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855 Posts
Discussion Starter #1 (Edited)
This is going to be a little long winded so bear with me, but I wanted to provide as much info as I could. :)

So don't know if anyone remembers me, but back in maybe 2008-2009 I had a stroker/maf project on here. Long story short I never got it running right and lost motivation and the truck ended up just sitting. I don't think it was smart of me to take on such a big project with what little knowledge I had about cars. ._.

Fast forward to 2013 and I got married, bought a house, and the inop Bronco came with me. Me and my husband got really into motorcycles right after so that's been my focus since then. After over 5.5 years sitting in the driveway my husband implored me to do something with the Bronco or get rid of it. Even though I had barely looked at it in years I still have a deep attachment to my Bronco and it finally regained my focus.

I've been working as a tech at Toyota for the past 6 years now so I consider myself a little more mechanically inclined than before. Lol

So last year I pulled the engine back out, intending to put it back to stock mostly, but still using a few small upgrades for a little extra power. I worked on it off and on over the year and finally got it back in the truck.

Now it has some idling/power issues that I've been trying to figure out, but haven't gotten it quite yet.

This is where you guys come in to hopefully help solve my issues. :D

First here's what I've done so far:

Got a factory engine harness/ecm from the junkyard and replaced all the broken connectors then repinned my ECM connector back to stock.

-351W bored 0.40 over
-Scat Crankshaft
-SpeedPro Pistons with Factory Rods
-Edelbrock Truck Intake
-BBK 56mm Throttle Body
-Reman. GT40 iron heads with Crane Cams stud conversion kit
-Crane CompuCam and Lifters CRN-444232
-Comp Cams 5/16" Pushrods (8.250")
-Ford Racing Double Roller Timing Chain/Gears
-Gibson Shorty Headers
-Bassani Y-Pipe (3" ID all the way back)
-Walbro 255lph Fuel Pump (about 10 years old, but been sitting in empty fuel tank. Fuel pressure seems ok?)

All engine vacuum lines replaced with thick-walled silicone hoses from siliconeintakes (triple checked for correct routing)

4gauge power and ground wires to battery/alternator.

Factory Distributor and Grey TFI Module
C1Z ECM (F1TF-12A650-ATA) from Junkyard
Factory Fuel Rail
MasterPro Ignition Coil

Timing 13° BTDC (with SPOUT disconnected); jumps to mid-high 20s with SPOUT connected

IAT @~55°F - 3.4V, 51.7K ohms (forgot to check it when warm)

NEW Parts

*Throttle Cable
*Fuel Filter
*Motorcraft PCV
*Harmonic Balancer
*Spark Plugs (Motorcraft AGSG32C) - 0.055 Gap
*Ford Racing Spark Plug Wires (routed as per factory tsb to prevent crossfire)
*MSD Cap and Rotor
*Bosch 4 Hole 19lb Injectors
*EGR Valve
*Magnaflow Catalytic Converter
*Engine Coolant Temperature Sensor
*O2 Sensor

*Fuel Pressure Regulator
Fuel Pressure at Rail:
-KOEO 40psi then drops to 34psi
-KOER First starts holds at 34psi, 36-38psi once it's idling a couple minutes.

*Throttle Position Sensor
-5V Ref
-Closed 0.93V
-WOT 4.66V

*Idle Air Control Valve
-Drops RPMs to ~450 when unplugged

So my very first starting attempt on the new engine it would crank strongly and sounded like it wanted to start, but wouldn't. After some poking around found the TPS resistance way out of spec.

Replaced TPS (set to 0.93V closed) and engine now starts, but will rev up for a bit and then drop in rpms and die. I usually have to keep my foot on the gas until the engine warms up then it'll idle pretty much on its own. Still will randomly drop idle, and then sometimes die out. Idle isn't that smooth, but is hanging around high 700rpms.

Once warmed up the engine felt ok when giving it throttle in Park. But when I tried driving it around the block it really lacked power and would hardly pick up speed when giving it steady throttle; eventually the pedal will be to the floor and still no change. This is when I decided to replace the FPR to see if there was any improvement (since my readings at the rail were borderline and it's a cheap part; vacuum line smelled a little like fuel also). Didn't seem to change anything. The hose on my fuel pressure gauge isn't very long so I was unable to view my fuel pressure when driving it.

No Check Engine light coming on, but here are the codes I get.



The GT40 heads I ordered did not come with thermactor holes (wasn't aware the later model ones didn't have them) so that may be the 44 code. I read the thermactor wasn't crucial for engine performance or to pass smog so I didn't worry too much about it. Tapped holes in the back of the heads though so I could still bolt up the pipe and all the other smog parts to make it look factory. So everything is hooked up as it should be, just no thermactor ports in the heads.

I did the WOT part of the KOER test when instructed, but still get that 77 code on multiple attempts. Don't know what that's about.

I was getting a KOER CM DTC 41 at first, but then I put in a new O2 sensor and I haven't seen that code come back yet.

So pretty much no codes to go off of.

A few other observations:

MAP may still be factory part (don't recall ever replacing it), but I've read they're not usually suspect for failure? Was thinking of grabbing a couple from the junkyard to see if anything changes.

My brake booster hisses when you press the pedal, but I'm not sure it's leaking. I tried pinching off the hose to it when the engine was running and didn't see any change on the vacuum gauge.

Keep forgetting so haven't checked my transmission fluid yet, but the pan has been leaking all these years so it's probably low. Not sure if that would cause any idle issues.

I don't have a lot of experience with vacuum gauges so I took a video for reference. At idle it's around 18-19hg, but the needle is quivering between. Blip the throttle and it drops to about 5hg then jumps to about 22-23hg. Again, I've replaced all the engine vacuum lines with thick-walled silicone lines.

EDIT: Videos still kinda aren't working, but if you go to the links and click 'Download Original Movie File' right below the player on the page you can view them.

Here's the other day when I first tried to start it; just cranks and cranks.

Then immediately after that I crank it again and it starts and will idle for a little bit then die (fuel pressure was holding at 34 even when it died). A lot of times though when it starts it will stumble bad and I'll have to give it some throttle to help keep it running.

A couple mins after it was running here's the fuel pressure up until it dies.

When warmed up it seems to idle ok for the most part, but still occasionally stumbles.

Let me know if the videos don't work! I just uploaded them.

I've been searching for days on the internet trying to figure out something, but no luck this far. I'm wondering if my fuel pump just isn't cutting it anymore? Or maybe I'm having some kind of ignition problem (factory distributor and factory TFI module). I checked spark coming directly from the coil and it was bright blue.

Thank you everyone who has read this far! Any help will be greatly appreciated!! Please help me get my Bronco back on the road! I really miss driving it.

18 Posts
Can't see the videos but with 18" of vacuum I would expect the fuel pressure under 32psi

When I first installed a 255 fuel pump on a Mustang, the stock FPR was causing high fuel pressure. Aftermarket adjustable fixed the problem. High fuel pressure will cause surging.

855 Posts
Discussion Starter #3
Can't see the videos but with 18" of vacuum I would expect the fuel pressure under 32psi

When I first installed a 255 fuel pump on a Mustang, the stock FPR was causing high fuel pressure. Aftermarket adjustable fixed the problem. High fuel pressure will cause surging.
Tried different links so see if they work now. 🤞 For some reason on Supermotors it's not showing the album when I go look at it from the public link; only shows when I try to edit my albums.

I don't know, I've seen other people on here run the 255 pump with no issues. I'm in California so I can't use an adjustable regulator anyways. Been toying with the idea of going back to the stock setup though.

The engine isn't really surging; rpms aren't going up and down like you'd see with a vacuum leak. It's more of a stumble then dying issue.

266 Posts
i had only two observations..... from my 5.0 efi: A slow unsteady idle can and has been the MAP SENSOR in my bronco.... 2) it looks like you have the MSD , plugs, wires, & coil trying to do the six liter tune.... I have read this is only for 5.0 motors as the 351 is a different animal. I think I would try to back down off the 13 btdc and see if it it improves.the idle after map replacement...Best Wishes Therin

Super Moderator
24,188 Posts
To Tiff,
WB and congratulations on your marriage!

Iggie 77 System failed to recognize brief WOT during Dynamic Response Test (user error). ...This is another code generated when the dynamic response or "goose" test as some refer to it is not performed during the KOER test. The KOER test requires that after a certain length of time the throttle be opened to bring the idle above 2000 rpm for a short period of time. If the dynamic response test is not performed or the rpm's do not peak ABOVE 2000 rpm's this code will be generated. (Computer needs to compare changes in sensor readings at different RPM's to determine system operation and efficiency)..."

Should nornally see DTCs 126-129 for MAP Sensor issues:
Manifold Absolute Pressure MAP Sensor Test @ Part 1 -Manifold Absolute Pressure MAP Sensor Test (Ford 4.9L, 5.0L, 5.8L)




See my Vacuum leak test in post #11 incl some jowens126 HVAC Control Panel pics/info @ Help with dtc codes and idle

The vacuum gauge should read between 15 and 22 in-Hg depending upon the engine condition and the altitude at which the test is performed. SUBTRACT ONE INCH FROM THE SPECIFIED READING FOR EVERY 1,000 FEET OF ELEVATION ABOVE SEA LEVEL.
The reading should be quite steady. .
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.
Vacuum Gauge Diagnosis @
1996 Bronco/F-Series
Similar to other EFI years.

Screenshot_20200327-103348.jpg Screenshot_20200327-103437.jpg Screenshot_20200327-103419.jpg

619 Posts
Did you do a MAF conversion yet?
You'll need that if you have a 408.

855 Posts
Discussion Starter #8
i had only two observations..... from my 5.0 efi: A slow unsteady idle can and has been the MAP SENSOR in my bronco.... 2) it looks like you have the MSD , plugs, wires, & coil trying to do the six liter tune.... I have read this is only for 5.0 motors as the 351 is a different animal. I think I would try to back down off the 13 btdc and see if it it improves.the idle after map replacement...Best Wishes Therin
I'm thinking of just throwing one in and seeing if it helps. I started at 10 btdc and advanced it a little to where I felt it was running best, but I'll see how it is once I put a new MAP in and then adjust it again. Thanks!

855 Posts
Discussion Starter #9
To Tiff,
WB and congratulations on your marriage!

Thanks! :)

And thanks for the info. I'd seen those, but I have no way to monitor the frequency on the MAP so that's why I've been iffy on it. Also thought if there was something wrong with it it would be more likely to throw a code, but so far I have no MAP codes. Was thinking of just throwing one in though to see if it helps. The Motorcraft ones aren't too expensive.

855 Posts
Discussion Starter #11
Ok, so just watched a couple videos on YouTube for testing MAP sensors and you can test it by viewing the voltage while applying vacuum; no frequency mapping.

Hope these pictures work, if not oh well. My Supermotors is being weird, probably because I haven't used it in so many years. Maybe I'l make a new one.

So I got the 5v ref.

Then with 0 vacuum at the signal wire it should still be showing about 5v to the ecu. I got 2.522v so already not good.

Then I added 18" of vacuum and no change with the voltage back to the ecu (it should be going down).

So this makes me feel better about buying a new MAP now if this test is pretty legit. Plus it definitely is the OG sensor. Lol So I'm gonna order up a new Motorcraft sensor and let you know how it turns out. 👍


855 Posts
Discussion Starter #12
Did some more reading and read that the MAP on Fords do need to be tested in regards to frequency. Luckily my Fluke does have such a setting.

So doing the test over here's what I got:

0" 158.1hz
5" 142.5hz
10" 128.5hz
15" 115.3hz
18" 108.2hz
20" 103.6hz

Not happy about that because they're pretty much what the chart shows it should be. :confused:A bad MAP shows exactly all the symptoms I'm having so was hoping this was going to be my fix.

From Here's How to Spot 7 Telltale Signs of a Broken MAP Sensor
  1. Lack of Power. If the ECM is reading high vacuum, it assumes the engine load is low, so it cuts fuel injection and retards spark timing. On the one hand, fuel consumption will go down, which seems like a good thing. However, if too little fuel is consumed, the engine may lack power for acceleration and passing.
  2. Rough Idle. Insufficient fuel injection starves the engine for fuel, leading to rough idling and perhaps even random cylinder misfiring.
  3. Hard Starting. Similarly, an excessively rich or lean mix makes the engine hard to start. If you can only start the engine when your foot is on the accelerator, you probably have a MAP sensor problem.
  4. Hesitation or Stalling. When starting from a stop or trying a passing maneuver, stepping on the gas might not give you any joy, especially if the ECM is giving you a lean mixture based on faulty MAP sensor readings.
Any input? I was planning on ordering a new MAP tonight, but now not so sure.
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