M20B29 hesitation at WOT above 5k

[sert57]

I've been sorting out all the kinks on my 2.9L build and am getting confused with the last little issue.

Issue:
The engine falls on its face at WOT above 5k rpm. It will continue to pull to redline but it is choppy and not nearly as powerful as 3/4 throttle. It is fine at 3/4 throttle going all the way to redline, and fine at WOT up to 5k. It almost seems like a fuel starvation issue but I cant identify a culprit that would do it ( see below)

Specs:
M20 2.85L ( S50US crank, eta rods, B25 pistons)
Bimmerheads 272 w/ nuk cam gear
IE HD Rockers
IE headers to custom midsection w/ O2 sensor bung
M52 pink top injectors
SSSquid custom chip to above mentioned specs

Potential culprits:

AFM: Potentially? It worked fine on my car before the build, but it has never seen the higher volume of air before that WOT would give. I could certainly see where there is a dead spot because it is not worn in on the higher airflow.
TPS: I tested the resistance and it works fine at idle and WOT
CPS: doesnt make sense with the issue only at WOT
ICV: doubftful because not idle
Vacuum leak: once again, doesnt make sense to me since it works fine at WOT from 4-5k
Chip: Initial instinct, but i had it reflashed for a few small adjustments i wanted to make ( redline and octane) and it still does it. i am doubtful
injectors: it has done the issue with the M52 pink tops, ford injectors, and known good oe b25 injectors, so i am doubtful
Fuel Pump: I installed a new meyle unit very recently, so I'm doubtful. I can T in a fuel pressure gauge and do a pull, but it gave ample psi at idle. Before the new fuel pump the old one would do the same issue
FPR: maybe? mine appears to work fine at all other situations though.
Spark Plugs: New Bosch units, i used the same type on my old b25 and they worked well, and I'm unsure how the WOT would affect this.
Ignition coil: potentially? I believe its still the original one, but it works fine at 3/4 throttle so I'm doubtful.

Does anyone have any ideas?

Thanks!
-Ken

[digger]

My gut tells me either an ignition issue (spark breakdown) or its way too lean e.g the wot switch Isn't being activated or fuel volume delivery issue . I don't play around with motronic anymore though.

[e30davie]

What guarantee is there that the sssquid tune is actually tuned for your engine? I mean i know the sssquid has a good name and all, but to me its the black box of unknown in the equation. Do you have a wideband o2 on the exhaust to see if its leaning out?

The WOT swtich is a good suggestion, but im not exactly sure how the motronic works, i would have thought that the AFM would still measure the air coming in and giving enough fuel regardless of the WOT switch.

Spark break down is the other option, have you gapped the plugs? what color do the plugs look like? do they look like the engine is lean or do they look black like the spark is breaking down and not burning the fuel?

[digger]

yeah, its easy to plug in a stock chip for a sanity check.

presumably the engine ran great before any of this? if so you are probably taxing the system too much so its either a mapping thing, fuel delivery issue or spark delivery issue which probably somewhat helps rule out AFM, FPR assuming those are carried over. i wouldnt rule out the coil. i wouldnt be looking at the ICV, CPS or vaccum leak. the TPS requires continuity checks. issues ive had with plug wires provided random missfires rather than this kind of issue.

had the exact same symptoms on a s50b32 just the plug gap being about 2mm, increasing rpm and cylinder pressure (load) taxes the ignition system more.

you will probably want to invest in an WB O2 sensor, as somewhere down the line you are going to need it anyway if you start upgrading stuff further (like the ECU to MS) and it will straight away tell you if its fuel or ignition with this issue otherwise a shop with a portable O2 setup will be able to help

[sert57]

Yeah before putting in the 2.9 I had a stock b25 that ran well with the same ignition coil, AFM, ICV, CPS, and ECU.
Just finish building the newer motor and have been shaking it down and breaking it in. New Bremi Caps/Rotor/plug wires and bosch plugs. different TPS that seems to work fine. Other than the described issue she runs very well.

Haven't picked up a wideband yet, but if i keep having issues I will.

Checked plug gap, it was all just a hair above spec so I bent them in spec. didnt fix it. Spark plugs look like its running a little lean but not too bad.

Changed out for the oem chip, didnt fix it, put the custom chip back in.

measured ignition coil resistance to be .6/6300 which isn't too far out of spec ( considering what others have gotten), but potentially worth looking at.

I recall when i changed the fuel filter out the original was backwards, so i double checked the orientation of mine and it was fine.

I T'd in a fuel pressure gauge and went for a spin, at idle I was getting around 37 psi and at WOT the gauge went a little hectic but seemed to stay above 25-30 psi for the most part.

I'm thinking ignition coil is the next shot

[digger]

Sounds like you have fuel pressure issues those wot numbers are way off what they should be

[sert57]

That possible, but its confusing to me since its a new meyle fuel pump and the FPR worked fine the B25 setup. I initially replaced the oem one because i suspected it running low too - at 37 psi, and due to a few other issues with the engine at the time that ended up being the injectors.

The gauge was laying on the engine during the test ( I was looking at it with the hood open), so I'd be hesitant to say the gauge going erratic is necessarily a fuel pressure issue.

I have another B25 FPR that is in rougher shape and a working M30 one I can borrow if needed to test. I'll probably try the M30 one at some point tomorrow and see.

[digger]

New parts are just new, not necessarily good. I'd suspect the fpr before the pump

[sert57]

Looked at FPRs today. did a quick bench test on bot hand they both held 3 bar fairly well. I went ahead and swapped the spare in and it didn't change anything, so i swapped the original back.

Tested the fuel pump as well. I used a valve after the pressure gauge to induce a backpressure or around 40 psi. At 40 psi the pump pumped 15.5 oz of gas in 15 seconds, translating to 110 L/hr or 1.8 L/min. This should be more than enough for my engine.

Only things i can think to check next are electrical related - swap in the known good but used plug wires, try to make the CPS air gap a little smaller ( i measured it in spec, but maybe there is a weird vibration of some sort), and triple check the engine ground and maybe add an alternator ground.

Heres a video to show it. it can be hard to hear it but if you listen carefully you can here the hesitation.

[sert57]

Did some more messing around today.

Changed the CPS gap from 1.1 to .7mm - no change.
Ran it with the tps unplugged - no change either. However the tps is tested and does work. Indicates to me that it's a fuel or spark issue as dinner noted previously.

I'll try to get my hand on a wideband setup and see what that tells me.

Afm still doesn't seem to be the culprit, as I'd be making peak hp between 4-5k and the problem is worse around 7k.

There seem to be a lot of people who don't like the Bosch spark plugs ( im running WR8LC) , is this something that i should be considering?

[Kershaw]

The oem motronic is not a very sensitive system when it comes to reading the CPS. You could be getting break up in the signal and the dme can't read through the noise. I had that issue at 4k rpm at wot and switching to megasquirt fixed it. It was a crazy issue to chase because I swapped everything at least 3 times. Engine harnesses, dmes, injector harnesses, coils, afms, tbs, etc... it's possible it was something with the resonant frequency of the chassis itself causing the cps signal to break up.

JLevie had the same issue on a race car and a brand new engine harness fixed it. Brand new shielding was enough to protect the signal. I considered this but 1. IX engine harnesses were NLA, 2. I wasn't willing to hack up a brand new i engine harness on a diagnostic, and 3. Megasquirt was the same cost as a new harness.

Mine was a much harder cut than anything in the video. But its something to keep in mind if you start eliminating everything else. And if its spark break up, then MS with coils may be the answer again.

[sert57]

Thanks for the reply! Yeah I saw your threads and certainly considered it, I think megasquirt will happen in the next year or two, but it would be nice if i can fix it before that. I hope to borrow a afm and ignition coil from a friend to run a quick test with to see if they affect it at all.

I haven't heard much about the sensitivity of the dme to the cps, but if it isn't too sensitive I could certainly see how interference could affect the readings. If that is the issue and the test above doesn't work i may play around with some quick and dirty shielding to see if it changes anything at all.

Got a better video of the hesitation, its much easier to see and hear in second gear.

[LowR3V'in]

i had a problem like that and it turned out the injector harness was really janky
and would vibrate enough at wot to where it would break contact for a sec.

[redlightpete]

i had a problem like that and it turned out the injector harness was really janky
and would vibrate enough at wot to where it would break contact for a sec.

+1. I skimmed the thread and didn't see where you had checked the injector harness connector under the manifold. If it doesn't look bad, you could test fire the injectors at the DME harness (per Bentley) and have someone wiggle the wiring to see if you can get it to misbehave.

In addition to vibration causing it to act up, the motor movement at peak torque might also contribute to it crapping out at higher rpm. My temperature gauge was also acting funny when this connector was a problem, since the sensor signal goes through it too.

[digger]

find a place that does dyno power runs and get them to log AFR