Engine 1990 Fox wont rev over 3500 RPMS

  • Sponsors(?)


Mustang5L5

i'm familiar with penetration
Mod Dude
Feb 18, 2001
34,647
10,820
224
Massachusetts
Those aren’t the right spark plugs for those heads, they will thread in, but look at the difference in thread lenght. The autolite 25s are likely not sitting deep enough into the chamber, if they even fit properly.

please pay attention here. This is important for you to realize. It appears very likely your car has had the long block from a non-HO 5.0 swapped into it. This is due to the incorrect firing order and the incorrect heads. This combo of parts is found on common passenger car engines like a crown Vic or an ‘88 thunderbird. It’s possible the upper intake manifold; and throttle body are also from the same non-Mustang car, but we haven’t gone searching for casting

if you borescope the cylinders, you’ll prob see the Pistons lack valve reliefs.

As you proceed with troubleshooting the car, keep that in mind. It may change the parts you end up using, like spark plugs, or it may present as evidence that the previous owner hacked the car together to kick it off to the next owner.
 
  • Like
Reactions: General karthief

AydenTv

Member
Nov 19, 2020
57
1
18
30
Alabama
Those aren’t the right spark plugs for those heads, they will thread in, but look at the difference in thread lenght. The autolite 25s are likely not sitting deep enough into the chamber, if they even fit properly.

please pay attention here. This is important for you to realize. It appears very likely your car has had the long block from a non-HO 5.0 swapped into it. This is due to the incorrect firing order and the incorrect heads. This combo of parts is found on common passenger car engines like a crown Vic or an ‘88 thunderbird.

if you borescope the cylinders, you’ll prob see the Pistons lack valve reliefs.
i actually went with the cheaper motorcraft ones but ima go outside right now and pull a plug really fast and see exactally which kind i have and take a pic of one right next to an old one which was in the car when i got it and both are same thread length.. i have said this before im not a mechanic and if it fr dont require money im willing to try anythij ng but will also say this the further you dig into this car the weirder it gets.. like example it wants to hook like way too good.. may be the tires or the aod cuz im use to t5s but it will launch for a mustang
 

Mustang5L5

i'm familiar with penetration
Mod Dude
Feb 18, 2001
34,647
10,820
224
Massachusetts
Well with E6 heads it’s likely down on power. Those heads were pretty weak. Non-Ho 302s were typically in the 180-200HP range. Throw an AOD behind that and it’s down on even more power.

what exactly is wrong with the car? This thread has gone all over the place.

take a photo of the spark plug hole with the plug removed. I can tell from the thread hole depth which plug it will need. But seeing as it is an E6, that info is already known
 
Last edited:

AydenTv

Member
Nov 19, 2020
57
1
18
30
Alabama
iyWell with E6 heads it’s likely down on power. Those heads were pretty weak. Non-Ho 302s were typically in the 180-200HP range. Throw an AOD behind that and it’s down on even more power.

what exactly is wrong with the car? This thread has gone all over the place.

take a photo of the spark plug hole with the plug removed. I can tell from the thread hole depth which plug it will need. But seeing as it is an E6, that info is already known
 

Mustang5L5

i'm familiar with penetration
Mod Dude
Feb 18, 2001
34,647
10,820
224
Massachusetts
You're last message didn't post.

More info if you want to identify parts. The upper intake on the 5.0 HO and non-HO differs as well. It's possible the HO intake was installed though as the lowers are the same but the upper is different.

Easiest way to tell is to remove the intake plate. Under the plate, you will see the HO cast into the manifold. If this is missing, it's not the HO intake.
06FCAC92-F68F-4C31-96F6-6631B904DB9F.jpeg



Part number for an 1989 HO upper intake is RF_E8ZE-9425-E4A. The 1990 HO intake started with an F0ZE part number.
93107D58-8B6A-4D67-90E9-116DDB455EE6.jpeg


Other info on what should be on the car

Upper intake: RF-E8ZE-9425-E4A
EGR spacer: E7ZE-9H474-AC 62mm TB side/64mm manifold size
TB: E7SE-HB (stamped on lever) 60mm

That's what should be on your car. You can compare to see how it matches. If the sizes differ, or the part numbers differ, then it's a good sign there are more non-HO parts on your car. I beleive the upper intake manifold part number for a non-HO is RF-E6SE-9425-



Just to be clear on the implications of this. It does appear that in some point of your cars life, the original engine was removed, and the long-block of a non-HO engine was installed. The reasons for this are unknown, but someone did repin the ECU to make it work along with most of the correct parts needed to get it running well. Maybe it was done out of necessity (the only long block they could afford) or maybe it was done to flip the car as cheap as possible. If it was known, it should have been revealed to you. WIth the E6 heads and non-HO cam and possible other non-HO parts, your engine might be making 180-200HP rather than the 225HP it should be making. E6 heads also tend to fall out of steam at higher RPM. They typically offer good low end torque, so they feel peppy on the street.
 
Last edited: