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1999 Ford F150 Bad PCM diagnosis.

Customer's question:  Can you diagnose if the vehicle has a bad PCM? Runs poorly all the time. Multiple codes are stored.

Mechanic's answer: Advised on how to test PCM and follow pinpoint tests for misfire codes. Verify pcm is bad before recommending replacement.

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Resolved Question:

Can you diagnose a bad ppm?

Mechanic's Assistant: What is the model/year of your Ford?

1999 f150 xl 4x4

Mechanic's Assistant: Are you fixing your F150 yourself? What have you tried so far?

Cop and plug replacement, aic valve, egr valve, dpfe sensor, cam position sensor and we are now trying to find out if the pcm is bad by ohming out each pin for the coil packs. we have a misfire code for cylinders 1,2,3 and 7 and cannot get the pcm to read anything as far as ohms

Mechanic's Assistant: Have you seen the misfire code before? Has the car been stalling?

Yes

Submitted: 4 months ago.
Answered in 20 minutes by:
3/16/2018
HDGENE
HDGENE, Ford Senior Master/Diesel/Trans
Category: Ford
Satisfied Customers: 6,576
Experience: 28 years Auto experience, Ford ,GM, Chrysler, Asian & European
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Hello, and thank you for using Just Answer! My name is***** me a few moments to review your question and Ill do my best to assist you.

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Hello, do you have codes for the coil drivers? ANy codes stored?

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Customer reply replied 4 months ago
p0131, p0132, p0133, p0137
Customer reply replied 4 months ago
that is the only codes stored in the computer

Do you mean P0301, P0302, P0303 and P0307? Are you sure on the codes? When does it misfire, at an idle or under a load?

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Customer reply replied 4 months ago
those are the numbers, and it runs rough from start and does not ever clear up...

Ok, are you sure it isnt a compression issue with the engine, is it a 4.6 or 5.4? How many miles are on the engine? Has it sat for a period of time?

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Customer reply replied 4 months ago
it is the 5.4L with 127k on the truck, about the only thing I haven't checked is compression on the motor. and yes it sat for almost 6 months
Customer reply replied 4 months ago
when this issue first started up it had sat for almost 2 months, it came up so I moved the truck where I needed it out of the way and left it there until now.

Ok, did it have this issue before it was shut down for the last six month? Is the fuel 6 months old or did you drain it and add fresh fuel to the tank? Why did it sit that long?

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Customer reply replied 4 months ago
from what I know it did not have this issue before it was parked. the truck belongs to a buddy of mine and he was working out of town in another vehicle. it has 10 gallons of fresh gas in it too.

Ok, if it didnt have the issue before, its either an issue with the valves sticking, bad fuel, or a wiring issue, a pcm doesnt just go bad. A critter could have made a nest in the engine and chewed up some of the wiring. But it would typically store other codes. Misfires all the time are either compression issues or injector faults.

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Customer reply replied 4 months ago
ok so what should be the next course of action, I have roughly 38lbs of pressure on the fuel line at the motor while running... there are no nests or any breaks in the wiring for the coil packs. I checked ohms between the coil signal wires and the connector to the pcm and all 8 read low resistance.

Pull all the spark plugs and inspect them, run a compression test, see if its good, if its washed out with fuel you will want to do a wet test on the misfiring cylinders to see if compression comes up or not. If compression is good, need to check spark quality off the coils and plugs, should be a good strong blue spark. Make sure all injectors and coils are firing using a noid light.

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Customer reply replied 4 months ago
I'll check that and have to get back to you... is there any way that since I have the pcm out of the truck that we can rule it out for tonight? I have my volt meter ready just need to know what to look for.

Thats not a test you can do with a meter of any kind, if you push current across the wrong circuit you can fry it. A pcm is tested based on trouble codes stored, if it has power and ground and if it doesnt fire a coil or injector. It will need to be installed to test those circuits with a noid lamp to rule it out.

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Customer reply replied 4 months ago
ok, the last time I was checking the coil packs, I checked voltage to the signal wires. on the four cylinders that are not throwing codes, I saw fluctuation in the d/c voltage on those wires, the other 4 I did not see anything other than the 14v

all the coils will have battery voltage with the key on/running. all the pcm does is supply ground to fire the coils on and off, thats where the noid light plugs into the coil or injector. if its firing it should pulse rapidly. If it stays light, the coil driver circuit is stuck on, indicating a probable bad pcm. If it doesnt blink at all, it could have an open circuit int he wiring from the pcm to the coil or the pcm itself is bad. It typically sets codes for the pcm if the coil or injector driver circuits have faults. Especially for multiple cylinders.

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Customer reply replied 4 months ago
what you're saying is that if the signal wire has constant power and does not "turn off and on" that the pcm could be the culprit

Yes thatd correct. check the operation of the injectors and coils, that they are being commanded to operate, run a compression test. check everything to rule it out. You cant just ohm the pcm.

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Customer reply replied 4 months ago
ok, we will reinstall the pcm in the truck, find someone who sells the noid lights, check operation of the injectors and coils, and check compression, and if all of that checks out then the pcm is to fault

Thats how you verify if its a bad pcm yes. If you need more help please let me know. If I was helpful, please use the 5 stars above to leave a positive rating.

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Customer reply replied 4 months ago
thank you gene

You are welcome, thank you!

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Most parts stores will sell or rent the noid lamp tool.

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Customer reply replied 4 months ago
that is what I'm looking up right now.

Harbor freight sells them cheap if you have one nearby also.

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Customer reply replied 4 months ago
good to know.

Im off the site for the evening, leave a reply and ill be notified if you need more help.

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Customer reply replied 4 months ago
I forgot that I have a test light that I was able to check signal to the coil packs and the injectors... the misfire cylinders have a constant power without blinking while the other 4 have pulsing. the injectors are functioning fine. I am at the point that I am going to order the pcm in the morning. again thank you for your help.
Customer reply replied 4 months ago
I checked the compression in each of the "bad" cylinders as well, and without having the gauge, stuck my fingers in the spark plug holes. each cylinder has strong compression and was pulling on my fingers as the cylinder stroked. as well I could feel the air passing as the piston was on the upstroke... everything I keep testing continues to point to the pcm. I had forgotten to mention that the truck had a parasitic draw on the battery when it was parked, if that could have been the cause for the issues that we are having now.
Ok good. Then it looks like a pcm. It will need to be programmed and the theft Pat's keys programmed for the truck to run. Draw will run down a battery. Probably a different issue.
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Customer reply replied 4 months ago
but if the draw caused a lower voltage, could it theoretically have caused a short in the pcm?
No, if t overcharged, voltage too high it would cause a failure of the pcm. Do you need more help with this concern?
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Customer reply replied 4 months ago
no just trying to make sure that it won't happen again when we put the new one in... I still have no clue what is the root cause of it going out... it was running and driving, while pulling a cargo trailer 6 days a week for my buddy's satellite installation company. just can't afford for it to get fried as soon as we plug it in. and the company we are ordering the pcm from is programming the pcm to the vin and mileage. idk that the truck has the security feature since it is the base model, with manual locks and windows
If I has a red theft light it does. They do fail over time. If I was helpful please leave a positive rating using the 5 stars above.
HDGENE
HDGENE, Ford Senior Master/Diesel/Trans
Category: Ford
Satisfied Customers: 6,576
Experience: 28 years Auto experience, Ford ,GM, Chrysler, Asian & European
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HDGENE and 87 other Ford Specialists are ready to help you
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Customer reply replied 3 months ago
is there a way to bypass the pats system? I'm just trying to ensure that the new pcm is what is going to fix the issues... I received the new one in the mail today...
No. Not at all it is not meant to be by passed
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A local locksmith can program keys for you . You must have 2 Pat's keys to program
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