I thought I had fixed this problem when I replaced the EGR & PCV valve and hose, but apparently that didn't fix the issue. When going through the gears (2000, 4.2, 5sp), when I depress the clutch the rpm does not go down. It's so effen annoying and I have no clue what's causing it. When I'm slowing down and I push the clutch in @ (for ex.) 1200 rpm, it'll rise. It'll rise to sometimes nearly 2k and then drop. I used to think that it was due to the A/C, but no. After I'd replaced said items above the issue went away, but now it's back. The throttle body is clean and the throttle cable isn't stuck. I've looked and listened to all the hoses but cannot hear/see a leak. This issue cannot be recreated if I simply rev the engine while not moving. It only occurs when going through the gears.

I hope someone out there has an answer to this nagging and gas eating issue!

Thanks...

 

Has it thrown any codes?


is the truck received any maintenance recently or just been neglected?

 

Haven't checked for codes, but will. Truck is very well maintained!

 

my old 2001 had this issue it was a vac leak at the back of the engine you would start it and it would rev high and stay high i found the problem which was a cracked vac line which was in a way that would of been impossible to see.. i used one of my small brazing wands hooked up to normal propane and just stuck it around the engine until the rpm's went up which gave me the general idea of where to look.. before it was found it would never idle below 1500 and the previous mechanics from the previous owner couldnt figure it out
mine was an auto tho so it would hit hard when putting into gear

 

So the propane acted as "extra fuel?" Do you have any recollection of the particular hose?

 

If idle speed is normal when the vehicle is at rest and the throttle and engine RPMs respond normally at that time, then the above probably doesn't apply. Suggest avoiding that rabbit trail at this time.

If the behavior occurs ONLY when the vehicle is in motion and when the clutch is depressed, then what you've got is a dashpot strategy that has either gone awry or is misunderstood. It is normal PCM strategy to keep the RPMs up under those conditions so that the engine isn't lugged down if the clutch is released. It sounds like yours may overshooting on the up side but I do not know the exact strategy employed on your truck. My old 89 SHO exhibited similar behavior but it's been too long ago and I do not recall the magnitude of the elevated coasting idle RPMs with the clutch disengaged.

This could be an IAC valve going bad or it could be a vacuum leak. However, if you had a vacuum leak, it would be expected that you'd have other symptoms such as "lean" codes or a high idle under other conditions, none of which were mentioned.

 

My 2003 4.2 manual does not exhibit dashpot strategy anywhere near to this degree, something is not right, it's not normal.

 

That's what I suspected. Thanks for the confirmation of what is to be expected.

 

I can confirm with glc. I've own a previous generation and a NBS with the same engine and transmissions. They both act the same with the dashpot strategy. It's there, but not annoying.

 

When I disengage the clutch, it does not speed up, but the revs gradually drop to normal idle speed.

 

mechanic pulled plug off the IAC to prove to me the IAC is the issue. Sure enough, the high revving stopped. Of course, the truck no longer idled steady. I took the IAC (Motorcraft) off and sprayed brake cleaner in there and it is much better. The IAC was only some months old. I could see the covering for the piston was still white. However, when I sprayed the cleaner in, the soot/carbon was obvious as the spray came out initially discoloured. Do IACs need to be flushed out every 6 or so months??

 

You shouldn't use brake cleaner, use WD-40 and work the piston back and forth with a small screwdriver. The lube properties of WD-40 will help prevent sticking.

My truck is 18 years old and has 200k miles on it, and the IAC has never been touched.

 

I'll take it off and do just that!

 

Ok , so I did as you suggested. At v. low speeds it seemed to have solved the issue. Today I went on the highway and it was right back to same damn thing :-/ Now the IAC LOOKED clean after I cleaned it and lubed it, but could it still be bad? When running/revving at high spds. the motor was fckn racing with the clutch in. Could the computer be bad? If it was racing and the mechanic unplugged the IAC and the engine stops revving, can we not deduct that the IAC is bad, or the computer is bad? How often does the computer go bad? How likely is THAT? I'm going to go check if it's throwing codes later, but this nonsense is getting OLLLD!

 

I'd suspect something like the TPS before I'd suspect the PCM.