Does there HAVE to be a check engine light for a code to be set? My truck does some irritating driveability things, but the check engine light has never come on.

Also, does any one know the name of the little plastic box that connects the IAC to the throttle body inlet hose? What would happen if I replaced it with a straight pipe? Would it make a seemingly sluggish (yet brand new) IAC more responsive?

My truck has the SLOW RETURN TO IDLE condition, a new IAC and a new TPS. The throttle plate snaps closed fine, but I haven't had the TB off and looked at the other side. I cleaned the MAF. When I disconnect the IAC the idle drops a couple hundred RPM and it returns to idle very quickly when I let off the gas. I think unmetered air is getting through somewhere, but I haven't been able to find any leaks by spraying throttle body cleaner on all the possible leaky spots.

99 F150 XLT 4x4 M/T

 

Did you disconnect the battery and do all that stuff turn the lights on for 5 minutes with the new parts? Then its gonna take miles b4 it figures everything out..Also is there an adj on the tps that you did? you would see a light if a emissions fault happened only..

 

I thought about that, but no, the TPS is not adjustable. I thought it might have to be clocked or something like that, but it actually has a return spring inside. Yeah, I reset the PCM, and I'm a few drive cycles in, but it still returns to idle slow.

Peas out.

 

Does there HAVE to be a check engine light for a code to be set?

peas,

The answer is NO. Most codes will not set a "CEL". Have someone with a good code reader check it out. Post the code(s) and get back to us. I'm sure you'll get some help.

Bill

 

We hooked up an OBD II scanner on the truck today. No codes. Now I'm back to trying to chase down the cause of my problem. PCV checks good, MAF just cleaned, TPS checks good, IAC recently replaced. I haven't found any leaks... I'm stumped. I'm going to pull the TB tomorrow and have a look on the other side of the throttle plate, but I don't expect anything to be out of order, as I cleaned the thing with a toothbrush and throttle plate cleaner already, and it was just carbon on it to begin with, rinsed right off.

Peas out.

 

I don't think you are supposed to clean any of that stuff....on the TB

 

A sealant/coating on the throttle bore and throttle plate makes the throttle body air flow tolerant to engine intake sludge accumulation. These throttle body assemblies MUST NOT BE CLEANED and have a white/black attention decal (Figure 128) advising not to clean.

I cut and pasted this from the manual.

 

I thought about that, and I think they're crazy. The factory service manual does say that, doesn't it? If the coating is so sensitive to petrochemicals, how do they expect it to hold up over the life of the vehicle? I mean, there is a chance that it might come in contact with gasoline... I used a cleaner specifically for throttle bodies that's o2 sensor safe and all that.

Tomorrow morning when I crack that thing off the manifold and scrub it out, I'll be using the same cleaner. I suspect whatever deposits are there will rinse right off. Last time I cleaned it from the outside, I didn't even really need the toothbrush. They made lots of aluminum TBs in the days before the special coating, so I'm not so worried. I'm looking for a lump of crud in there holding open the throttle plate. If it's there, the sludge resistant coating didn't work, so it won't matter if I accidentally wipe it off.

Peas out

 

I kinda agree. I just thought you might wanna know.

 

as I say, good lookin' out...
I appreciate it, most definitely
Peas out.

 

Try testing the resistance of your IAC plug and other things with an ohmeter to see if everthing is working right. The IAC should have 6.0 to 13.0 ohms. If it isn't showing these numbers replace the IAC.
Oh, and be sure your gaskets are in good shape. I had a gasket go bad on the IAC on my Bronco II and the idle on it fluctuated alot.

Chris

 

10.2 ohms. Where did you find this spec? I didn't see it in the factory service manual.

 

I got it out of my Haynes Manual, its a must have for anyone who works on their own car or truck.

 

Okay, i found it, chapter 4 section 14. I'm within spec. It's a new IAC, so I'm certain it's free of carbon deposits and internal short circuits.