911URGE

Been searching the forums but need some assistance.

Recently my 89 964 C4 has developed a significant intermittent ignition issue. When cold it seems to runs fine. Once warm it gets a intermittent significant loss of power like its running on half the plugs. It does not stall but idle is low when this occurs. Almost has a "chugging" feel and sound. This will go on for a few seconds to several minutes then will run fine instantaneously. Stopping and restarting the car has little effect.

Replaced the DME with a new unit and no change.

Decided to test coils and secondary dizzy. I did the following:

• Pulled ignition harness from primary coil (secondary coil/dizzy connected) runs fine and proves belt is intact
• Pulled ignition harness from secondary coil (primary coil/dizzy connected) runs poorly
• Swapped primary ignition harness to secondary coil (secondary disconnected) runs poorly
• Swapped secondary ignition harness to primary coil (primary disconnected) runs fine

It appears that the car runs fine only when the harness for the secondary circuit is connected to either coil pack and primary harness is disconnected. With both connected I sense a very slight ignition miss.

When trying to connect my Scantool to the car I cannot connect to Motronic, only ABS connects so I have no error codes to chase.

At this point I'm unsure of what I have proven and what my next steps should be. What should I be looking at next? ECU connections? Grounds? Am I missing something else? Does this sound like an upstream issue?

Thanks in advance.

Hobbsie

You should find out why your Scantool doesn't see the PCM (powertrain control module). Have you tried someone else's?

alexjc4

I was doing some similar testing myself recently and I think you may have made the same fault assumption I made. The plugs from the loom that connect to the ignition control units mounted infront of the coils are hardwired to a specific coil, so swapping these plugs from one ignition control unit to the other will not test the coils, only the ignition control units. So to test the coils independently from the dizzies you need to swap the king leads over.

newsboy

Even if you test your coils with a multi-meter, and they are within specs, they can still have internal faults which could cause problems. Not too expensive to replace them, so you might start there.

Duck

If you decide to change your coils, you might want to do this:

https://rennlist.com/forums/964-foru...-to-964-a.html

Vegas993

You may want to check your Fuel Pressure Regulator...

https://rennlist.com/forums/964-foru...placement.html

alexjc4

If it was the fuel reg wouldn't you expect it to run bad on either/both distributors. The op indicates it runs fine on one and not great on the other.

Feudal Serf

Hi 911URGE

You have identified that the primary coil ignition "system" is the problem.
and your dizzy belt is OK.

Swapping the coil harness, suggests that both your coils function fine.

So you are left with Dizzy cap on primary, primary wires or plugs supplying the lower side.

You should look for arc in the wires, run vehicle with low light and mist the engine bay with water.
If nothing, switch out the king plugs to rule that out.

I had one lower plug that wasn't seated!!! and i idled the engine only on the affected coil, running rough.
Wear some insulated gloves, and pull a wire from the dizzy cap one at a time. The engine will stall with 2 cylinder misfire. The one you pull that doesn't stall the engine is the problem.

I had the exact symptoms as you and it turned out to be a mis seated plug wire.

Have you been underneath working on the exhaust recently?

Good luck

cheers
FS

Silvertarga

When mine did this it turned out to be the coil(s)

They were ok until things got warm and then failed. Tested ok when cold as well .Sent them back and was told they tested OK and nothing wrong, told them to test them warm/hot...refund issued and I did the 993 coil mod

The new Silver bosch coils are worthless..

alexjc4

This is a reasonable assumption but wrong, swapping the coil harnesses does not swap the coil being used, only the ignition control units. I made the same assumption, which gave some baffling results until i checked the wiring diagram and found that each coil harness is hard wire to one coil. The only way to see if one coil is the problem is to swap the king leads, getting the primary coil to run the secondary dizzy on its own and viceversa.

Feudal Serf

Thanks for this.
I never did it this way myself, just made the assumption. I swapped the king leads.

Ill take your word for it

The 964 ignition system sure is fickle.

Cheers
FS

backitoff

If this is any help, I had similar symptoms, turned out to simply be a burnt out rotor arm.

911URGE

All,

Sorry for the delay in a response. I've been on vacation since Thanksgiving. I appreciate all of your helpful replies and experience.

If this is the case did I demonstrate my primary ignition module is a potential issue?

This is the current theory I have as well. I am looking at the 993 coil mod.

Thanks - I will swap coil to dizzy leads and retest.

911URGE

Finally got Scantool to connect to ECU - No Error Codes. But I already guessed that.

Timing is running at 0.8 deg (0.0 +/- 3.0 is spec) at idle so ignition is not being retarded due to any limp modes. Will continue checking

911URGE

Just a quick update on my ignition issues. I tried a lot of tests and nothing made sense. I gave up and took the car to my indy pcar service center and they found the secondary distributor was not in sync with primary. The advance was off from the primary by as much as 15 deg. Distributor belt is intact and the two units are in physical sync. They state the internal advance of the secondary is not functioning correctly. In addition looks like I had three plug wires fried. My wires are original and 27 yrs old.

Replacing the distributor with a rebuilt unit and getting all new plug wires and spark plugs. At this time all ignition issues resolved.