Just bought a 1991 Formula 350 yesterday. Car was not running when purchased. Got it home and found that if I unplug the tach signal wire near the coil the car will start. I've spent the day working through some bugs. After checking all the fluids, I started it, set the timing to 8* BTDC, reconnected the ESC wire, and tryed to let it run to full temp. After a few minutes It started idle surging and stalled. I pulled the IAC motor and found the pintle and housing were caked with carbon, so I cleaned the housing and installed a IAC that was in the garage. Reassembled and tryed again with the same result. Code 43 is present in the ECM, which I think relates to a ESC problem, I'm just not sure what. Is the tach symptom related to the code 43 as well?

Just a heads up- the car has a few issues. Majority of them are due to the prior owner adding a bunch of "Hot Rod" parts. Car still has a factory ECM and PROM (they were laying on the floor).

 

Well for starters if thats a TPI set up i tought you timed it with the EST connector still connected? Could be wrong though you might want to check on that. Now as for the car starting only with the tach dissconnected I would check for a short on the tach wire. The reason i say that is because as you know the ignition coil must be switched on an off to create the collapsing magnetic field that induces the voltage that causes spark ect. Now after a quick look at the schematic it seems the ignition control modual switches the ground of this coil on and off to accomplish that however it also happens to be in parrallel with the tach output. Now if the tach output was grounded in essence the ignition controll modual is bypassed and the coil has a continuouse path to ground meaning the coils power is no longer being switched. Further more because its not being switched its not induceing the high voltage in the other coil so that means no spark. As for the EST trouble code i dont think thats related. That code is set when the ECM detects a knock condition pretty much continuously. In your case ide bet thats because the EST modual is dissconnected which also means the computer is constantly retarding your timing to try and stop the possably non exsistant knock.

 

Thanks for the info Rolling Thunder.

I was just looking it over some more and noticed that the knock sensor wire is burned and contacting the header. Any idea how the knock circuit feeds back to the spark module and would grounding this send the continuous knock signal you referred to?

I was going to move it and give it a try, but I also noticed that the plug wires are a bit funky. They have the spark plug boot end on the dist cap and at the plugs I noticed that they used points cap style boots. So the terminals are wide open. I just washed the car so I think i'll wait until it drys up.

Yes it is a TPI. I thought timing was set with EST disconnected. Can anyone confirm?

 

Well the way it works is theres 3 parts the the ESC circuit. Theres the knock senser itself which feeds the ESC modual which acts kinda like a buffer monitoring the signal looking for anything abnormal and if nothing is detected then sends a i believe 8-10 volt signal back to the ECM to say everythings ok if a knock is detected or the signal is some how lost the comp sends a signal to the ESC modual in the distributor to retard the timing. Now the output of the knock sensor itself is very small only a few milivolts and if it was shorted against the header i would expect it to short out the signal but then again it could be induceing noise into the signal wire triggering a knock responce as it wouldnt take a whole lot to trigger a knock responce from the ESC.

 

I'll fix the knock wire tomorrow.

Runs great for a few minutes and then rich & crappy. Guessing here, but does the ECM ignore knock signal during warm up (closed loop?)? Seems like at some point it changes to another calibration or something. I would just like to understand the why now that we know the what (hopefully).

Thanks again....
ST

 

My understanding is the only sensor that is ignored in open loop mode is the oxygen sensor. Heck even the factory manuals deffinition of open loop mode is " describes ECM fuel control without use of oxygen sensor information." Now Thats not to say it has no effect. For example if the timing retarded as result of the ECS module than that could effect the O2 sensors readings. That in turn would have no effect in open loop as that sensors not used anyways but in closed loop would make a difference.

 

Further inspection of the knock wired showed that it was not shorted to ground, but the starter wire bundle with the knock circuit was melted and the wire was frayed and touching a larger red wire. Fixed that and it still has the same condition.
Today I performed two diag tests for code 43. One from Mitchell repair and the other from a Chilton manual. Both tests have a step to check voltage on the knock wire with key on. Each reads that the wire should only have 5 volts on it. My reading is battery voltage = 11.87 volts. Both tell me that I have a shorted wire or a faulty ECM. I checked the wire from the knock sensor back to the ECM (visually and DVOM)and found it to be OK ,as far as resistance (haven't load tested it yet). And the ECM was just replaced with a reman unit before I got it.
-Can someone confirm that the 11.87 volts should be 5?
-Since the ECM has already been replaced could this be a memcal issue?

 

I think I found something!!! The prior owner finally emailed me the cam info. It's a Comp Cams Thumper series:
Thumpr™, 283THR7
Choppy Idle, High Performance Street
PART #08-600-8

I looked up the specs and noticed that it only has 107* LSA and 227/241 dur. This cam doesn't sound like it was meant for a TPI engine! It fits and everything, but I don't think it's for an EFI application.

Does my code 43, bad idle, and rich exhaust make sense now? And does anyone have a smaller cam they would like to sell? Stock or a little larger would be fine with me.