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Old 09-16-2013, 05:03 AM
Alpha Alpha is offline
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Default 91 Ranger DTC codes and Idle issue

Sorry guys/gals but this will be a long one, but I desperately need your help!

My father in-law brought me his 91 3.0 Ranger (non-EGR model) with manual transmission for a high idle problem. Where I'm at with it now, is Getting TPS range codes (DTC 23) and DTC 12-13 (Idle range high/low). Below is what I've done to this point and why:

My first findings was that the air intake hose was deteriorating and had many holes allowing un-metered air coming in causing the vehicle to run lean. I don't know how long it was this way, but later I found that this completely destroyed the bosch Platinum 2 spark plug in the number 2 cylinder.

I've checked over and over for vacuum leaks and cannot find any. At idle, vacuum holds around 16-17 hg. I've sprayed carb cleaner around every hose, fitting, junction, around the intake and plenum and no changes in idle to indicate a leak.

I rebuilt the air intake hose system, so now all air is coming through the MAF.
I cleaned the TB butterfly (the teflon coating is now gone, but that may have happened long before my cleaning, regardless, it's not there)
I replaced the IAC valve (though I found the original was fine, just had a piece of the original intake hose stuck in it).
I replaced the IAT sensor
Yes, the Idle screw has been adjusted (a no no... I know)
I replaced the O2 sensor
I replaced the spark plugs with motorcraft platinums (OE) gapped at .44
I replaced the distributor cap and rotor
I replaced the TPS (the original was reading .57 - 4.23 v; which seems to be the proper range for this year).
I replaced the master cylinder (spongy brakes) and RABS valve due to the original failing closed.
I replaced the ABS Speed Sensor
I replaced the temperature sending unit for the gauge and thermostat
I replaced the belts
I replaced the battery positive cable
I converted the A/C system to 134a from R12

Many of these sensors have been replaced needlessly, and I'm pulling proper voltage from the ECT sensor, MAF, new and old TPS, new and old IAT.

Yet.... My CEL remains on, and I keep getting DTC 12/13. My RABS light is also on after replacing the brake components and I'm not having any luck getting it to blink it's trouble codes (by shorting out the RABS jumper under the dash behind the fuse panel). I was also getting DTC 77 (failed to detect goose test), but I think that was gone on the last code scan.

All in all, it runs great, but I can't get this CEL to go away! Numerous clearing of the EEC via pulling the negative cable for 15+ minutes.
The only way I've been able to get it to idle properly (barely running with IAC disconnected) is to take some "Quick Steel", block off the factory bypass hole in the TB butterfly and drill it out with a smaller diameter (5/32). Only then do I get it to barely run without the IAC, and run ~900 RPMs with the IAC. However, when I turn on the A/C, the RPMs drop to ~600, the AC clutch disengages, the idle shoots to ~1200 RPMs, the AC clutch re-engages, idle drops, and this annoying pattern repeats over and over and over unless I bump up the idle screw, which just causes more problems with proper idling... Maybe the 5/32 hole in the butterfly is still too big????

Most threads that I've seen are for 92s, 94s and newer... the whole concept of things like the TPS range being .98-4.7ish volts don't apply to this truck, but I'm having a hard time finding 91 specific info.

Any help, thoughts, experience in this matter would be greatly appreciated.... I'm pulling my hair out trying to get this thing running right and without a CEL.

Just a moment ago, I removed the Quick Steel patch over the TB hole in the butterfly..... Come to find out, the factory hole is 5/32"... So I disconnected the negative from the battery and re-did the Quick Steel plug and made a 1/8" hole in it to accommodate the blow by from lack of the factory teflon coating.. I moved the Idle screw out until it was no longer touching the TB mount, TPS reads .579v. I moved the screw back in until the TPS voltage read .588v.
I then re-connected the battery and started the truck, it idled up to between 2-3K RPM and within a few seconds (after throwing a CEL) the idle slowly dropped down to about 600 RPMs, where it climbed back up to ~900 RPMs and then held steady.
I turned on the AC and the idle dropped again to about 600 RPMs and up to about 1100 RPMs, where it gradually settled down to ~900 RPMs.
Now, I can turn the AC on/off at idle and it holds pretty consistently at 900 RPMs, no wild fluctuations.
The pattern I described above seemed to me as the computer was relearning how to handle it's idle with the IAC and load.

I'm crossing my fingers that about 50 miles on road is all it takes to clear the CEL.. I really hate to keep throwing these codes when it appears to handle idling just fine. Maybe I was scanning too soon after reconnecting the battery before....
91 Ford Ranger 3.0 XLT 2WD manual

Last edited by Alpha; 09-16-2013 at 08:08 AM.
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Old 09-17-2013, 05:53 AM
Alpha Alpha is offline
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Default Re: 91 Ranger DTC codes and Idle issue


So I drove it around about 30 miles.. Drives and runs wonderfully, great idle, great response, no complaints... Except... The dang CEL is still on!

I'll have Oreillys pull the codes today to see what is lingering..

Also, I can't seem to do the KOEO test, the scanner simply says that there was no response from the DTC.. I really don't wont to contemplate replacing the computer.. Anyone have any experience with this scenario?
91 Ford Ranger 3.0 XLT 2WD manual
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Old 09-17-2013, 09:02 AM
Alpha Alpha is offline
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Default Re: 91 Ranger DTC codes and Idle issue

So after ~50 miles, it still runs fine, but here is what I'm still dealing with
DTC Codes
12 - IDLE RPM High
23 - TPS voltage out of range (new TPS)
26 - MAF out of range
41 - System Lean
77 - System did not receive "goose" test

I marked the 10 deg BTDC line on the harmonic balancer and checked timing with a light.. While running in neutral (manual transmission), timing appears to be 40 deg BTDC.... yet it runs FINE!

again... I'm completely confused.. I tested the voltage on the MAF sensor, and it reads voltage that changes with air passing over it.. Anyone have any thoughts or ideas?

Anyone actually using this forum?
91 Ford Ranger 3.0 XLT 2WD manual
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cel, dtc, idle, sensors

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