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Ask Arench Your Own Question
Arench
Arench, Master Technician
Category: Chevy
Satisfied Customers: 1490
Experience:  30-years experience auto and truck repair.
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I have an 85 Pontiac Fiero, 2.8 6cly. GT, manual transmission.

Customer Question

I have an 85 Pontiac Fiero, 2.8 6cly. GT, manual transmission. I rebuilt the motor and installed it. It has not been on the road yet. There's a very difficult problem some where in the electrical system, possibly. A local mechanic has the car and can not get it to run normally. It runs rough and stalls. In order to start it I have to pump the gas pedal. At times when the motor is cold, it will start right up and run smoothly a little. Eventually it will stall and be difficult to start again. From there it will run roughly and stall. The codes I get are temperature, map and mat sensors. I don't think these codes are correct. I replaced all the sensors as well as the ECM. Although the new ECM causes it to run even worse. So I'm using another one I pulled from a parts car. We (my mechanic and myself) have tried quite a bit. We have corrected vacuum lines. Inspected and replaced the fuel regulator. We have double checked the wiring for the injectors to be sure that they are not crossed. As well as spark plug wires. My mechanic felt that the engine was getting too much gas. That's why we replaced the fuel regulator. He felt there was too much fuel pressure. We can't say if it did any good. We don't believe there's any thing wrong with the injectors. So, that's out. There certainly aren't any vacuum leaks. At this point in time my mechanic feels that electrical pulses to the injectors are not synchronized with the engine ignition timing. This function would take place between the injectors and the ECM. I'm looking for any real and tangible information on this issue and if it turns out to be correct and solves the problem I'll go ahead and render payment.
Submitted: 1 year ago.
Category: Chevy
Expert:  Arench replied 1 year ago.
My name is ***** ***** I will pass on some info that may help you.I can't honestly imagine doing anything from my chair here that might help directly because I can not touch the car, the car is old and it has a lot of paw prints on it already. However, I have some insight that might be useful. I had gone to the Delco training center in Tarrytown NY about the time when these cars were being unleashed in the public. The courses I took were all about computerized engine controls. I clearly recall the instructor dancing around singing a tune he made up about powers and grounds (PAG).If we were diagnosing some fiddled with problem car he would not even look at our work without a list of voltages for the powers and grounds to all the ECM terminals. We had to write down the connector color, the terminal number and the voltage with the key on engine off and the engine running. The overall point to this story is that your staring point has to be knowing if the ECU has all its power supplies and proper grounds while under load. You will need a wiring diagram and a pinout chart for the ecu. You can get some easy ground measurements at the TPS and the MAP sensor under the hood. The grounds should be very close to zero with the engine running. Have a look at the small red wire going to the battery positive. This is a critical power for the ECU.Have a look at the ECU ground which is supposed to be at one of the transmissions bolts if my memory serves me. Remove it, clean the bolt and sock it down. Make sure there are no unhooked grounds or broken ground wires in the harness where it is supposed to ground. Make sure the negative battery cable is both clean and tight at the engine bolt.One thing I know for sure is a car with a bad ground often runs rich (too much fuel) because of erroneous sensor readings.I don't think I have anything else to add but my gut tells me it's a harness issue because back in the day it seemed like everything was a harness issue with this generation of cars, like bad connections, broken terminals or bad pin drag (loose female terminals.)I suggest you print this out and show it to your mechanic friend who has the car.
Customer: replied 1 year ago.

Thank you for your input. I'll know the answer in a day or two. Until then.

Expert:  Arench replied 1 year ago.
Best of luck, I'm pulling for you!
Customer: replied 1 year ago.

OK. Here's an update: We did find disconnected ground wires from the ECU and harness. We're uncertain that connecting them has had any effect. What we do know is this: We connect diagnostic equipment to the individual injector plugs. When we turn the key, all the plugs pulse current simultaneously, rather than individually. Your thoughts please.

Expert:  Arench replied 1 year ago.
As designed. The mpfi is non-sequential that's why there is a fuel line muffler in the fuel supply line.
Customer: replied 1 year ago.

Hello. Did you get those numbers I sent you?

Expert:  Arench replied 1 year ago.
No, sofar this is the only reply I have from you today.
Customer: replied 1 year ago.

OK. My mistake. Here are the numbers: A8: 14.00 A9: 4.79 A4: 13.37 D8: NOT FOUND. D7:.21 D6: 2.72 A11: 4.52 or it could be .10. D13: .03 C11 and C12: There are 4 solid green wires, all alike. Two have a voltage of 13.33 and the other two, -.08. C14: 4.95 C13: -.05 D12: -.06 C10: 4.92 A12: -.06 B3: .08 B5: 1.45 B4: 1.43 C5:12.71 C6: NOT FOUND. C4: 12.65 C3: .70 A10: 13.30 B10: NOT FOUND. D1: -.14. There is one bright orange wire that fluctuates between 3.25 and 4.15.

Customer: replied 1 year ago.

OK. My mistake. Here are the numbers: A8: 14.00 A9: 4.79 A4: 13.37 D8: NOT FOUND. D7:.21 D6: 2.72 A11: 4.52 or it could be .10. D13: .03 C11 and C12: There are 4 solid green wires, all alike. Two have a voltage of 13.33 and the other two, -.08. C14: 4.95 C13: -.05 D12: -.06 C10: 4.92 A12: -.06 B3: .08 B5: 1.45 B4: 1.43 C5:12.71 C6: NOT FOUND. C4: 12.65 C3: .70 A10: 13.30 B10: NOT FOUND. D1: -.14. There is one bright orange wire that fluctuates between 3.25 and 4.15. Did you just throw the question open? If so, why?

Expert:  Arench replied 1 year ago.
Two big problems here. C10 coolant temp sensor 4.92 volts. This is the yellow wire at the coolant sensor again. This is way too high. Measure volts at yellow wire at the cts with everything plugged in. Replace coolant sensor and wire connector with new and retest C10. Measure black and yellow wires at coolant sensor with engine running record results. Problem number two is D6 tan wire engine ground 2.6 volts. This could be close to zero. Replace wire to engine ground or backup the ground to this wire. I have no idea what throw the question open? If so, why? means, sorry.
Customer: replied 1 year ago.

A message said that the question was put open to all experts. As if to say that you were stumped and were asking other experts.

What is "cts"? When I measure it what SHOULD it read? If it reads normal should I still replace the sensor and connector?

There are 2 or 3 other wires that look like tan. I measured the one that looked the most tan. ARE YOU SURE that I have not confused the tan wires?

Expert:  Arench replied 1 year ago.
You should not confuse the wires because you are supposed to be looking at the cavities, not the wires. The diagram shows where each cavity is. cts is coolant temp sensor. The coolant sensor black wire should be close to zero and the coolant sensor yellow wire should read 1.5 to 1.9 volts with warm engine both at the sensor and wire C10 at the ECM. I don't get to see system generated messages to you, I have no idea what that is about.
Customer: replied 1 year ago.

I still don't know how to read the cavity diagram. Can these wires be pulled from the connector or should I splice a back up to the existing?

Expert:  Arench replied 1 year ago.
You must learn to read the diagram or get help from your mechanic friend. You should cut the wire two inches back from the connector to splice in a new wire with solder or a but connector. the big connector shows two rows, row c and row d. empty slots count. and they go in order as pitured starting with D1D1 black/whtD5 tan/blkD6 tanD7 PPLD8 greenand do on
Customer: replied 1 year ago.

Gotcha. I'll get back to you tomorrow. Talk to you then. Good night.

Expert:  Arench replied 1 year ago.
okay good luck
Customer: replied 1 year ago.

OK. Removed CTS and tested. NO GOOD. I told that clown at the shop to test it and he refused. He charged me good money for nothing. Now, I put in a new sensor and spliced in a new wire at D6. It tests at -.06. The CTS yellow wire tests at .04. The black at .00.

Expert:  Arench replied 1 year ago.
with engine running and everything plugged in the yellow wire at the coolant sensor and terminal c10 should have 1.5 to 2.0v recheck.
Customer: replied 1 year ago.

I will. Does it matter if the black and yellow wires are crossed? Was the D6 reading correct?

Expert:  Arench replied 1 year ago.
D6 is a ground and should be close to zero yes.The coolant sensor is a simple thermistor so the order of the wires does not matter the computer sends out a 5v reference on the yellow wire and the coolant sensor grounds it partially.
Customer: replied 1 year ago.

OK. I retested the yellow and it's about 190 at the sensor and at C10. I must have misused the meter. Although, with a brand new battery and alternator it's charging at 14 volts.

Expert:  Arench replied 1 year ago.
that sounds right. clear the computer, drive the car and check for codes.
Customer: replied 1 year ago.

Will do. I'll get back to you by Friday. Thanks for all the help.

Expert:  Arench replied 1 year ago.
okay good luck