broken green?white wire at ecm

[94conv.]

I unplugged the ecm to check connections still trying to get to the bottom of my stumbling and dying problem. Still only does it after it sits after running for about an hour. After I plugged the ecm back in the problem started again, stumble and ran rough, didnt die this time though. I looked closer found a cracked green and white wire, not broken all the way but one strand is definately broken. This wire is on the ecm on bank "red" which is top left or inside connector. From what Ive read its the fuel pump relay wire. Would this cracked wire cause my intermittent stumble after it sits? If we wiggled all the connectors the car would act up also.

[94conv.]

What codes are displayed when you have the running problems ?


Highly unlikely if only one strand broken.


I would be more concerned with bad contacts on the ECM plugs if you can generate the miss by wiggling the plugs
Possibly heat related since problem only appears after engine has been running for a while and the ECM has heated up.

Known problems with heat and the underhood ECM's on C4's
Post # 25
Which is failing? ECM or CCM? - Page 2 - Corvette Forum

Thanks, I read the post, I'm not getting any codes and its intermittent. I can run fine around town but after its driven for about an hour and then it sits for about 30 or 40 minutes and I go to restart it, it stumbles and dies and wont restart till it cools down for a couple hours to over night. I noticed the ECM was getting hot but when I rechecked it, it really wasnt any hotter then anything else under the hood. The last time this happened I tried restarting after an hour and it just cranked but wouldint fire so then I unplugged the maf sensor and it fired right up and ran fine ( supposedly limp home mode, Im assuming). I drove it a block back to the house and plugged the maf back in and it started and ran fine. Yesterday I had a guy come and look at it and we unplugged the ecm's four connectors and they looked fine but when we plugged them back in and restarted the car it acted up, missing and stumbling. Shut it off and restarted it and it ran fine. We did find a half broken wire, one strand was actually broken on the red connector, from what I read it goes to the fuel pump relay. If this wire was causing it, it doesnt seem it would be intermittent or only when hot or would run fine in limp home mode. Ive replaced the fuel pump, regulator, filter, coil and module and Maf sensor. I appreciate your response so thank you and if you have any other ideas please let me know. Ive been dealing with this for a year.

[94conv.]

Possibly a injector problem?
Are known to fault with heat (break down due to old age )
How old are your injectors?

they are the original injectors but I can hear them pulsing while its cranking but not wanting to start. Ive ohmed them out cool and they're right at 16 and 13.4 when they're hot and the with a pressure guage on the rail, it'll hold pressure for almost an hour.From what Ive learned, the nicked green/white wire goes to the fuel pump relay and activates the pump for 2 seconds to prime the fuel pump, after that the oil pressure sending unit activates the fuel pump. I still need to fix the wire but its not my problem I dont think.

[John Robinson]

The wires are very fine in the wiring harness especially the ones going to the ecm. Remember the ecm depends on the signal from those wires to decide how to trim the running of the engine. I would guess if you have one broken strand in a wire you have more. For some reason the wires in our cars are now getting tired from age. As stated many times in post here on this site problems with wires has been the major source of many problems. I personally have had to splice in whole new sections of wire and connectors for the tps, iac. maf. temp sensor on the block and the ground bundle for all of them. My car now runs as it should without replacing any of the sensors. Oh I almost forgot I also have spliced wires going into the ecm connectors that were bad.

After rereading your description of how the caar behaves I think if it were mine I would start with the wires coming from the maf sensor and follow them back through the wiring harness untill you get to the common ground bundle for the sensors on that side of the engine. I think you will find that it is in a wad of black electricle tape. When you unwrap this bundle you will find that it is heavely corroded and the ground are no longer good. What makes me think this is the problem is that what you describe sounds like a problem with the iac. The iac can cause high idle, rough idle, or a no run condition. At least that is what happens if the ecm does not get a good clean signal from the grounds in that bundle. Good luck in finding what is really causing your problem. I see you have been a member of this forum a lot longer then I have and I am probable rambling on about things you have read many times before.

[G Winter]

Have you checked all grounds ? Check connections on all the sensors, I recently found my temp sensor connection badly rusted.
Seems that some sensors have to be totally nonfunctional before they throw a code.

[94conv.]

First, thank you for the replies. This problem is frustating . The ground were visually inspected but I didnt tear into any tape so I need to do that and also examine the bundle of wires going into the ecm closer. The problem does seem to be heat related and will run fine in "limp home mode" to get myself back to the house. The temp switch on the water pump definately needs to be looked into as well. These parts are expensive so Im trying to narrow it down the best I can. I bought I fuel gauge which I believe is faulty which led to the new fuel pump and regulator. Its reading 20lbs at the rail and after the pump and regulator its reading 18lbs. then goes slowly up to 32lbs when the car is running. Gordon Killebrew told me it wouldnt start on 20lbs so thats why Im thinking my guage is off and havent been able to borrow a known working guage. It wouldnt start at all at one point so I unplugged the maf, it started right up, which led to the new Maf. Then I learned that by unplugging the maf it sent the ecm into limp home mode and didnt mean that sensor was bad. So I'll check the wires closer and check the temp switch or have someone with a scan tool check it, I dont have one. Ill post my findings and hopefully all of this will help the next guy that has this problem. Thanks again and keep posting any ideas of what could be wrong.

[John Robinson]

lOOP STATUS - Tech 1 ~isplays Open or Closed -
"Closed Loop" displayed indicates that the ECM is
controlling fuel delivery according to oxygen sensor
voltage. In "Open Loop," the ECM ignores the oxygen
sensor voltage and bases the amount of fuel to be
delivered on TP sensor, engine coolant, and MAP
sensor inputs only.
SHORT TERM FUEL TRIM - Range 0-2SS -

This right from my 93 FSM. You will note that depending upon which loop the ecm is in it uses different sensors to adjust the engine tune. So when you first start the engine it is in closed loop and therefore the ECM is only using the 02 sensor. When the engine gets up to about 138 degrees the the ECM switches over to Open loop were it uses the MAP sensors along with the others listed. So if it runs good in closed loop but bad in open loop you have a problem with one of the sensors or it's wiring in that loop. When you disconnect the MAP sensor with the engine in Open loop the ECM takes the engine tune back to the Closed loop. In other words it reverts to a program that does not need in put from those sensors.

Now here is my disclaimer I always get confused which loop is first in this information but I think you will get the idea about how the loops change the way the ecm maintains the engine tune. I hope this helps you to undeerstand why the engine runs good cold but changes when it gets warmed up.

[boomdriver]

First, thank you for the replies. This problem is frustating . The ground were visually inspected but I didnt tear into any tape so I need to do that and also examine the bundle of wires going into the ecm closer. The problem does seem to be heat related and will run fine in "limp home mode" to get myself back to the house. The temp switch on the water pump definately needs to be looked into as well. These parts are expensive so Im trying to narrow it down the best I can. I bought I fuel gauge which I believe is faulty which led to the new fuel pump and regulator. Its reading 20lbs at the rail and after the pump and regulator its reading 18lbs. then goes slowly up to 32lbs when the car is running. Gordon Killebrew told me it wouldnt start on 20lbs so thats why Im thinking my guage is off and havent been able to borrow a known working guage. It wouldnt start at all at one point so I unplugged the maf, it started right up, which led to the new Maf. Then I learned that by unplugging the maf it sent the ecm into limp home mode and didnt mean that sensor was bad. So I'll check the wires closer and check the temp switch or have someone with a scan tool check it, I dont have one. Ill post my findings and hopefully all of this will help the next guy that has this problem. Thanks again and keep posting any ideas of what could be wrong.

Assuming you;ve changed the fuel filter on the frame ??? I do not see any reference to that along with the other parts parade....any MAF relays?

The grounds that you are interested in are a stack of 5 that are all on one bolt on the block somewhere just over the oil filter.

No, it won;t run on 20 psi fuel pressure. Might start but won;t run. This sounds like a plugged filter. Sits for a while then starts after sediment settles after pressure bleeds down. Fuel flows for a while then it stalls from low pressure and the cycle starts all over. Getting into open loop (limp home) is coincidence. If it were truely a closed loop problem there would be a code set and THEN it would shift to open loop on its own.

You're never going to see fuel pressure at the rail if the inline filter is bad. New pump or otherwise...the filter blocks the flow inbetween.

Take the filter off and see what falls out. If you see crumbs coming out of the inlet side...thats your problem. Even with no chunks falling out of the filter, its still easy to clog with trash that cannot fall out. Replace. Do not try to reuse.
Fuel filter problems do not set codes.

BTW...
the 2 second prime is independent of the normal run fuel system. Once the engine fires the dist sends a signal to the ECM that closes the fuel pump relay circuit which completes the path to the fuel pump. As a back-up power source there is an oil pressure switch that will also close and supply power IF there is oil pressure. Just a different means of telling the ECM that the engine IS running so it knows to run the pump. If there is no signal to indicate that the engine IS running the ECM stops the fuel pump so it does not flood the engine.

[BigJimZ16]

lOOP STATUS - Tech 1 ~isplays Open or Closed -
"Closed Loop" displayed indicates that the ECM is
controlling fuel delivery according to oxygen sensor
voltage. In "Open Loop," the ECM ignores the oxygen
sensor voltage and bases the amount of fuel to be
delivered on TP sensor, engine coolant, and MAP
sensor inputs only.
SHORT TERM FUEL TRIM - Range 0-2SS -

This right from my 93 FSM. You will note that depending upon which loop the ecm is in it uses different sensors to adjust the engine tune. So when you first start the engine it is in closed loop and therefore the ECM is only using the 02 sensor. When the engine gets up to about 138 degrees the the ECM switches over to Open loop were it uses the MAP sensors along with the others listed. So if it runs good in closed loop but bad in open loop you have a problem with one of the sensors or it's wiring in that loop. When you disconnect the MAP sensor with the engine in Open loop the ECM takes the engine tune back to the Closed loop. In other words it reverts to a program that does not need in put from those sensors.

Now here is my disclaimer I always get confused which loop is first in this information but I think you will get the idea about how the loops change the way the ecm maintains the engine tune. I hope this helps you to undeerstand why the engine runs good cold but changes when it gets warmed up.

John, you have it exactly wrong, the car runs in open loop until it warms up enough to get reliable information from the O2 sensor. As in any servo control system, it is open loop until you introduce feedback (the O2 level), then it becomes closed loop as the feedback signal corrects the error signal introduced by the throttle position command.

Servo control systems are my professional area of expertice...

[John Robinson]

John, you have it exactly wrong, the car runs in open loop until it warms up enough to get reliable information from the O2 sensor. As in any servo control system, it is open loop until you introduce feedback (the O2 level), then it becomes closed loop as the feedback signal corrects the error signal introduced by the throttle position command.

Servo control systems are my professional area of expertice...

Thanks for the information I can never remember which is which. Lets see If I can remember it this way Open loop is like the old cars choke is pulled out as it warms up push in choke until it is closed.

[Hib Halverson]

I've read every post in this thread and it seems all but "vetteoz" missed the last sentence in the OP

(snip) If we wiggled all the connectors the car would act up also.

Based on that, the first thing I'd do is fix the existing problem with the wire going into the red connector then check the other four connectors and road test. If you can wiggle the ECM harnesses where they go into the ECM connectors and still have the problem, I'd disconnect all the harnesses, remove the ECM and inspect all the pins on the connectors with a magnifier and a bright light. If they are dirty clean them with contact cleaner. If any are bent, attempt to straighten them. If can't fix the connector pins, replace the ECM. If you find no problems with the pins, reinstall and reconnect the ECM, road test and still have trouble when you wiggle those harnesses, replace the ECM.

[94conv.]

I've read every post in this thread and it seems all but "vetteoz" missed the last sentence in the OP

Based on that, the first thing I'd do is fix the existing problem with the wire going into the red connector then check the other four connectors and road test. If you can wiggle the ECM harnesses where they go into the ECM connectors and still have the problem, I'd disconnect all the harnesses, remove the ECM and inspect all the pins on the connectors with a magnifier and a bright light. If they are dirty clean them with contact cleaner. If any are bent, attempt to straighten them. If can't fix the connector pins, replace the ECM. If you find no problems with the pins, reinstall and reconnect the ECM, road test and still have trouble when you wiggle those harnesses, replace the ECM.

Thank you. Im going to check into it this weekend and Ill post back. Theres to many post where people post problems and it doesnt get to the fix.