its not necessarily the unspeakable cracked head. a good compression test would have ruled that out, and you could have continued to search for external problems. but - since you have low compression in that cylinder you need to pull the head and inspect the head and headgasket.

you might have a simple head gasket leak, or a cracked head. no way to tell w/o removal.

 

Update: head is fine. No pressure loss. The problem was with my friends compression tester. The pressure release button was busted. Went out and bought my own just now and the cylinders are holding 150 psi without an issue. Now what? ha.

 

that's good news.

has your plenum ever been fixed ?
how does spark plug #5 look? is it fouling or does it look any different from 3 or 7?
does it ping/spark knock under load?

assuming those are ok, and since you already have a full tune up fuel pump, then my move would be to test injector 5 with an ohm meter, and swap it with #3 to see if the problem moves.

 

Plenum has never been replaced that I know of. I've only owned it for 10,000 miles and it has 160k now. Spark plug #5 isn't fouled. It's actually lighter in color than the rest. Supposing that maybe combustion isn't occurring at all within. Not sure what a ping sounds like (probably a *ping* ha) but when it kicks down and the light starts it's flashing, there's just a massive whooshing sound and a lot of vibration, rockin' and rolling if you will. And no real acceleration.

 

pinging sounds like someone threw a handful of screws into your engine. you'd notice it. its caused by lean condition, excessive cylinder temp, or timing too far advanced. those first 2 are classic leaking plenum symptoms.

vibration sounds likes the cylinder is not firing. that makes sense, even though we don't know the cause yet.

i'm trying to figure out why it would make a whooshing noise. if it only does this at higher rpm's - you may have a weak valve spring thats floating... how many rpm's are we talking about. does it sort of sound like the engine is farting - like the compression is blowing out an open exhaust valve ?

 

What that most likely is, is due to excessive heat burning off all the deposits. It sounds to me like a CAT issue. Partially restricted CAT. The whooshing sound and all.

If it were just a plug wire- (which it could be) you MUST MUST MUST keep No 5 away from No 7 and keep the boots away from one another to prevent induction crossfire.

Now, here's what I think is happening. When it is cold, it's most likely fine, then, when it's hot, and, you step on it, the CAT is choking off the output section thus, the engine can not breathe thus, the whooshing sound - as it's building pressure and releasing it.

If the output is clogged, there's no flow. If there's no flow, there's no input at the intake thus, it retards the process thus bogging it down.

Also, to avoid killing your valves on the rears, you'll want to fix that ASAP before you fry a valve or two along with killing the plugs and overheating the head thus possibly cracking it at the spark plug hole or valve seat which can happen if it's overheated. Again, if there's a restriction at the exhaust, all that thermal load is going to back up into the cylinders and heads starting in the rear, then, your going to have major work to do at that time.

My lady keeps driving her M-300 like this and, I've told her a hundred times to let me at least remove and replace the CAT before she kills the engine. Nope, stubborn woman is DETERMINED to kill the engine despite being informed many times about this.

Check the CAT at the very least and wire the No 5 away from No 7. Also, get some Magnecor wires www.Magnecor.com on your truck so that you never have to worry about induction cross fire again.

CM

 

My cylinder 5 misfire is back after doing my plenum and tuneup. Mine makes the farting sound at idle. Is this a floating valve?

 

i don't think so. floating a valve spring occurs at high rpm, not at idle.

 

valve spring could be broken. they usually break like a coil spring, right around the base or top.

 

i was guessing valve spring was ok based on good compression test, but now i just noticed there's a 2nd poster in here... dang i can't keep it straight