WOT dead spot - Dorkiphus.net

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Old 04-17-2019, 08:47 AM
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Default WOT dead spot

Something I’ve noticed since the HPDC this year is I have a dead spot at 5k in the 964. It basically feels like the engine dies for a moment then comes back full power. It only happens when I am WOT and always at the same RPM. It seems correlated to ambient air temp as the colder it is the “harder” the engine dies.

I have:

1) verified the resonator flap is working through a Durametric.
2) smoked the intake but found no leaks.
3) replaced plugs, wires, caps, and rotors.
4) replaced the full B&B exhaust with a stock exhaust, minus race cat and j pipe
5) I was getting fault codes for o2 sensor and right knock sensor. After 3 and 4, I am no longer getting codes.
6) gone through tanks of gas and just added fuel cleaner.

Other than the dead spot, the car feels awesome. (And the new/stock exhaust is so much better. Awesome sound, no drone, no noticeable difference in power).

My next steps seem to be to replace th fuel filter and maybe the coils, since they are still stock. Does anyone know how I can test if it is weak spark, fuel, or something else? Anyone seen something like this?
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Old 04-17-2019, 10:06 AM
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Quote:
Originally Posted by Modian View Post
Something I’ve noticed since the HPDC this year is I have a dead spot at 5k in the 964. It basically feels like the engine dies for a moment then comes back full power. It only happens when I am WOT and always at the same RPM. It seems correlated to ambient air temp as the colder it is the “harder” the engine dies.

<snip>
My next steps seem to be to replace th fuel filter and maybe the coils, since they are still stock. Does anyone know how I can test if it is weak spark, fuel, or something else? Anyone seen something like this?
Hmm. Those seem unlikely, as the engine power comes back to high rpm. A clogged fuel filter or bad coil would both likely not behave that way (e.g., bogging would continue).

It doesn't look like this engine has variable cam timing, but can you confirm?

ed
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Old 04-17-2019, 10:32 AM
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What about the chip? What fuel are you running? Air/oil cooled cars and their fuel/air ratios fall off right in that rpm range. My bet is natural behavior showing itself. Our cars HATE today's gas. Go see John Behe and have him adjust the air/fuel ratios it will make a world of difference and you will see exactly what your motor is doing. I did this for my car and the difference is quite impressive. Bad news is 100 octane is expensive (kinda like HoHos). Good news is a tune will even with 93 octane post air/fuel tuning will be very different. My HP/Tq numbers are closer to table top post 4k rpm versus the backside of a rollercoaster!
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Old 04-17-2019, 10:43 AM
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Is is 5k everytime, load or no load? Belt good in distributor? Also, airflow meter clean? Can also check the voltage to check operation.
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Old 04-17-2019, 02:01 PM
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Quote:
Originally Posted by Dandelion View Post
Hmm. Those seem unlikely, as the engine power comes back to high rpm. A clogged fuel filter or bad coil would both likely not behave that way (e.g., bogging would continue).

It doesn't look like this engine has variable cam timing, but can you confirm?

ed
No variable timing, but around that RPM, a resonator flap opens in the intake to let in more air. My theory is a clogged fuel filter could be reducing the fuel pressure, or bad coil reducing spark, so when the extra air comes in, it can't handle it for a moment. Though I see your point that it probably wouldn't recover so maybe those are dead ends.

Quote:
Originally Posted by Hunter View Post
What about the chip? What fuel are you running? Air/oil cooled cars and their fuel/air ratios fall off right in that rpm range. My bet is natural behavior showing itself. Our cars HATE today's gas. Go see John Behe and have him adjust the air/fuel ratios it will make a world of difference and you will see exactly what your motor is doing. I did this for my car and the difference is quite impressive. Bad news is 100 octane is expensive (kinda like HoHos). Good news is a tune will even with 93 octane post air/fuel tuning will be very different. My HP/Tq numbers are closer to table top post 4k rpm versus the backside of a rollercoaster!
I'm running 93. I've been debating tune and I was going to see if I could hook it up to a wideband to see if I could get any clues to what is happening at that moment.

I've also been curious what chip I'm running. I'm almost positive I have to be running one because it has a LWF but doesn't stall, and the throttle response is very good. How can I tell?


Quote:
Originally Posted by cmartin View Post
Is is 5k everytime, load or no load? Belt good in distributor? Also, airflow meter clean? Can also check the voltage to check operation.
Yes, 5K every time and only at WOT. If I go past 5K not at WOT, it doesn't happen. Belt is good. I unplugged each ignition module separately and it ran fine on either.

I'm not sure about the AFM or the intake. I know I should take the intake apart and check that and the flap, but I've been trying to avoid it. I'll look up how to check voltage.
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Old 04-17-2019, 10:12 PM
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Could be a dead spot on the AFM wiper board, very common. This happened to mine, same symptoms. If you unplug the harness from the meter and put a voltmeter on it then manually move the vane through its range of motion you will see a sudden drop in continuity at the affected spot, once past it it will pick back up close to where it left off. It can be corrected by moving the wiper arms into a new unworn area of the wiper surface. Easiest way to do it is to loosen the screws and shift the contact board slightly downward rather than mess with the actual wipers. There a few videos on YouTube showing the process.
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Old 04-17-2019, 11:27 PM
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^^This. If you don't have an analog ohm meter, find one to borrow. The old black Simpsons are the best. Watching the needle move, and then dip as you manually move the door will speak volumes as to the health of the carbon trace rheostat inside the AFM. a good cleaning and realignment of the wiper arm to a new spot can solve the issue.
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Old 04-18-2019, 10:14 AM
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Re: dead spot on the AFM board - I thought the same thing too. Problem is, Aaron says this is happening at WOT. There are two switches on the throttle body - idle and WOT - that are enabled in those positions (pins 52 and 53 of the DME). At WOT the ECU ignores the variable signal from the AFM (pin 7) and switches to the WOT map based on the input from the WOT switch.

So... do you know if you're hitting and enabling the WOT switch? Unhook the DME and test that with a multimeter. Should be continuity to ground at WOT.
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Old 04-18-2019, 03:49 PM
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Thanks, guys. I had a shop check the AFM voltage and they said it was good across the board, but I can test again.

I've checked the WOT switch by moving the throttle by hand, and I can see and hear it click, but I have not tested that A) the throttle pedal moves it that far and B) that the switch is working. My assumption is it is, because if I am just below the WOT threshold, I don't have the problem. It's only once I cross into it.

I did find a mention on rennlist about an oddity at 5K in the stock fuel maps (See https://rennlist.com/forums/964-foru...l#post11493599), where it drops fuel to near idle levels for a moment, right where I am seeing the issue. This makes me lean towards Ken's thoughts that it needs a professional tune.
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Old 04-18-2019, 04:56 PM
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We can always test the theory quite easily, pull the chip out of my car and put it in yours and go for a drive. PM me if you want to try.
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2003 996, (Speedy) Grey sold


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