Supe

Hi all. Recently picked up a former Chump/Lemons car to run SCCA ITS from a friend. When the car was parked a few weeks ago, it was running fine. New to Porsches, so hoping you all can help me figure this out before the TT event at Charlotte a few weekends from now.

When we go to start it last week, it starts no issues. Half a turn of the engine and it fires right off. There are no issues at idle - no stumbling, no hunting for RPM, no overheating.

However, if you bring the RPM up to 3000 RPM, it hits a wall. The missing/popping is very apparent, though it doesn't sound like any given cylinder is missing each time. It will do this at idle, or under load, no difference in the RPM it misses at or the frequency of it.

Some details:

• No apparent grounds we have found issues with, including the braid at the back of the head.
• Plug wires/distributor cap look OK, albeit some play in the cap indexing with that upper spring terminal that holds it.
• Pulled the #1 plug - not beautiful, but not seeing signs of being excessively fat/lean. Smells faintly of fuel, but wasn't soaked.
• Pulling the vacuum lines to the fuel pressure regulator/damper resulted in no change. I have not checked pressure at the rail yet, as I need to find a gauge that will fit (anyone know what thread size is under that cap at the end of the rail?)
• Pulling the TPS sensor lead - no change in performance or idle.
• Reference sensors - Disconnecting the sensor on top stops the car from running if you disconnect it. Disconnecting the sensor on the bottom, the car keeps running/idling as normal. The bottom sensor runs to "B" on the block, towards the front of the motor.

If any of the above description means anything to you guys, some help would be much appreciated! Video link below so you can see what it's doing.


http://vid144.photobucket.com/albums/r162/Supermatch1813/Porsche%20Idle%20Rev_zpsqgdic4gx.mp4

konakat

You got like 30 replies on your Facebook post, have you tried any of them yet? You're going to get the same responses here.

Supe

Had posted here first before a friend posted up to FB.

Things that haven't been looked at yet are coil, FPR (need an FPR gauge for that goofy thread), individual injectors. Rotors/wires looked fine, though we will have to check at night to see if there is any arc jumping. We did the usual wiggle/move of wires on connector pigtails and plug wires while testing with no change. Trying to exhaust the free options before throwing parts at it. I've read mixed things on the speed/reference sensors, one being that it should run with either sensor plugged in at a time, and this one did not. When the top sensor was unplugged, the car would die. When the bottom sensor was unplugged, the car would continue to run.

snb13

AFM

Supe

What would the symptoms of a bad AFM be?

Tiger03447

Although it seems remote, check the heat on the Catalytic converter with a laser thermometer..If it's plugged, or something went awry therein, you could have a problem there..You did check it for dead squirrels, right? How about on the induction end of things? Is that OK too? Should be something kinda simple causing this..IMHO.

Supe

No cats on this car, race only. Air intake is clear w/ K&N type filter.

snb13

I had similar symptoms. I couldn't rev past 3,500, but the car drove fine. Just seemed like 3,500 was the ceiling.

The electrical strip inside the AFM gets a track worn in it. There's a procedure on Clarks for addressing it, however it's temporary. This is a good excuse to upgrade to a true MAF.

Supe

Thanks for the feedback. We'll open the AFM up this week and look at sliding the wiper arm forward/back if the strip is well worn. Trying to get this thing up and running for the rest of the season, can look at MAF upgrades from Rogue, Miller, etc. over the winter if they're class legal.

harveyf

Not sure you're going to see anything when you look at the AFM. You need to get a copy of the WSM and check the terminals designated in the WSM for the correct resistance response. In general, you will see a smooth rising resistance as the mechanically operate the flap in the AFM.

There are also how-to articles about checking the AFM here on Rennlist and at Clark's Garage. Do a search.

odonnell

Here's how I check the AFM, you'll need to remove it from the car to do this test:

Tiger03447

Thanks, Michael...great post! Neat video..I'll check mine out one of these days!

Supe

Good video, thanks. I'll have to pull out my meter and leads. I did pop the cover off the AFM today. The typical worn grooves, with two spots in particular about 1/4 way in being more worn than the rest. I moved the wiper arm down on the shaft to give it fresh material to ride on.