dimmer

Just getting the shark back on the road after winter and I need help diagnosing a problem.

Last year I had one on the cams break at the timing belt sprocket thus causing some major work. Rebuild one head, T/B & W/P, refinish intake manifold & cam covers, replace plug wires, and replace distributor caps & rotors. Not long after getting it together, an intermittent problem started.

The car starts up and idles fine at all times. However, when stepping on it and accelerating quickly, it sometimes seems to lose half of the cylinders. A slow and gradulation acceleration is typically fine. It's almost as if pushing the accelerator beyond a certain point, causes the power loss. When it loses power, the pitch of the engine sounds lower and more of a growl (guessing lower pitch with half the cylinders working). The problem isn't too consistent other than it happens only under hard accleration.

Could the problem be related to a bad coil ( just because I've replace many of the other elecrical components) ? Are the any tests to verify the these comonents,

John Speake

Are you quite sure the cam timing is OK ? Have you checked it since the rebuild ?

Vacuum/inlet leak ? A weak mixture on one head or bad cam timing could give the effect you mention, especially under high load.

If you swap coils, also remember the output stages that drive the coils (to the right of the hood latch).

dimmer

I checked the cam timing on both sides after the work. Timing on the head that wasn't worked on was perfect, so I'm fairly confident in my procedure. Also. the problem is inconsistent. Sometimes it doesn't happen at all, sometimes it's worse than other times. Sometimes as it is happening there are short bursts (1/2sec) where the power kicks back in. Is it possible for a coil to be fine at idle, but not be able to provide enough juice under acceleration and just go dead under those conditions and then be fine again back at idle?

John Speake

A weak mixture will make combustion harder, especially if there is some other issue.

I would suspect igntion as you do. The favourite problem is the lead from the coils to the distributors. They can sometimes corode under the rubber boots, exp. the coil end.

I have known too large plug gaps/wrong type plugs to cause these type of problems.

Does it do it even when the engine is cold ?

blau928

You may have a bad knock sensor, as it happens under accelleration..... Coils either work or do not work...

HTH

John Speake

Hi Richard
Good thought - though it sounds as though the problem is lower down the power scale...although he does not state rpm...

Knock faults retard the ignition only at high rpm and engine load ie above 3000rpm - 6 degrees

blau928

Hi John,

Thanks... I was thinking his acceleration term meant hot footed like mine.. !

Flott Leben

dimmer,

Maybe a faulty O2 sensor? Do you have access to a Bosch Hammer of John Speake's Spanner?

worf928

Which head? P-side or D-side?

Edit: Right-side or Left-side? (Don't know if you're LHD or RHD)

dimmer

Dam ... it might be knock sensor. I replaced the rear knock sensor because it was cracked, but not the front one. However, when I was putting it together, I noticed the cable on the front one was brittle and cracked. At that point, I was too anxious to put it back together (against my better judgement). It might be coming back to haunt me. As far as the RPM's... I can be cruising at around 1800RPMs and if I floor it, it happens (that is, RPMs are still around 2000). When it does happen, it's like someone flips a switch to put the engine into low power mode ... it's an abrupt change. Could O2 sensor cause something like that??? If I slowly accelerate, it doesn't seem to be a problem.
Also, when I changed all of the ignition wires, I did notice the corrosion on one of the coils (I simply cleaned it out with some sandpaper). Maybe this has had an affect on the contact.

As far as engine hot or cold, it doesn't seem to matter. One time it's rough when it's cold and goes away when it warms up; another time it's just the opposite. The only things consistent are 1) it idles fine, 2) it doesn't happen all the time (but seems to be getting more often), 3) it happens when I open the throttle beyond 1/4 to 1/2, and, 4)it's an abrubt change/power loss.

Tomorrow is supposed to be sunny here ... I'll go through some of your ideas then.

Please add anything else you can think of ... I do appreciate everyone sharing their experience and knowledge.

ErnestSw

Could be a bad coil wire. Could be a Hall sensor (that's why the question about which side). A knock sensor or O2 sensor will cause retarded timing under load and a loss of power at the top. Chances are you wouldn't even notice the loss of power. Your car is bogging down under load. I'd try to find an LH controller to swap. I don't know whether a bad MAF could cause this type of problem. It's highly unlikely, but you could have blocked cats.

Bill Ball

I think knock or Hall would not be THAT noticeable. I ran with the Hall/rear knock connector detached for a while before the Spanner discovered it. A Spanner scan would be a good idea.

Dimmer, where are you located?

Also, I have recurring corrosion to one coil lead. Last time I cleaned it off, I put some dielectric grease on the boot. Hopefully it will seal better.

worf928

Toasted Hall sensor can be very noticeable. The failure mode seems to vary from car to car - not sure why; it may simply be progressive. On my '89 the dead Hall sensor caused a simple lack of top-end power (~80-100 hp), car ran smooth as silk otherwise. On Doc Mohr's car - one of them anyway - the dead hall sensor manifested as above with an eventual mode like that described in this tread.

So, if the left-side (p-side on LHD) head was the one touched I would carefully check the harness to the Hall sensor and the connection to the Hall sensor. The three-pin connectors (2x knock, Hall, Flywheel) turn to dust eventually.

Edit: Meant to write Hall sensor not knock sensor, Post changed.

WallyP

The Hall sensor affects all cylinders, and failure simply retards the timing. Failure of the Hall circuit or of the knock sensors is usually not really noticeable to most drivers.

The amount of voltage required to punch a spark across the plug tip depends upon the resistance in the secondary (high voltage) circuit.

The fuel/air mixture between the plug tips acts as a resistor - the more mixture between the tips, the more resistance. The amount of mixture between the tips depends upon the throttle opening and the filling efficiency of the induction system. The maximum amount of fuel/air mixture occurs at the torque peak of the engine.

It is easily possible for the secondary ignition system to furnish enough voltage to fire the plugs at idle and low throttle openings, but not enough to fire the plugs at full throttle, especially near the top of the torque curve. With plug gaps around 0.035", a typical required voltage might be around 8-9 KV at idle and 12-13 KV at torque peak. I haven't measured a 928, so these numbers may be different for our cars.

Ignition misfire can be caused by many things, but the primary suspects should be:
- Corrosion at the terminals of the coil wires.
- Faulty coil wires or coil wire routing. Keep the coil wires away from any metal or wire harnesses.
- Dirt or moisture on the coil, distributor cap or rotor.
- Faulty coil or plug wires. Not very likely on a 928, since it doesn't use the fiberglass/carbon strings of American cars.
- Faulty coil. Possible, but less likely than other problems. It is possible for a coil to be able to furnish enough voltage for idle and light throttle operation, but fail under high throttle operations. It is also possible for a coil to work when cold, but fail when hot.

It is possible to have dirt, a crack, or a carbon track that is higher in resistance than the plug at low throttle, but lower than the plug at high throttle. This will cause a condition such as you are experiencing.

I would start by carefully checking and cleaning the coil tops, all coil connections, the rotors, the distributor tops, and all distributor connections. Check for any cracks or carbon tracks on the coil tops, the distributor caps and the rotors.

dimmer

Thanks for all the help everyone ... problem is solved!

I checked the coil to distributor wires and found that with the aftermarket wires I got, the metal clip in the connector that goes into the coil has 1) little spring to it and 2) had some diaelectric grease on it. Whereas the orinal connector was fairly stiff, this one deforms easily. I think the combination of a weak connection and grease made it a poor contact. I cleaned the contact and bent the clip out a bit and it seems to have done the trick. Wally's comments about the spark resistance changing due to fuel mixture makes sense. The poor connection worked fine for idle but caused problems under the load of acceleration.