marky522

Ok, here is an issue that i have been reading about, no one can figure it out so can anyone here figure it out?

ok here are the details:

only runs on cyl 1 & 4
replaced plugs wires cap and rotor
coil only recieving one fire signal every revolution instead of two 2 & 3 no spark
all 4 have injector pulse
scoped flywheel sensors, voltage over 8v
Scope ignition 1 & 4 get good spark but 2 & 3 none
replaced with a known good DME
occasionally #2 will spark on startup just once

This is all second hand info, but i can ask questions, any help is appreciated.

Mark

Mamooguy

I dunno ... how are the valves?

jpk

The output transistors in the DME are paired 1-4 and 2-3, meaning that one transitor fires the coil for 1-4 and another fires the coil for 2-3. The output transitor for 2-3 is bad. That's the problem. Bad connecton on the board (cold solder joint), toasted transistor, or in more general terms, bad DME.
Transistors don't usually just "go bad" or wear out, so, the more difficult problem is finding what caused it. If it's just a bad solder joint, no biggie, just re-solder it. If it's a toased transistor, replace it, but it may very well fry again and again and again untill the root problem is found and corected.

tomrc

When I had that problem it was my ref sensors. They scoped good and resistance was good but still it (barely) ran on two cylinders.

You can get the ref sensors from the junk yard out of older BMW's, I forget the models but if you do the search you'll find them, I paid about $10 for both.

Calmchaos

I had the same kind of problem when I got my car... My rotor wasnt bolted to the cam. Don't even ask how long that took me to figure out. :-D

StoogeMoe

You're close. The coil is fired by one transistor. You're thinking about the injectors. They are fired in pairs as you describe. I think the problem is in your one injector transistor, not the coil driver. Resolder both and see if the problem persists.

The engine SHOULD be firing the coil once per revolution as you described, so that's not your problem.

skilpatrick

Please let me know what you found! Mine is doing EXACTLY the same.

kingston944

Had a 6 cylinder volvo and installed new plugs major tuneup cap rotor ect, and within a month it was only running on 5 cylinders and it was a bad spark plug that ended up costing me $ 500.00 because the shop said couldn't be the plugs they are brand new !!! I think that is why I do all my own work now, besides the pleaser of being in the garage.

944Ross

He said he replaced the DME, no change.

A 4-cyl fires a plug every 180 degrees, so 2x per rev.

I tend to think it's the sensors, although he says it scopes good. Maybe they are gapped too wide, so they give a pulse on the scope but not enough to trigger the coil circuit.