[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

 

[Vochy]

Sounds like vacuum leak but you confirmed thats not it. 3 more things come to mind. Throttle plates not closing all the way, bad ECT sensor, new IAC is bad. Pull codes to look for electrical issues.

Sent from my SM-N960U using Tapatalk

 

[miesk5]

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

 

[90.CaliBronco]

Thanks all, I tried my code reader again just to make sure was still receiving nothing and i actually received codes 34, 41, 67

34 and 41 were "continuous memory" codes
67 was KOEO

Continuous memory = ^C
KOEO = ^O

Per the code reader's user guide the codes are:

34^C - EGR control circuit fault (ex. V8 models) question does "ex." mean excluding, existing or example
34^C - EVAP control system fault / voltage higher than closed limit
34^C - Defective EGR pressure transducer sensor

41^C - HEGO (HO2S) sensor signal out of rang/always lean
41^C - No HO2S switching detected

67^O - Neutral Pressure Switch (NDS) circuit failure, cicuit open (3.0L MAP, 3.0L MAF-SFI, 3.8L SFI)

Note code 67 for trucks when continuous memory is shown, says air conditioning compressor clutch switch fault, but i got 67 with KOEO not continuous memory, the only reason i bring it up is that i had the A/C switch on max air when running the KOEO test.

Anyway can someone layman's term these codes. I am thinking i should replace my EGR and that would probably help with the O2 sensor codes?

Thanks,

 

[rla2005]

Code 34 (CM): EVP circuit has intermittently failed above the closed limit of 0.67 volts

That code indicates the EGR Valve Postion (EVP) signal has intermittently been too high when the EGR valve is supposed to be closed. This could be caused by the EGR valve not fully closing or the EGR Valve Regulator (EVR) may be allowing vacuum to get to the EGR valve when it is supposed to be closed. Either of those can cause unmetered air into the intake. Or the EVP sensor is going bad.


Code 41 (CM): HEGO sensor circuit indicates system lean

Unmetered air/vacuum leak can cause this. Low fuel pressure, exhaust leak can also cause this code. Given you have the Code 34 as well you need to investigate if the EGR valve is closing completely.


Code 67 (KOEO): Neutral safety circuit failure.

Make sure the Neutral Safety Switch (NSS) C6/AOD or the Manual Lever Position Sensor (MLPS) E4OD is properly adjusted.

 

[rla2005]

Another quick test to rule out a vacuum leak versus bad feedback signal to the computer is disconnect the IAC. If the idle RPM drops very low or the engine stalls the problem is electronic. If the idle RPM stays high you have a vacuum leak or someone has jacked with the throttle stop screw on the throttlebody.

 

[90.CaliBronco]

Thank you!!

I found this post from SeattleFSB on how to test the EGR, I'll take a look tonight.

I'm going to have to research the MLPS a little more, found this article from Miesk5 seems to put me on the right path.

 

[90.CaliBronco]

Ok so replaced the EGR and sensor, the bronco started running better, still revs up to about 1100 rpm after i put in park. Ran codes and received no KOEO codes, memory code 41 and KOER codes 13, 18, 21, 25, 42, 72, 73, 74, 77

I did the "goose test" wrong so the 70's codes were gone on my second try.

I replaced my MAP sensor anyway.

After replacing the MAP sensor I only get the 41 memory code, and KOER 18 and 21.

I replaced my engine temp sender for the gauge a couple months ago as it tested bad. It tests fine now, but seems like the engine just wont get up to temp. The only time i got it to touch the N on the gauge was after 20-30 min of driving. Wondering if i have the right thermostat in there?

The engine was also stalling out on me at stops, when i would give it gas then let of the pedal quickly. Researched on here and replaced the ICM sensor on the distributor. wow what a difference that made, no more stalling, i guess the stock ICM going over 400k miles will do that to you. but it feels like its running rough.

I borrowed a timing light, so going to check the timing then rerun codes.