oz928s4

I had finally completed the install of a heap of upgrades including porken, cam chains,seals and tension pads and a twin screw blower to the s4.
I had it running and was sorting out a few vacum issues and did'nt think to keep an eye on the temp guage....which had crept up due to fans not kicking in.
I am going to work this issue out first...BUT I was just about to turn the engine off and it cut out.
I walked to the front of the engine bay and noticed no fuel pressure...I thought that it had gone into limp mode, although having said that the temp guage had gotten to around 3/4 so it didnt even boil.
I let it cool down for 20 minutes and tried to start it again I have fuel pressure but it wouldnt start.
At this stage it had been a long day so i walked away from it after disconnecting the battery.
If limp mode comes into play does it drop spark and fuel or do I have something more diabolical to work thru.
I will work thru the fans first and get them up and running...I keep blowing fuse 28 the A/C fan..I suspect a fan motor there but the main fan should have cut in and does with the A/C on but dies when 28 blows.
Anyone have a comment on the engine cutting out as it got hot.

Mrmerlin

relays

dr bob

'Limp mode' is a default fuel map mode that allows you to ghet off the highway when a mass airflow sensor (MAF) failure is detected by the LHA brain. That is the ONLY time that 'limp mode' is engaged.

Start your diagnosis with the basics, as if you just received a car that won't start. Pay particular attention to the timing belt, looking through the vent holes in the top belt covers to make sure that the belt is on the pulleys and tracking reasonably. If you need to rotate the engine, do it by hand, slowly, and stop if you feel any resistance. Plugs out of course. After that, check for spark, fuel pressure, etc., as you would for any car. Did you overheat to the point of losing coolant from the reservoir cap vent?

oz928s4

Not sure wot happened but it started right up next day

Mrmerlin

check the starting relays look on the pins for corrosion, any that have it should be opened, start, fuel pump, ignition, EZK.

WallyP

To expand on the "limp mode" subject a bit...

If your '85-'88 32-valve 928 suddenly loses power and appears to be running on four cylinders, STOP! This is NOT a "limp mode" to get you home! The 32-valve cars effectively have two ignition systems. If one fails, the other will continue to run the engine - but all eight injectors will still be pumping fuel into the engine. This excess unburned fuel from the non-running four cylinders will cause the cats to overheat - we're talking glowing red! This can and will set the car on fire.

Starting in 1989, Porsche installed an ignition monitoring system to shut down the affected four injectors, preventing the cat fires. If your monitoring system is still installed and functional, there should not be a problem...

Again - running on four cylinders is NOT "limp mode", and can destroy your '85-'88 928!

nsantolick

I have an 89 S4 and just ran it about 25 miles on four cylinders. For no apparent yet reason, after that, it DID start easily and run perfectly, just occasionally "bumping" into 4 cylinder mode for a fraction of a second, every few miles.

Anyway - I DID experience more or less normal driving on the return trip of 25 miles. I'm starting to diagnose the "problem" and am probably buying one of those code reader software thingies, but I AM concerned that I've done MORE DAMAGE by running it 25 miles, wide open, just to keep with traffic.

Interestingly enough, as slow as it was, it would often chirp the tires while shifting. (Automatic transmission...) Maybe this adds a hint to my problem???

I assumed when these symptoms first started quite recently, that it was the fuel pump. I hadn't heard of the ignition monitoring system and have just started "reading into it" on here.

Hopefully I haven't done damage by driving it as such... Oh well...










Nick

Hazleton, Pennsylvania

1989 S4 Auto - 80K miles - White/Gray Leather - Pristine - Upgraded Stereo - Maxed Out Aftermarket Currency/Serenity Black Hole Simulation Package

James Bailey

Driving on 4 cylinders if said by Porsche to stress the diveshaft leading to it possibly breaking from the pulsing of missing every other cylinder....also many later 32 v have had the "ignition protection" relay jumpered and it might cause a fire just like the 85-88 cars experience.

AO

Nick, that sounds strange. If you're going into 4-cylinder mode, there's a reason for it. I would inspect your coils, coil wires, caps, rotors, and even plugs... oh and final stage ignition thingamabobs. IIRC, the Ignition Monitoring Relay system only resets if you turn off the ignition and restart.

Bill51sdr

Driving the car in that manner is known to be very hard on the driveshaft in the torque tube & they have been known to break. I would not drive any further than the time it takes to get the car safely off the road again, if I were you. You may have a faulty exhaust temperature sensor in one of the exhaust manifolds, causing the IMS to activate if the engine seems to be otherwise operating properly. Porsche also changed the location of these sensors on later engines due to false readings. Next time this happens to you, have a look at the relay and see which LED is on. This will tell you which ignition system has been deactivated, which will tell you where you can begin troubleshooting.

SharkSkin

-Your wife is disappointed
-You are disappointed
-You start actually reading some of the spam in your inbox

AO

nsantolick

Thinking about this pulsing causing driveshaft damage potentially. I'm not disagreeing - just trying to understand.

How would this be different than say a 4 cylinder car running normally???

Does this mean that a V-12 would be inherently smoother on a driveshaft???

V-16???

Not interested in dumping a Jag or Bugatti engine into a Deutscher car or anything, but it has me wondering about the logic...

dprantl

Because a 928 V8 in 4-cylinder mode is not the same as a 4-cylinder engine. When a V8 is running on all cylinders, there are twice as many spark fires as a 4-cylinder. When that V8 is only running half the cylinders, half are firing while the other half are not in one cycle. Each distributor cap is connected to two cylinders on each side of the engine. This fire-not fire switching is what causes the stress on the driveshaft.

Dan
'91 928GT S/C 475hp/460lb.ft

WallyP

Let's assume that are motoring down the road normally in your automatic transmission 928. It takes a given amount of power to hold that speed. The engine is firing once every 90 deg of crank rotation, with each pulse giving enough power to accelerate the driveshaft enough to drive the torque converter (which acts as a flywheel to smooth out the torque pulses) which drives the transmission, which drives the rest of the drive train which drives the car.

Now assume that the driver's side coil wire arcs to ground, killing four cylinders. If you have a properly-functioning Ignition Monitoring System, it promptly kills the injection on those cylinders, preventing the cats from overheating and burning the car. You just drive on...

The engine is now firing every 180 degrees, and each pulse must put out twice the power in order to maintain the car's speed.

This power output induces twice the cyclic stress on the driveshaft, and cyclic stress is what leads to fatigue failure.