Ed Hughes

I'll take full credit for "fixing" this car. Hey, I did ask him about what gas he runs, and got him thinking about stopping off and getting some good stuff.

chaadster

Ernest,

I was just commenting that the symptoms don't sound like implausible effects of low octane/"bad gas."

I was not trying to invalidate your concerns; I certainly do not know enough about the ignition system to do any such thing.

IcemanG17

Wow
Bad gas.........that must have been some really old low octane junk gas or something.......at least you know the computers are still doing their job protecting the engine!

Bill Ball

Yeah, that's what you would expect, but MAYBE one of the two test cylinders, 4 or 8, was more on the ragged edge than the other and it dropped while the other continued to fire somewhat more regularly. If one dropped entirely, it wouldn't take much firing on the other to get a 150 degree delta. Who knows? Anyway, I would finish the tuneup (plugs, wires, cap, rotor) and keep the relay jumper in the glovebox.

littleball_s4

So the official version is that bad gas burned better in one testing cylinder than in the other so ICM measured delta T and shut ignition off?

Not impossible, but sounds a little odd to me.

Bill Ball

Sunds a bit odd to me too. It's certainly not official. Any other ideas?

littleball_s4

Nop, anything solid.

Probably something in the intermittent failing ignition half self-fixed. It happens to me "all the time".

I had a very similar problem (car sputtering slightly when throttle was opening) on a L-jetronic 5cyl VW. It happened all the time, all kind of gas, and everything checked ok. Fully warmed was ok, though.

The only thing that cured that was new battery, but only for two days, and new plugs, only for one week. Looked like ignition weakness (possible due to transient lean condition), but still very odd problem.

One day it self-fixed for ever. Owner selled it two years later, in perfect shape.

Bill Ball

Yeah, this is a 928 after all, so we need to use a lot more imagination than I have.

Jim bailey - 928 International

The ICM shuts off the INJECTION to the four cylinders fired by one of the two ignition systems which is TWO cylinders on EACH BANK. ( note how the plug wires from one distributor are routed)...the 89 temp sensors used cylinders 4-8 BUT that was found to sometimes give false readings and for 90 > cylinders 3-7 get the temp sensor near the exhaust ports and 89s were to be updated as needed. Is the mass air from Roger's GTS still on the car ? One failure mode of the mass air seems to be temperature related and when hot the signal is such that the LH Brain does go to" limp home mode" the true limp home stored default program just like when you unplug the mass air the car still should run...just very low power not rev much just enough to get it out of harms way.

worf928

Under the assumptions that:
1) each cylinder is operating exactly like every other cylinder
2) whatever's making the gas 'bad' is homogeneous throughout the mixture
then you might be correct.

I would offer, however, that rarely are these two assumptions correct.

Old cracked plug wires and/or a marginal distributor will result in different operating constraints for each cylinder.

'Bad' gas can be many things. Wrong octane. Water. Separation.

I think the bad gas may have been, in this case, the straw the broke the camel's back. We shall see.

ErnestSw

I agree with all the arguments as possibilities. As with a belt tension warning my approach to the problem would be to ASSUME the worst case scenario until PROVEN otherwise.

Earl Gillstrom

A few thoughts:
If you don't use Chevron gas or Chevron Techron additive or similar in your gas, your injectors get dirty and won't atomize properly. Bad gas adds to the problem picking on the dirtiest injector. This is very difficult to anaylize short of pulling the injectors for a cleaning. The bad gas picked on the cylinders with the weakest injector or ignition.
I run Techron in all my vehicles and record when used. My '93 F150 135K miles started to run and idle poorly. I checked my records and found I had not used Techron in 2 years. Put in a bottle and in a few miles it was running perfectly. I normally use it every 3 months.

Randy Carter

The symptom and the temp cure of restarting sounds like a problem I've had. It was cured (?) by replacing the Ignition Relay manual (928-618-175-00). Unfortunately, it's reoccurring some 3k miles later. Twice, so far.

danglerb

Would a partially clogged injector cause this?

Bill Ball

Randy:

Did you try the procedure of loosening and turning the sensors, then measuring the voltage delta?

I had 3 false alarms in 2 months, so I bypassed the relay. I felt OK doing this, thinking I would recognize a malfunction serious enough to be dumping raw fuel into the cats, but smarter people than me have had cat fires. I'm going to get under there and fiddle with the sensors and reinstall the relay, probably this weekend. However, I have had the relay bypassed for 2 years with no issues, although I can't recommend it except in an emergency and when it is clear that the system is providing a false alarm.