A Miss (in heat!)

rlane · 12-01-2001, 02:27 PM

Ok..now that I got your attention....

I have a persistent miss , under load, after my 78 911 SC heats up. It takes 25-30 minutes (45 minutes today as it is cooler outside) to develop, runs superbly til then then gradually misfires under load. Today i replaced wires with Magnecor...thought it was gonna be OK but after 45 minutes it became severe enough to pull over. Oil pressure and temp are normal. Idles fine. Cooling off for 5 minutes allowed me to drive it home. I have replaced the wires as above, cap, rotor, plugs (they looked good). I have run a Techron tank and replaced fuel filter. I checked and worked over all grounds. Timing is right on at idle..I have not gotten courageous enough to do the 6000 rpm thing. I do have a MSD 6AL ignition sitting ready but the permatune box is just 2-3 years old.
I am pretty new to Porsches, got this car 2 months ago so I may have missed something really dumb. It sure seems to be some component that does not like getting warm. I have a MSD Hi-vibration coil that I have not put in use (it came with the 6-AL)..is that my next option.
ANY...and ALL ideas appreciated!

Jorgeman · Jorgeman's Garage

Rich--would you characterize the miss as random or occuring at a timed frequency like it would if it was in a specific cylinder? Since you've done an excellent job of already replacing most normal ignition potential problems, I was thinking of maybe a sticking valve. The problem is heat oriented--which causes expansion--which might be pinching a valve and slowing or stopping it's closing and sealing. Any different sound at the exhaust when it's missing?
How hot is the engine after 40-45 mins? Which mark on the temp guage?
IF the problem IS a valve, you should be able to narrow it down by removing each ignition wire and looking for the one that DOESN'T change the miss pattern. You'll get some other ideas from this board also--these guys are great!

George 86T

rlane · 12-01-2001, 03:19 PM

I think Miss is really an understatement..it is...perhaps..a stumble...not really a frequency thing ..from my experience with BMW's it would be at least 2-3 cylinders involved. Today it became so severe I barely made it up a hill (downshifted 4th-3rd-2nd--pulled into a drive at perhaps 15mph!. Before it seemed to rev OK without load but today I noted it missed even while free-revving.
I haven't really listened to the exhaust because as I noted it had been doing OK except under load before today.
The temp gauge shows steady about 3-4 needle widths above the top of the cold area. Oil pressure between 4 and 5 at 3200 rpm. Full oil by check today.
Richard

stormcrow · stormcrow's Garage

Richard

You either have a bad spark plug or your permatune is starting to fail. Not sure if you replaced the plugs or not, but sometimes new ones do fail. The Permatune may be failing after things get warmed up. Could be a resistor failure, capicator or some other component on the board that may be the culprut.

However, I would check my plugs first and make sure the plug wires snap onto the plug when you install them.

Steve

Jorgeman · Jorgeman's Garage

Well that adds a different slant to the problem, maybe more fuel oriented than ignition. How about checking the fuel pump inlet filter screen? At the age of your new toy, there's a good chance the gas tank may be crudded up--especially if the car has been stored without STABIL in the tank or not driven during winters. You may be getting fuel from the pump in surges Rich.

George 86T

Early_S_Man · 12-01-2001, 03:40 PM

If the 'miss' is repeatable, and can be heard when reving the engine from idle at a standstill, after the engine is warm/hot ... then take it to a shop that has a Sun Ignition Analyzer and put it on the scope to find out if it is an ignition output problem.

rlane · 12-01-2001, 04:12 PM

As usual wonderful help.
First Stormcrow. All plugs are brand new 2 weeks ago...no change in miss. I may try the trick of using a can of that photo compressed air on the Permatune box....

Jorgeman. I will pull out my friends bentley manual and check the fuel pump screen. Would that occur such that it runs great for quite a while then stumbles?

Early_S_Man: I am thinking you may be right. I may take it to the local Porsche specialist and see if they have a Sun tester. Can these devices tell if the coil or the Cd unit is to blame? Or (I keep fearing this is a fuel problem. As I noted replaced the fuel filter but I see that there is a filter at the fuel pump and i think I recall there is one actually in the tank.

Tyson Schmidt · 12-01-2001, 06:12 PM

How old is your coil? The original coil in my 72 decided to turn into a really effective heating element on me. After about the same 45 minutes, it would stumble and miss and the tach would occasionally bounce. I opened the hood and grabbed the coil to see if it was hot. It was hot. Real hot. I think I still have the Bosch part number backwards across the palm of my hand. Replaced the coil. Problem fixed.

Ex-girlfriends Saab had the same symptoms. Saab shop threw many parts at the situation to no avail. She brought it to me, (How we met.) I put in a new coil and voila!

It's the one part that seems to get ignored everytime since it is much less common of a failure.

rlane · 12-02-2001, 06:09 AM

Good point Tyson..I don't know the age of the coil (bought the car 2 months ago..records that I have going back 5 years do not show a replacement. I may try and switch in my new MSD Hi-vibration coil today. Do you or any of our other experts know if this couil is OK for use with the Permatune box? Like I said before I guess next step is to get it to a Sun diagnostic computer or else go ahead and swap out the Permatune for the MSD 6-AL.
Anybody know if it is safe to try the MSD coil (I guess I worry because I was told to use this coil..or the Bosch Blue (both relatively inexpensive) with the MSD 6AL...versus the quite expensive standard Bosch coil.
Richard

Tyson Schmidt · 12-02-2001, 12:39 PM

That one should work. It's the stock "voltage transformer" coil that will only work with the stock Bosch CD unit. And vice-versa.

Permatune should work with most conventional coils.

john walker's workshop · 12-02-2001, 01:24 PM

the CD units often fail when hot. it sounds like poor spark, but did you see if your fuel filter is plugged? take it off and blow thru it in direction of flow.

rlane · 12-02-2001, 07:04 PM

I tried the new coil...same exact problem..but the new coil looks good (without that silly label MSD puts on it!

John...I haven't tried the old filter but did replace the one that was there..no change. Looks like I am gonna put that MSD 6AL to use. Even though Waynes 101 Projects book makes it sound easy I may let the local Porsche shop do this switch..my mechanical skills are way past strained...Still this has been a good cjhallenge and I have learned a lot doing what I have done so far...