Computer issues with IAC...

[Z Bronco]

Hello,
My Bronco is having some idling issues, it is currently in a shop as I am not able to work on it where I now live. The IAC has been checked and is good (new when I did the EFI conversion), there is perhaps around a 1000 miles since I did the conversion. The problem, if I understood the mechanic correctly, is that the IAC is not getting accurate information from the computer (ground issue?), so it doesn't really know what rpm to idle. I have an A9T that was gone through a few years ago by a member on this board (his work is outstanding), and an RJM harness. The shop's suggestion is another computer.

When the motor is started, it starts at a higher rpm, then kicks down and will surge between 600-800 rpm. Once you start driving, the idle will be a steady 650rpm...for awhile. However, after freeway driving the idle rpm drops to 600rpm, change from D to R, or turn the wheel, at it will drop to 550-500rpm. There are no vacuum leaks.

Thought I would check in with the brain trust on this board.
Thanks in advance,
Z Bronco

 

[Timmy390]

What cam you running?

Have you checked the T-body blade for carbon buildup?

As for vacuum leak, power brakes by chance? I had an idle issue on my rig....turned out to be an internal booster leak. Found it by taking the hose off the booster and capping it....

Tim

 

[EricLar80]

Need to first find any potential unmetered sources of air:
- Do you have breathers on your engine?
- Do you have a crack in the evap connection?
- Do you have a leak in the power brake booster?
- Do you have a leak anywhere along the manifolds?
- Does your oil cap seal tightly to the valve cover?

Secondarily, can you provide the specifics for your engine? Engine size, injector size, which throttle body, manifolds, MAF sensor. Are you running two O2 sensors and where are they placed in the exhaust?

Third - are you getting any engine fault codes? if so, what are they? Did the engine always run like this, or did it just start happening? Did it happen after a modification?

If all else checks out, I have had good luck with installing an idle air adjustment plate, even on a relatively stock engine.
http://https://lmr.com/item/LRS-9939A/1986-93-Mustang-50L-58L-Idle-Air-Control-Iac-Adjuster

 

[Broncobowsher]

Did you install the IAC backwards? Often for clearance. There are 3 different IACs. Find which one you have and make sure the flow is in the correct direction.

They act weird and don't behave if backwards.

 

[jim3326]

I had similar issues with mine. Installed an idle air adjustment plate and the issue went away.

https://www.cjponyparts.com/ford-idle-air-control-adjuster-5-0l-1986-1993/p/HW2451

 

[Timmy390]

I had similar issues with mine. Installed an idle air adjustment plate and the issue went away.

https://www.cjponyparts.com/ford-idle-air-control-adjuster-5-0l-1986-1993/p/HW2451

I run one of those too. My SN95 EFI can't compensate for the larger cam I'm running. It solved my hunting/surging idle.

The A9T ECM is a bit of a hybrid. Some info below Key note....The A9P code is missing some idle control logic that the A9L has. The A9T is just a hotter tuned A9T

The A9T is another EEC that follows the same memory strategy as A9P. The A9T was tuned for the State Patrol 5.0L Mustangs in the late 80s and early 90s. They are rich-tuned in Closed Loop, but at WOT run a bit leaner than stock A9P. The stock 5.0L GT EECs are notorious for being overly rich at WOT to the point of loosing power. Presumably Ford made the assumption that people would put mods on the engine and to be safe, it is rich-tuned...better to be too rich than too lean. Another benefit the A9T has over the A9P is a higher spark advance at WOT and non-WOT conditions. I have no clue what mechanical differences there were between the stock GTs and the State Patrol 5.0Ls, but the tune is certainly different and obviously directed to improve performance. The only non-performance difference in the tune is a slightly higher EGR flow likely to help with NOx emissions due to the higher spark advance.

The A9P code is missing some idle control logic that the A9L has. And the settings in the code that's common between the two are fairly different. It's quite possible for someone to notice the difference between an A9P and A9L, but surprisingly MOST people with manual trannys don't. In fact, the only people that notice the difference are people with automatics that replace their A9P (tuned for automatics) with an A9L (tuned for manuals). What they notice is when they put the tranny in gear, the engine could stumble or completely conk out when the tranny loads up the engine in drive/reverse. With a tuning device, this is 100% tunable for in the A9L because the A9L has ALL the code the A9P has...just different settings/parameters.

Tim

 

[Z Bronco]

Hello everyone,
I appreciate all the feedback.
There are no vacuum leaks, all sources have been checked. The IAC is installed correctly. My setup is a 351W with lightening lower and explorer upper w 1' spacer, Explorer TB and a MAF that corresponds with the airflow, 19# injectors, C4 transmission. It actually idles smooth once you get going, just has the drop in rpm after freeway driving. It still runs better than any carb I have had, just trying to figure out a few things. The shop where it is at has a great track record with the Ford EFI, though I will admit, it is weird not doing the work on my Bronco now.

 

[Timmy390]

Who did the initial tune? I ask because Gary had to update my ECM for the Explorer parts. Running the same setup minus the spacer.....

I will say this.....It's normal for it to have a 1500 or so idle for a couple seconds on start up and it should then drop to under 1K. Mine does that......It then runs in loop on set parameters till the engine warms up. Then all the sensors take over and adjusts to what's being being seen.

Taking that a step further...IF the base loop parameters were not updated to the add on parts it's trying to idle when cold with bad information. Then once warm it's fine.

Mine wouldn't idle after it warmed up. I added the Idle Air Control Adjuster to correct the warm idle surge because the IAC couldn't keep up once it came out of loop.

Tim

 

[Jebus]

Two things I experienced with my setup. Before I had the VSS hooked up, M/T ECU, I would pull up to stop and the engine would be up at 900 - 1000 for a few seconds before settling down. Hooked up the VSS and it went away. Second event was idling very low and rough. I discovered a fouled MAF sensor. Had a chunk of AF stuck to one of the wires. Cleaned and eventually replaced. Shure ran better after that...

 

[Z Bronco]

The idle starts high then drops down, then that is where it surges about 200 rpm for a bit as it warms up. Once driving there is no surge, idles smooth at 650rpm. The problem is after driving at freeway speeds for a bit, then it wants to idle at 600 rpm (smooth), toss in going from D to R and turning the wheel to park it in its spot, and the idle drops to 500-550. I have the VSS hooked up. The IAC was new when I hooked everything up with around a 1000 mile on it now, since there isn't any carbon buildup, that is why I haven't installed the adjustment plate. The MAF and TB are clean and functioning correctly. The shop says that the computer itself is the faulty component, we will know as soon as another computer gets installed. The shop checked the wiring in the harness, no issues there. There was no tune on the computer that was in it, and other then the idle stuff, my Bronco has never run better. The shop is going to do a tune with the new computer. Things are close, I am being positive that it will all work out. All feedback is appreciated and I am happy that the shop when I talk to them has already gone through the processes suggested.

 

[Jebus]

Well, it is possible a signal conditioning capacitor has let loose in the ecu...