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Running rough at low rpms, poor gas mileage

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Got a bit of a weird one here, it’s a 1989 5.0 and it is running rough only at certain times. If you do a light acceleration from a stop, it will miss/hesitate all the way up to speed, but if you get on it pretty good and keep the rpm’s up it won’t do it. Also, if you’re cruising down the road at say 40 mph in overdrive and give it a light acceleration, like 1,000-1,500 rpms, it will also do it. Going up a bridge, if I give it enough throttle to drop a gear and keep the rpms around 2,000, it’s fine, but if I just try to creep up the bridge while still in overdrive it misses. Can not get it to duplicate while power braking. KOEO and KOER test only came up with a code 33 and I tested the egr, egr sensor on top of it, and evr and they all test good. The other problem that I’m sure is related is the fuel mileage has dropped significantly, from around 14-15 to 10-11 mpg

I had recently done the plugs, wires, cap, and rotor, but it’s still got worse over time. I pulled the plugs again to check and make sure they weren’t cracked and still gapped properly. The wires are ran good, no rubbing or touching. I did a water sprayer test but didn’t see any arcs. Did a vacuum test and it was good too. I’m sure I’m forgetting a thing or two but any help/advice is appreciated.

oh also new fuel pump and fuel filter, replaced after it was already acting up, didn’t make a change although I replaced it because it went bad, not just to throw parts at it.

Edited by j-rod

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Yo j,

Hello again J!

Let me post these tests again for your review and posterity;

DTC 33 is triggered when the EVP sensor is not closing;
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..."
http://www.fullsizebronco.com/forum/...placement.html by Seattle FSB 
EGR Valve Test; "...these procedures are based around a typical OBD II Ford system but the overall principals should hold on most vehicle lines. Engine running, so be careful and use proper care! Find the EGR valve and disconnect the vacuum line going to the top of the valve. There should be no vacuum there at this time. If there is then check for proper EVR (EGR vacuum regulator) operation and vacuum line routing. Repair and continue testing. Hook up your hand vacuum pump to the EGR valve and slowly apply a vacuum. If the EGR valve is functioning then the engine should begin to run poorly and stumble. If you apply full vacuum and notice no RPM change or can’t pull a vacuum at all, then check for a faulty diaphragm in the valve or a restriction of the EGR tube, exhaust or intake manifold EGR ports. I have run across many concerns where the EGR passages in the intake manifolds become plugged with carbon and prevent EGR flow, so try to keep this in mind during testing. Repair and continue testing..." by ?

Damaged EVP sensor pull vac hose off @ EVP - I pull it off and use the straw sucking test; one finger over one end; and... ya get the idea? Or grab a hand held vacuum pump with gauge availableat local parts store for a refundable deposit.
•Corroded or dirty connector
•Damaged EGR valve
➡️Faulty Vacuum system
•Broken wire in harness
•Grounded harness
•Damaged Computer

 Misfiring/Pinging due to Improper Spark Plug Wire Routing for 5.0;  Diagrams for 5.0, 87-93 & 94-96 5.0 HO - INCLUDES 5.8 
Source: by Ford via Tank92 (Tank) 

Try my vac leak test @ http://www.fullsizebronco.com/forum/...Test%3B+idling
post #11

Inspect PCV valve for correct operation and its vacuum hose.  Pull it and shake it.  Should hear a rattle if good.

Air filter; is it relatively clean?
Air inlet atop radiator support to filter box and to throttle body; look for obstruction; damaged tubing, openings in tubing, or loose fit at throttle body. 
Fuel quality; Oxidized fuel often turns darker over time and may even smell sour. You can check stored gasoline by pouring some into a clear glass container and comparing it side-by-side with known fresh gasoline. If your old sample looks noticeably darker than the fresh gas, you have strong evidence the gas has gone bad. May need to test pressure and filter if all other possible perpetrators are cleared of this crime.
Electrical connectors; inspect for corrosion, etc at coil, firewall, ICM, distributor, PCM, etc. especially those with broken locking tabs.

  • Check for correct base timing.
  • Check distributor cap, adapter, and rotor.
  • Inspect spark plugs and plug wires..
Power and Grounds
  • Check from batty post, terminasl to starter relay  and alternator .
  • Check both battery posts, and cables, connectors for looseness and corrosion. If you see corrosion on a cables wires strands between connectors or lugs and leading into the insulation, peel back the insulation to see how far back it reaches.
  • check grounds from battery to frame, block, upper intake manifold and small black wires on  inner fender near hood hinge on driver side. G100 @ LH front of engine compartment on upper radiator support-in front of WS washer/Coolant overflow tank; G101 front of fender apron-between starter relay & headlight, etc.

Restricted Exhaust is much less likely: Inspect the exhaust system for crushed, bent or otherwise restricted pipes. Replace or repair as required. If pipes look good, temporarily remove the O2 sensor ahead of the converter. If symptoms are still present inspect for internal air gap pipe restriction ahead of the converter. If symptoms are no longer present, reinstall the front O2 sensor, remove the O2 sensor behind the converter and retest the vehicle. If symptoms are still present the converter is causing the restriction. Check for rich condition, excessive oil consumption, misfires or other root cause of failure. If symptoms are no longer present with the rear O2 sensor removed, the restriction is in the muffler or resonator. Inspect and replace resonator or muffler assembly as required.

Here is the 1989 EVTM, Emissions & Pre-Delivery Manuals, Partial via mrnewland1 in Google Drive @ https://drive.google.com/drive/mobil...?usp=drive_web
Slow scrolling for me with lousy Comcast service.. Find page # of item desired in contents, then flip screen flip down like spinning a bearing (a no-no) then stopping periodically to see which page you're on.

1989 Bronco Dealer Brochure @ 1989 Ford Bronco

Free registration for some wiring diagrams (86 through 96) and Technical Service Bulletins, (80 through 96), same as by Ford @ BBB Industries- Premium Alternators, Starters, Power Steering Products: http://www.bbbind.com/free-tsb/

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I have checked all those items and still haven’t found any smoking gun, that’s why I’m at a loss here. 

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Also, what do you mean by “dotted needle”, I’ve never heard that phrase before

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Something else I just realized, the catalyst sticker on my air filter box says the firing order is 1-5-4-2-6-3-7-8, but my wires are routed in the configuration for the 1-3-7-2-6-5-4-8. I even swapped it the the 1-5-4 config just to see and it ran like crap, so it’s definitely the 1-3-7 config. I know for a fact that the truck is an 89, but I’m wondering if someone has swapped the motor with a newer 5.0.

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A previous owner or shop may have swapped a 94 through 96 5.0 or even a 5.8 in.

Check the casting number; on the small block V8, it's  is above the starter.

For example;


F4T = 94 Truck: 4C1 = March 1 1994

See http://reincarnation-automotive.com/casting-numbers-page-2-ford-casting-numbers-explained.html


94 through 96 5.0 and All 5.8 have same firing orders, 

1-3-7-2-6-5-4-8 . On these vehicles, the #1 and #3, or #5 and #6 spark plug wires must be separated to eliminate the possibility of an induction crossfire. 
To eliminate the possibility of the coil wire becoming disconnected, route the coil wire under the spark plug wires at the distributor cap.

If you still have DTC 33, it must be solved. DTC 33 is triggered when the EVP sensor is not closing.... To prevent the EGR valve from opening when the engine is cold,  vacuum is not allowed to pass to the valve until the engine is warm. EGR isn't needed when the engine is cold, only when it is warm and under load. 

"Dotted needle"; is in my vacuum leak test hot link;

such as in #2, "NORMAL READING DURING RAPID ACCELERATION AND DECELERATION... "engine is rapidly accelerated (dotted needle), needle will drop to a low (not to zero) reading."  Note that the dotted neeedle is shown near zero on the gauge.

Section 03-00: Engine, Service
 Workshop Manual

Intake Manifold Vacuum Test

Bring the engine (6007) to normal operating temperature. Connect Rotunda Vacuum/Pressure Tester 164-R0253 or equivalent to the intake manifold (9424). Run the engine at the specified idle speed.

The vacuum gauge should read between -51 and -74 kPa (15 and 22 in-Hg) depending upon the engine condition and the altitude at which the test is performed. SUBTRACT 5.5 kPa FOR EVERY 500 METERS ABOVE SEA LEVEL. SUBTRACT ONE INCH FROM THE SPECIFIED READING FOR EVERY 1,000 FEET OF ELEVATION ABOVE SEA LEVEL.

The reading should be quite steady. It may be necessary to adjust the gauge damper control (where used) if the needle is fluttering rapidly. Adjust damper until needle moves easily without excessive flutter.

Vacuum Gauge Readings, Interpretation

A careful study of the vacuum gauge reading while the engine is idling will help pinpoint trouble areas. Always conduct other appropriate tests before arriving at a final diagnostic decision. Remember that vacuum gauge readings must be interpreted with care.

Most vacuum gauges have a "normal" band indicated on the gauge face.


Following are potential gauge readings. Some should be considered as normal; others should be investigated further.


  1. NORMAL READING: Needle between -51 and -74 kPa (15 and 22 in-Hg) and holding steady.

  1. NORMAL READING DURING RAPID ACCELERATION AND DECELERATION: When engine is rapidly accelerated (dotted needle), needle will drop to a low (not to zero) reading. When throttle is suddenly released, the needle will snap back up to a higher than normal figure.

  1. NORMAL FOR HIGH LIFT CAM WITH LARGE OVERLAP: Needle will register as low as -51 kPa (15 in-Hg) but will be relatively steady. Some oscillation is normal.

  1. WORN RINGS OR DILUTED OIL: When engine is accelerated (dotted needle), needle drops to 0 kPa (zero in-Hg). Upon deceleration, needle runs slightly above 74 kPa (4 in-Hg).

  1. STICKING VALVE(S): When the needle (dotted) remains steady at a normal vacuum but occasionally flicks (sharp, fast movement) down and back about 13 kPa (4 in-Hg), one or more valves may be sticking.

  1. BURNED OR WARPED VALVES: A regular, evenly spaced, downscale flicking of the needle indicates one or more burned or warped valves. Insufficient hydraulic lash adjuster clearance will also cause this action.

  1. POOR VALVE SEATING: A small but regular downscale flicking can mean one or more valves are not seating.

  1. WORN VALVE GUIDES: When the needle oscillates (swings back and forth) over a 13 kPa (4 in-Hg) range at idle speed, the valve guides (6510) could be worn. As engine speed is increased, the needle will become steady if the guides are responsible.

  1. WEAK VALVE SPRINGS: When the needle oscillation becomes more violent as engine rpm is increased, weak valve springs (6513) are indicated. The reading at idle could be relatively steady.

  1. LATE VALVE TIMING: A steady but low reading could be caused by late valve timing.

  1. IGNITION TIMING RETARDING: Retarded ignition timing will produce a steady but low reading.

  1. INSUFFICIENT SPARK PLUG GAP: When spark plugs (12405) are gapped too close, a regular, small pulsation of the needle can occur.

  1. INTAKE LEAK: A low, steady reading can be caused by an intake manifold or throttle body gasket (TB gasket) (9E936) leak.

  1. BLOWN HEAD GASKET: A regular drop of approximately 33-50 kPa (10-15 in-Hg) can be caused by a blown head gasket (6051) or warped head-to-block mounting surface.

  1. RESTRICTED EXHAUST SYSTEM: When the engine is first started and is idled, the reading may be normal. As the engine rpm is increased, the back-pressure caused by a clogged muffler, kinked tail pipe, etc., will cause the needle to slowly drop to zero. The needle then may slowly rise. Excessive exhaust clogging will cause the needle to drop to a low point even if the engine is only idled.

When vacuum leaks are indicated, search out and correct the condition. Excess air leaking into the system will upset the fuel mixture and cause conditions such as rough idle, missing on acceleration, or burned valves. If the leak exists in an accessory unit, such as the power brake, the unit will not function correctly. ALWAYS SERVICE VACUUM LEAKS.


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Thanks Miesk5 as always, you give great information. However, I should have posted last night that I found the problem to save you from posting all that. So being that everything looks original on the truck, including the motor and catalyst stickers, I just assumed it all was. When I did the tune up recently, I never bothered to check a diagram of how the wires went on the distributor cap because I just change them one at a time and it’s usually not a problem to do it that way, but I did make sure the #2 & #4, #7 & #8 wires weren’t close enough to crossfire since I know that’s a common issue with these years. Well last night I was out there double and triple checking to make sure all my wires were routed correctly and that’s when I noticed that the supposed firing order vs the way they were configured on the distributor cap wasn’t the same. So I pulled them all off for shits and giggles and put them in the configuration of the 1-5-4 firing order and it ran very roughly, which let me know this motor for whatever reason is supposed to be the 1-3-7 configuration. So where I thought I had to separate the 2&4 and 7&8 wires, I actually needed to separate the 1&3 and 5&6 wires. I did that and the problem is gone. Lesson learned, with an old vehicle, never assume the proper or original parts are still being used. Hopefully this might help someone who’s having a similar issue.

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