OK, I've read all kinds of threads and have measured and checked until I'm blue in the face.

Truck starts fine. It drives fine, save for an annoying problem:

When I press the gas and then let off, it accelerates fine and then stumbles on deceleration and then recovers. Sometimes, when it recovers, it over-revs then engine.

I read the thread regarding setting the TPS voltage and I have it at .991 volts DC.

I removed the IAC and cleaned it really well. If I remove it's connector while the engine is running, it idles fine and does not exhibit the stumble upon deceleration , but - when I put the truck in gear, the idle drops unacceptably. If I leave it connected, it maintains the idle speed when in gear but the stumble occurs on sharp deceleration.

I've checked and checked for vacuum leaks, broken lines, etc. and am loosing my mind.

Mods are the typicals - 6lb. lower, CAI, Accufab, Magnaflow cat-back, Predator & custom tune, etc.

Truck is an '04 with 58K on the odometer.

Any ideas would be greatly appreciated by me and my hairline !!!!

TIA !!

 

Truck is an '04 with 58K on the odometer.
another 04 with 58k plus miles on it. good. my truck was doing the same thing, i changed the oil, fuel filter, plugs and cleaned the intercooler and the bs went away.

 

whens the last time you changed the plugs or fuel filter? how long have you been using the current tune?

other possibilities are coil packs or the iac even though you cleaned it may be going bad.

your blinker fluid may be low as well i would toip that off.

good luck bud

 

Sounds like coil Packs to me. Change them and I'll bet it goes away.

 

I changed the fuel filter at the 30,000 mile mark - I'll change it tomorrow.

Plugs were changed this past spring - I ate an Iridium in a race with an BMW...

I've been using this tune for over 2 years now.

I am leaning towards the IAC - I plan on getting with Art-Man tomorrow to try and do a little comparison checking and maybe swap the IAC.

I'm having a hard time understanding how coil packs could cause a strange idle under deceleration, yet function perfectly when under any other condition ?

Thanks for the ideas - keep them coming !!

 

An IAC valve can get worn and cleaning it won't help. Be nice if you could substitute a known, good one before you buy it...........


Dan

 

+1 on the IAC. Get a new Ford one. I put in an aftermarket one and it lasted about 4 months...new Ford one has been good for the last year or so.

 

A lot of people (including myself) have had idle problems with and Accufab TB. TPS adjustments had no affects on clearing it up. Have you had the Accufab for a while with no problems or did the problem appear after installing? Just a thought! Try switchin the stock TB (if available) back on and check drivability.

 

Single Blade AccuFab on mine.
Following install of built motor I had Idle issues.
Truck would travel at 20mph without any pressure on pedal.
Knock it into N and would get 1500rpm
If you stopped or ran engine from start up the idle would settle after a short while.

Cleaned IAC
Cleaned and check throttle body
Checked all hoses etc etc
Scratched my nuts ...

Still idled too high....

Replaced IAC with new Ford one - perfect - Job Done.

 

Now that's one I haven't tried yet !!!

Seriously, thanks to all for the responses.

I scratched the coil packs, fuel filter and tune from the list because I re-installed the factory TB and it runs like it should again.

I think I am going to try a new IAC - since it is directly involved in the idle circuit of the truck, I guess it could be the problem.

If that doesn't do it, I'm sending the Accufab back...

Off to Ford for a new IAC....

 

OK, it's fixed...

For the purposes of this write-up, I had put the truck back to the original config (factory TB installed).

For anyone that cares, here's what we did to cure it:

1. Purchased the new IAC from Ford, after 2 trips...

If you need to get an IAC from Ford, have them look up the unit from the Harley trucks - the unit that shows up for an '04 Lightning is incorrect.

2. Installed the new IAC on the truck, with the factory TB. No difference in how the truck ran.

3. We disconnected the negative battery cable and did a lengthy inspection (again) of all the vacuum lines, connections, etc.

4. Re-installed the Accufab. Re-connected the battery, turned the key on and set the TPS voltage to .990 VDC.

5. Now, a REALLY important tip: Be sure to turn the key to the OFF position after you set the TPS sensor. Every time you start your truck, the ECU reads the closed throttle TPS voltage and uses it as a reference for what is a closed throttle. If you don't turn the truck OFF after you adjust it, it uses whatever the setting was before you adjusted it.

6. Started the truck and it idled a little bit high. We removed the large hose on the IAC and it was barely flowing any air. This lead us to believe the TB stop was set too far open. We turned the truck off, adjusted the stop on the TB, re-adjusted the TPS, turned the truck OFF and back ON again, verified the TPS voltage was .990 and started the truck.

7. Idle improved slightly and the IAC was flowing more air at idle, indicating that the TB flow was too high previously. We repeated Step 6 and the IAC now flows more air and the idle is now better than it has been in ages.

Hopefully another friend with access to a Ford Service Manual will help us figure out how to properly set the baseline idle speed. Then we might find out how to measure the IAC to determine if it is operating in the middle of it's permissible range.

So, in summary, the factory TB worked with both IAC valves, but the Accufab didn't work right until we replaced the IAC.

Thanks to everyone who responded - I really appreciate your help !!!

 

Good info

WHAT THE HELL DID YOU SAY
JK but I have to admit, first time I've heard of this one, GOOD FIND B and good Fix

 

Thanks RonBo (I think ???)

Just finished a 45 minute ride - she runs like she used to !!

WOOOO HOOOOO !!!!!!!

(Sorry, I had to !!!)

By the way - I like your new beast...

 

ding ding ding.

sorry i was late reading this post. did you try your old iac after adjusting the throttle blade?

when you disconnected the iac and the idle never changed should have been an indicater that the blade was cracked open a little to much. theoretically(sp?) the truck should have stumbled or died when you disconnected. the iac can only allow more air to bypass through it.

 

I did try the old IAC - and the crappy idle characteristics returned with it.

The old IAC made the idle stick high. After 15 seconds or so, it would settle down, but driving around it would stay at 30 MPH without using the pedal and it would stumble if you got on it hard and then quickly let off.

If I disconnect the IAC completely, my truck idles fine. I think it is because it remains open at whatever point it is when I disconnect it ?

I guess to really prove it out, I'd have to rev it hard and disconnect it while the throttle is open and the RPMs are up - theoretically the IAC should be closed at that point, right ?