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129 Posts
Discussion Starter #1
Well, still confused....
SO, still working on the idle, no power issue now getting very confused...
I have replaced the MAP, TPS, fuel pressure regulator, complete new distributor, new motor, new transmission.
Testing the fuel pressure, everything looks good, good idle pressure, 32psi and 42psi with no vacuum.
Here is what I am dealing with, the truck idles rough and has no power, it backfires thru the intake when you try to drive it, the more gas you give it the worse it runs. When it idles rough the vacuum bounces between 10 and 15, then it will all of a sudden idle great vacuum goes up to 18 it will run like that for a few min, then back to rough idling. This is the confusing part, when it is idling rough (stinking like gas also), if I disconnect the vacuum line from the MAP sensor plug the like with my finger the idle straightens right up (it did this with the old MAP as well) and does not smell like gas. I did an experiment with that in mind, I disconnected the vacuum line from the MAP, attached vacuum pump to it and as I applied vacuum the truck began to go back to rough idling.
I am leaning toward the processor but I am not quite sure, I don't want to throw parts at it as I am teaching my16 year old (it is his Bronco) about cars/trucks so would like to find out what part is bad. My other thoughts are the O2 or the intake air temperature sensor. Any ideas would be greatly appreciated

95 Posts
Are you sure your timing is correct? And after you installed the new dizzy did you tighten the hold down bolt good? I ask because I just went thru these exact symptoms and it turns out after I checked/set my timing the bolt wasn't tight enough and the dizzy rotated 20 degrees on me. Took me forever to figure out.

207 Posts
With all you have done, and still have the same problem, I would have to think it is the computer module. (ECM)
Heck, for around $40 for a salvage one it would be worth a try IMO. Always good to have a spare anyways. :doh0715:

Super Moderator
26,089 Posts
Try a Self Test for Diagnostic Trouble Codes (DTC)s by my pal, BroncoJoe19

A helper is good to assist in reading Codes; best is to take a cell fone vid and replay it.
Some basics;
Visual Check
Battery is fully charged
1.Inspect the air cleaner and inlet ducting.
2.Check all engine vacuum hoses for damage, leaks, cracks, blockage, proper routing, etc.
3.Check EEC system wiring harness for proper connections, bent or broken pins, corrosion, loose wires, proper routing, etc.
4.Check the Powertrain Control Module (PCM), sensors and actuators for physical damage; can use these two diagrams to locate ACT, TPS, EVP above EGR), Ignition Control Module (ICM) on distributor mounted years (Ford re-located the ICM from Distr side to driver side hood hinge area on inner fender liner in 92-96); to MAP;
Sensor Locations Diagram ECT, IAT and Water Temperature Gauge Sender; Sensor Locations Diagram ECT & Temp Gauge sender, PCV by Gacknar..

5.Check the engine coolant for proper level and mixture.
6.Check the transmission fluid level and quality.
7.Make all necessary repairs before continuing
8. Check headlights

The engine temperature must be greater than 50° F for the Key On Engine Off (KOEO) Self-Test and greater than 180° F for the Key On Engine Running (KOER) Self-Test.
Run it around to heat the engine up and shift thru all gears including Reverse.

Make sure A/C is off and transmission is in Park (automatic); or in Neutral for a Manual & release clutch.

Turn off all accessories; radio, lights, A/C, heater, blower, fans, etc. (close driver's door)

Then turn off engine and wait 10 seconds.

Do KOEO test First

Post Code(s) here according to:

This is by Ford for a 94, but same for your 88; troubleshooting without Codes;
As you can see, Timing is Mentioned as advised by R Hudson
and Vacuum Leaks are #1;
See my Vacuum leak test, Acronyms & gauge pic @;+idling

As Ford cites; "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."

Vacuum Gauge Readings, Interpretation in a 96, but = to your 88

Vacuum Line Routing Diagram in a 88 5.0

Source: by Jem270
without Calibration Number; it is a close approximation to your 88 5.0
see your Vehicle Emission Control Information (VECI) Decal on hood, top of rad. etc.

EEC - how to check for capacitor leaks by jowens

129 Posts
Discussion Starter #5
Thanks for the help for sure, I am leaning toward the ECU also, looking at Ebay to get one for this weekend.
As far as the vacuum leak, when the engine is idling rough the vacuum is low and fluctuating but when it decides to idle good it as great vacuum. I did use a can of starter fluid to look for any vacuum leaks that I might have but could not find any. Also when the truck is idling rough the timing is good so keep coming back to ECU.
I did check for resistance between the negative battery post and the grounds in the system and the resistance is between 2 and 13 ohms, which seems good to me but I really do not have any specs on this. the only sensor I have not changes if the air intake temperature sensor, I did check it and it gave the correct resistance readings for the temperature according to the chart I have in the manual. I may just change it anyway since it is only 15 bucks, the parts store did not have one this weekend when I got the MAP and O2 sensor.
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