968 Engine Knock sensor on full tank

dav968 · 04-16-2017, 07:02 PM

Hi all,
I've been having a problem for the last couple years with my 1995 968 and have taken it to a several dealers/mechanics to try to figure out, but no luck so far.
To try to get to the root cause, I recently bought Durametric so I could read engine codes myself.
Here is what I'm seeing.
Mainly when I have a full or nearly full tank of fuel, my car hesitates when I try to accelerate after cruising at a constant speed. Sometimes it comes out of it in 1/2 second, other times it runs rough for a few seconds. Sometimes the engine light comes on, other times not. It does seem to be getting progressively worse, though slowly.
After scanning with Durametric, I saw I had 3 error codes registered, Hall Sensor, (23) Oxygen Regulation / Stop, and (32) Knock Sensor 2.
I've replaced the Hall sensor and both knock sensors, reset codes, took it out for test drive and all seemed great, even seemed to have a little more power (maybe in my head), until I filled it with fuel, then the (23) Oxygen Regulation / Stop, and (32) Knock Sensor 2 error codes were back within 10 minutes.
My assumption is that the (32) Knock Sensor 2 error is leading to the (23) Oxygen Regulation / Stop error, if this assumption is wrong, please correct me.
Has anybody else experienced anything like this?
Any help is much appreciated.

Jfrahm · 07:40 AM

No, but one would assume it could be related to the evap system, have you tried leaving the gas cap loose? Disconnecting the evap system and capping it off at the vacuum side? You cannot cap off the tank side as it needs to breathe somehow.

Knocking would seem to be indicative of leanness maybe from a leak via the evap system.

dav968 · 04-17-2017, 06:27 AM

Hi Jfrahm,
Thank you for the response. That makes a lot of sense. I just went down and loosened the gas cap, and had a small amount of pressure in the gas tank that was released. I'll try driving it today with the cap loose since that's quick and easy. If I still have issue, I'll figure out how to disconnect evap.
One of the items I had done in the shop was replacement of the tank pressure valve as the tank was building up pressure on hot days, and I had to fill tank slowly because of back pressure. Those seem to be resolved now, but I still think this is the right track.
On a side note, I'd love to see a pic of the supercharged 968. That sounds like a fun ride.
Thanks again,
David

flash968 · 04-17-2017, 01:58 PM

can we assume that you have premium fuel in there?

it is not uncommon for these cars to develop tank pressurization issues. a number of people have been chasing down the same issue over the years. some found the charcoal canister jammed up. others found the vent line buggered.

here's a shot of my supercharged 968 engine bay:

dav968 · 06:56 PM

Wow, that is AWESOME!!!
Maybe one day my wife will agree to getting a coupe 968 and doing the same. A man can always dream.

Yep, I do use premium from large brand gas stations only. I've had the car for about 10 years now, so don't think it's a fuel issue. Last year I replaced the pickup and level sending unit in the tank, and saw that the tank looked nice and clean. I did put in 2 bottles of Techron to clean injectors/combustion chamber when I filled last time, something I do once or twice a year to try to avoid buildup. I pulled spark plugs when doing sensor replacements last weekend, and all the plugs looked really good still, have never had an issue with buildup on them in the past.

I think something in the evaporative emissions system is the culprit, but that is still just a guess.

So, tried driving yesterday with the gas cap loose. 40 mile trip to a client location in morning produced no error codes at all. On return trip though, I got an error code on both knock sensors, but no hesitation while driving.
As a little more back info, before getting the Durametric, I also changes out the cap, rotor, wires and plugs, twice, trying to resolve the hesitation issue. Other than the coil, I think the ignition side of the equation is not the source of hesitation.
This weekend I'll see if I can figure out how to disconnect the evap system and see if I can isolate that as being or not being the cause.

I have a long trip up to an autocross this Saturday, so we'll see how that goes. Running hard and on a less full tank doesn't seem to cause any hesitation, so think it will perform fine during autocrosses again this year. Be interesting to be able to read codes and see if all is doing as well during the event as I feel it has by seat of pants.

dav968 · 09:31 PM

I had a few minutes and curiosity got the better of me, so I ran down to the garage to see if I could figure out how to disconnect evap. I think I found it. On driver's side, between tower and firewall, I see a plastic vacuum valve with a hose going into fender wall. I disconnected it, and could smell gas. Just for the heck of it, I blew into that hose to see if it was obstructed, and could hear air coming out in the fender. Is this normal? There did not seem to be any resistance to air flow.
Have I found the right spot to disconnect, and do I need to plug either the valve or the hose before starting engine? --edit, just re-read your post and see you say to cap off valve side, so will do so.

thomasryan · 04-19-2017, 06:41 AM

You might remove the tire and the fender liner to get to he canister. Give it a shake to see if it has failed internally.

Emissions systems are petty simple. Expanding vapors get trapped in the charcoal then get purged through the motor once it is warm. The pluses valve under the brake booster will open once the engine is warm and the dashpot valve allows flow under high vacuum. Check your little rubber elbows to see if they are petrified or have turned to mush after 25 years.

If your drive time is short and you drive on bumpy roads, there may not be enough time to pull down the pressure from the expanding fuel.

dav968 · 04-19-2017, 07:18 AM

Hi Thomas,
I'll do that, would like to visually inspect the canister also, just to know it's ok. Looks like they are a little under $200, so probably won't replace unless I see something wrong.
I did check the vacuum and other lines, and they appear to be in good shape still. The car has had a pretty easy life, mostly sits in garage until a nice day comes.

dav968 · 04-19-2017, 07:23 AM

I think while I'm doing this work, I may also pull the air flow meter and throttle plate to give them a good cleaning. I've never cleaned either, and I've put about 60k on the car since getting it.

Jfrahm · 04-19-2017, 07:58 AM

I fixed a typo in my above post.

You have to cap off the line to the evap system or have a big vacuum leak. The evap/gas tank side can be stuck into a fuel filter so it can breathe, this is not a big deal for testing. It could be as simple as doing nothing (not a lot of air will go in) or putting a coffee filter or something over the line and ziptying it into place. Air has to go in to replace the gas that flows out or the tank will go under vacuum resulting in various problems.

The assumption is that you are going lean somehow as that will cause knock. You may also have a problem with fuel flow causing the lean condition or maybe a vacuum leak from the Evap system And they could be related.

You might have multiple problems, maybe it's a MAF issue in general or an O2 sensor issue or both and maybe an evap system issue also. Hard to imagine a MAF or O2 sensor issue could be related to the amount of fuel in the tank. Ultimately you will want to restore the evap system to normal operation but simpler is better when problem solving.

It's not a bad idea to also look for other intake or vacuum leaks, particularly ones that might open up under acceleration like you might get from the intake boot after the MAF, there is an elbow there that can crumble or crack. If you have had bad engine mounts that can cause cracks in stuff like that.

If the knock issue remains you might disconnect the O2 sensor in order to run in open-loop and see if in that condition it does not knock. The evap system might be a red herring but we can take it out of the game for troubleshooting.

Cheers,
-Joel.

thomasryan · 04-19-2017, 09:23 AM

this.

check the hard plastic vacuum line and elbow that is mounted on the top of the throttle assembly housing to the purge dashpot. if you have a mightyvac, test the dashpot also.

dav968 · 04-20-2017, 03:15 PM

Last night I pulled the AFM and cleaned it and the throttle plate. Both looked good, but cleaned anyway. I inspected the vacuum line running to the throttle plate from the dashpot, and it looked good. Thanks for the suggestion Thomas. Today, with the evap line disconnected and plugged as Joel suggested, I took the car out for about an hour drive. No codes, and the throttle response was great, no hesitation at all. Since this was an intermittent problem, I'm not ready to say I've found the issue yet, but it's looking good. I'll continue driving with evap line disconnected and plugged for a few days, and do another fill up with it disconnected. If I don't see the issue, I'll reconnect evap line and see if it comes back. That should narrow down where the issue is, I think.
Thank you both for the suggestions!