Full Throttle Loss of Power

Pyrenee2005 · 11-06-2013, 08:57 AM

I have a tough problem I would appreciate input on. 85 Carrera, with headers and open exhaust. About 4 months ago I developed a partial loss of power only at full throttle, can happen from 6000 to 7000 RPM, when it happens you feel the car stop pulling as hard and hear the pitch in the exhaust change, when it happens sometimes after a second or 2 it will clear itself out and if it doesn’t 50% of the time lifting the throttle and going back to full throttle will clear it, sometimes I have to lift off the throttle a few times to clear it. When it happens I can also lift off the throttle say 400RPM and hold it there and it clears out and then going back to full throttle also clears it out 50% of the time as well. It can happen once a lap to several times a lap to not happen all day to happen all day, it can happen in the first session of the day or the last session of the day, it can happen if the air temp is 60 degrees or 100. It feels fuel related as the loss of power is smooth, does not miss and stutter like an electrical miss might. Very hard to isolate because it is so intermittent and won’t happen on the dyno.

I have a fuel pressure gauge and when it happens the fuel pressure is unaffected.
The DME has been checked out and I have tried a different DME,
I have also replaced: coil, fly wheel sensors, temperature head sensor, throttle position sensor, air meter, DME relay, spark plugs, plug wires, rotor, cap

Bob Kontak · Bob Kontak's Garage

Rev limiter. Betcha.

Carrera 3.2L Rev Limiter Problem - Speed Sensor Solution

The 3.0 SC's were the only models allowed to rev over 9,000 from the factory. I mean, that's what I have heard.

steely · 11-06-2013, 02:51 PM

Bob is usually right - but for fun as a test, you could disconnect the WOT position sensor and see wot (what - sorry) that does - if it replicates the problem or creates new ones - it'll be another clue.

Bob Kontak · Bob Kontak's Garage

Quote:

Originally Posted by steely

Bob is usually right - but for fun as a test, you could disconnect the WOT position sensor and see wot (what - sorry) that does - if it replicates the problem or creates new ones - it'll be another clue.

Dan, Thanks.

My guess is only based on post in the thread I linked that says that the rev limiter will kick in over 6k. The symptoms sure sound like it, though. Alternatively, maybe a spark jumping at the coil at high RPM's?

steely · 11-06-2013, 06:03 PM

Bob,

I think you've nailed it already with the rev limiter. I saw this in work, couldn't post, came right home and posted and split for a meeting before I even read your link.

Dan

Pyrenee2005 · 11-06-2013, 06:42 PM

Thanks for the replies, to clarify this problem is very intermittent, and it does not feel like hitting the rev limiter, car just stops accelerating as fast, but it is still accelerating and you can hear a different pitch in the exhaust, but it is smooth, there is no stutter or pops like a rev limiter or electrical miss, it always happenes at WOT in the 6k to 7k range. Many run sessions the car runs perfectly, some sessions this happens several times a lap and there are sessions every where inbetween. When it occurs backing out of the throttle a little clears it out and makes it stop 50% of the time, sometimes you have to back out a couple times to get it to clear out.

I did disconnect the throttle postiion sensor and tried a new one and it made no difference it did the same thing.

The big clues are its intermittent and when it occurs you can fix it by backing out of the throttle for a second and then back on the throttle, usually once will do it but sometimes it takes a couple of times

Tippy · 11-06-2013, 07:07 PM

I wouldn't recommend disconnecting WOT switch doing a WOT run as the richer AFR's and retarded timing will not occur as it's supposed to.

steely · 11-06-2013, 08:42 PM

Quote:

Originally Posted by Tippy

I wouldn't recommend disconnecting WOT switch doing a WOT run as the richer AFR's and retarded timing will not occur as it's supposed to.

Not to sound like a jerk, but isn't this the point - since it already isn't running as it is supposed to?

This is just a question out of a fair (and admittedly large) degree of ignorance, but would leaner AFR's and lack of retarded timing yield the symptoms that Pyrenee2005 is experiencing?

because...

Quote:

Originally Posted by Pyrenee2005

I did disconnect the throttle postiion sensor and tried a new one and it made no difference it did the same thing.

if you never ran with it purposely disconnected, how would you know you didn't have an intermittent connection to the WOT? (read: harness) Unless something on the TB was preventing complete closure of the micro switch - like mechanical vibration. Or is it plainly obvious that the WOT -> DME interface is golden?

forgive me if I am over-simplifying - but I just figured this if it wasn't the rev limiter or DME you could have a physically jumpy signal from the WOT due to an open wire or switch.
Tnx

ben parrish · 11-07-2013, 06:39 AM

Look at the AFM wipper..could have a bare spot in the carbon track..can verify with an ohm meter.

Pyrenee2005 · 11-07-2013, 07:20 AM

I put a new AFM on at the track and it did the same thing, there was no change.

The biggest clues are:
-Its intermittent, can happen once a lap or several times a lap or can go can 15 laps without it happening
-only happens at fuel throttle from 6k to 7k
-50% of the time it clears itself out after a second
-if it last longer that a second can clear it 50% of the time by lifting off the throttle and going back to full
-if that doesn't work lifting a few times takes care of it
- when it happens there is no miss of the motor, it just doesnt accelerate as fast, maybe 30% less, but still accelerating and you hear the pitch in the exhaust change.

I have a fuel pressure gauge and when it happens the fuel pressure is unaffected.
The DME has been checked out and I have tried a different DME,
I have also replaced: coil, fly wheel sensors, temperature head sensor, throttle position sensor, air meter, DME relay, spark plugs, plug wires, rotor, cap

Pyrenee2005 · 11-07-2013, 08:30 AM

Questions: My fuel pressure gauge is located right after the fuel pump and my fuel pressure is constant when the motor looses partial power, could the pressure regulator mess up and not show up on the fuel pressure gauge with the gauge hooked up in this location? I would think since the fuel is a looped system it would show up? as I stated backing above backing out of the throttle and going back to full throttle clears the issue, backing out of the throttle also increases vacum and moves the diaphram?

Has anyone heard of a broken valve spring causing an intermittent problem as explained previosly above, maybe when it vibrates into the right position at full throttle the loss of power occurs and then lifting off the throttle and and back on again moves it into a different position?