Rattle whenever engine is high rpms then gas is let off

[cp2295]

I have a 98 4.0 and whenever I put some power in it then push the clutch in I hear a throaty rattle.. Definitely not throw out bearing that's brand new, I can make it happen whenever it's at 2750 rpms or so and I give it a light throttle (very light) and it rattles

 

[RonD]

Get a rubber mallet and bang on some of the exhaust shielding, see if you can hear similar rattle

 

[ab_slack]

I was going to say it sounds like maybe the cat shield. That a resonance is getting hit. So what Ron said.

Usually if I hear a rattle at some rpm I am able to reproduce it by slowly going up and down in rpm till I hit the sweet spot. Doesn't even have to be something loose in the sense if you wiggle it you notice as it can just be something in sympathetic vibration and a spot that normally doesn't hit anything (or even hard against something) is able to vibrate enough to bang against that surface.

Sometimes they are trickier like only under load or accel because part of it is related to how the engine shifts on the mounts.

You describe it as throaty rattle. I wouldn't usually associate those two terms. What I have had happen is an exhaust leak someplace that isn't particularly loud resonate at some RPM and get louder. You could have something like that going on as well even at the same spot.

 

[cp2295]

I'll try that but it sounds a lot deeper than an heat shield rattle, the tone is somewhat deep

Also it's pretty hard to get it to do it in neutral so it's related to load and strain being put on the frame and transferring to some loose heat shield, or more pressure in the exhaust... Something like that?

 

[cp2295]

Okay that exhaust leak make sense, I'll try crawling around underneath while I have someone replicate the noise

 

[cp2295]

Truck does it once the temp gauge starts moving so around 100+ degrees. Before then I can't get it to make the noise. That would indicate the chain is fine right? I checked the chain when it was out a month ago and it looked fine, checked for play and it was within 1/16" (I didn't even see any movement), and power is good so timing isn't off. Leaning more towards an exhaust leak once it warms up (which is completely reverse of what I always thought) or some other thing. Could it be the belt tensioner? I'm gonna try and record the noise I don't know how well I'll be able to get it though..

 

[cp2295]

Just learned what it was, just it backfiring. Went up to the exhaust and had my dad rev up the motor and heard it backfire

 

[swynx]

Try running higher octane gas. Mine sounds like its eating bearings or something under load. I mostly notice it dogged out going up hill. Put higher octane in and its gone.

 

[RonD]

Just learned what it was, just it backfiring. Went up to the exhaust and had my dad rev up the motor and heard it backfire

One of the immediate fuel saving benefits fuel injection brought to the table was the ability to shut off fuel flow when it wasn't needed.

When you take your foot off the gas pedal the injectors are shut off, if engine is above 1,500rpms, with a carb any air flow sucked fuel out of the jets, so coasting down a hill in gear cost you money.

If you don't push down on the gas pedal again the injectors come back on, at "idle levels", at around 1,200rpms.

So there should not be any backfires unless some fuel is leaking in to the intake.
I would check that TPS is below 1volt with throttle closed.
And if possible check fuel pressure with engine on then with key off to see if it is dropping way down, i.e. a leaky injector.

 

[cp2295]

Okay that's good to know, I'll check the tps, I touch that wire (I've done it before can't remember which color wire, ill look at the diagram) but I touch that with the hot lead and the negative lead to the ground?

And it'll say volts, which volts DC right? And what if it doesn't say less than 1 volt?

I've been thinking about running a pressure test, so I'll try that too. I had to take a rubber hose in place of the steel line when I saw the braided line was rubbed almost to death and so I was freaked out because it was aimed at the exhaust and so I didn't want it to catch my engine on fire.. So I cut the braided line out around the steel line (with pipe cutters) and then attached fuel injection hose on that end, used a fuel hose clamp and then went to the junkyard and cut out a braided line with the quick connect plugin and attached that to other side of the hose. I haven't smelled any gas but I am skeptical.

I'll let you know the results tomorrow, hopefully it's just the tps, it's been dropping idle weirdly, like very slow sometimes, I thought it was just the autozone iac valve I got was crap. I read it was, but I also had to screw with the throttle screw because when I first got the motor running it idled crazy high so I just unscrewed it and it fixed the problem, don't know if that's good or bad

 

[cp2295]

It will idle at 750 but it takes a while to get there

My dad told me it was backfire because one of the valves wasn't seating properly, and needs a valve job, get a 3 or 4 angle grind, check the springs, that sort of thing, clean valves or get new ones.

I replaced the guides and cleaned them the best I could when the engine was out, but they were caked with carbon just completely stuck like gorilla glue times a million. So I believed him on that but that'd be cool if it was just a sensor. I was thinking about running some fuel injector cleaner, and decarbonizing with either water (don't know if it's a bad idea I've just seen cars with coolant leakes have valves that look brand new) or the way ford reccomends to do it with their tsb. (Anyone know which one it is?)

One head is from a junkyard 98 4.0 expo with valves that are very clean compared to the other head, so idk if that could throw something off? I have a 98 ranger 4.0 btw so I know the exhaust ports are the same size, figured that could be an issue

 

[cp2295]

It does it too when I'm in the snow 4 wheeling and I hear it like crazy because your always feathering your throttle and it does it so much I'm gonna go insane.. But on the streets it just does it when I shift like I said, when I put my foot down the noise goes away completely and it sounds clean, it's so weird it creeps up around 2200 and gets worse the high it goes up to about 3000.. Weird stuff, I bet it's computer because it's just fine when the power goes down, if the valves weren't seating before they definitely wouldn't be seating properly with more intake,

 

[cp2295]

Try running higher octane gas. Mine sounds like its eating bearings or something under load. I mostly notice it dogged out going up hill. Put higher octane in and its gone.

Read this:
http://www.explorerforum.com/forums/showthread.php?t=57953

Don't put higher octane in after you do this, builds up carbon easier

 

[swynx]

Hmm, makes Sence.

 

[cp2295]

Yeah just did it to mine and I know I cleaned the sh*t out of it! Smoked up the whole neighborhood but doing that didn't help any with the noise.

My vacuum reads a healthy 18.5" hg at idle. Flutters a tiny tiny bit down at idle. How could I tell with a vacuum gauge if the valves are operating properly?