Eli_Swer

I built a Braineack MS3x for my 1994 and have gotten the car running very well in stock form. This one issue however has alluded me. When I am low rpm and let off the throttle to slow down to say a stop sign in second gear, then get back on throttle around 1250 RPM or so the car bucks a little bit, but not like the car is cutting out, more like my foot is bouncing on and off the throttle which is I think what is happening. I just hadn't noticed it before but maybe thats because if i am very careful and roll into the throttle rather delicately the bucking will not happen. Am I in my head? Is something wrong with my tune? I have messed around with the accel enrich but ended up changing it back to near brains settings. Here is a log of the incident. you can see the waves in the rpm as they rise. The PW is bouncing as well, this makes me think I am just bouncing my foot on the throttle accidentally? My throttle cable is VERY tight and the slightest touch opens it. I suppose tomorrow I will try to back it off some and see if that helps.

Edit: This is only apparent in second and third gear it seems, maybe because those two have the torque neccessary to bounce my foot at low RPMS?

Eli_Swer

Ruled out the bouncing pedal, the car still does it even if I am wide open. I loaded it brains exact accel enrich settings. I did a couple more logs and it looks like when I can pedal through the fluctuations it's because my tpsdot is not fluctuating too much either. Maybe I have a bad tps? The pw looks steady on wot as well as the AFR. Maybe I'll check the plugs and wires? Although they only have like 5k on them. It has been probably two years tho because I just don't drive the car a ton, especially with these cold northeast winters.

Rrrracer

Looks like it is related to AE, there are definitely big PW spikes. Can you post the tune please?

Eli_Swer

I can try driving the car without AE and see what changes but I really dont think thats the issue. I wasnt having any problems with it about a month ago and havent changed it. It seems like this issue has gotten progressively worse without tune changes which makes me think its mechanical. I tried cleaning the ground near the throttle body and to no avail. Here is another log where the pw looks fine to me, some spikes where TPS AE and MAP AE go off but otherwise steady. There is also a pull in there where I managed to finesse the pedal enough to not get a ton of bucking. Its the third ramp up in RPM.

I am going to try the classic plugs and wires even though theyre not very old. I did see a little bit of moisture (oil) in the spark plug well of cyl 3 and 4 and read its possible for the plug wires to soak that up and arc to the plug well wall instead of the plug itself. The plug wires dont look different and it was a small small amount of oill but I'll try anything to get this to stop.

Eli_Swer

The bucking doesnt feel violent, Its like a smooth oscillation up until it goes away. I've dealt with a dying CAS and that was much more violent, which makes me think this has to be tune related because a timing/spark issue wouldnt feel this way. So I am just back and forth on this and cant seem to get a bead on it.

Rrrracer

If it was working fine before, then the issue appeared, and then progressively got worse with no changes to the tune, it would almost certainly be a mechanical/electrical issue... but the key would be that it was working fine before.

To me it looks like it is dumping quite a bit of fuel on TPSdot AE; the threshold and breakpoints are fairly low (max TPSdot in your log was 490 and the curve tops out at 200) but more importantly, the added % is based on Req Fuel and your numbers are pretty large for stock injectors and a Req Fuel of 11.4. As it sits, it is injecting 30-80% more pulsewidth during AE events... that's a lot. With smoother throttle inputs, the AE would not be as aggressive and it would not be overfueling as much.

You could just turn AE off, but that most certainly would not help your driveability LOL. Check out the attached revision, all I have done is changed the AE curves to something a little more appropriate for your setup. Give it a shot and report back.

The rest of the tune could use some love as well, but most notably, the WOT pull in that log is very lean; it's hovering between 15.5-16 AFR (if everything is calibrated and those values are able to be trusted) and target is 13.8 (also too lean.) You should probably fix that before really putting the screws to it again.

Eli_Swer

Thank you so much! I will give those AE settings a shot. I tried to do my own rendition of a better curve so I'm interested to see how my newb tuning skills compare haha. Also you're right on the quick WOT pull it gets a little lean although up there in the afr table the target goes from 15 to 14.6 to 13.5. I will definitely have to get back into those cells a few more times with auto tune but I promise the rest of my map in higher rpm's is tuned much better.

edit: I do agree brains AFR targets are a little lean in most areas but I'd rather just trust him than monkey with it. Worth mentioning the maps look like I am turbo but I am still NA, soon to be turbo so I'm not resizing them just to have to change them again.

Eli_Swer

Unfortunately those AE settings were not nearly enough fuel. I went back to Brains settings and did a little tweaking of my own through MLV.

This is much easier to drive around now, but still bucks a bit on harsh throttle applications and let-offs. I think I am going to chalk it up to a slightly off AE tune as well as driveline lash issues that are highlighted by the extra torque this car makes on Brains tune. Also it had been a while since I drove the car so maybe I just need to get use to that gas pedal finesse.

The guy I bought the car off of a couple years ago did have diff bushings he gave to me so maybe he knew something I didn't, might get a GoPro under the car to take a look while driving.

I also got the VSS working, for whatever reason brains tune had it set to 40000 pulses per mile instead of 4000 which is correct. Got my wheel and tire diameter in there correctly, and now my car doesn't randomly go into idle at low speeds. Thus makes putting around parking lots and such so much easier because the car doesn't need to transition in and out of idle at low speeds, it simply will only go into idle under the threshold I set of 5 mph.

Still going to try new plug wires, if nothing changes I won't be surprised and I'll just return them to Amazon.

Eli_Swer

Update, wires did nothing. Not surprised though. The car is pretty drivable anyhow just gotta make sure to finesse it at low speeds, still a beast carving up backroads. I do have a weird scraping sound from the trans sometimes. Got a video of it while I was sitting still in neutral and clutching in. Bad throw out bearing?? If it's seized enough I feel like that could be a culprit for the bucking. Good news is I plan on getting a Supermiata organic clutch in the car this winter anyhow.

Also I am not sure how/if I can move this thread from megasquirt as it seems to not be a tuning issue anymore. Help??