Multipass

Thank you for the offer Darren, but looks like Porsche changed the AFM type in mid 85:
http://www.clarks-garage.com/shop-manual/elect-22.htm

Wipeout

There's not a CEL - a bad O2 sensor wouldn't trigger the ! light, if that's what you're asking, but the car would run like crap after starting.

I've got the O2 sensor out of my '85 (three wire, as it was a California car) as well just sitting in my garage. You can't solder O2 sensors because of how they work...it's over my head but effectively they "breathe" through the wire and soldering prevents that from happening. You can use crimping tools (I believe) to connect the generic sensor instead.

Bummer, was hoping the AFM was still the same. Guess that'll go on my "things I can sell" list.

drift a 944

Well, I never had to glue mine back on actually. I just press it down and the old silicone holds it in place really well. It takes a bit of force just by hand to remove the cover now, but I can take it off and on all day without worrying about it falling off.

However if yours falls off too easily once you try to reattach it, you can always just put a dab of silicone or something in one or two of the corners, so that you won't have to pry like a mad man again should you ever have to remove the cover a second time.

Hope it all works out!

Multipass

Thanks for the tips.
I found OEM replacement O2 sensor on Amazon, of all places, for $76. It needs to be changed every 60K miles, so I figure it is time anyways.

I'll crack that AFM tomorrow.

gtroth

I'd be looking at the AFM too. Maybe a big vacuum leak (I'd measure vacuum). Stuff that would misread the mount of air being used and convince the DME it needed less fuel than required
If you use silicone under the hood, let it cure for 24 hours since I hear that stuff can foul O2 sensors.
And you should not solder O2 sensor harnesses (it should come with special crimps). Believe it or not, O2 sensors need O2 to slowly diffuse down the harness through the strands to act as a reference O2 level, and solder would be air tight.

Rock

I dunno about vacuum leaks here. Usually when the engine sucks in more air at higher speeds, it would mask the problem. Its at the lower RPMS that vacuum leaks tend to show their ugly heads.

Now it could be an air restriction. Did anything get sucked up into the AFM? I had a BMW 735i suck up a piece of paper and wouldnt revv past 4k at all.

Also, I had a BMW 740IL that wouldnt rev past 3k. This was because the catalytic converter was clogged. A good way to check for this is to remove the 02 sensor, creating a hole in the exhaust. If the cat is clogged the exhaust will have another place to escape from. You can also CAREFULLY tap the post cat side of the the exhaust. If the piping after exhaust is cold, while the cat is burning hot, then that could be a problem. Just because you pulled air with a vacuum, doesnt mean that it cant be clogged. An engine at 5k rpms is going to be spewing a lot more than a vacuum cleaner can. It doesnt have to be completely clogged for it to restrict the engine at higher RPMS.

FWIW- You could disconnect all of those little vacuum lines and the 02 sensor, and have it still rev to redline.

Multipass

I opened up the AFM and cleaned it up. The voltage reading steadily rises as the flap opens (as expected).

Rock, you are right, I guess I still don't know if the cat is clogged or not. Right now the exhaust is off the car, I am waiting for the new O2 sensor and muffler hangers to arrive. I wonder if I can soak the cat in something to clean it, without damaging the function of it.

When I had the head gasket leak, the exhaust got effectively steam cleaned, so the cat looks pretty clean, I can only see the back of it. I have less then 1000 miles on the car after the top end rebuild. So for the cat to be clogged, the steam from the coolant either didn't clean the cat or it got clogged shortly after the rebuild.

One option is to replace the cat but that involves lots of money or a generic cat with muffler shop work. I was hoping to only do this when I am certain the cat is the culprit. Maybe I can borrow a clean cat or a bypass pipe from somebody.

I am concerned that removing the O2 sensor and leaving the opening will allow hot gases to go places they shouldn't. Have you tried this, driving over the road?

Rock

Ive done it many times. In fact I just did it today on a turbo car. You can drive it like that around the block and be fine. It will be louder no doubt, but if youre able to exceed 5k then id start looking into your cat being clogged.

Rock

Hey just an update for ya. Had a problem with my Turbo today, wouldnt rev past 4K for the life of it.

Ended up being the faulty TPS. You could always try disconnecting or wigglin the connector to see if it makes any difference.

Multipass

Thanks, I'll check that too, once I put it back together. Fortunatly this is not my DD.

gtroth

Hey Rock - just trying to understand here - if it sucks in too much un-metered air through a vacuum leak, won't the DME set the fuel injected way too low (run way too lean)?

Multipass

Well it is not a clogged cat, I finally put the exhaust back in without the O2 sensor and the CO2 sample tube and drove it with pretty much with the same results but more noise. I also cleaned and tested the AFM. I am now officially out of ideas on what could be causing this.

When the revs get to 5,000 the cars jerks pretty hard in 2nd gear, there is a poping sound and if I stay on the gas it revs a bit past the 5K and then jerks and pops again.

Is there any way to data-log the missfire or whatever is causing the 5K stall? The ECU must be seeing something wrong from the sensors...

carlege

test the TPS (throttle position Sensor)

Multipass

Checked the TPS, per http://www.clarks-garage.com/shop-manual/fuel-06.htm works as it should, basically a couple of on-off switches telling the ECU when throttle is open or closed. Car idles fine. Clark's Garage is awesome, by the way.

I am at a loss again as to what to inspect.

jwade944

Check rod bearings. Could be heating up and binding.