Strange 944 misfire

[Jacquiles]

I have a 1986 944 and after replacing two of my lifters, my car runs very poorly after the first two miles of driving anytime i drive it. I don’t think it has anything to do with the lifters. i may have put something back together wrong or something like that but i have double checked all my work. Not too long ago i replaced all my spark plugs, rotar and cap, spark plug wires, coil and reference sensors. i believe it is not related to fuel, but rather the reference/speed sensor. as you can see in the video my rpm is a little crazy.

[Tom]

Yikes, that is a weird one. At least it's not intermittent, or subtle, so that should make it a little easier to find. I'd start by going over the basics -- cam timing ok? Plug wires in right order? Plumbing correct within mixed-up AOS and ISV hoses? Cap and rotor on tight? AFM connection tight? Any fuel in fuel pressure regulator lines? I'd just give it a good once to start, and probably not drive it like that....

[Jacquiles]

Yikes, that is a weird one. At least it's not intermittent, or subtle, so that should make it a little easier to find. I'd start by going over the basics -- cam timing ok? Plug wires in right order? Plumbing correct within mixed-up AOS and ISV hoses? Cap and rotor on tight? AFM connection tight? Any fuel in fuel pressure regulator lines? I'd just give it a good once to start, and probably not drive it like that....

Hey tom thanks for the reply. Yea this one is very interesting. I checked all those things you were talking about and that all seem to be good. Cam timing is correct, cap is on tight, plugs are going to the correct spot, etc. I replaced my reference and speed sensor with a pair of old ones i had lying around and the problem immediately became better. Although it still occurs, just now it’s rare. happens once or twice per drive instead of a constant problem like in the video. Although i don’t think replacing the sensors fixed it, i’m thinking that maybe it’s a wire issue since i had to get in there behind the motor and moved stuff around a little in the process of changing the sensors. later today i am going to go through every injector plug and sensor plug and make sure there are no loose connections or broken wires. That’s the only thing that makes sense to me because i never ever had this problem before. this car never skipped a beat. then i replace the two lifters i had and it was immediately obvious something was wrong. since the cap and rotor. check out ok the only other electrical connection that i “messed with” was just moving the fuel rail out of the way while it was still connected, so maybe there is a wire messed up in there.

[Tom]

Hmmm... Try letting it idle and rattling the speed/ref and injector wires. See if you can make it stumble that way. They fray inside the harness side of the connectors... you need to peel back the rubber boots to see it.

[Tom]

See this thread if helpful...

https://www.carpokes.com/viewtopic.php?t=565

[944er]

I would suspect the speed or reference sensor gap is just slightly too big. The signal is marginal to the DME, and when the DME doesn't know what is going on it shuts down the ignition and fuel injection. When the ignition shuts down, the tachometer starts dropping.

[Tom]

I would suspect the speed or reference sensor gap is just slightly too big. The signal is marginal to the DME, and when the DME doesn't know what is going on it shuts down the ignition and fuel injection. When the ignition shuts down, the tachometer starts dropping.

Possible, but at running speeds you need to be way (way) off to kill the pulse. I did a lot of testing a few years ago. Here's a video showing my test rig and how the gap affects the pulse amplitude...

[Jacquiles]

I would suspect the speed or reference sensor gap is just slightly too big. The signal is marginal to the DME, and when the DME doesn't know what is going on it shuts down the ignition and fuel injection. When the ignition shuts down, the tachometer starts dropping.

Possible, but at running speeds you need to be way (way) off to kill the pulse. I did a lot of testing a few years ago. Here's a video showing my test rig and how the gap affects the pulse amplitude...

[Jacquiles]

As far as the gap goes for the sensors, i double checked the gap was set correctly yeasterday, and found that with the bracket all the way down, it still wasn’t close enough for the sensor with a .8mm washer on it to touch the teeth on the ring gear. But this is the same way it was set up all last year and never had this problem so i don’t think it’s related to gap

[944er]

. But this is the same way it was set up all last year and never had this problem

But, you replaced the sensors. Potentially with one that due to manufacturing tolerances needs a closer gap. If you still have the old ones, put them back in and see if the problem goes away.