Dan Martinic

Dan Martinic

BTW what does the part highlighted in red mean? I DON'T get a reading when testing pin 2 at the DME, but I DO get a reading (3-4 ohms) at the TPS itself. How do I "Check power flow from plug to control unit"?

Tom M'Guinn

I think the red box is just saying to make sure the harness wire has continuity all the way to the DME/KLR. As for the exhaust popping, that's unburned fuel igniting in your exhaust. Some people like it -- sounds race car-y. My brand new 911 does it to some extent, though fairly muffled with cats and mufflers etc. If you have black soot on your plugs, that is an indication that you are running too rich (i.e., more fuel than needed), and the extra fuel is burning in the exhaust. If you are running off-the-shelf maps, it's probably rich by design to prevent detonation/pinging. Do you have a wideband and/or knock counter? Both are good investments. Also, do you have a catalytic converter? They tend to help reduce popping in the exhaust. You don't really want to tune a car without knowing the AFR and knock situation, but sounds like less fuel and/or more timing advance is in order. Beware, however, that too little fuel and/or too much timing, and you can blow out another head gasket. Did I mention that a wideband and knock counter are a good idea?

I suppose there's always a chance that an exhaust valve isn't sealing, but seem to recall you just had the valves done and, in any event, you'd probably notice other symptoms...

951and944S

I know mixture is spot on when I hear pop and see fire under the car from trackside with lift/braking zone.

Dan, you could experiment with the DME timing/fuel setting, this may stop the popping.

T

Dan Martinic

Thanks gentlemen.

I'll check continuity on that wire.. but I notice that the VOM sometimes reads a small ohm reading then gradually goes to infinity, so that's weird. But, clearly the DME is getting the idle signal as the cut-off is working (slight bump when it turns back on at 1200 rpm)

I've got the original DME with original chip (28 pin--one of the earliest ones I think; it's got very european-looking handwriting on it the chip itself!). I am dreaming of the A-Tune--I've bought the 3.0 fpr already--but I guess getting a wideband is my next move.

Yes, the head was brought in for a valve job. However, I have since been told that the place that did it has a reputation for.. ...let's just say the guy in the back likes to drink and screws up a lot of jobs. I never had this popping thing before I re-assembled the top end, though nothing else behaviour-wise has changed. Is there a quick way to tell if the exhaust valves are sealing?

To properly install the wideband, I understand it goes in the cat pipe (forward of cat) and the cat needs to be removed, correct? This is holding me back as I've just put the exhaust all back together with new seals etc.. a drag to take it apart again.. then buy those oddly expensive sealing rings so soon

951and944S

If you wind engine to 4000 and let engine brake your exhaust note will interrupt with random pops when your exhaust valves aren't sealing.

On the WOT then slam throttle plate shut pop you originally described,

Turn up volume and listen at 0.54, 1.14 and 1.30 The instance at 1.30, watching from trackside, I can see a flame before the pop 3' x 3' in size every lap (we run turn down just in front of transmission)

Master cylinder went out and we didn't have a spare so my son split a 1.5 enduro with a friend in his 944S2 in SP3 class, made a vid of both fastest laps SP2 vs SP3 that switches car view at half/video.

T

Tom M'Guinn

A popping exhaust is pretty common for these cars (like that great video), but if your machinist was drunk and you have bone stock engine management with a cat, then it might be worth doing a compression check or leak-down test just to make sure the valves are sealing. Also worth triple checking the cam belt alignment.
You can run a wideband without removing the cat. Just weld an O2 port up in the starter area of the pipe-- see picture in the zetronix install write-up linked in my signature below. But no need for a wideband really if you are using all original engine management (though it's always useful as a diagnostic tool even if you are not tuning).

951and944S

New YouTube channel - 'The Drunk Machinist'



T

Dan Martinic

This morning it was quiet so I drove around the 'hood trying to get it to pop. I must have looked like a superstar

I couldn't get it to pop. When it does, it's usually a 1-2 or 2-3 shift at *low* rpms and at half-throttle, not WOT or the racing revs in your video. But, when it happens, the pop is the same as the last one in the above video, albeit heard only with window open (and surely no flame). Not this morning though.

I even tried the suggested 4000rpm engine braking (after a low 1-2 shift):


Hey check out how the boost doesn't start doing anything during my brief trial.. .bet you guys forget how the stock setup runs lol

Tom, I checked out your WB install. It has to be "pointing down", meaning installed near the top of the pipe. You think it's possible while mounted? Seems hard to get a drill in there for the hole?

It would be nice to have some clear AFR numbers, esp given the aging afm. Someone said you can get an idea from reading the regular 02 sensor, but I don't buy it, and neither do the many threads I've read on the topc.....

Tom M'Guinn

Re the wideband, you need to mark the location with a Sharpie or something while it's in the car (so the tip of the sensor is slanted downward at least a bit), then remove the pipe/cat, drill a hole, and weld on an O2 sensor bung (with threads for the sensor). There may be people who could do all that with the pipe in the car, but I'm not one of them. Your car seems to be driving nicely. Cold morning air might help the gas burn in the motor instead of the exhaust.

Dan Martinic

Ahhh.. now I see it: I mis-read this as 'you can install a wideband without removing the cat'. Ugh. I JUST put it all back.

I guess the car does run nice.. but.. very sluggish & slow revving off-boost; no pedal response. Always been that way I guess

My buddy has an '86 951 similar mileage with a Lindsay Racing chip and LSD transmission; not sure if the LSD tranny has much shorter gearing or it's the chip, but his feels double the responsiveness from standstill and at low-revs around town. Just pulls nice at all speeds

I'm hoping it's the chip

Humboldtgrin

Dan Martinic