Engine 1990 Fox wont rev over 3500 RPMS

alex.sweitzer

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Oct 27, 2020
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Hey guys,

I am new to the StangNet forums and I am helping a buddy work on his 1990 Mustang Foxbody 5.0. He purchased the car a couple of months ago and we have been working on this issue occasionally in the evenings.

Heres the problem: The car starts and drives fine, with no idling issues. When you get on it, it runs up the RPMs and then falls flat on its face around roughly 3500 RPMs. It feels like the car is just starting to get up and go and then boom, nothing. It almost seems like the ECU is retarding ignition like its hitting a rev limiter.

I have searched far and wide on the internet for a solution to this, including the thread on here talking about the 94 Stang with a similar problem (Thread "Still won't rev over 3500 RPMs" that resulted in a faulty distributor)

Here is what was listed on the sale when the vehicle was purchased:

302 bored over 306.
Forged pistons and crank.
Edelbrock intake setup with 70mm throttle body with spacer new throttle response.
Msd coil pack.
SVO heads roller rockers and rods.
Water pump Bracket kit
spark plugs.
Cold air intake
electric fan powered by a switch
stage 1 clutch
e303 cam
aluminum driveshaft
gauges in hood water temp boost and oil press and tach
grant gt steering wheel
24-pound injectors
rear seats removed - front racing seats
8.8 rear with 4.10s lower
upper tubular control arms
tubular k member and control arms
Solid motor mounts
new front bumper and fenders
new headlights tinted no fogs
has dumper with 10 series mufflers
Catless H-Pipe
No AC or Heat
new oil change and new gear Fluid
1inch springs in back and Viking coil overs in the front
5 lug convert Jegs 28 spline axles and brakes
sn95 front spindles with new hubs drilled rotor and new pads and calipers
3.5-inch cowl hood
rear cobra bumper

I know most of the above items have nothing to do with the issue but there's the full list at the time of purchase


So far, we have done the following as we were working through the thought process:

Electrical
Checked TPS at idle - ~0.94v
Current idle RPM - around 800

Fuel -
Replaced in-line fuel filter
Replaced in-tank fuel pump with 255 GPH pump and pickup foot, line, etc.
Checked fuel pressure at the rail - around 40 PSI

Ignition
Spark Plugs do not seem to be fouled
Replaced MSD distributor (Hoping issue was bad PIP sensor as was the issue in the thread mentioned prior)
Checked base timing with spout connector out - 14*

Went to check for vacuum leaks but all vacuum ports on the car are plugged/capped and the vacuum tree removed

We have not touched:
ECU
Ignition Coil
Injectors (Currently 24lb)


I am hoping this is something obvious that we are missing (very possible - we are glorified DIY at home mechanics with not a little but not a ton of knowledge)

Any help or ideas with this would be greatly appreciated!

Thank you
 
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jrichker

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High speed miss on a warm engine

Revised 24 June 2019 to add new source & part number for TFI removal tool .



The TFI module mounted on the distributor is one of the culprits for a high speed miss on a warm engine. The other suspect is the PIP sensor inside the distributor. If the problem does not occur when the engine is cold, the TFI module or PIP is definitely suspect. Dumping the codes may help determine which one it is. You may need a special socket to remove the TFI module, but most auto parts stores will have one for $5-$7.



It is not uncommon that the replacement parts may be defective; TFI modules made by non Ford manufactures seem to have a high failure rate.



Be sure to use plenty of the heat sink grease on the new TFI and clean the old grease off the distributor.

10550469_lis_64650_pri_larg.jpg


Lisle P/N 64650

See https://shop.advanceautoparts.com/p...0kH8DTh2LmTcgnmc_E4aAuCIEALw_wcB&gclsrc=aw.ds



attachments\.586550




diagram courtesy of Tmoss & Stang&2Birds

TFI_5.0_comparison.gif








tfi-module-troubleshooting-gif.586550
 
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Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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Here's my hunch.

Do you have your original coil, or a coil you can borrow? Take the MSD coil out and try another one.

I had this same issue. At higher RPM the car would break up. After pulling my hair out for a year or so, it ended up being my Accel coil.
 
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jrichker

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We haven't tried the coil - I'll report back on that.

The replacement MSD distributor came with a new tfi module, could that still be the issue?
Yes, MSD has a bad reputation for reliability. Borrow a TFI module from a working distributor. Sometimes even new out of the box TFI modules are bad - their Chinese like Covid-19...
 
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Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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Yeah, new TFI modules these days don't get a great rep. Quality has really suffered these days considering the number of TFI-equipped vehicles on the road has dropped, so likely the lowest bidder on the planet is now making these. I would trust a 30-year old junkyard TFI over a brand new one if I was to be honest.
 
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alex.sweitzer

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Oct 27, 2020
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Centreville, MD
UPDATE:

Okay guys, I finally gathered some time to check out installs on these recommendations. We had trouble sourcing a functional TFI out of another vehicle that we could use as a baseline, so we went to the auto store and picked up a replacement.... Still no luck, after two distributors that both had new TFIs and an additional one from the parts store, unless I have extremely bad luck, it would seem to me that one of these should have fixed the issue if that was the problem.

After this, we replaced the ignition coil with a new one (Old one seemed pretty beat up) - still same issue.


Could it be possible that the timing chain is out or not correctly installed, causing this issue? It does have an aftermarket cam in so I imagine there could have been a mistake when that was installed possibly?

If not, next step I believe would be to replace ECU and see if that has any effect.


Sorry for the delay and thanks for the quick responses from the community!
 

jrichker

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UPDATE:

Okay guys, I finally gathered some time to check out installs on these recommendations. We had trouble sourcing a functional TFI out of another vehicle that we could use as a baseline, so we went to the auto store and picked up a replacement.... Still no luck, after two distributors that both had new TFIs and an additional one from the parts store, unless I have extremely bad luck, it would seem to me that one of these should have fixed the issue if that was the problem.

After this, we replaced the ignition coil with a new one (Old one seemed pretty beat up) - still same issue.


Could it be possible that the timing chain is out or not correctly installed, causing this issue? It does have an aftermarket cam in so I imagine there could have been a mistake when that was installed possibly?

If not, next step I believe would be to replace ECU and see if that has any effect.


Sorry for the delay and thanks for the quick responses from the community!
If the timing chain was defective, it would show up long before you got to 3500 RPM..

Check the coil wire and spark plug wires for the resistance; you should see about 1000 Ω (ohms) per foot of length.
 
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Mustang5L5

Put lubricant all over the balls
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I assume the new distributors you tried also had a new PIP?

Do you have anyway to monitor fuel pressure while you go WOT? Underhood camera and light? A piece of braided line to run the guage up to the wiper? Electric gauge with remote display?

What is your spark plug gap? Set to 0.052" - 0.056"?

Have you dumped the engine codes and performed the cylinder balance test?
 
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alex.sweitzer

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Oct 27, 2020
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I assume the new distributors you tried also had a new PIP?

Do you have anyway to monitor fuel pressure while you go WOT? Underhood camera and light? A piece of braided line to run the guage up to the wiper? Electric gauge with remote display?

What is your spark plug gap? Set to 0.052" - 0.056"?

Have you dumped the engine codes and performed the cylinder balance test?
Each Distributor had new PIP Sensor

I will add line to see Fuel Pressure at WOT and report back

Will also check plug gap and modify if needed

Dumped Engine Codes a while back but will redo and make a note of them

Have not performed a cylinder balance test - How is this test performed?



Thanks for all of the help and for bearing with me on this!
 

Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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Is this what Im looking for?


Post 5. After you dump codes, you give the throttle a 50% blip. The engine will idle up, and start disabling fuel injectors 1 at a time. It will then report if any cylinder is weaker than the others.

You should get a code 9 or 90 (depending on your reader) which means everything good. I don't think this will come up with anything other than a 9, but it's one thing to check off the list.
 

DxFox.0

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Dec 27, 2020
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I had this same problem years ago. Unplug the mass air fuel sensor and take the car for a test drive. Turned out it was MAF the whole time apparently thats a common issue for Foxbodys.
 

alex.sweitzer

Member
Oct 27, 2020
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2
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23
Centreville, MD
Re-Gapped Plugs, tightened to ~0.055” - No Luck

Fuel Rail Pressure WOT maintains ~50 PSI

Pulled and cleaned all injectors, they were fairly dirty but still no luck

Disconnected MAF sensor & immediately noticed a difference in the idle, still same problem while driving so I’m assuming the MAF was working properly

Running out of items to check, ordered new tunes for the car - maybe that’s the problem.... time will tell

Thank you everyone for suggestions & input this far
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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I didn't see where there was a chip in the car on the list so I doubt that will fix the issue(s),
You need to put the work into running the checklist.
NOTE: did you do a sweep test on the tps?
 

alex.sweitzer

Member
Oct 27, 2020
15
2
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23
Centreville, MD
I didn't see where there was a chip in the car on the list so I doubt that will fix the issue(s),
You need to put the work into running the checklist.
NOTE: did you do a sweep test on the tps?
Have not done a sweep test on the tps... the car is living at my house now until we get this resolved so I’ll check that out in the next couple days and report back. Thanks
 
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Dan02gt

10 Year Member
Mar 2, 2003
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Greenville, NC
Re-Gapped Plugs, tightened to ~0.055” - No Luck

Fuel Rail Pressure WOT maintains ~50 PSI

Pulled and cleaned all injectors, they were fairly dirty but still no luck

Disconnected MAF sensor & immediately noticed a difference in the idle, still same problem while driving so I’m assuming the MAF was working properly

Running out of items to check, ordered new tunes for the car - maybe that’s the problem.... time will tell

Thank you everyone for suggestions & input this far

You have 50 psi at WOT? You should be around 40. You ether have a bad FPR or that 255 fuel pump is over powering your fuel system. I would fix this before messing with anything else.
 

alex.sweitzer

Member
Oct 27, 2020
15
2
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Centreville, MD
You have 50 psi at WOT? You should be around 40. You ether have a bad FPR or that 255 fuel pump is over powering your fuel system. I would fix this before messing with anything else.
Yes, I videoed the pressure gauge that I ran off the rail into the car and it maintained 50 PSI from idle all the way to WOT
 

Dan02gt

10 Year Member
Mar 2, 2003
745
317
113
Greenville, NC
Yes, I videoed the pressure gauge that I ran off the rail into the car and it maintained 50 PSI from idle all the way to WOT
You should be in the low 30s at idle. Do you still have the stock fuel pressure regulator with the vacuum line connected? At 50psi across the board you are way to rich.