waveflux

Oh boy, I don't like the sounds of this. My MAF was ordered from 928 Int. Should be here before the weekend. Now I have to cross my fingers and toes and hope this cures the problem....or it's off into the abyss again.

VehiGAZ

It's not that bad, waveflux...

As Jim said, it is a not-uncommon problem to have a faulty ignition protection activation kicking in intermittently. As I mentioned, all I did was feel up the exhaust temp sensor wire from under the exhaust header up to the connector mounted to the back side of the valve covers, then unplug/replug the sensor at the connector and my then-persistent ignition protection problem went away. I might have had a semi-dirty or corroded connection or something going on - who knows what was going on after 19 years?!

A "valid" reason the engine may go into 4-cyl mode is if the MAF is very out of whack and you are getting an extremely sub-optimal fuel mixture, leading to misfiring in one of the exhaust-temp-monitored cylinders, which would cause a "valid" engagement of 4-cyl mode by the ignition protection circuit. (Personally, I agree with Jim in that is a fairly unlikely scenario, but this is what I've been told, and let's face it - the MAF is one of the most critical sensors in the car)

Try replacing the MAF - it fixed me up good last year! And get the bypass relay - don't get stuck driving 60 miles home from DE in a bogus 4-cyl mode like I did!!

Alan

I like the simple idea to check for an ignition control issue (relay LED's on?) - if yes check for spark on that bank - maybe the coil/distributor/plug wires really have a problem and the ignition control is doing exactly what it is supposed to do - stop you from pumping unburned gas into the hot cats... seems like a good idea to me...

Alan

no doubt

If you are going to bypass the cat safety, please shoot your $10 laser thermometer onto each cat first to insure that you don't have an overheating cat.

Should a catalytic converter superheat, it will torch your car. Hence, the temp safety sensor. That's what you are bypassing.

So if you do bypass it, be smart about it.

no doubt

Do we have a picture of this relay's location, lights with proper operation, left bank inop, and right bank inop?




*and is the relay cutting fuel...or is it cutting spark?

waveflux

My car died in a friends driveway a few blocks from my house. Went over and tried it out tonight. Still in limp mode. Except it won't idle this time around. I went into the footwell...did not see any lights whatsoever. I did check all the connections in that area..everything is secure. My MAF will be here Thursday, in the meantime my friend has a nice shiny yard ornament.

no doubt

What does it do with the MAF completely disconnected?

waveflux

I'll find out tomorrow.

John Speake

The relay is mounted on the same plate that holds the LH and EZK at the side of the footwell, passenger side.

A faulty MAF will not trigger 4 cylinder mode. To trigger the relay, you need an imbalance in heat between the two sides of the engine (or of course a faulty temp sensor/wiring to one of the heads).

Imbalance in heat could be faulty igntion wires, an injector not flowing too well.

On a recently rebuilt engine i saw the relay being triggered due to the cam timing on one side being out.

Mrmerlin

remove both of the coil wires and check the ends for seating and corrosion

Koenig-Specials 928

This is important Operating experience: My car did the same thing last year and it was after I gave her a bath. Yes I was able to re-create the symptoms. Both times it went to limp mode. So now I use a gentle spray on the hood to prevent water puddling in the engine bay. On each occassion it was the left bank. It hasn't happened again since then.

VehiGAZ

The relay is on the right side of the passenger footwell, behind the carpeting, by the LH controller/ECU (as noted). It has a clear plastic case and is mounted low on the side-wall by a brass-looking band, which is held in place by a screw.

No LEDs = Normal operation
Green LED = driver's side bank (IIRC)
Red LED = passenger side bank (IIRC)

The relay monitors the exhaust temp sensors on one cylinder per bank (don't recall which ones for which years, but that info is posted in Stan Smith's thread from this summer on his rough-running GT). When it detects a significant temperature difference between the two monitored cylinders (>300*F) or it detects a signal loss from the sensors, it shuts down two injectors per bank (not sure about spark, though) and throws the LED to show you which side has the problem.

Yes, I realize that the ignition protection circuit serves a useful function to protect the cats, but for most people who have this problem intermittently it seems that it's a fault of the protection circuit (sensors or relay) rather than a "real" problem with the ignition. So check your coil wires as suggested to make sure you are making spark everywhere to make sure your phantom problem is not a real problem.

And while I understand that it is hard to accept that the MAF could have anything to do with this problem, replacing mine last year pulled my shark out of the 4-cylinder mode (at least until this year). Swear to G-d, cross my heart, and hope to die. It really, really happened - instantly and unmistakeably, and I'm not the only one either.

Alan

It cuts fuel (injectors) if it cut spark only it wouldn't really reduce risk...

It is located on the bottom of the frame that holds the EZK & LH ECUs. Normal running is no LEDs illuminated. Red & Green LED's indicate bank failures. Banks are: 1,4,6,7 & 2,3,5,8

I don't recall which color maps by bank.

Alan

John Speake

Red is 1-4-6-7
Green is 2-3-5-8

Jim bailey - 928 International

As noted it does NOT shut down one bank but 2 cylinders on each bank those which are on the same coil, same distributor cap. Also running on 4 cylinders is NOT LIMP HOME MODE !! Driving on 4 cylinders is said by Porsche to be very hard on the drive shaft inside the torque tube. Porsche felt the need to add that system in 1989 because the dual ignition cars ...(two coils ) were sometimes experiencing a failure in one circuit. The car would run poorly have little power BUT BIG BUT people would still try to "limp home" so with four cylinders not firing and pumping unburned fuel and air into the exhaust the catalytic convertor would glow red hot and the undercoating would catch on fire and the flames would burn up everything on top of the engine. So to avoid that stupidity Porsche added the protection circuit to shut of the injection to those 4 cylinders 2 on each side) which were not firing. However like any other device which can shut off it sometimes will fail and do it at the wrong time.....it is after all a RELAY getting information from temp sensors which passes through wires ! The real limp home is a stored program in the LH computer which under certain failures of the mass air sensor allows the engine to run at low engine speed without the information from a mass air sensor.....which is why most cars will run with a disconnected MAF just not rev up well . Just run well enough to pull off the freeway.