Twice on the way to Sharktoberfest (and once on the way back) my car went into four cylinder mode. At least that's what I think it was; I didn't know to check the LED light on the relay near the LH/EZK at the time.

Each time it happened I was near the bottom of a long grade when I noticed it, and the first time I had no idea what was going on. I limped off the highway to a service station and shut it down, waited a minute or two and started it back up. To my great relief it started and ran normally, and I figured that the computer 'fixed itself' (yeah, right!)

The second time it happened I wondered if it had something to do with the engine temperature since the only unusual symptom was that gauge dropped as I was going downhill and was nearly to the bottom line at the end. All three times were cold mornings with temps around 55 or lower.

The team of Sharktoberfest consultants that listened to my sad story concluded that it was the LH sending the engine into four cylinder mode due to a difference in cylinder temperatures. I also understand that in later models than my '89, the temperature sensors are in a different pair of cylinders, perhaps for this reason. Someone suggested that I shift into neutral when it happens and cycle the ignition on the fly - which I did on the Grapevine on the way back and it worked fine.

I came back from SF with a new rheostat and seal that I'll install in case the old one's defective. The flap was removed from the car when the Devek radiator was installed so that may be letting the engine get colder than it should.

Another suggestion was to install the non-US relay bypass plug and disable the four-cylinder mode altogether. I'm not sure that I could recognize when a distributor fails, so if I do that I'll remove the cats at the same time since there's not an emissions test requirement in my area.

Another interesting piece of data is that this Spring on a trip over the North Cascade Highway when the weather was much colder, I did not experience the problem. That was before the car was Sharktuned and the fuel pressure changed from 30 psi to stock 55 (after we discovered the FP gauge reading of 90#(!) was incorrect.)

Has anyone had a similar experience? Any comments on the diagnosis or suggestions? Other suggestions?

 

My '90 S4 used to do the same in the same / similar circumstances - crest a hill under power, coast down the other side and chances are that it'd trigger the 4 cylinder mode. In due course the central shaft got sick of this (i.e harmonic spikes) and snapped. As part of the repair I replaced both cylinder temp. sensor probes. Problem has now been fixed for many years having done this.

 

Good to hear someone else has had the same experience. I did a search and found out here that a Technical Bulletin suggests that rotating the sensors and/or moving them to cylinders 3 and 7 will help, so I think I'll try that first.

Sensors are listed at over $300.00, so maybe the cats will disappear if that doesn't work.

Since running in the four cylinder mode can break the central shaft in a automatic, I need to fix it one way or the other pretty quickly.

 

I don't follow. You will need new sensors with longer wiring for cylinders 3 and 7, but they have nothing todo with cats.

 

Aryan, oh yes they do. the sensors were Porsche's cunning way of stopping cats catching on fire.

 

928s made w/o cats do not have the ignition monitoring system or it is disabled - Porsche sells a bypass jumper box that replaces the relay (see below). The system is there to cut fuel off to half the cylinders if it believes they have lost ignition for some reason, with the principal reason to prevent cat fires from the dumping of raw fuel. No cats, no great need for this system.

 

I believe the wires are long enough to move to 3 and 7.... nothing too fancy about the sensors..just like those oven meat thermometers ...long metal probe up into the exhaust stream. If it senses an imbalance between the two cylinders, it shuts off the fuel to 4 cyl, until you re-start. You can always cap all the ports and tie the temp probes off to the side so they never get close to operating temp.

I'm running the bypass in the GT and was running it in the track car but don't use it anymore after the conversion to megasquirt..(so I've got a spare relay if you want to try it out Bill)...

 

One more thing Bill...your 928 shouldn't cool down that much when not under load (and it's cold outside)....my track car did that and it ended up being a stuck thermostat..the small part of the thermostat had lodged itself into the hole in the block (corroded seal) and was always diverting coolant to the rad. When I replace the thermostat and seal (both seals)...it worked better..

 

In my experience of running down hills of any length against the engine, the temp drops quite quickly, and several degrees. The ECU monitor on my Subie (showing actual coolant temp) will drop at least 3-4 degrees on one specific hill I know - gauge shows only a small drop. Thermostat setting is 83C like the 928, and I have seen 79C on the display on a cold day.
jp 83 Euro S AT 54k

 

It's possible even with things operating normally for the coolant temp to get below the T-stat level. On a very long downhill grade with foot off the gas, fuel is cut-off above 1400 RPM so no heat is being generated. Even with fuel, if you have reset that feature with Sharktuner, the heat generated is small and the block or at least the water bridge where temp is sampled can get quite cool.

 

Tom,
Thanks for the offer of the spare relay; it's certainly possible that the relay could be bad. Given the long grade that it takes to make it fail, the troubleshooting process could take years, especially since it didn't happen on the last two grades on the way home. On those grades I was cycling speed to try and pump a little heat into the engine. Seemed to work but I think the outside temp was warmer than before as well.

I have a new T'stat and seal that I picked up as SF to try, as well as moving the sensors (if they'll reach) or rotating them per the TB.

 

I was talking about the "bypass" relay...if you want it... but also have a couple of the regular ign monitoring relays too

 

This recently happened to me... turned out to be a bad final stage module.

Mine too, after shutting it down -would 'fix itself' and drive fine. Each time the amount of time it took to recur got shorter, until it was only running on 4 cylinders all the time.

Not good at all... the fuel from the 4 cylinders that are not firing can accumulate in the cats and start a fire.

Good luck & Godspeed for the repair!

 

Bill, obviously you are shifting beyond 2nd gear when on the freeway.
This goes against all things natural and sacred to an S4 A/T owner when traveling below 95mph.
Even with the lower final ratio in your '89, you shouldn't need to shift out of second until you are above 80mph.
The fact that you are having these problems while trolling along in "D" is no suprise to me. What do you think you're driving, a mini van?
Set the keys down, and get your head straight man.
"GT cams, AMG Trans, 968 intake valves, Ott X-pipe, 2.5" exh, Sharktuned" - I'm not sure what part of that description aniticipates lollygagging around at idle when going down a steep grade.
Take it like a man, leave it in second, engine brake to the max, get those pipes rattling - sure you'll get crappy mileage, pulverize your brake bands, and push your valve stem seals to the limit - but that's what 928 S4 A/T ownership is about. This is what we do.
If you can't handle it, get yourself a 5 speed and join all those other panty waist, viagra poppin', ballsocketrearcouplerslavecylinderbluehoseintermediateplate - challenged losers out there who don't have to deal with our manly 928 S4 A/T issues.

Get with the program.

-Adam

 

Tom,
Ahhh... yes the bypass. Thanks for the offer; it could come in handy.

Barry,
Did your final stage module fail on long downgrades only?

Adam,
Yes, I see the error of my ways. I can only say in my defense that like most newbies I didn't know the correct question to ask.

I will need a demonstration of the proper operation of our 5-speed delete models. When can I schedule a time that you can take me for a drive in your car so that I can learn from the master? Oh, that's right - like proving my diagnosis it's only virtually possible at the present time. Minivan, track car, what's the difference when they're used as an excuse for avoiding all things natural and sacred.