3.6 motor problem... Help an idiot contest.

[Moses]

Well, I tried a few suggestions: First I disconnected the battery for 10 minutes (because that was easy and unlikely to hurt anything) No effect.

Next I disconnected the O2 sensor. Interesting. Compare the motors before and after performance:

Before: The tachometer is crazy. At 4,500 RPM the needle dives to zero. There is nowhere near the power I expected. Not much different than my old 3.0. I know, something must be VERY wrong. The idle is relatively steady at around 700 RPM or so. The motor died twice on the freeway after coming to a stop after cruising at 70 MPH. The motor sounds great, but the throttle response is sluggish. There is definitely no "slam you back in your seat" pull.

After disconnecting O2 sensor. Still have the wacky tachy. Now have much better throttle response and pull to about 5000 RPM when engine begins to stumble. No top end power. Engine seems to be missing badly above 5000 RPM. Motor dies at every stoplight, but restarts and idles steady at 700 RPM. Interestingly, with the O2 sensor connected, the motor backfires constantly on deceleration. Without the sensor, backfiring greatly reduced.

My throttle cable does NOT give me WOT, but I have a Patrick Motorsports unit on order. (Getting wide open throttle is not the fundamental problem at this point.)

What does all this mean? Any ideas?

Help!!!!!

[pbs911]

The problem with the tach is between the DME and the tach. The O2 sensor will not effect that. Check the connections under the seat and at the back of the tach to ensure a good connection. It does sound like the tach is wacky at the same time the engine loses power. humm. Power to the fuel pump is also received from the DME box. humm. Check to make sure you have the correct DME box part nuber for the 993 motor. It really sounds like a lose/improper connection somewhere.

Check to make sure you are not having a fuel starvation problem. It is my understanding that the motor runs full rich with the O2 sensor disconnected. Since your motor runs better with it disconnected it seems like with it connected the mixture is too lean and the DME is retarding the timing to the point you have no power. These motors do not run better with the O2 disconnected so this shows something is wrong with power from the DME brain, fuel and/or timing as a result of fuel mixture. Also try another DEM relay if you have one. If not you might as well get one b/c it is nice to have a spare.

I would hook up a emissions tester and see what the fuel mixture is at the different RPMs with the O2 connected and disconnected.

Don't worry about dying when coming to a stop yet. I would bet that when you get the rest of the system worked out that will resolve this issue.

Also do another check to make sure all vacuum lines are either conencted or plugged.

FWIW, I just had my exhaust completed yesterday and conencted the O2 sensor. Before, the car would backfired sometimes on deceleration. the fuel was till full rich which was evident by teh flames out the tail pipe when lifting off the gas. Now that is gone completely and the motor runs better.

It sucks I know.

[k9handler]

do you have a fuel pump from a 3.6 or the original pump? I would think if the fuel system does not get the fuel and pressure it needs then everything would be downhill from there. Just a thought.

[tryan]

i would check your mixture. an extremly leann mix would backfire throught the intake.

fuel quauntity restriction or old fuel stagnated durring the conersion process.

i have my money on disconnecting the tach to see if that is killing your dme. do i win a scooby snack?

[ischmitz]

It seems you found already one problem after we talked:

You are running on only one coil. The engine should idle O.K. on either one coil. If it dies by disconnecting coil #1 but does not die by disconnecting coil #2 you need to trace why one coil is not firing.

Once you run on one coil only the DME will sense severe knocking and retard timing to get rid of it. The net result is a komplete loss of power.

This could also cause the ignition to shut down at higher RPM's to prevent detonation damage.

The tach going crazy at higher RPM's could also be as simple as having +12V and tach signal hooked up reverse. That'll cause crazy things to the ignition system/DME. I did this to a 2.7 CIS motor and the engine ran rough and erratic. And the tach behaved erratic.

After you have both coils firing check how the car runs. If the tach still does crazy things go ahead and disconnect the tach altogether and see what'll happen. Keep us posted on the results.

Ingo

[ischmitz]

Mark,

Any luck yet on your 3.6? I hope the mechanic got the second coil going by now. Keep us posted.

Ingo

[Moses]

Ingo,

Just took it to the shop today. We'll know soon. Thanks!

[cary]

Just checking on the progress.
Hope you get her up and running today.

[dad911]

Originally posted by pbs911
These motors do not run better with the O2 disconnected so this shows something is wrong with power from the DME brain, fuel and/or timing as a result of fuel mixture.

I have not had an O2 sensor on my car since the 3.6 was installed. Used for track only, seems to run fine. Should I bother cutting one in? Will I get more power? Is there a specific place in the exhaust it should be? I have stock exchangers with custom pipes/mufflers. No Cat.

[Tinker]

Moses,

Has someone mentioned checking those ignition control modules next to the coils? I think most/all 964/993 motors use these?? When they have a short/go bad, the car behaves crazy. Runs fine, then misses, tach goes crazy, stalls, wont start, then starts. I had a VW that did this. There was a simple test in the VW shop manual to check them. Probably a similar test for a Porsche.

Just a thought!

Tinker

[pbs911]

Alan, I got that tid bit of information off Rennlist 964 board. With mine the motor did not run as well with the O2 sensor disconneted, but mine is a street car with occasional track use. Something about running full rich all the time. I would get poping out the tail pipes upon deceleration with flames from the unburn fuel. Too bat mobilish for me. Maybe different on a track car where you are running high revs all the time since the O2 sensor would put the motor in full rich anyways.
I'm no expert, and I may be passing off not totally correct info. But that is what I found to be true in my experience.

[ischmitz]

O2 sensor explanation for 964,993:

The throttle position on those engines is sensed by two microswitches. There are three areas:

1. Idle 0 - 2%
2. part throttle 3% - 66%
3. WOT 67% - 100%

The areas are adjusted my adjusting the microswitch positions correctly.

The DME will use close loop operation (Oxy sensor controls mixture) only in 1. and 2 when it is at operating temperature and the code plugs are set correct. It will sense the exhaust oxygen concentration and adjust the injected amount of fuel accordingly. In WOT the system does not care about the Oxy sensor. It uses a different fuel map and runs in open loop.

When the oxygen sensor fails (disconnected or shorted oxygen sensor) on those engines the DME will switch into a mode that does not use feedback but a preset value for mixture.

As a result a disconnected Oxy sensor might result in more unstable idle. It does not change performance. For a track car that runs at WOT most of the time the Oxy sensor is not important. For cruising in a street-driven car it should be there

Ingo