Bucking when accelerating from stop

drew10101010

drew10101010

Greenie N00B Member
Getting hard bucking to where the car stops violently for a second, let go of the gas and it idles normally. Can't drive it normally unless I keep rpms extremely low and low pedal depression. Log attached. Anyone notice anything?

Only 3k miles on brand new 1 step colder NGK iridium plugs (standard one Justin suggests). The other day I blew a fuse ENG BAR2 and the car wouldn't crank. New fuse and it works normally again. But now bucking issue.

I did add a new downpipe, vented catch can since the last tune. The new downpipe is slightly better than my old. Both 3" catted.

Cleaned MAF, all intake hoses connected. Log is idle to light pedal and it bucks hard.
 

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Things to go over:
1. Doublecheck plugs, just to be safe. I'm pretty sure these aren't the issue, but maybe you'll find a loose coil or compressed coil spring or something.

2. Doublecheck for post-MAF leaks. Double check the air filter too, and try rotating it 90 degrees (maybe do this first because it's easier).

3. I'm not really a fan of double slit straighteners. If you have the chance on a windy day, start the car with your hood up and see if the idle changes randomly; this will be a big clue if true.

4. Clean the MAF; also let the car idle and wiggle the wires about 4" away from the MAF. I've had this issue where it would change signals when wiggling and cause the idle to change. Fix for this was buying a replacement connector off Amazon and wiring it in.

5. The vented catch can could be a culprit. How is it routed? Does it have a checkvalve to prevent un-metered air?

Next log, after the above if the issue persists: Replicate the issue once and then stop the log immediately after it recovers. This helps isolate what's going on in the log to make it easier to see rather than having to guess where it's happening.

Your airflow is all over the place though so I'ma guess air/MAF issue, which if I'm right is a fairly easy if not annoying fix considering the housing you have.
 
Got a new log, it's short and right when it happens. I noticed the maf goes very low. MAF problem?
1. Re-gapped plugs, cyl 2 or 3 was a little wide at over .026. And another a little less. Re-gapped all to .024.
2. Will check again. Boost leak test next.
3. Haven't done this.
4. Cleaned MAF a few days ago. Wiggled maf and harness hard while idling -no change.
5. It was running fine with the new catch can setup. It's a Damond plate and I'm using all their check valves. 1 for pcv inlet with arrow towards manifold. Two to a vented can with check valve arrows towards can. It ran for roughly 500-750 miles no problem.
I noticed the maf voltage shoots up to 2 and the g/s also goes wonky.
I drove it after doing the plug gapping and spring stretching. Springs were 81mm before now all stretched to 84mm. Also installed new oem main relay.
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springs.png
 

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Please post pic of engine bay; emphasis on intake side.
Preferably fully assembled and recent (after the above photos were taken).

Edit:
Looked over the log pretty quick:
upload_2024-4-4_14-26-37.png

This looks like airflow reading error; probably less of an issue at higher throttle inputs.

After posting the above requested pic for perusal, you can try running the car without the MAF plugged in; it won't be fast but if it drives "normal" at a lower throttle input, you will probably want to change your filter at a minimum or preferably resolve the straightener issues.

Can discuss this further upon the above being tested.
 
I'll try with MAF unplugged. So what does it mean I can do if it's an airflow error? Retune? My filter is huge and only slightly started to get a little dirty.
20240404_174126.jpg 20240404_174134.jpg 20240404_174241.jpg
 
It means air is getting pulled across the maf inconsistently due to crooked air filter, lack of straightener or airflow through the front of the car disrupting it
 
I did run it for a bit without the air filter on, could that have messed it up? I've had the same intake setup for about 4k miles now and just recently it started bucking.
 
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Did boost leak test and found only a small leak at the clear plastic hose that goes to the valve cover from the intake but it's always leaked a tiny amount. I think something else is wrong. Air was coming out from my valve cover breather and when that is closed my new catch can which was pretty fun to see. Also reflashed to an older map, map 3 and same. So weird. I'm getting no codes.
 
Yeah, when boost leak testing, it all comes out of the oil cap breather. If I put my hand on it it builds up and when releasing it gushes out.
 
I misunderstood. Chances are one of your PCV / check valves might be sticking. Maybe pull them and clean them out?
Would be a weird issue but would explain some things.
 
U know what I could try is blocking off the two vented ports. My pcv valve has unknown miles on it. Can a bad pcv valve cause this? When I installed the Damond pcv plate I cleaned it out with brake cleaner and listened for the spring and tested it with air and it was good. Also when boost leak testing I can hear what sounds like check valves activating. lol
 
The sudden burst in g/s reading could suggest that you are no longer getting crankcase gasses in that instant, suggesting that one of your check valves is closing. What catchcan setup are you running? Post the DM catchcan pic here, or let us know what number it is.
 
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