Skip to main content ↑ ↓
Topic: 4k shutoff (Read 5052 times) previous topic - next topic

4k shutoff

will not having o2 sensors in place cause an 88 5.0 bird to sputter and no longer accelerate at 4000 rpms? i have talked to several people who said no, but i have replaced every sensor on the engine since i have had it (except for bunging my exhaust for o2's) and nothing helped. it acts like it hits a rev limiter or something.

also will no o2's cause poor economy?
Currently Birdless but never Foxless

86 Mustang GT

4k shutoff

Reply #1 –
Missing O2's will certainly cause poor economy, but if it's a stock non-HO 5.0 you're not gonna get any more than 4k RPM out of it anyway. They simply can't breathe at that RPM.

Now, if your sputtering and refusal to accellerate at 4K RPM happens at around 110 MPH, that would be the speed limiter kicking in.
2015 Mustang GT Premium - 5.0, 6-speed, Guard Green - too much awesome for one car

1988 5.0 Thunderbird :birdsmily: SOLD SEPT 11 2010: TC front clip/hood ♣ Body & paint completed Oct 2007 ♣ 3.55 TC rear end and front brakes ♣ TC interior ♣ CHE rear control arms (adjustable lowers) ♣ 2001 Bullitt springs ♣ Energy suspension poly busings ♣ Kenne Brown subframe connectors ♣ CWE engine mounts ♣ Thundercat sequential turn signals ♣ Explorer overhead console (temp/compass display) ♣ 2.25" off-road dual exhaust ♣ T-5 transmission swap completed Jan 2009 ♣

4k shutoff

Reply #2 –
Isn't your 4K prob, O2 sensors are ignored by the computer at WOT...

4k shutoff

Reply #3 –
it happens in every gear and trust me, it will run WELL over 110 :D

it has an HO intake, CAI, mustang headers, gutted cats, and no ler. should be able to breath.

what could be causing this? the car pulls hard up to 4k, then its like it hits a rev limiter......
Currently Birdless but never Foxless

86 Mustang GT

4k shutoff

Reply #4 –
forgot that it also has a 65mm throttle body.

scanned it at autozone today and the only codes it was throwing were TPS out of range and something about the IAT. i put 3 different IAT sensors in it and none helped the way it does. going to get a digital multimeter soon so i can set the TPS
Currently Birdless but never Foxless

86 Mustang GT

4k shutoff

Reply #5 –
what cam and heads have you?

those affect breathing too ;)

Also, what eec do you have?
'98 Explorer 5.0
'20 Malibu (I know, Chevy, but, 35MPG. Let's go brandon, eh)

4k shutoff

Reply #6 –
If you have the stock cam, your not going anywhere. You probably can't even hit the  rev limiter with the shiznitty stock truck cam shaft in there.
Quote from: jcassity
I honestly dont think you could exceed the cost of a new car buy installing new *stock* parts everywhere in your coug our tbird. Its just plain impossible. You could revamp the entire drivetrain/engine/suspenstion and still come out ahead.
Hooligans! 
1988 Crown Vic wagon. 120K California car. Wifes grocery getter. (junked)
1987 Ford Thunderbird LX. 5.0. s.o., sn-95 t-5 and an f-150 clutch. Driven daily and going strong.
1986 cougar.
lilsammywasapunkrocker@yahoo.com

4k shutoff

Reply #7 –
That tps might be confusing the ecc into thinking it hit redline. I had a similar prob. but it would break up and not go between 3-4 grand. I changed the ignition switch and hooked my vss signal wires up and it went away. not sure what fixed it, also mine would only do it sometimes.
Old Grey Cat to this.88 Cat, 5.0 HO, CW mounts, mass air, CI custom cam, afr165's, Tmoss worked cobra intake, BBK shorty's,off road h pipe, magnaflow ex. T-5,spec stage 2 clutch, 8.8 373 TC trac loc, che ajustables with bullits on the rear. 11" brakes up front. +

4k shutoff

Reply #8 –
Quote from: Haystack;273806
If you have the stock cam, your not going anywhere. You probably can't even hit the  rev limiter with the shiznitty stock truck cam shaft in there.

To add to this: If you've still got the stock cam AND the stock E6 heads you're not going anywhere. The stock cam and E6 heads are what makes a non-HO not HO. All the exhaust and intake in the world is not going to help you. With all that you might, just might, under ideal conditions, eke another 10-15 horses out of the engine, but it'll be at an RPM you can't use (because the heads and cam won't allow the engine to spin that fast) and you'll lose an equal amount of torque down low (for example, Ford gained 5 horses when they added dual exhaust in 1988, but lost 5 lb-ft of torque).

You might be experiencing valve float as well. The weak-ass stock valve springs may not be capable of pulling the valves closed fast enough at that RPM. When brand new they'd have been hard pressed to do it. 20+ years old they're lucky to pull the valves closed at idle. There's a reason the non-HO engine had a 4500 RPM redline.

..and unless you've done something to disable the speed limiter in the 1988 EEC-IV ECM (such as replacing it with an '87 or cutting the VSS signal to it, which would remove the limiter but cause other drivability problems) you're certainly NOT going faster than 110.
2015 Mustang GT Premium - 5.0, 6-speed, Guard Green - too much awesome for one car

1988 5.0 Thunderbird :birdsmily: SOLD SEPT 11 2010: TC front clip/hood ♣ Body & paint completed Oct 2007 ♣ 3.55 TC rear end and front brakes ♣ TC interior ♣ CHE rear control arms (adjustable lowers) ♣ 2001 Bullitt springs ♣ Energy suspension poly busings ♣ Kenne Brown subframe connectors ♣ CWE engine mounts ♣ Thundercat sequential turn signals ♣ Explorer overhead console (temp/compass display) ♣ 2.25" off-road dual exhaust ♣ T-5 transmission swap completed Jan 2009 ♣

4k shutoff

Reply #9 –
Could it be as simple as the stock fuel pump cannot keep up anymore, and it's running out of fuel? :hick:
95 Ranger Splash 2.3
88 Tbird Sport :ies::ies:
5.0 SO, stainless shorty headers, w/ Magnaflow lers. KYB struts, KYB shocks. 5lug conversion from sn95 Mustang, subframe connectors, drilled and slotted rotors, 03 Mach 1 wheels. sequential taillights.140 speedo

4k shutoff

Reply #10 –
Quote from: cougarman;273829
Could it be as simple as the stock fuel pump cannot keep up anymore, and it's running out of fuel? :hick:

Could be a possibility, but usually a weak pump will show at different RPMs, depending on gear but always at WOT... Probably wouldn't hurt to install a fuel pressure gage and see where pressure is at 4K...

You could install a 6cyl tach, it will read 6K, problem solved... :hick:
If you're feeling really frisky, get a 4cyl tach and spin it up to 8G...

If it's the "brick wall feeling" you described, then it's probably either hitting the speed limiter or you have a valve float issue... I doubt it's valve float, I've seen some sorry springs in my time, but never seen a SB Ford that wouldn't rev at least near 5K... SO heads ain't great at RPMs, but it should rev more than 4K, my '86 Stang with E6 heads would pull hard to about 5300, then run in to the wall you described... Didn't have a speed limiter so had to be the springs...

Sooo this leaves speed limiter but if it does it in any gear, puts us back at fuel pump, valve springs or maybe a iginition problem...

4k shutoff

Reply #11 –
Quote from: Thunder Chicken;273823

You might be experiencing valve float as well. The weak-ass stock valve springs may not be capable of pulling the valves closed fast enough at that RPM. When brand new they'd have been hard pressed to do it. 20+ years old they're lucky to pull the valves closed at idle. There's a reason the non-HO engine had a 4500 RPM redline.

..and unless you've done something to disable the speed limiter in the 1988 EEC-IV ECM (such as replacing it with an '87 or cutting the VSS signal to it, which would remove the limiter but cause other drivability problems) you're certainly NOT going faster than 110.


get some heads of a 91-93 bird or cougar. could probably use the cam too. There alot easier to find if your in mustang territory.

The tbird I have right not won't even rev to 4k. I can just sit there and hold down the gas and it stops about 3800.

You can get the stock speedo to read over 110....:mullet::burnout:
Quote from: jcassity
I honestly dont think you could exceed the cost of a new car buy installing new *stock* parts everywhere in your coug our tbird. Its just plain impossible. You could revamp the entire drivetrain/engine/suspenstion and still come out ahead.
Hooligans! 
1988 Crown Vic wagon. 120K California car. Wifes grocery getter. (junked)
1987 Ford Thunderbird LX. 5.0. s.o., sn-95 t-5 and an f-150 clutch. Driven daily and going strong.
1986 cougar.
lilsammywasapunkrocker@yahoo.com

4k shutoff

Reply #12 –
it has new plugs, plug wires, cap, rotor, module, and the distributor shaft feels like new so i doubt it's ignition related.

i'm still on stock cam and heads, but the car has turned 6500rpm's when my brother had it.

we have a fuel pressure guage and it has good fuel pressure.

as for the 110mph limit, i made it 25 miles in 12 minutes, 2 miles of that was stop and go and about 4 miles was at 45mph. averages out to 125mph if my math is good....
Currently Birdless but never Foxless

86 Mustang GT

4k shutoff

Reply #13 –
Quote from: Dougy_Fresh;273861
as for the 110mph limit, i made it 25 miles in 12 minutes, 2 miles of that was stop and go and about 4 miles was at 45mph. averages out to 125mph if my math is good....

Well I'm calling BULL shiznit on this... Six miles of stop/go & 45 mph averaged into 19 miles would probably mean you'd have to be running 150-160mph the for the 19mi... AS I SAID, BULL shiznit...

As far as prev turning 6500 another BS, the computer limits RPMs to 6250... Ain't happinin' with STOCK ford valve springs anyway...

Wanna try another story???

4k shutoff

Reply #14 –
Quote
The weak-ass stock valve springs may not be capable of pulling the valves closed fast enough at that RPM.


Like Tom said, I'm doubting it's the springs as well -- at 4K RPM anyway...
Anyone know if there's a difference between the springs from HO to non HO?  Part #'s or anything like that?

Also remember that a BONE STOCK AOD WILL NOT GIVE 4th/OD @ WOT.... 

Gotta back off it a bit.
-- 05 Mustang GT-Whipplecharged !!
--87 5.0 Trick Flow Heads & Intake - Custom Cam - Many other goodies...3100Lbs...Low12's!