355 engine cut off at 2500 RPM

Hi,
I have a 1997 355 GTS. The following problem cropped up out if the blue. As soon as I reach about 2500 RPM, the engine cuts off, check engine and slow down lights come on. Release the throttle and everything back to normal. All lights off. Accelerate again and same thing. Same thing hot or cold.

I don't have cats and have disconnected the sensors. So my check engine light has always been flashing. Only recent work was to change oil and put in a new battery.

Mechanic saw a leaking breather and fixed it. Problem continues but at 4000 RPM and when warm. Plug wires ok. No error codes. Still diagnosing but drawing a blank. HELP!

Regards
Hameed

 

Does it do it while revving the engine in neutral? it's interesting that it moved from 2500 to 4000. That alone should indicate something. I'm not sure but it might be possible that one engine bank is cutting since the slow down light is on. Is it blinking or is it solid?

 

is it possible that 1 of your thermocouples is giving a wildly inaccurate temperature reading because it is faulty? that might send the engine straight into limp mode where it is running on four cylinders only. A simple thing you can do is unplug all of the associated ECU's and clean the connections.

 

Two things:

First I would look for a plug wire to be touching a header or pipe somewhere.
Second I would look for water down in the spark plugs (under the covers).

 

I dont know what you mean that you have disconnected the sensors.

My testing shows that an unplugged Thermocouple will will output 5V from cat ecu and trigger a bank shut down. If you unplug the ecu there is a bias voltage of 5V as well.

How long has the Cat monitoring system been disabled and when did you unplug things>

 

Look in your engine compartment and make sure all the air intake pipes are connected on top of the engine, if one is loose or disconnected, you will have a problem.

 

Yes it happened in neutral also. It is not one bank. I know how that feels. Light was solid.
Thanks

 

Thanks. mechanic checked that and fixed a minor leak. That is how the RPM moved from 2500 to 4000

 

Thanks. CAT monitoring system disconnected for a year now. That is what I meant by disconnected sensors

 

Thanks all. Will check and keep you updated

 

You will get limp mode voltages sent to the ecu with the system disconnected. I cannot explain why you haven't had this in the past year but if you check the voltages on the signal lines to the main ecu from the disconnected cat ecu s you will probably find 5V.

5V will trip the slow down system. I suggest you look into this first.

 

Yikes

May have fried the ecu

I would check every connection. When cold it should not go into limp and should rev normally

Try disconnect battery for a few min and see if you can get it to rev, if not your best bet is an SD tool or try swapping a computer with a known good one

 

What about a bad throttle position switch. My 95 would go into limp mode even when stone cold when I had a bad cat ECU but you have yours disconnected.

 

NO luck Yet! Can't diagnose