Electrical guys, please help

[Black1987Stang]

Ok so im trying to find my missing probelm, Ive changed the plugs, wires and coil and still the same. With the key in RUN i put my red voltmeter lead on the positive coil harness (green/yellowstripe wire if i believe) and i put the black voltmeter lead on my negative battery terminal and i got 12v but If i put the black voltmeter lead on the coil ground (black wire) I have around 11.4volts. Shouldnt I have 12 volts there? Tomorrow Im going to check the injector power. Im going to look for 12v at the injector harness with key on and about 2ohms of resistance within each injector right? How can I check to see if the injectors are operating right, I no the pin numbers for each injector #1injector=pin 58 #2injector=pin59 and etc. I cant reach the injector harnesses with the engine running so i was going to check them at the computer instead, I dont have a noid light, could I use a voltmeter or test light at the computer to see if each injector is turning on instead? Shouldnt each injector pin have 12v at the computer with key on and engine off? BTW this only started missing after i changed the lifters and yes i have the correct firing order

 

[carbed87]

ttt

 

[90mustangGT]

Are you sure it's a spark miss? Why do you suspect fuel? You changed lifters, maby one is hung open? You need to find why it's missing first.

 

[Black1987Stang]

The missing isnt consistent, one day it will run like its running on 5 cylinders and another time i got it to only miss under a load (in neutral it was fine) and it only felt like it had only a slight miss, then later on it went back to normal. And another reason i think its electrical is my voltmeter readings...i got 12v at the coil with it grounded at the battery but when i used the coil ground for the voltmeter ground i only got around 11 volts. Maybe this is normal since it has to go through the computer but im not sure.and i know there is a resistor in one of those circuits so maybe it is normal What im not sure is this...With my ohmeter i checked from coil power+ to the negative battery terminal i got 6 ohms of resistance, I dont think this sounds right to me so hopefully someone can clear this up for me. Im hoping its just a bad ground or loose connection or something.

 

[RsStanG1987]

Check your grounds.

 

[Black1987Stang]

Check your grounds.

Will do tomorrow

 

[jrichker]

With my ohmeter i checked from coil power+ to the negative battery terminal i got 6 ohms of resistance, I dont think this sounds right to me so hopefully someone can clear this up for me. Im hoping its just a bad ground or loose connection or something.

This is the wrong way to check the resistance of a powered on circuit. The proper method is to measure the voltage drop across the circuit. If you want to check the ignition circuit for resistance, turn the key to the run position and measure voltage drop between the coil power and the Positive terminal on the battery. You should see less that 1 volt.

Checking grounds:

1.) The main power ground is from engine block to battery: it is the power ground for the starter & alternator.

2.) The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges. Any car that has a 3G alternator needs a 4 gauge ground wire running from the block to the chassis ground where the battery pigtail ground connects.

3.) The computer has its own dedicated power ground that comes off the ground pigtail on the battery ground wire. Due to it's proximity to the battery, it may become corroded by acid fumes from the battery. It is a black cylinder about 2 1/2" long by 1" diameter with a black/lt green wire.

4.) All the sensors have a common separate ground. This includes the TPS, ACT, EGE, BAP, & VSS

5.) The O2 sensor heaters have their own ground (HEGO ground) coming from the computer. This is different and separate from the O2 sensor ground. It is in the fuel injector wiring harness and comes out under the throttle body. It gets connected to a manifold or head bolt.

6.) The TFI module has 2 grounds: one for the foil shield around the wires and another for the module itself.

7.) The computer takes the shield ground for the TFI module and runs it from pin 20 to the chassis near the computer.

8.) The computer's main power ground (the one that comes from the battery ground wire) uses pins 40 & 60 for all the things it controls internally.

See http://www.fluke.com/application_notes/automotive/circuit.asp?AGID=1&SID=103#volt for help troubleshooting voltage drops across grounds

 

[Black1987Stang]

Cool thanks, I no my tps has a ground but ill check them all out in a bit.

 

[joeyd0730]

I agree check grounf to motor and gorund to body, had simialr problem with bad motor ground

 

[mustang-a-holic]

What did your plugs look like when removed?


Check your injector harness. Sometimes those clips break holding the injector connectors on. Especially 6,7,8 under the upper plenum. Also, check for vacuum leaks under there...rotted hoses are hidden well under the plenum.

Your DVM won't be able to check if your injectors are firing correctly. You need an oscilloscope for that. Plus, with no sync signal it is difficult to track the signal, especially if you have an erratic idle, vacuum leaks etc.

 

[TheUser]

I don't think you can check anything about the injectors at the computer; the computer supplies the ground for the injectors - the power is supplied by VPWR if I remember correctly.

I would definately check all of your grounds like the others say.

Have your reset your TPS? I assume you don't have any check engine codes. Have you checked your timing w/ a timing light? Did you put your SPOUT connector back in?

Maybe check for vacuum leaks like one person mentioned.

What did you "do" to get it to idle fine in neutral, but still stall under load?

 

[bcd]

could be a plug wire too. my truck started missing under a load a few months ago after an oil change. i went through and presses all the plug wires down firmly and it quit

 

[Black1987Stang]

I FOUND THE PROBLEM!!!...With key on, I was checking all my pins at the computer for each injector for 12v with my DVM and found #8 injector had no power, Last year at one time i was tightening down the lower intake manifold and the #8 injector wire got squished between the intake and block so i had to resplice the wire together. When i did my lifters the wires must have seperated and VOILA No Power to the injector, tonight i took off the upper in my college parking lot , found the power wire wasnt connected, reconnected it and now I have all 8 injectors firing , im so happy i havent been pushed back in the seat in over a month b/c of various problems...lifters, fuel pump, and this problem Only thing im not liking is one rocker/lifter is ticking under load, hopefully its just not enough preload or something. At least I wont get passed anymore by regular people and now i can start work and get some money coming in. BTW I also found my pcv valve out of the hole in the lower, can u say vacuum leak, i guess ill put some silicone around the grommet and stuff to help it seal in the hole. Thanks everyone for the suggestions

 

[HISSIN50]

I am glad you got it. In one of your other threads, I thought you did a CBT to rule out a cylinder miss. My bad [memory] or I would have suggested doing so.

Glad you got it whipped.

 

[Black1987Stang]

I am glad you got it. In one of your other threads, I thought you did a CBT to rule out a cylinder miss. My bad [memory] or I would have suggested doing so.

Glad you got it whipped.

Yup i did do one and it came clean but i think it might have been before this problem i dont even remember, i did one the other night and i did get cylinder 8 Now just got to play with my timing the best I can and try to figure out why one of my rockers/lifters are ticking under load. Remember my timing problem, i was thinking of running a direct wire from the computer pin to the tfi just to rule out that my spout wire isnt shorting out anywhere since i got a code saying Spout Circuit grounded or somehting like that. This couldnt mess anything up right? Im trying a friends maf saturday to check if it will fix my bucking problems, same as DarkWriter and new maf fixed his so im trying the same. Who knows maybe this could be my timing problem too