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Swordfish94x

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Hey all,
Been a while since I've been on here, but you've helped me in the past so why not try again. My 96 4.0 was running great with almost 150k on it. I went to dinner one night and stopped by my buddies house on my way home. When I started it up, could almost immediately tell is was missing. It misses at idle, and basically when it's not under any load. It doesn't seem to miss much under a light acceleration, but it doesn't like to hold the same speed. It misses the most when It's idling and I rev it, or when I let off the gas just a little.

Fast forward a few days and it finally gave me codes P0300, P0351, and P0353. My buddies scanner said it was the ignition coil, replaced that and didn't help at all. I checked all the plugs and wires and everything seems to look fine. Few more days go by and now its throwing P0122. I checked the throttle position sensor with ignition in the run position. Ground came out with like 12.5v, referance came out with 5v, and signal wire went from like .98v up to ~4.7v with no breaks or jumps going either way. Mass air flow seems to be fine, can't find a vacuum leak anywhere, and I don't see any burnt, broken, or corroded wires.

After all this I'm beginning to believe my PCM is bad. I'm kind of out of other ideas. Is there any way to test the PCM other than just like a smell test? Please, I could use some guidance.
 



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rasouth

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I would do a compression test to make sure you don't have a blown head gasket or cracked head. I believe that the PO300 indicates a misfire somewhere. A blown head gasket could let compression out or let coolant into cylinder, causing the misfire.
 






Swordfish94x

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I would do a compression test to make sure you don't have a blown head gasket or cracked head. I believe that the PO300 indicates a misfire somewhere. A blown head gasket could let compression out or let coolant into cylinder, causing the misfire.

Stupid excuse, but I don’t have a compression tester lol. 0351 and 0353 say that there’s an issue in the ignition system between pcm and spark plug. 0122 is for throttle position sensor, and it passes all the tests. The fact that wires look good and it’s affecting multiple systems is really making me suspect pcm.
 






rasouth

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When I had a misfire on mine awhile back I went through the same scenario. Turned out that my problem was that a plug wire was not routed properly, not in the little plastic clips, and one wire was shorting out. Routed the wire properly and no more misfire.
When I went looking for the misfire I found other problems and ended up replacing a cracked head, head gaskets, intake gaskets, wires, plugs... None of these other problems corrected the misfire until I routed the wires properly.
Good luck on your search.
BTW... Most auto parts stores will loan you tools, such as a compression tester.
 






Cuco

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1996 Ford Explorer XLT
Hey all,
Been a while since I've been on here, but you've helped me in the past so why not try again. My 96 4.0 was running great with almost 150k on it. I went to dinner one night and stopped by my buddies house on my way home. When I started it up, could almost immediately tell is was missing. It misses at idle, and basically when it's not under any load. It doesn't seem to miss much under a light acceleration, but it doesn't like to hold the same speed. It misses the most when It's idling and I rev it, or when I let off the gas just a little.

Fast forward a few days and it finally gave me codes P0300, P0351, and P0353. My buddies scanner said it was the ignition coil, replaced that and didn't help at all. I checked all the plugs and wires and everything seems to look fine. Few more days go by and now its throwing P0122. I checked the throttle position sensor with ignition in the run position. Ground came out with like 12.5v, referance came out with 5v, and signal wire went from like .98v up to ~4.7v with no breaks or jumps going either way. Mass air flow seems to be fine, can't find a vacuum leak anywhere, and I don't see any burnt, broken, or corroded wires.

After all this I'm beginning to believe my PCM is bad. I'm kind of out of other ideas. Is there any way to test the PCM other than just like a smell test? Please, I could use some guidance.
Check your vehicles fuel pressure

image.jpg
 






Swordfish94x

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Check your vehicles fuel pressure

View attachment 173934
Someone had lost my fuel rail adapter so I’ve had a hard time finding one without spending 50 bucks. Key on engine off it was running about 38 psi, while at idle it held 30 psi without moving, while still missing. I’m pulling the injectors today to see if I can find someone to flow test/ clean them. I can’t seem to find a kit that comes with O rings and filters and what not or I’d do it myself
 






Cuco

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Someone had lost my fuel rail adapter so I’ve had a hard time finding one without spending 50 bucks. Key on engine off it was running about 38 psi, while at idle it held 30 psi without moving, while still missing. I’m pulling the injectors today to see if I can find someone to flow test/ clean them. I can’t seem to find a kit that comes with O rings and filters and what not or I’d do it myself
Even though you are within specs you are at the minimum fuel pressure. 40 psi‘s is ideal , change fuel filter see if that helps , might need fuel pump
 






410Fortune

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30 psi is too low
you will not atomize the fuel properly and it will misfire
You are looking likely at a fuel psi issue not a pcm issue, seems like the pcm is actually doing its job

good job getting a fuel pressure tester and checking it out
I would plan to replace fuel pump and filter
Should be 36-42 psi, 30 is toooooo low under load there is no way the pump can deliver enough fuel with that line pressure
 






Swordfish94x

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30 psi is too low
you will not atomize the fuel properly and it will misfire
You are looking likely at a fuel psi issue not a pcm issue, seems like the pcm is actually doing its job

good job getting a fuel pressure tester and checking it out
I would plan to replace fuel pump and filter
Should be 36-42 psi, 30 is toooooo low under load there is no way the pump can deliver enough fuel with that line pressure


Not saying your wrong, but everywhere I’ve read says about 40 with engine off, then about 30 at idle is all within specs for these things. The only reason I’m skeptical about it behind fuel pump is when I’m accelerating lightly it won’t miss, but if I try letting off to where it keeps a constant speed it’ll start lurching.
 






Cuco

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Not saying your wrong, but everywhere I’ve read says about 40 with engine off, then about 30 at idle is all within specs for these things. The only reason I’m skeptical about it behind fuel pump is when I’m accelerating lightly it won’t miss, but if I try letting off to where it keeps a constant speed it’ll start lurching.
Are you getting any check engine lights and what’s the code? If you do not see a check engine light on check it anyway you might have a pending code
 






410Fortune

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does the psi jump up when you detach the vacuum hose at FPR?

30 psi is at the bottom of the scale,
the 4.0 trucks I have built and work on over the years run higher then that, if I see 28-32 psi I suspect a problem, I like to see 36, yes at warm idl


I do not mind being wrong!
There are plenty of other reasons for a random misfire, I have seen my share of faulty PCM's too these trucks are getting older.
You might be on to something there are other areas to check and you DO have fuel psi in the minimum range
So lets go back a little and look at your symptoms


P0300 random misfire multiple cylinder misfire,
more then one cylinder is key here

p0351 ignition coil A circuit malfunction
P0353 ignition coil C " "

then later P0122
TPS sensor circuit low voltage


This could be a voltage issue
Check your battery/ alternator voltages
Are battery cables clean/corroded/tight
The ground near the pcm is it clean and tight
the EEC relay is suspect as is the power diode in the power dist box
check all blades for corrosion, visual, consider swapping EEC relay with AC WOT relay or fuel relay

with those Coil codes you should take a close look at your plug wires also I know you said they look good but they do wear out over time
interference from radio, noise in the electrical system can cause those codes in the PCM when the plug wires or spark plugs start to wear and allow arcing/resistance, etc. Plugs and plug wires very important in a waste spark ignition
 






Swordfish94x

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So 99% sure its the injectors. Ran the canister thing through the fuel test port and it seems to have helped. I tried taking the intake manifold off, and had everything disconnected that I could see. Something was still holding it in, and after I broke a vacuum line I said screw it and put it back together. I can't really find a set to rebuild the injectors so I think I'm gonna have to wait until I can afford to replace them all.
 






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