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Weak cylinder/injector #7

No new codes and passing the balance test!

Truck is still idling pretty rough, it’s probably a vacuum leak. I’m going to try to track it down and if not I’ll start another thread.

Thanks for the help!!


don't play well w others
Look where the plastic lines go behind the plenum. They always crack and break. Vacuum reserve can on fender could be rusted through if not the upgraded plastic one. There is another reserve can on driver side under air box for cruise control. Could be rusted as well

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don't play well w others
Some pictures. On this 5.8 the vacuum lines run across the top, not behind. Reserve can for cruise control under overflow by air box, I replaced the other reserve can with the more modern upgrade.
Last picture is a/c expansion chamber has nothing to do with this post but to piss off Doc




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don't play well w others

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So I’m not seeing any broken vac lines or rusted cans. I also cleaned the air filter, MAF, IAC, with no improvements, although the IAC looked pretty rough. I tested the TPS and got normal readings from 0.97v to 4.66v. Still no codes either but I have a pretty intense knocking/tapping noise and my code reader says the engine is running RICH.

Should I still start taking apart vac lines or is there something else worth looking at?


Staff member
I had one years back that was rough running and rich due to the coolant temp sensor not working, the sensor for the ecu not the guage


don't play well w others
Missed this somehow.
The coolant temp 2 wire on the heater hose to the pcm. Think of it as a choke on a carberated motor. When cold adds fuel to richen until sensors throw closed loop. If it sticks open truck runs like crap when first started, probably quits 2-3 times. Stuck closed will run rich like the choke is on.that would probably trigger o2 sensor code.

I lean more towards a vacuum leak.
Take carb cleaner, spray all over any gasket surface, or vacuum port while the truck is idling. Once the carb cleaner hits the leak it will raise the idle. As soon as it burns off the cleaner idle will return to normal. Pay special attention to both ends of the pcv,, and the back of the intake. Listen inside the cab for a hissing. Brake booster could be headed out.
Vacuum leaks suck almost as bad as electrical issues to find, but we know you, you will persevere

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Dusty you are too good! I thought they were unrelated and I’m kind of all over the place. However, I did hear a hiss about a week ago, but it’s not always there. I ran the tests from Big Dog and the only issue I really found was that the check valve slightly allowed air to flow in the direction that would be out of the brake booster and that the brake pedal decreases more than “just a little” when held while starting the truck. However, when testing engine manifold vacuum (~20Hg) my engine still idled poorly leading me to think that the leaking check valve/brake booster was not the culprit, incorrect?

And the PCV line…literally just learned what the was a few hours ago because while running the test and looking for leaks, I noticed this odd hose going to the back was wet looking? I also randomly had a puff of white smoke/steam come from the middle hood gap (closest to windshield) the other night while parking and thought WTH?

You know I have carb cleaner so I’ll try that test tomorrow. Let me know if you think of anything else.



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Pull that pcv valve out and have a look at it. Give it a shake, make sure it rattles. My bet it needs replace. Not aftermarket motocraft from ford. Pull the air cleaner filter out and look at the vent filter in the air cleaner box. My bet it needs replaced too

I'd replace that check valve on the booster. That's a issue.

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So. much. Oil. Sludge. The PCV valve slipped right out of the elbow hose, wasn't even being held in there. I then barely moved the elbow hose and the elbow connector popped right out of the engine. Took everything out and cleaned it. The valve appears to be an aftermarket (has a plugged port) and still rattles. Can it rattle too much? If so, it definitely is. But the truck still runs the same...rough.

Didn't get to the carb cleaner vacuum test yet.

Going to try the dealership or NAPA tomorrow for the booster check valve, rock auto is out.


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If they dont rattle you have a problem. Make sure the hose from the left valve cover to the air cleaner box is clear and hooked to the filter. Make sure the filter is clean. If the pcv valve was stuck it would allow blowby to toast that filter, causing no vent, and backpressure from the blowby that will blow out gaskets, starting with the oil pan

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So new check valve is in, no more hissing, but still a rough idle. I did the carb cleaner test and couldn't get increased idle out of any spot I sprayed. Air filter is good. I used a stethoscope to listen to 5 of the fuel injectors I could reach and they all are clicking away, so I guess not stuck open?

With my scanner saying the truck is running rich, no codes, and ruling out all I've checked, I can only guess the IAC valve. Any other ideas?
I distrust sensors... over time they can drift or read incorrectly. Which then has you chasing your tail. What sensors feed the "running rich" codes? Perhaps Charlie can weigh in.
It is very possible I am chasing my tail.

My Genisys scanner says that both O2 sensors are reading rich.

I am also thinking about taking apart and cleaning the throttle body.


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Iac is mostly idle control, and shouldn't cause a miss.
They do carbon up and can cause some issues, mostly say when you coast to a stop, could even cause a stall.
Years ago ford had a tsb on these, and made a kit, was a plate between the plenum and the iac. I put one on my 90. It has screws in it to adjust like a carberator. I never saw a change, left it but wouldn't add another.

I would take the iac off and clean it, check it with small voltage to make sure it operates, after seeing the pvc was gummed up.

As far as the throttlebody, you can stick your finger in and look at the back of the plates and see how dirty they are. Use caution cleaning the plates, they have a coating on them that melts off, and then the engine really runs like poo.

Ask me how I found this out.....

Spray carb cleaner where the throttle cables hook to the throttle body. Your shaft could be worn.

My guess at this point, change the fuel filter, and see if it runs better.
Could be a filter issue, even if a new filter.

One bad thought you dont want to hear, could be something was in the fuel, and has an injector or 2 contaminated, and firing a bad spray pattern.

Injectors for this run about $45 each, and you have to do all 8, so if it comes down to that, I'd run some fuel injection cleaner through it.
You hook the can to the fuel rail, start the truck, then shut down the fuel pump. The truck runs until the can is empty. Then fire up the fuel pump, start the truck and see what you have.

I had an areostar van with the 3.0 that ate fuel injectors after it hit 100,000 miles. Hope you don't have that issue

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Thanks dusty, I'm going to check a few more things today.

Do you have a recommendation for the fuel injector cleaner through the fuel rail? I'm only seeing gas tank ones or full kits.

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