Engine 1990 Fox wont rev over 3500 RPMS

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

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?

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 said:

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.

Click to expand...

Thanks! I'll check and report back

Mustang5L5 said:

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?

Click to expand...

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!

Is this what Im looking for?

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