M20B29 hesitation at WOT above 5k

[sert57]

I tried the oem chip and it didn't change anything, and have had this chip reflashed once and still no change.

[Kershaw]

It's too bad you can't swap out another chip/dme. Did you ever try to run it on the stock motronic? Since you have AFR, maybe try it and see if it has the same issue? I would back off pretty quickly if I hit 15.9+ AFR at wot.

[sert57]

I was finally able to get a good video of the issue. AFR is in the lower right corner. You can see it run fine until about 5k when it leans out very suddenly.

[digger]

What is the afr like immediately prior to the issue ? Ignition issues can show up as lean.

[sert57]

brand new bremi cap / rotor / plug that seem to work fine at all rpm at partial throttle and up to 5k at WOT.

Applied some dielectric grease to the C191 connector and that didn't change anything.

While sitting in a parking lot i had a friend hold the engine around 4k rpm and i wiggled the connectors for each injector but couldn't get any issue to pop up

Hooked up a wideband AFR. At the problem area the mixture leans out instantly ,so much, it maxes out the gauge ( using an AEM setup, so > 18).

This seems to rule out ignition system and point towards an electrical gremlin.
In thinking either CPS signal as previously noted. I may look into some simple emf shielding next to see if I can get any change.

The issue isn't consistent in terms of what rpm it hesitates at each time, but its always above 5k.

[Slicktop_serg]

Cap and rotor?

Have you checked the cap and rotor? i had the same issue after changing bunch of parts like fuel pump, plugs, wires, coil,vaccum lines, etc changed it all and still had issue changed cap and rotor and she drives like a bat outta hell.

[digger]

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

[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.

[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.

[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.

[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]

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?

[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.

[digger]

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

[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.