WOT power dip issue
03-20-2019, 10:44 PM
#1
Rennlist Member
Thread Starter
WOT power dip issue
I'm trying to track down an issue and could use some help. A week ago at the track, and since then, I’ve been hitting a momentary power loss at around 5000 rpm only at WOT or close to it. It’s just a split second where power seems to die and then it’s fine again, and only around that rpm. If I use less than full throttle, it goes through that range with no issue. This wasn’t happening before the track and the only thing I had done recently was an oil change and brake flush.
other notes:
other notes:
- I had driven in the rain the day before going to the track. The last time I did this and was having issues, one of the ignition modules went bad. I’ve unplugged each module and the car runs fine on either.
- I have checked the resonance flap with a diagnostics tool and it works fine that way. Haven’t checked with vacuum, which I think is how it normally works?
- I’m getting a code for knock sensor 2. I don’t know how recent that is. I’ve tried cleaning the connector a couple times, but it looks new and I don’t see much change. Maybe I should try contact cleaner and grease?
- i noticed a vacuum line near the knock connector is dry rotting. Maybe should do a smoke test?
Any other ides on why I would get a power dip there and only there and only at full throttle?
03-21-2019, 11:43 AM
#2
Race Car
Work the vacuum issues first- they are a low hanging fruit. Also pull the right handle intake manifold off and see if the resonance flap is sticky when fully closed. Clean the flap and the walls of the throttle body there, and sticky is a common problem, it may be that it sticks for a moment and then after being under full vacuum for a bit, let's go and works properly.
Also the check valve for the vacuum is commonly gunned up and can be replaced for 20 dollars. If that isn't functioning or barely functioning - you exhaust the vacuum in the canister that the resonance flap uses to open, and you will have intermittent flap function.
Dry rotted vacuum elbows and hoses- just replace. Easy enough. Once that is done, if the issue persists...then move on to the next...
Also the check valve for the vacuum is commonly gunned up and can be replaced for 20 dollars. If that isn't functioning or barely functioning - you exhaust the vacuum in the canister that the resonance flap uses to open, and you will have intermittent flap function.
Dry rotted vacuum elbows and hoses- just replace. Easy enough. Once that is done, if the issue persists...then move on to the next...
03-21-2019, 09:33 PM
#3
Rennlist Member
Thread Starter
Thanks a lot. I’ll work on the low hanging fruit.
After a lot of search, the vacuum line I see that is cracked is for the charcoal canister.
After a lot of search, the vacuum line I see that is cracked is for the charcoal canister.
03-21-2019, 10:57 PM
#4
Rennlist Member
Originally Posted by Sirenty
Thanks a lot. I’ll work on the low hanging fruit.
After a lot of search, the vacuum line I see that is cracked is for the charcoal canister.
After a lot of search, the vacuum line I see that is cracked is for the charcoal canister.
You can test the vacuum with a cheap tester and cycle the ignition to operate the solenoid and test the flap operation.
https://rennlist.com/forums/964-foru...refurb-65.html post #962
04-14-2019, 09:51 AM
#5
Rennlist Member
Thread Starter
Still trying to track this down. I did a smoke test and there were no leaks. What I have noticed is the colder the ambient air temparure, the worse it gets. I drove the car in 30 dog weather, and the engine basically died for a second when it hit 5k. As the air warms up, the effect lessens.
This makes me think the flap is working but the engine can’t handle the extra air? Replaced plugs, wires, caps, and rotors thinking it might be weak ignition (plus didn’t know how old they were) and it does run a little smoother but no change in the issue. Also changed the exhaust and got the o2 sensor working again.
Next steps are to replace the fuel filter in case it’s weak fuel, and inspect the flap and intake while in there. Sound good?
This makes me think the flap is working but the engine can’t handle the extra air? Replaced plugs, wires, caps, and rotors thinking it might be weak ignition (plus didn’t know how old they were) and it does run a little smoother but no change in the issue. Also changed the exhaust and got the o2 sensor working again.
Next steps are to replace the fuel filter in case it’s weak fuel, and inspect the flap and intake while in there. Sound good?
04-14-2019, 10:19 AM
#6
Three Wheelin'
Knock sensor code is unusual. You’ve gone through more than one tank of fuel to rule out bad gas? If the knock sensor pulls timing, you’ll feel the power loss. Though it won’t feel like hitting a wall.
Running lean would cause a massive power drop, and likely trigger the knock sensor.
How are cold and hot starts? The cylinder head temperature sensor, or air intake temperature sensor could cause lean running. You would likely notice rough hot or cold starts.
Good luck!
Running lean would cause a massive power drop, and likely trigger the knock sensor.
How are cold and hot starts? The cylinder head temperature sensor, or air intake temperature sensor could cause lean running. You would likely notice rough hot or cold starts.
Good luck!
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04-14-2019, 07:27 PM
#9
Rennlist Member
Thread Starter
Yes, gone through a tank of gas, and I just added more to make sure. Thought that might be it as the first two times it was fine. But then I punched it at 4100 and nailed the hiccup at 5 again.
Hot and cold starts seems fine. I did have a problem this weekend during an autocross. Before my fifth run, the car did not want to catch. It would crank and crank. Finally gave it a bit of throttle and it caught and ran rough for 5-10 seconds and then evened out. This was after sitting for 5-7 minutes. I let it idle for a bit before shutting off after that and didn’t have any more problems.
I’ve scanned after three drives now and I’m not getting any more faults.
WOT switch looks fine.
Power seems fine. It pulls strong. I was fourth fastest at the autocross despite this issue.
Hot and cold starts seems fine. I did have a problem this weekend during an autocross. Before my fifth run, the car did not want to catch. It would crank and crank. Finally gave it a bit of throttle and it caught and ran rough for 5-10 seconds and then evened out. This was after sitting for 5-7 minutes. I let it idle for a bit before shutting off after that and didn’t have any more problems.
I’ve scanned after three drives now and I’m not getting any more faults.
WOT switch looks fine.
Power seems fine. It pulls strong. I was fourth fastest at the autocross despite this issue.
04-15-2019, 04:44 PM
#11
Rennlist Member
Thread Starter
I should’ve mentioned I unplugged each ignition module and it started and ran fine on either one. Coils are still stock black coils though.
I replaced the DME relay yesterday for giggles. The one in there was made in Hungary and had a different diagram on it. New one is from Germany and has the 964 part number on it. Might be my imagination but it feels like it runs even smoother and has more power. Still a small stumble at 5k WOT though.
I could swear it didn’t do this last year, and the only thing I had done before the issue arose was an oil change.
I replaced the DME relay yesterday for giggles. The one in there was made in Hungary and had a different diagram on it. New one is from Germany and has the 964 part number on it. Might be my imagination but it feels like it runs even smoother and has more power. Still a small stumble at 5k WOT though.
I could swear it didn’t do this last year, and the only thing I had done before the issue arose was an oil change.
04-20-2019, 06:29 PM
#13
Rennlist Member
Thread Starter
Fixed the issue. Thanks to some locals and a thread on the 944 board, I checked the AFM internals even though I had been told the voltage was good. There was a worn spot near the end of the wiper arm travel for 2 of the 3 contact points. I adjusted the arm travel to use a different path and no more dead spot issue. She's purring now.