I have a 1998 5.4L and notice the throttle body has a hole which appears to be an air bypass with an adjustable allen wrench bolt on the back. There is a hole before the throttle blade and after the blade with the allen bolt on the back. Does anybody know if the flow is adjustable via the allen wrench bolt? I am having a horrible idle surge when I tap the throttle and release. A/F ratio goes rich the instant I tap the throttle to 10:1 and then 20:1 when the throttle blade is closed. From 20:1 it goes back to 17:1 then back to 20:1 then to 14.7:1. After it reaches 14.7:1 it stabilizes. I have replaced the iac and tps and also tested about every sensor on the vehicle to no avail. Surge only happens in gear, but I am noticing the a/f ratios do the same in neutral/park but no surge.

 

How are you reading the A/F ratios? Do you have a programmer? If so, how does it run if you put the stock tune back in?

 

I'm trying to recall the allen head screw he's referring to, - but I can't lol. Maybe I just don't remember.

EDIT: I know now, he's referring to that set screw with thread locker blue on it. Yea, you shouldn't ever need to touch that.

 

I have an X3, autotap and wideband autometer gauge. This problem is very perplexing. Thanks jbrew for the advice, I will leave the screw alone. It seems to be worse when the engine is cold, but does not happen in open loop (20 secs or so after start). I did notice the idle is higher in open loop, but right when the idle drops down and the truck enters closed loop, this is when it is happening. Tap throttle, split second goes to 10.1 then to 20:1 then 17:1 then 14:1. This whole time the truck is surging till it stabilizes at 14.1. The a/f ratios will happen regardless if in gear or not, but the surge is only noticeable in gear. My transmission is on the way out but I do not think it is causing the issue now.

 

I wouldn't even troubleshoot that way. It will drive you crazy lol.

 

Does this sound like an intake manifold leak. That is the only thing I can think of, or it is in the tune. I have checked the egr with a rubber hose/vac, unhooked the egr vac line, changed the tps/iac. No check engine lights to help out either.

 

did you clean the throttle body? Especially the IAC? do that and see if it fixes it.

 

I have cleaned the TB and replaced the IAC twice. Bank 1 is running about 5 percent leaner than bank 2. I do not know if this enough to cause an issue. I have checked the pcv hoses also and replaced the pcv valve.

 

I would try cleaning the throttle body if its never been done. You'd be surprised at how plugged up the ports get.

 

I have already cleaned the TB and ports. I have checked EGR function via vacuum test and by unhooking it completely. The PCV is non-motorcraft and purolator brand. I am wondering if this is the problem.

 

Again - have you returned the tune to stock? You need to do that - if it's a bug in the tune you are gonna be chasing your tail till hell freezes over.

Have you cleaned the MAF? If you have an oiled air filter that can foul it.

 

It could very well be, since 3rd party valves lack the correct margins of the Motorcraft valves. Their way to restrictive and stress the system. Also cause oil usage. They can get noisy as well.

 

Possible, but a lot less likely than it would be with a Fram valve!

 

Yes. Purolator makes some of the motorcraft filters from what I hear. May be just some bad blower surge and bad values in the tune.

 

Right, that's more directed to the Fram valves. I've heard that Napa sells decent PCV valves, but I can't confirm that one way or the other.