Landseer

Gas makes a lot of sense in a case like this when the symptoms develop overnight.

dr bob

There are TWO coils, one on each side of the engine bay. There are TWO distributors each with a cap and a rotor, each attached to a camshaft drive belt sprocket/"gear". Follow the coil wire from the bottom of each distributor cap back to the coil that feeds it. Because of the location on the passenger side, the access is somewhat blocked by all the coolant hoses, the suction hose for the refrigeration system, and the front-of-engine wiring harness that loops through the area. But there is a coil and a coil wire in there.

!!DANGER!!: Driving with one failed or failing ignition system means that raw/unburned fuel is passing through the engine into the exhaust catalysts. They aren't really ready to take much extra, so the additional load will cause them to get hot enough to CATCH THE CAR ON FIRE.

Porsche recognized the problem early on, and by the 1989 models had fitted the "ignition protection relay" to the LH controller to block injection outputs to cylinders served by a single failing ignition. I cringe when I hear folks recommend bypassing that protection relay when they get odd symptoms. Your pre-'89 car does not have that protection.

The engine will run fairly smoothly with one ignition disabled. There's enough power available, as you may have discovered, to allow reasonable performance with one ignition system disabled or faulted. More than a lot of 928 drivers have only discovered that the car is running on four cylinders when they notice the carpet and undercoating on fire under their feet and in the passenger's footwell.



Perhaps-Interesting Side Note: Porsche looked very seriously at active cylinder management for the later (S4+) 928, disabling fuel flow to four cylinders under specific conditions to improve low-load fuel consumption numbers. Think about two-plane manifolds and an airflow-blocking butterfly.


Anyway, you really don't want to drive the car until you can positively rule out a weak or failed ignition.

safulop

Yeah I kind of thought there had to be another coil somewhere, but the WSM is not very clear with showing the location of stuff. I will check that one as well, but honestly this car does not seem like it is running on '4' most of the time. It idles like a 928, has a good sound from the RMB going down the freeway, and will even give a nice fast rev-up to red line (sitting in neutral) with a roar that would blast the ears off the most die-hard Harley rider. The problems begin when you give it lots of gas. Then it bogs down so badly, it actually will not accelerate at all at WOT. You could drive down the street at 40 mph steadily with your foot flooring the gas, and it just kind of chokes and bucks instead of accelerating, as if it were getting flooded or starved of fuel, it is hard to tell. It is the strangest thing I have felt, and that's why originally I thought someone had pranked me at work with a bottle of water into the gas tank (I don't usually lock my cap, poor me). It had only 1/4 tank at that time, so I filled it up and put a bottle of HEET into it to try to address any water contamination. The symptoms now are somewhat better than they were when the problem first cropped up. Now at least it will accelerate rapidly with almost no problem, so long as foot is not all the way to the floor.

Now I would have thought that by driving it around for a while that any gasoline contamination would clear out fairly quickly, but now a previous post states that it took several tank-fulls to clear out a gas problem.

I realize how dangerous it is to drive an older 928 with misfiring going on, but I honestly don't feel or sense any misfiring 95% of the time. If only 4 cylinders were working wouldn't it (A) sound funny, (B) idle poorly, and especially (C) continue to be able to accelerate at wide-open throttle on the remaining 4 cylinders ?

safulop

I got a new set in August for like $400 so I hope they are good. They are Beru, but it does seem like the coil wire just plugs in like milk toast, which is probably why the driver's side one had worked its way partly out of the coil. You have to peel back these hefty rubber boots to check the attachments at the end of the wire I guess?

I found the passenger side coil, facing the other way so the plug end is underneath where a hose is clamped; this hose would need to be disconnected in order to inspect that coil plug or to unplug it, so my shop will have to take care of that if it is determined to be necessary.

safulop

Since this started here is my first theory of what went wrong. Water got into the fuel system somehow, with the result that the octane content of the fuel is way too low, so at wide-open throttle there would be lots of knocking. The anti-knock sensors kick in to prevent that and retard the timing so much that the engine loses 90% of its power. Is this even a possible scenario? Or do the anti-knock systems not have the capacity to do that?

James Bailey

nope

safulop

OK I dug in and inspected the battery connections, they look great and very tight and clean, battery is quite new. The ground wires have been replaced by PO several years ago to install a kill-switch that interrupts the ground. So now there is a new-looking 6-gauge wire from battery to switch, and then a short flat braided strap from the switch to the body ground.

dr bob

Sean--

Check the fuel flow and pressure. Please?...


On the pass side coil wire, it shouldn't be wrapped up around anything. If it is, normal engine motion risks pulling in loose. Those hoses are "in the way" for your hands getting to the coil and wire, but you should be able to find and feel your way while the engine is cold.

Water in the tank causes interesting problems, but "reducing the octane" isn't one of them. Water settles to the bottom with the car sitting still, so if there was much in there the car wouldn't even start. When you do the delivery test and have a few liters of fuel in a can, you'll be able to see if there's much water in it. Let it sit for a bit and it will usually separate back out in layers. Ethanol doesn't help though, as it gloms up water and holds it a little more in 'solution' than 'suspension'. Vintage boaters trick is to pass water-contaminated fuel through a chamois. Gas will pass, water not so much.

safulop

Yeah she is going to shop Wednesday; my instructions will be to check the fuel system first, then move to possibilities like loose engine grounds and bad coils. We will see what comes up. I suppose it is possible that the ground interrupt kill switch is to blame? Might be useful to bypass the switch with the ground strap.

I really was not confident that if I unplugged the passenger side coil I would ever get it plugged back in, so I just pushed it to make sure it is plugged in firmly, for the time being.

FredR

Sean,

Losing one cylinder leaves the car feeling "sick" so losing 4 must be utterly diabolical and I very much doubt you would not know about such were it happening. What can happen is that one coil [or both] starts to break down and the impact of this becomes more noticeable under full load conditions [WOT] and higher revs and most likely will be erratic as to when it does break down- does this sound familiar?

Bottom line- as the system loses spark energy at some point of attrition the motor will start misfiring. If someone can lend you a spare coil you can test by process of elimination changing them out one at a time.

safulop

Well, you guys were correct it is definitely a main electrical circuit issue. Since I played around with the battery I must have moved something because now the problem is far worse. In fact it is now exposed as basically the same problem that had this car sidelined for 3 months over the summer.

Now the car starts but runs so poorly you basically have to just shut it off. This is the behavior my shop reported over the summer, while I was out of town. It had them baffled, they checked everything they could think of, never found the problem, and when they put it all back together it worked fine. We never knew why, and of course now the problem is back. Doesn't that always happen when a problem is never really solved?

It appears to be grossly overfueling, as if the LH circuit is open or running with the Temp sensor unplugged. I kind of doubt that the battery ground strap on its own could cause this? But I guess we are going to chase after ground circuits. It was already overfueling since, well basically this car has always been overfueling, no one has ever found out why.

safulop

The Temp II sensor wiring boot is really bad, the wires are all exposed, I'm going to start there with the troubleshoot this week. I could get the engine to hold idle today by hanging on to those Temp sensor wires as it ran. Possibly way too much resistance in those wires, may also explain the high cold idle I've had.

FredR

Given your last statement one thing you might check is the clearance between the battery lid and the various smaller wires leaving the battery positive terminal. There is little clearance at that point and it is not beyond the realms of possibility that you are arching to earth.

Take a look at the under side of the lid adjacent to the positive terminal to see if there are any signs of scorching. I wedge a piece of inner tube rubber in there to prevent such. Lesson learned after I once forgot to fit the battery holding bolt and the thing suddenly arched big time and literally the lights and everything else went out- the battery welded itself to the lid and I could not get in there! Obviously this has not happened but I would not rule out some intermittent leakage to earth at that point with whatever consequences that may have - simple enough to eliminate from the search list.

Rgds

Fred

Mrmerlin

have you ever had the computers rebuilt?

safulop

Oh yes, unfortunately the LH and MAF are eliminated as culprits, they were both checked out in July. It is purely a connections issue, not a components issue it seems.