... no matter how slowly you let up the gas... as soon as it hits that point... jerk... decell...

The WORST part, however, is the transition back to acell.. if i'm coasting in gear (basuically slowly slowing down) with my foot off the gas, and I want to start accelerating again, I touch the gas, and not matter how slowly I press the gas pedal, as soon as it's out of decel mode, (maybe pressed 2-5%) and into aceleration, there is a crazy buck, like the car is being bumped from behind with a mack truck to start accelerating again... then it's smooth after that... almost feels like there is 360 degrees of play in the U-joints or something- but not quite.. the driveline is definatly nice and tight.

Just that damn transistion point.....
It makes it very jerky and uncomfortable to cruise city streets at a steady speed.
PLEASE HELP!!
Could possibly having my clutch switch disconnected contribute (for remote start)- it mentions the clutch switch in the 90 FSM associations chart as an input for decelleration..

Please!... *crawling... digging nails into floor.... slowly... dying....*

 

My car did something similar, and it turned out to be a leak in the intake pipe after the AFM. I'm sure you probably checked this already, but it couldn't hurt to double check.

Are you throwing any codes?

Also try lubing the internals in the afm. If it's at all sticky, it could throw the mix off off. Since you've changed ECUs, TPS engines etc, it seems logical that it has to be one of the parts still in the car (AFM, inlet duct)

Also, Ignition timing, bad motor, trans, diff mounts etc.

Also, check the tb. The main butterfly (on the bottom) is supposed to open before the 2 secondaries when the throttle is depressed. If the clearance for the primary is off, it could cause what you describe. The spec is in the FSM

Well. that's my brain dump on the subject... any other ideas?

 

No codes (exept for the resultings ones for the ACV being removed... exept the light doesn't come on... they just come up when I check the codes.. odd)
NO vacuum leaks either- timing is fine (been at many different points...) plugs are fine (new or old, still jerks)
I just replaced an engine mount. It defnitaly isn't a drivline prob- I was just using the feeling as an analogy, but if you were in the car you would know it's coming form the engine.

Anyways... the AFM moves smoothly, though I have yet to check if there are any spikes in the voltage sweep...
One thing I did notice, is that when giving it a bit of gas, and watching the AFM cone (no filter on) it dives in like crazy for a split second when I give it just a bit of throttle off idle, then comes back out... I though it was the normal vacuum being releived...

I havn't checked the throttle plate stops... I'll do that post haste, I have a feeling about that! I'll send you a cake if you are right! Truffle or Lemon Merangue?

 

I have the same problem that you do... i've been trying like hell to figure out what it is also, however, it's not on the top of my list Even so, my dad and i hypothesize that it's the U-joint possibly... you said you thought about it, but didn't replace it, right? i'm looking at ordering one very soon (prolly this week), along with some toher things. just fer the record, mazdatrix.com has the u-joints for 86+ for a whopping 17.55 http://www.mazdatrix.com/getprice.asp?partnum=2651-60 if you wish, i can let you know if it fixes the problem (though no garuntees when i'm going to do it!

PS - It appears to me to be the throttle linkage or something related, though... (again, this is un-tested)

 

I have checked my driveline...
There actually is a tiny bit of play in the U-joints- but nowehere near enough to create a jerking sensation like it has. It isn't driveline anyways.

 

oh.. well, the throttle plate seems promising too I'm gonna take a look at mine when i have her up on the stands t/w

 

I had a similar problem actually... my TPS was misadjusted. What would happen with me: I would have very little throttle on, and then I would get the engine braking, with the jump when I got back on it. Well, my TPS was pretty far off, I could tell by my boost gauge; when I would be almost all the way off the gas (like 10%) the needle would jump, and when I was almost fully off it would jump the other way, like I had a dead spot. I adjusted the TPS, problem went away. If you already checked it, disregard

 

No acv, eh? Well, when I bought my 89, I swapped on a set of dual headers from my 87. These headers have a pickup tube for the 6p. When I swapped the headers to the 89, I just ran the pickup to the 6 ports like I did on my 87, and installed a block off on the acv.

This caused 2 things:

1. No VDI
2. Massive bucking when letting off the gas and re-accelerating (at rpm's below 3k)

I just decided to put the acv back on, and let the air pump vent into the atmosphere. At the time, i didn't have the patience to see what was necessary to bypass the acv. Fixed both problems for me.

Could the ACV code be causing this?

 

I have this problem on my RX-7, and also had it on my previous car, a 12AT-powered ‘84 Cosmo, which has an identical TPS to the FC. I firmly believe it’s related to the TPS, and since yours is new, it may also be an ECU-related problem. The TPS is the only input the ECU uses to determine deceleration fuel shut-off, so I can’t think of anything else it could be. I think it may just be a poorly designed system overall.
I’ve noticed that FC owners who install an aftermarket programmable ECU, which requires the fitment of that brand’s own TPS, report this throttle transition issue disappears and drivability is much improved, reinforcing my belief it’s TPS-related.
I’ve driven with this problem every day for the last six years, so I’ve sort of gotten used to it, but I still hate it as much as you do. So if anyone comes up with a proven fix, there’ll be a lot of happy FC owners out there!

 

Can i have my ecu back yet? CJ

 

Just making a guess here but maybe the dashpot? Since that is what drop your gas slowly while you let out so it doesnt just drop to no gas right away. right?

Nick

 

Dashpot- ya, but I can let off the gas SOOO slowly, it's just as son as it hit a certain switchover point where the electronic transistion is, it jerks...
The decell isn't so bad, its the transistion to acceleration that gives the big buck.

The TPS is new.... and well adjusted....

I'll check the throttle plates tonight...

As for the ACV valve... I ruled that out because my car bucked from the day I bought it, (full emmisions intact)but I just remebered that it didn't work though!!!
I'll check into that..

 

I have a stupid noobie question, where is the TPS at? my haynes doesn't say anything about it.... thanks!

 

yeah i have the same bucking problem with my '89 n/a. I've adjusted my TPS a few times, it reads right at 1.0V, but the resisance readings are a little off for the closed throttle position. Do you think this is a problem, or does the CPU not care what the resistance is as long as the voltage is 1.0V? My throttle cable is a *little* sloppy, I haven't tried tightening that up, but I don't think thats the problem because, like you, I can just barely press the gas (slow enough to take any slack out of the cable) after coasting, and it will buck like crazy. If you figure it out, I'd be more than happy to know what you did!
Jake

 

have you done the tb mod? also have you tried different settings for decell on your s-afc? try also taking adding some slack to your throtle cable. and maybe adjusting your rich lean screw. i'm pretty sure it has nothing to do with the tps or ecu and such because mine started to do that a bit when i did the tb mod, my throtle cables were a little tight, the rich lean needed adjustment, as well as my timing hope some of this helps you.

james

 

Anyone know the fix for this because i also suffer from this issue.

 

2 things wrong in your first post. You need to set the TPS by the narrow range plunger. The full range doesnt affect fuel delivery. Second, it needs to be set at 1V at idle, not .7V. Ground the initial set connector, and then set the TPS to read 1V at idle on the narrow range TPS.

 

It's funny though... whenever I set the TPS using the different methods- the other 2 of the three (lights, ohm, V) will be way out....
I'm going to ground the TPS now...back later....

 

The ohms dont mean a thing as long as the voltage is correct. The ohm method it just to get you close. The voltage is THE way to adjust it. If the voltage is right, your problem isnt in the adjustment of the TPS.