Engine 87 Foxbody Hesitation at High RPMs

It’s certainly possibly previous owner went with an aftermarket engine management system like a MegaSquirt EEC, Some people don’t use the stock DLC connector as
It’s possibly relocated, or if aftermarket-eliminated easily done with a usb plug-in to a PC based laptop and easy to do.
Pull the passengers kick plate cover, and you’ll find wa the Computer’s type & track
it’s Diagnostic connector.If not present.
Ensure you have a proper connection to (if you have one) your while tuning your 86-95 Mustang with a MegaSquirt PNP Gen 2 with USB to your tuning PC/laptop doesn’t have a DB9 Serial connector.
Talk to the previous owner if they have any info, it’s useful.
I’d start your troubleshooting by pulling the MAF and blasting it clean with CRC MAF cleaner, check for any vacuum leaks, and all lines have good, tight connections . Any air leaks can affect your performance, A/F ratio.
set fuel pressure at 35lbs w/regulator attached to vacuum, check for dirty distributor posts, check your battery to block grounds, clean and retorque, same with chassis and computer grounds, any intake piping clamp leaks, filter, etc.
Running an MSD ignition module? Bypass it and check how long the motor holds fuel pressure upon shutdown. If it drops like a rock, may have a sticky or dirty injector or injectors. Check all your wiring terminations.
This ran fine already, and just started a high speed miss? How much vacuum is the engine pulling at idle, what is it’s idle?
Is it idle hunting, idle wavering up and down slowly, unstable?
Or was getting progressively worse.

Fuel pressure regulator must be adjustable, 40lbs? What is it at the rail with the vacuum line connected? I’d shoot for 35lbs with the regulator hooked up., first.
New build? Ran like this when you bought it, or always had a hesitation? It left a clue.
Certain sensors do not function until the car is running Closed loop, (operating temperature ). 02 Sensors are examples.

If you’re setup for a dynotune, any extra time it’s there for that purpose, it’s going to cost more for them to work out issues. Just FYI

Sounds as if the previous owner is running a MS or MS2 (Megasquirt) Computer.
No DLC connector under the hood?(as pictured below)..
A posted pic is 1,000 words! If you’ve got it ready to get dynotuned, it’s best to repair any issues first, then tune.
Having a list of all internal parts, I.e. Cam, Heads, compression ratio, etc., will save time and money.
Was this was done well with pin&sleeve connectors, neatly loomed up with labeled wires with soldered shrink tube, etc?
It’s no longer an SD motor, that’s good, there’s certain aftermarket engine controllers that run only on SD platforms, some use both, some MAF only. (you mentioned both a MAF sensor and MAP sensor).
An FYI that may help now or in the future, It’s widely understood among the Mustang community that many ignition issues are created as a result of aftermarket ignition components.
I’m not inferring they’re all bad, but test and keep that in mind is all. if you have an issue,
replacing aftermarket with reliable stock components sometimes really helps.
Best!
John

Hi,
Ok, picture becoming slightly more clear. Sounds like the saving grace was this guy didn’t have the car for longer that 3 weeks, or who knows what it would be like now.
You have an appointment with a Dynoshop? Might not be a bad idea for them to at least eyeball it, get some information. Seems this guy did a number on this.
The diagnostic connector is actually removed from the harness? still can’t quite make sense of that, lol. Well, although the guy was vague, the motor sounds fairly sound (no thanks to him), more electrical issues. (As described).
So, it’s running OK until it hits closed loop and the other sensors are making decisions. Open loop, it uses preset parameters, and a few sensors. Closed loop, it’s also relying on all the remaining sensors. The 02’s are inoperable.
I’d not add any other aftermarket parts, especially now...it’ll only confuse the issue(s).
Running certain 5 speed EEC’s in an Automatic Trans Car will fry the traces Internally off the EEC’s PCB.
Ironically, the 02 sensor circuit is involved in this occurrence. So, the 02 sensors were purposely not added to avoid this, or he was desperately trying to keep it running in open loop, where it ran OK.
Seems a solid bit of wiring needs to be corrected.
1) What is the EEC’s ID number?
2) locate the diagnostic connector wiring. It will need to be connected correctly.
3) It’s likely the 02 sensors not being connected that’s giving you a portion of your closed loop issues, it’s unable to make air/fuel adjustments as there’s no reference.
I’d not connect anything until at least you know what EEC you’re running with the specific drivetrain combo.
So, it’s running a stock EEC and also a MAF sensor you confirmed as functioning, which means it’s been converted to running the MAF. Also running the SD platforms MAP sensor, or just erroneously connected?
This is up to you in regards to where you want to go with it, you can certainly do most of the work, but you may consider having the shop you have a visit setup at give it a once over and get their opinion at minimum. Then move forward with the project yourself, then have it tuned as it needs to be with collaborative efforts..
Do you have any underhood pic’s?
Let’s check it out, get some more opinions...
Best!
-John

Hi,
A9P is a good EEC & was intended for the AOD trans. The Chipset sticking out of the back is the “J3 Port”, where a modified program was created and is running instead of the stock program, based on the specifics the engine build combination is comprised of. Looks like an SCT 6600 Chip, they’re sold at AM, many shops also program them.


Many times a rotary switch is connected to the chip, they have capability to run 4 programs, you select the one you want to run. See anything like that?
(Sometimes folks wedge them between the passenger seat and console).
02’s are the feedback that the EEC requires along with the other sensors to help in calculating the Air/Fuel ratio, and is missing.
Many ways these can be setup, but if that 02’ wire connector in your pic. is coming from the EEC, a reasonable thought is it’s intended to be used.
The pic you’d posted of that 02 sensor wiring is the mate of the connector I’d posted below and includes the wires terminating at the 02’ sensors, Connector pictured, hyperlink to site below..


Idle is at 900RPM’s? Really lopey (Cam)?
Cannot say for sure if this is your only function impediment, but certainly plays into it.
There’s many reasons drivability issues such as this occur, everyday. Fixated on the EEC as it seems it was started, but not completed.
Things seem to be looking up, however. Some really butcher these poor Pony’s, but this is lining up much better now, a little work and you’ll have a great running Pony!
How’s the rest of the wiring on the car, everything work?
Best!
-John
P.S., 02 Link is for a 5 speed trans, not auto- -was to show you the connector. Can delve in further when you reply

Matthewhh said:

Everything else works it just seems like someone got frustrated with car and just kind of left it as is. The wiring is poorly done I’d have to say.

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Hi,
Yeah, I’d agree with that. And it’s going to require quality workmanship to do it right. Gorgeous car, sounds like a solid build, suddenly interrupted. But Believe me, I’ve seen much, much worse!lol!
Best!
-John

Hi,
Aside from the EEC...
The car never reached operating temp and ran closed loop when you test drove it— correct? It’s only when the car gets to operating temp when you have the “miss” and idle hanging, or is that at any temp?
No, doubt the issues is a bad EEC. Sounds more like it needs it’s 02’s, and a good dynotune to me.
Yes, I think the Tuner will be able to tell you what’s necessary within a few minutes. Little handicapped here, if you read me.
Much easier if you can touch, really eyeball, and feel things. I think you should at minimum- see what that Dyno tuner has to say once they look at it & establish a connection to the EEC.
If the shop deals with Mustangs only, they’ll likely have it taken care of in no time, leaving the rest of any wiring that doesn’t impact how she runs to you.
On this end, I’m looking at uncertain wiring- with the right thing to do being to correctly repair it.
The shop may see old wiring not removed that isn’t interfering with function and simply leave or remove it, leaving you the rest.
If it’s running and driving as it is, IMHO, it’s really worth the eyes & hands on opinion.
It’s going to need a solid tune regardless, let them look at it closely, get information from that, ask questions, make your decision.
I’d say, get a shop manual, and locate a harness, carefully and meticulously repair all bad connections, starting from square one.
Sometimes an idle that hangs..then drops is a result of how programming was done & can be easily improved by slightly modifying the program on the aftermarket chip.
When is your appointment setup with this Dynotuner?
Consider what I said, in this instance, that’s your most direct path, hands on.
Does this car have a Wideband 02 sensor?

Other...
What does the car have for engine vacuum at idle(900rpm’s?).
Pull apart your salt & pepper shakers as all the important engine management connections run through them. Clean the pins with electrical contact cleaner, Carefully close any female pins that may be loose when the connectors assembled with a jewelers screwdriver for a tight fit.
Spray out your MAF with MAF cleaner?
Distributor phasing correct? The PIP is a Hall Effect sensor that triggers both spark and injectors, is located in the bottom of your Dizzy. The Reluctor wheel within the Dizzy rotates and if it’s not lined up correctly to #1 Cylinder (should be located at 12:00 when standing in front of the Car).
If the reluctor is in the wrong spot the injector timing will be off.causing usually minor drivability issues.
Best!
-John

Hi Matt,
Did you want to explore further options, and wondered where your vacuum is at, etc..
This sounds as if it’s having the issue due to the 02’s not in the loop, however some-cars have been tuned to run without them. Not a great idea, but if it’s running as well as you explain, yes- there’s certainly some wiring that needs to be done, but there is the possibility that a small component failure is causing this other anomaly.
You’ve also likely noted that there’s a spot for a check engine light, but there’s no bulb behind it nor is it connected to the EEC on the SD Mustangs (88’ Ca. 2.3l Mustangs did have them). Ford began fully using them in 89’, the first year of the MAF equipped motor, end of the SD platform.
Note the SD platforms were great for what they were, they have issues mostly when you start swapping Cams, airflow- which is why the MAF conversion was done. PRO-M makes the harnesses, and may be why it’s also running a PRO-M MAF was used. Excellent quality parts, as the rest of the car appears to have been built by someone who actually took pride in their work, it seems they would’ve done the same quality work with the wiring.
Things happen, folks move, etc, I’d suspect that’s what occurred here. I just cant picture the same person doing the mechanical and electrical work. The guy who built the mechanical end took pride in his work, the other....well- you get my drift.
I can make the lack of 02 sensors work, but still quite puzzled about the Diagnostic connector. Is there an Rs-232 or USB plug near/on the computer I’m not seeing, or is it pretty much ‘as is’?
There are other methods to control the A/F mixture, normally 02’s partake in this, but even bone stock EEC’s have been successful in adapting to run without them. Funny, had an 86’ that a guy brought in years back, id realized the wires were AT the 02’s, but not until later did I realize neither were actually connected. But, 1,000’s of miles later- the Car ran fine.
Gas mileage went up considerably when-he finally had the $$ to build it, do a MAF conversion, etc., but it actually ran good.
On a Track-only Car, I’d had no issues with setting up cars using the (no 02’s)programming approach. I’d at least run a Wideband 02 sensor so they’d be able to SEE what the A/F mixture was.
At WOT, and when In open loop, the motor doesn’t use the 02’s anyway. It sticks to the preprogrammed tables within the EEC, and uses other than one strategy to run decently as things get worn, situations change, etc.
Are you 100% positive your car is running in Closed loop, not remaining in open loop? If your Car was remaining in open loop all the time, it wouldn’t be using the 02’s at all. Certain sensors, like the MAF, TPS, etc
are used whether in open or closed loop.
Once it warms up, you have this issue of the “miss”-Correct? Some components start to fail in the presence of heat. The TFI is one, but you replaced it.
Advanced auto, etc tests TFI’s for free, bring a heat Gun with you in the off chance it’s faulty as well. It may work fine when cool, but add a little heat to it while it’s being tested, and see if it continues to pass the function test, other sensors are also prone to failing at temperatures.
If you remove the engine ECT, I’m pretty sure it will not enter closed loop. You can (temporarily) test this and see what occurs with your “miss” first.
As for your idle hanging, it’s usually the staple proof of a Tune that wasn’t done well.
Almost sounds like “tip in” for a manual transmission setup, only a guess...
What else have you tried since the initial list, in trying to get it running better?
Since the CPU doesn’t use the 02 Sensors at WOT throttle, nor in open loop- if your problem is a result of the 02 sensors, you shouldn’t have the hesitation at WOT while driving it. Not telling you to beat on your Car- have you run it WOT yet? If you jump onto the highway, car to operating temperature, dip into it WOT safely for a couple seconds to get to highway speed and see if the issue does not occur, but still does occur at say, 1/2 to 3/4 throttle. Be sure you’re in a gear that the rear tires won’t break loose on you, driving cautiously, back off when you hit the speed limit. (Just be safe about it is all).
Don’t do it if you feel uncomfortable.
1) Pull/clean/add electrical grease and re-terminate all your grounds, Block to battery, block to chassis(Rear I’d b- strap), battery to CPU should have a dedicated ground.. Another ground next to the passenger side fender from the battery. Check them all, this is a huge issue with these cars. Make them all solid.
2) Pull & clean all your plugs, look for cracks in the porcelain and note the colors of each plug. Use electrical grease when terminating.
3) I’d be suspect of the aftermarket Coil, Fire the car up in the dark and look for any stray arcs from the wires, anywhere the secondary coil voltage is.
3) is #1 Cylinder stamped on the Cap pointed towards right around 11:00-1:00 if standing in front of the car?(injector timing).
4) What Octane are you running in the motor, blast the MAF elements with CRC MAF cleaner, don’t touch them, only spray them, anything else may leave a residue.
5)If you pull your MAF connector off, engine running, it should just about (if not) stall.
6) Really need to pull the Codes from the EEC. Will look at the pics closer to see if there’s a connector you may be able to view codes, and dump them from.
7) Have a full radiator, top it off (coolant temp sensor).
8) Does your “hiccup” occur during the same position on your gas pedal, or is it at any pedal position...RPM, low rpm’s- midrange?
Pull the connector off your TPS. Using a VOM, set on Resistance, connect your leads across the 2 connectors to look for any dead spots along the windings from idle to WOT. Go slowly. An erratic jump as your opening the butterfly indicates the TPS may be worn out. It’s a simple Rheostat, like a volume knob in an old stereo. A wiper runs along the windings inside, if there’s a worn spot- just like a volume knob, you’d hear an annoying “crackle”, the EEC will receive this as an interruption and throttle may hiccup for a brief second. If your hiccup is in the same throttle position, it’s quite possible. Use a little heat from a Heatgun/hairdryer and see if you notice anything unusual while testing. If so, replace it. Buy OEM parts.
9) Your Plugs will allow you a good peek to what your motors running like, mixture, timing, etc. As I’d mentioned, it’s a very good idea to view the colors on each plug as you remove them carefully. If they’re new, then any differences should be easy to spot. Snap pictures of them and compare them to a plug chart (100’s online). As this is one of the rare diagnostics you can run on this, utilize it. Clean them, regap at 0.052, add a little never seize to the threads. Torque to spec’s.
What plugs are you running(?)
10) Pull your radiator cap off, engine cold, start the motor and observe the coolant in the radiator, are you seeing any small to medium sized bubbles in the coolant rising to the surface as the engine gets warmer?
10) Have an adjustable gap spark tester? They’re about 6-7$ at the auto parts store. It’ll show you how strong the spark is by its ability to jump a larger gap by spinning the thumbscrew & making it larger than it needs to be, warming the motor up & viewing the spark, checking each until you’ve checked them all, simply pulling one wire pulled off & starting the motor, and repeating. (Engine cold test, then try it warm).
Pull your Dizzy cap/rotor and blast it out with compressed air, any rust? Moisture? Blast it with a little MAF cleaner, in case there’s any crud inside.
11) If you remove the SPOUT connector while the engines idling, does your spark become stronger, visually. Check it on the spark tester at the correct gap, then check it on the plug.
While you’re checking around, do a wiggle test on the wiring and connectors to make certain all are tight, connectors clean.
If we shift the focus off the 02 sensors. the previous should catch it.
There’s also a Fox 5.0 checklist to run through on Stangnet that may be beneficial, would you like to try it, I’ll end up typing the whole thing up- referencing the list makes more sense.
Catches most common issues quickly, highly effective. Yes/No(?)
The Chipset you have is an SCT, lower end of the budget- but when tuned properly-do work well. They ran fine 15yrs ago, and the same today.
Other chipsets, like a Moates Quarterhorse- has some advantages (datalogging- for one). And the list goes on with similar products available, programmed with the right software and a laptop. But yes- programming in binary or hexadecimal code is certainly a learning curve. Newer software has made the process easier, but I’d not begin on this Pony with a 6K$+ motor.
After they created some Chips that could be added to the EEC, everyone was digging in as the EEC is nothing but a PLC, simple version that would run anything from a large buildings Central air/heating system to an older CNC Mill. A common chip burner was one of the first 0BD0, OBD-1 options, folks created software after trial and error and once you had your preferences in your Computers software,, transfer (burn) the program onto the Chip, prep your EEC for the new socket, enable it, few other components added, you’re done. (Chip, components= 6-8$)
A slew of aftermarket options began coming, best like the Megasquirt stand-alone EEC’s that replace the OE EEC, software more user friendly, allowing you to tune your own car, a learning curve- but much easier.
Currently, it’s the MS2/3 vs Holley Systemax or Terminator X, latter having a Dizzy that is controlled by the same microprocessor, so timing curve/max adjustments are a snap, an 02 sensor to constantly keep your target A/F mixture in check.
All of this is visible on a display that plugs into the harness it comes with. I’ve got a pic of a 331 I’d just built with a Systemax (Sniper) TB sitting on top, the Terminator is slightly different; improved.
The Systemax Sniper has a Throttle Body with (4) 100lb injectors inside it, supports about 600HP. I could also run standard injectors, I’d had this one sitting unboxed for a while, price was right for the Customer at 800$, complete.
Note it would sit on a standard Carbureted intake manifold. I prefer the intake type pictured, it’s an EFI Performer RPM2. it’ll run a Carb, or TBI, or injectors and a regular Throttle body, with a 90’ sweep & off to the MAF and an air cleaner. Injector holes are blocked off if not using them, or a perfect spot for D.P. NOS.
Useful for running with a Centrifugal Supercharger, as you have large CFM’s of unrestricted airflow available.
I’m just listing the typically used electronic components required when a motor is built in a computer controlled Ponycar, if not done- it will run awful if heavily built, or have fairly minor issues with a mild build. The stock program can be made much stronger with a few minor mod’s, a build to its full extent with the same.
Back to the 02’s, this is what I’d located on the 87’, credit for the post below to JRichter & additionals listed..
Diagrams courtesy of Tmoss & Stang&2birds

O2 Sensor wiring harness. Check the part numbers on the drawing against the 87 O2 sensor harness and see if they are the same. If so, the O2 Sensor harness interchange and modification tech note below applies.




O2 Sensor harness interchange and modification

Originally Posted by 302EFI

Revised 16-Oct-2011 to add O2 sensor harness warnings
The wires for the 02's and low oil did not change throughout the years, they are all in the same place.
The main ones you need to worry about are (on the harness end (ECU) that plugs into the 02 plug) is:
\- 1. Lightblue / yellow
- 2. White / Purple
- 3. Purple / Yellow
The White/Purple & Purple/Yellow gets looped for a automatic ECU
The Purple/Yellow & Lightblue/Yellow for a manual ECU



See Oxygen sensor Harnesses - Manual/Auto differences and year differences - Ford Mustang Forums : Corral.net Mustang Forum for more O2 sensor wiring harness info

Basic premise to use with transmission swaps:
Only run a 5 speed trans O2 harness with an A9L. Do not run an Auto O2 sensor harness with an A9L. Doing so will damage the computer’s internal signal ground.
Only run an Auto trans O2 sensor harness with an A9P in a car that has an Auto trans. Using a 5 speed trans O2 sensor harness with an Auto trans may cause no crank problems.
See Computer issue? | Mustang Forums at StangNet for Joel5.0’s fix to the computer internal signal ground.
The 4 cylinder O2 harness uses 4 wire O2 sensors. It probably won’t work correctly without modifying it.
Good luck!
Best!
John
Matt: This build from start to finish, an 11:1 T.F. 190cc Comp ported, Internally balanced Steel Crank/Carillo SIR’s, forged 331, SR Custom ground Cam, without the “extra’s” cost the Customer near 11K$ (Dynotuned) included in a month.
Point is,, a fresh motor as yours put together well now will be on the north side of 7.5K (Top end is 3K$ alone in parts). Take care of it, it will do the same for you.
Very nice find, BTW!

I see one item in the specs I don't like.
Set base timing to 10degBTDC. 14 is a bit too much.
08GT500's advice is solid.

Matthewhh said:

Thanks for the reply, I’ll give that try. When I got the car timing was set to 25’ I have no idea why.

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Hi,
Just a note about the 25 Deg.Base timing.
When the SPOUT is removed, the Computer is locked out from altering the ignition timing per it’s programmed values as it normally would. So, it runs only at Dizzy (base) timing.
Example..If you pull the SPOUT and set your Dizzy (base) Timing with a light to 10 Degrees, locked down the Dizzy but didn’t replace the SPOUT, the maximum timing would remain at 10 Degrees at all RPM’s.
Was the SPOUT installed when you first checked the timing?
Best!
John

Mustang5L5 said:

This diagram is not correct.

It should be

87 and early 88: No jumper
late 88-93 AOD Pin 5 to 6
Late 88-90 5-spd: Pin 1 to pin 5
91-93 5-spd: Pin 1 to Pin 6

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Appreciate the clarification, will update resource. TY!