Misfire, where do i go from here. (Page 13/18)
sanderson231 MAR 01, 03:27 PM

quote
Originally posted by zkhennings:

Check power to the ECU, there are two pins that splice together into one wire that goes all the way to the junction block below C500 on an 85. There is a single pin connector inline at the junction so you can reset the ECU. This line went bad on me and caused all kinds of weird intermittent problems.




Wires 10 and 15 on the white connector provide power to the ECM from a fusible link (unswitched power). The 84 does not have the junction block by the battery. I don't remember where the fusible link is connected. In addition the C500 connector is located in front of the engine on the firewall not by the battery as in 85-88.

With the ECM connectors removed use a 12V test light to check wires 10 and 15 on the white connector. The bulb should light brightly. If it doesn't there is resistance in the power supply wiring to the ECM.

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formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged

Spoon MAR 01, 08:26 PM
A lot of issues going on here. Perhaps a common source is causing multiple issues totally unrelated to one another. To keep this short, how many ground connections have you tested? Body, engine, chassis, etc. This is where a factory service manual comes in handy.

I recently had an issue with a 2011 Cadillac. Started it one morning and drove 40 yards and check engine light came on along with Anti-Traction, etc resulting in limp home mode. I connected my scanner and nearly every code in the book popped up virtually over night. If I was to replace every component that was affected I would of spent a fortune and still have a problem. It turned out to be a damaged wire harness that some hungry mice chewed apart and some wires within the harness were chewed together causing cross voltage, etc.

In your case a high idle could be caused by a bad or weak ground causing unstable voltage. A probe may reveal 5 volts to the component, lets say a O2 sensor but without a dependable ground to regulate it all bets are off. I once solved a high idle (2500) rpm on my 2.8 Fiero by adding wide braided ground straps from battery (-) to frame to engine and body.
Just my 2 cents, hope it helps.

Spoon

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"Kilgore Trout once wrote a short story which was a dialogue between two pieces of yeast. They were discussing the possible purposes of life as they ate sugar and suffocated in their own excrement. Because of their limited intelligence, they never came close to guessing that they were making champagne." - Kurt Vonnegut

Dukesterpro MAR 02, 10:20 AM
Well here's what currently happening behind the scenes.


I have been driving the car using ALDLdroid on my radio headunit so I can always monitor the ECM. It is turning up all sorts of weird quirks that I am starting to think may be a ground issue.

The car gets really funky driving in the rain. Alternator will randomly kick on and off when wet. Watching voltage on the screen go from 14.8 to 12 to 11 then back to 14.8 for a while. The headlights dim aswell during the voltage drops. So that's not good, never happens dry.

BLM is all over the place. The more throttle the more BLM. At idle its 128/127 and just keeps increasing.

Car is flooding when cold. If I dont give it gas to start it wont. If I keep cranking without gas, I have to put my foot to the floor to start it.

New, random code 24s while not moving.


I am starting to think there has got to be some sort of ground issue. But at the same time, the high idle only began after I changed by TPS and my intake gasket. Which doesn't involve a ground. But who knows. I'm going to start grounding the snot out of everything and see if It alleviates some symptoms.
zkhennings MAR 02, 01:39 PM
Yes I agree with your grounding plans, I just highly recommend also checking all power to the ECU with a test light as mentioned above to verify good power to the computer. If your alt is not working great in the rain, makes sure the alt itself is not bad. There is a wire going to the alt from the battery light on the dash, if that is not putting a drain on that pin in the alt, it will not create a field and therefore will not charge. Check continuity and connections. Your alt is clearly giving up if you are dropping down to batt voltage, and I doubt it is an issue on the output side, it is most likely either on the input side, or internal to the regulator.

Does not take much low voltage to get electronics to freak out. I have an open source tuning cable for my WRX and it would not work at first, it kept reporting back that system voltage was like 8volts when trying to flash a new ROM. I was puzzled as the battery was at like 12.8V. Since it was open source I found the schematic and looked up the data sheet for the 12V regulator in the cable, turns out its minimum voltage was 14V, and since I load a tune with the car not running the voltage regulator was breaking down and only outputting at about 8V. I bypassed it and have been fine since. But just an anecdotal story that slightly lower voltages can cause serious headaches in the performance of the sensors and ECU. These 80s electronics are especially sensitive to this.

[This message has been edited by zkhennings (edited 03-02-2023).]

sanderson231 MAR 02, 02:26 PM

quote
Originally posted by Dukesterpro:

BLM is all over the place. The more throttle the more BLM. At idle its 128/127 and just keeps increasing.

Car is flooding when cold. If I dont give it gas to start it wont. If I keep cranking without gas, I have to put my foot to the floor to start it.

New, random code 24s while not moving.


I am starting to think there has got to be some sort of ground issue. But at the same time, the high idle only began after I changed by TPS and my intake gasket. Which doesn't involve a ground. But who knows. I'm going to start grounding the snot out of everything and see if It alleviates some symptoms.



Seems like multiple problems:

High BLM with engine under load but not at idle could be a sign of fuel pressure decreasing as fuel demand goes up. This could be partially clogged screen on the fuel pump suction or a partially clogged fuel filter

According to the FSM a code 24 that is setting even though the speedometer is working could be caused by a faulty ECM connector or ECM

The TPS can be bench tested. You'll need a 5V power supply like three 1.5V battery taped together or a power supply from a computer. Also need a multimeter to check voltage on the sense wire as the TPS is stroked.

As I mentioned earlier start by checking the ground resisyance on the ECM plugs. These are critical.

[This message has been edited by sanderson231 (edited 03-02-2023).]

zkhennings MAR 02, 02:46 PM

quote
Originally posted by sanderson231:


The TPS can be bench tested. You'll need a 5V power supply like three 1.5V battery taped together or a power supply from a computer. Also need a multimeter to check voltage on the sense wire as the TPS is stroked.






Try and hook it up to a scope if you have access, or use an analog multimeter so you can see if the needle dances at certain points, can be hard to pick that up with a digital one.
Dukesterpro MAR 02, 03:56 PM
I'm gonna have to keep working at it. A lot of what you said has already been done.

New FP with a spotless tank.

New fuel filter.

Only thing I haven't opened is the injector.

I will keep trying y'alls suggestions and keep you updated. Still moving into my new shop so I may be a bit spotty over the next few days.
sanderson231 MAR 02, 06:00 PM

quote
Originally posted by zkhennings:
Try and hook it up to a scope if you have access, or use an analog multimeter so you can see if the needle dances at certain points, can be hard to pick that up with a digital one.



I agree with use of analog multimeter. I think they are also better for resistance checks. Digital ohm meters sometimes give ghost readings

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formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged

sanderson231 MAR 02, 06:22 PM
There are thirteen ground connections on the 1984 Fiero. However not all of them are pertinent to engine related problems. The ones to focus on are:

G501 - this is a size 19 wire that connect the negative battery post to the engine
G502 - this is a size 3 wire that connects the negative battery post to the sidewall
G502 - this is a braided strap that connects the location where G501 is on the engine to the trunk hinge
G503 - several small wires connect to this ground. It is locate behind and just below the water neck. This is the ground for two ECM wires
G504 - vertical stud driver's side of the engine just ahead of the valve cover. This is the ground for the other ECM wire
G202 - under the console passenger side below the ECM

To clean ground connections I like to disconnect the wires and then polish the eyelets and contact surfaces with a wire brush on a Dremel.

I would also clean the connectors and the pins at the ECM with CRC QD electronics cleaner or a similar product. Then work the plug in and out a bunch to brighten up the connections

You can do a visual check on the spray pattern of the fuel injector by removing the hat. Should see a nice spray pattern if there are droplets that's an indication of partial plugging.

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formerly known as sanderson
1984 Quad 4
1886 SE 2.8L
1988 4.9L Cadillac
1988 3800 Supercharged

zkhennings MAR 03, 08:18 AM

quote
Originally posted by Dukesterpro:

I'm gonna have to keep working at it. A lot of what you said has already been done.

New FP with a spotless tank.

New fuel filter.

Only thing I haven't opened is the injector.

I will keep trying y'alls suggestions and keep you updated. Still moving into my new shop so I may be a bit spotty over the next few days.



I would still hook up a fuel pressure gauge and record it with a gopro while you drive, if nothing else to eliminate fueling prior to the injector being a cause. I have had the submersible fuel hose fail that couples the pump to the sender, you could have wiring/connector issues preventing enough current getting to the pump with higher fuel demands, etc.