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Still high rev after putting in park!

2103 Views 7 Replies 4 Participants Last post by  90.CaliBronco
1990 XLT, 5.0

Symptom: High engine rev constant at (1400 rpm) when putting bronco back in park after driving, when driving idle is perfect at 650 rpm. After engine is warmed up though, stopped at a light i do get random surging small but noticeable and today it just stalled on me at a light. Started back up but seemed to struggle like it was binding or had no power. No check engine light warning.

Tried Fixes:

Replaced all vacuum lines with silicone lines (a/c works great now) didn't help

Replaced the stock IAC (400k miles and never replaced) - didn't help

Saw that the throttle cable plastic was broken, so replaced the throttle cable thinking it could be sticking - didn't help (also my cable was abo 38" but could only find 40" so now its really hard to pedal down on the accelerator, any help?)

Fed up took it to a local shop that always has box mustangs, broncos and f150s out front, they found the lower intake manifold gasket was causing an air leak, so got the gasket kit and fixed that. still no dice.

Could I be missing something?

Other things to note, whether helpful or not:
installed new PCV and hose to intake as mine was completely missing
installed used TAB as mine was missing and smog pump went straight to TAD (flows fine)

Mods to engine when I rebuilt it in 2004 (50-60k miles on engine since) abo 40k miles on the E4OD tranny
Comp Cams Ford EFI Truck Cam (can't find the info on it anymore, the receipt only states this)
K&N Intake
Electric fans
Basani Headers, hi-flow cat, Basani exhaust
130 amp alternator
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Yo 90,
This is by member Nelbur; "I have spent some time this week trying to set up an air bypass around the IAC valve by cutting away some of the gasket between the IAC in and out air holes, rather than pay big bucks for Fords spacer kit. I cut away the center of the original IAC gasket from the outside of each hole to the outside of the other hole, giving about 1/2" gap for the air to pass through. I noticed an immediate improvement in the engines starting behavior, but it would still die occasionally.

It would rev up and then drop the idle so low it would die unless I would catch the idle with the accelerator. ... ...
I decided that more improvement could be had if I had a thicker gasket, because the original IAC gasket was very thin (0.018"). I had some 0.030" gasket material so I made my own with the same 1/2" cut out. This gave enough bypass to noticeably raise the idle speed and almost eliminate the dying. After maybe 50 starts in the last few days, it only died twice. After so many years of catching it with the accelerator it is darned hard to leave my foot off it. It is clear to me that by trial and error one can tune the air bypass without the need for the expensive Ford kit. I may combine the two gaskets for more bypass, but the idle is about as fast as I would want now, especially for driving in snow."

Although no CEL, try a Self Test for Diagnostic Trouble Codes by my pal, BroncoJoe19 @ Code Reader.....
DTC 13 is Cannot control RPM during Engine Running Self-Test, low RPM check. possible causes are:
IAC did not respond properly:

Idle Air Control (IAC) Sludge; Poor Idle TSB 91-25-07 for 85-92 Bronco & F Series & many others; "...Hard cold starts, hesitation and stalls on initial start-up or during idle or decel may be caused by sludge in the throttle body and/or idle by-pass valve. Sludge deposits or oil film on the throttle body bore and plate or the idle air by-pass valve may cause one or more of the following conditions. Hard Cold Start, Stall On Initial Start-Up, Stall During Idle, Stall During Decel, Rough Idle, Rolling Idle, Hesitation During Acceleration. A new idle air by-pass service kit (F2PZ-9F939-A) is now available for service use to correct sludge contamination concerns of the throttle bore and plate only. It eliminates the need to clean the majority of past model throttle body applications. Cleaning is not required on sludge tolerant throttle body designs released for 1991 and newer model years..."

Was the new IAC Motorcraft unit?

Lastly, Sticking throttle linkages or throttle body plate is stuck; we had this occur recently in our 96. Sprayed throttle body cleaner on linkage atop throttle body and then light coat of WD 40 on linkage. Don't try to lubricate the cable internally.

Throttle Body top view pic in an 88 5.0 by jem270
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