HELP! car starts on 4 cylinders (no slow down lights)

I have a 90 348 that would shut a bank down intermitantly.
sometimes a slowdown light would come on, sometimes just flash once, sometimes shut a bank down other times just the light would come on full.
the cats were not over heating.
you'll know if that is happening, it's obvious.

turned out to be an exhaust ECU.
Ricambi had one,
the bad one I had was black, an the epoxy was separating from the casing.
the new one was green.
I found the bad one by switching things around.

I also disconnected them one at a time when the problem was happening.

 

Just to say it again, on an 89 t with M2.5, the cat ECUs CAN NOT shutdown a bank. The wiring diagram supports this, as does discussion with an experienced mechanic.

The symptoms as described by Steve indicate a lack of crank angle sensor signal to the 1-4 ECU. It isn't bank 5-8 because the tacho is not affected and it ias driven by the 5-8 ECU. It will be the sensor (not likely as it's supposedly new), the connector (possible as they are not the best), or the wiring harness (possible also coz it happened on my car). If it's not any of these problems, then the next place to look is power to the 1-4ECU, and finally the ECU itself. Until all these are checked and proven, I'd put my money on this area.

Dave

 

Reset the ECUs, takes 10 seconds to switch off the battery and turn it back on, then 10 minutes to let the car just sit there and idle. As before, if it starts cutting out during this process, abort it. No point continuing....

I am 99% certain it is to do with the crank sensor circuit, power, or the ECU itself, and almost certainly on bank 1-4.

There are of course other possibilities such as a bad fuel pump, or fuel pump circuit. I'd also check the ignition coil and controller. Intermittent failure here would result in no spark, but fuel still being injected. If the car is stationary when this happens, you'd certainly smell gas at the exhaust pipe. If it was a few cylinders cutting out, then the cat would get real hot real quick resulting in slow down light and possibly that lovely red glow.

Give me a call if you want to talk it through. One of these days I'll sit down and attempt to create a trouble shooting flow chart. The WSM has one, but it's only for M2.7 cars, and they have far superior diagnostic capabilities to the M2.5. In other words they have SOME diagnostics built in!

 

Well then I am seriously suspecting a bad cat ecu, especially since the problem jumped sides (twice?) when the ecu's got swapped.


Try this,

Disconnect BOTH cat ecu's.
Run the car, and watch the cats.
If they BOTH get hot then I'm with Dave, the cranks sensors. You can change out the crank sensors yourself. They are WAY easy to change. 1 - 10mm nut, 1- allen bolt, and disconnect the clip. It can be done in 5 minutes. But before you change out the crank sensors peal back the rubber boot over the back of the clip (on the wiring loom side, it's the female clip) and see if you have any busted wires.

 

I have a 89
Mondial T with a 1-4 bank light intermittantly coming on and off, with no decrease in performance or bank loss.
Just a drop in RPM'S. The Tach appears to have lost 1500 rpm's and reads lower than it should. Weird

Does this sound like the same problem?

I guess if it is not dropping banks now it is not if it's when?

Thanks for any help.

Mark in Danbury CT

 

A good starting point would be to pull your ecu error codes.

 

Steve, great to hear everything is well again. Now, enjoy the drive!

Dave