Low rpm bucking

[Varinn]

I've got a bit of a problem that is more of an annoyance.

S52 with OBD2 and M50 manifold. It bucks HARD at low rpm/low throttle crawling. Basically any time I'm cruising in stop and go traffic it'll at random cut all throttle for 1/4-1/2 second then kick back in. It happens frequently, and I drive in stop and go traffic frequently. I do have a decent diag suite, but not sure how to use it to solve this (I've got ISTA, etc). Engine has no codes.

I'm thinking my TPS, throttle stop, cable slack etc might be wrong or out of sync causing it to dance between detecting fully closed throttle and 1-5% open. Anyone have a source on the procedure to set this up properly? (My TPS has been replaced, no change).

I think I have a small exhaust leak on my rear header flange after the o2 sensor, possible this is distorting the readings? Doesn't leak at idle, only under load.

MAF? ICV? Any other ideas?

[sweet3]

Mine did something similar after I had just done my swap. If you have a K&N filter or something similar, try put the MAF sensor connector on the top and see what happens. That fixed my problem. I thought that was strange, but it did the trick for me.

[Varinn]

Simple enough to try, I'll give it a shot

[328ijunkie]

What tune? What maf? Any codes?

[Varinn]

Stock S52 MAF that came with my motor. I can double check the P/N on it. No codes last time I checked but I can hookup INPA today and run the diag again. I'm actually running your tune Dave. O2 sensors are new. I do remember seeing something along the lines of my throttle reading in INPA showing as being 14% open or so even when fully closed (cable disconnected to confirm it's not running out of slack). I can double check that today too.

[328ijunkie]

Last 7 of vin on ecu?. That throttle reading is normal. It will read 15% to 85-90% max

[Varinn]

Ey73444

[328ijunkie]

That's an old flash from April 2015 sold to a "John.." Has it always done this or it just start?

[ak-]

Maybe something between you and the tuner but I had this problem on my m20 which has a variable tps just like your 24v motor (same sensor actually).

My WOT wire was cut/disconnected.
Second time it happened, my tps wasn't adjusted correctly when screwed in (or maybe got loose?).
Anyways, in my experience, it was tps related on both occasions.

[Varinn]

That's an old flash from April 2015 sold to a "John.." Has it always done this or it just start?

From what I can see in my emails it's always done it, ever since I first installed the motor in my car.

Original tune was by TRM in 2013 for stock obd2 manifold (before you started doing your own flashing)

Updated tune by yourself in early 2014 to add the m50 manifold.

Nothing done flash wise in 2015 and my name definitely isn't John


ak- I do still feel like it's something along these lines but 24v motors don't have that slotted TPS mount like an m20 has. They're fixed position and non-adjustable. I've already replaced mine with a new one at some point and it didn't change anything. I can definitely verify my wiring is intact though on the MAF but I suspect I would be having more issues if I had anything as drastic as that. Car dyno'd well at over 230whp, never stumbles or behaves strangely above the bottom rpm "button" throttle issue I'm having.

Maybe something between you and the tuner but I had this problem on my m20 which has a variable tps just like your 24v motor (same sensor actually).

My WOT wire was cut/disconnected.
Second time it happened, my tps wasn't adjusted correctly when screwed in (or maybe got loose?).
Anyways, in my experience, it was tps related on both occasions.

[ak-]

Okay. Good to rule it out then.
My last low throttle bucking issue was my fuel pump. I didn't use to be able to let go of the clutch in 1st gear so the car can roll on its own. It would just start bucking. Low throttle inputs would also make it buck - makes it look like you're new to driving a manual.
Megasquirt confirmed my fuel pressure was fine on throttle, idle, and on ACC but I replaced the pump anyways as I've ruled everything out.
Car drove like brand new after and realized my new pump was much quieter than the old one.