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How did I get 6 Rigs??
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When I travel around town, the CEL doesn't come on. However, when I get on the freeway and get to around 55/60, the CEL will come on and typically stay on, until either I lift off the gas OR if I accelerate.

I have played with a slow acceleration and the CEL will NOT come on, as long as I keep some (even the very littlest amount) of acceleration. When the CEL comes on...all I have to do is lift off the gas a little and then give it a little acceleration...and then the light goes off for a few seconds. So, as I travel down the freeway...it is Off-On-Off-On....kinds freeks my kids and guests out.

I did a KOEO test, and I get a 34, which is:

PFE or EVP circuit has intermittently failed above the closed limit of 0.67 volts.

Before I go replacing all the vacuum hoses (which is on the list of things to do), I wanted to check and see if anyone else had any ideas. I have the EGR sprayed with PB to try and get it off, because it didn't want to come off. I was going to clean it to see if this got rid of the problem, but like I said...I am in a holding pattern until the PB does its thing.
 

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yo,
repair those vac lines 1st; Vacuum Leak Test; On an idling engine check for vacuum leaks using a mechanic's stethoscope with the probe removed, or a ~3' garden hose section. On COLD ENGINE only, use propane torch w/rubber hose attached, UNLIT or spray carb cleaner, when it gets to the the leak the RPMs will rise. Also check: vacuum hoses; intake manifold gasket & throttle body; PCV line; vacuum reservoirs: EGR Sys Vacuum Tank & 2ndry air (coffee can); AC, heater, defroster, vent control ckt & vacuum tank (plastic ball type), under dash & lines to heat/blend/etc. doors; power brake booster; cruise control ckt, etc. Where applicable; BEWARE OF BELTS/PULLEYS, FAN AND HOT ENGINE Source: by miesk5 at FSB

DTC 34 "...in Key On Engine Off (KOEO) or Engine Running (KOER) Self-Test indicates that the EGR valve and/or EGR Valve Position (EVP) sensor may not be fully seated in the closed position. The EVP sensor voltage is greater than the closed limit voltage of 0.67 volt. Because of the preload on the installed EVP sensor, it is very difficult to determine whether the EGR valve is seated or the EVP sensor is in contact with the EGR valve stem..."
Source: by rla2005 (Randy) via miesk5

DTC 34 - EGR voltage above closed limit - Failed sensor, carbon between EGR pintle valve and seat holding the valve off its seat. Remove the EGR valve and clean it with carbon remover. Prior to re-installing see if you can blow air through the flange side of the EGR by mouth. the egr is not closing properly which can cause detonation. remove the egr and clean off any carbon built up on it with carb cleaner and a brush if necessary.
Source: by miesk5

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EGR Valve Removal Tip;

in an 89 5.0; "...BE CAREFUL HERE. USE PENETRATING FLUID HERE PRIOR TO REMOVAL. i BROKE ONE ON MY LAST BRONCO AND I HAD TO BUY A NEW ONE FROM THE DEALER FOR ALMOST $100..." Source: by Waltman at FSB
"...If you plan on replacing yours go buy a 1 1/16 wrench you'll need it to get the egr pipe off the egr itself..."
Source: by Bobby (blue) at FSB

Stuck open will act like a vacuum leak and cause a rough idle and stalling
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DTC 34; Next isn't listed by anyone for the 34 Code, but is a good test anyway to rule the EVR out & is a simple multimeter test; The EGR Vacuum Regulator (EVR) controls the opening and closing of the EGR valve. The EVR is an electromagnetic solenoid and should have between 20-70 ohms resistance between the pins. +12volts should be constant on one side from the EEC Relay, the computer controls the ground signal when EGR flow is needed. When the EGR Vacuum Regulator (EVR) is off, both ports vent slowly to atsmophere. To test it mechanically check to see if vacuum is present at the EGR valve with the electrical connector unplugged from the EVR. The top port should not have vacuum! Because that would open the EGR at the wrong times. With everything connected and the engine running ground out the pin 33 side. The EGR vavle should open and the engine RPM should change.
Source: by miesk5
Testing & Operation; "...The EGR Valve Position (EVP) sensor monitors the position of the EGR valve pintle. The EVP sensor converts the mechanical movement of the pintle into an electrical voltage signal which is relayed to the PCM. The EVP sensor is a linear potentiometer in which resistance varies with the EGR valve pintle movement.Voltage is fed to the EVP by the signal return circuit. As the EGR is opened the EVP directs more voltage to the EEC and less down the voltage reference circuit. The EVP sensor provides the PCM with information on EGR flow and EGR system failures. The EVP should read between 0.24 and 0.67 volts at idle with a closed EGR valve..." read more
Source: by Ryan M (Fireguy50) at http://fordfuelinjection.com/index.php?p=35
...........

Check wiring for open high resistance, short to ground between EVP & EEC,
using this Wiring Diagram from Ford Bronco 1989 EVTM:
by SeattleBronco
Engine off, key off; backprobe EEC using long straight pin (I use my better-half's sewing kit pins with large plastic balls on end and clip one meter's lead to it) and inserting it into back of EEC connector
BR/LG to Pin 26
O/W to pin 26 (through a splice hidden in harness) this is the Ref signal
and B/W to Ground




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Connector Pin Outs, Bronco & Ford Truck & Van: 4.9, 460, 5.0, 5.8; miesk5 Note, see his Connector Pin Diagram Link above)
Source: by Ryan M (Fireguy50) at http://fordfuelinjection.com/truckpinouts.html

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&
PCM will not operate (OPEN) the EGR valve unless it sees: The engine warmed up to normal operating temperature. TPS at partial throttle. TPS not at Wide Open Throttle (WOT). MAP/MAF must be indicating light or moderate load. A certain amount of computer clock time has to be elapsed

& Broncos & Trucks never used a Pressure Feedback Exhaust (PFE) Sensor
 

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When I travel around town, the CEL doesn't come on. However, when I get on the freeway and get to around 55/60, the CEL will come on and typically stay on, until either I lift off the gas OR if I accelerate.

I have played with a slow acceleration and the CEL will NOT come on, as long as I keep some (even the very littlest amount) of acceleration. When the CEL comes on...all I have to do is lift off the gas a little and then give it a little acceleration...and then the light goes off for a few seconds. So, as I travel down the freeway...it is Off-On-Off-On....kinds freeks my kids and guests out.

I did a KOEO test, and I get a 34, which is:

PFE or EVP circuit has intermittently failed above the closed limit of 0.67 volts.

Before I go replacing all the vacuum hoses (which is on the list of things to do), I wanted to check and see if anyone else had any ideas. I have the EGR sprayed with PB to try and get it off, because it didn't want to come off. I was going to clean it to see if this got rid of the problem, but like I said...I am in a holding pattern until the PB does its thing.
TNT,
yep I chased that code for months but it was an incentive to replace vacuum lines and test all my components. Finally figured out what it was - not sure if you got one of these on your 351 but mine is the same setup as yours. Hope this helps ya.

http://www.fullsizebronco.com/forum/showthread.php?p=2727710
 

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TNT
yep found a pic you posted. If you're sure your vacuum lines are good and your EGR components are good, just as an experiment try removing that thermistor and running without it. I removed mine when I kept getting code 34/35 and never could get rid of it. I've been running without it now for a few months and my engine has never been happier. Getting around 16 mpg on long hauls at around 3200 rpm.
 
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