Please help me diagnose a problem

[Ryleighsdad]

Hey guys. New-b here. I have bashed my brain in trying to diagnose this issue. Here is what I know.

I picked up a 95 GT two days ago, and I've been going through the car fixing small problems. This one is turning out to be a large problem. The car has a BBK intake, and flowmaster cat back. Otherwise stock.

Symptoms:
Drives fine under normal(conservative) driving, even got 21 mpg.
It will rev fine all the way to the limiter if you rev it slowly
If you stomp the pedal from an idle it will rev perfectly until 4,300rpm then start "missing?" and backfire through the intake.
If your driving normally and stomp the pedal it will pull fine until 4,300 and do the same
Bogging the car at slow speed in any gear and going full throttle, it will just pull no problems, until...you guessed it, around 4,300rpm, then hesitation, missing, studdering, and popping from the intake.

Things I have done:
New motocraft plugs gapped at .049
New MSD cap and rotor
Run premium fuel
Checked timing at 10* advanced to 12* with no change.
New 8mm MSD plug wires
Ignition module tested ok by mechanic
Coil tested ok by mechanic
Fuel pressure is 39-45 psi depending on rpm/load
Cleaned MAF with proper cleaner
Verified sparkplug wire firing order on dizzy
Ran cylinder balance test and got 9 flashes = everything ok
NO CEL!!!!

It does this every time you get to 4,300 rpm at full throttle. Doesnt matter what gear Im in or if Im sitting still in neutral.

Do you guys have any ideas?

Thanks in advance!

 

[toyman]

And you pulled the codes and only got a 111? A CEL isn't always present to have a stored DTC. I would first make absolutely certain the battery to engine and engine to chassis grounds and power leads are solid. Important as all the engine sensors need a good ground. Next I would consider changing the MAF. It cannot be effectively tested in neutral.

 

[Ryleighsdad]

I pulled codes and got 111.....111 but IDK what that means. Thanks for the info on the MAF, a friend of mine has one that is tuned for 30# injectors, could I run this on my car with 19# injectors to see if the problem is fixed?

 

[95Vert383AOD]

Do you have a STOCK Maf installed? I had the same exact problem when i had a C&L installed. Sounds like your meter may be pegging.

Chris

 

[Ryleighsdad]

Yep stock maf.

 

[Ryleighsdad]

Swapped MAFs today with a new one, let the engine have some seafoam through teh brake booster vacuum line, and cleaned the intake temp sensor...No change.

 

[Ryleighsdad]

Can a weak valve spring cause this, but run fine under moderate throttle even at high rpms?

 

[ratio411]

Run premium fuel

Edit: My money is on a dirty fuel filter after some thought.

If your engine is "otherwise stock" with only those bolt ons, you are hurting yourself by using premium.
Only use higher octane (harder to burn) fuel when you have the need for it. Stock 95 5.0 engines don't have what it takes to need it.


As for your 4300 rpm 'wall'...
That sounds like valvetrain issues to me.
Weak valve springs, weak hydraulic lifters, worn lifter bores, or even lack of adequate oil pressure to supply hydraulic lifters at high rpm.
There are others, but what you describe sure sounds like what is commonly called 'valve float'.

Are you sure the previous owner didn't install a rev limiter or some sort of valet device on the car?

Edit:
Does the car have cats?
Could be clogged cats.
Could also be a dirty fuel filter now that I think about it.

 

[Ryleighsdad]

Thanks for the input ratio. I should back it off of premium and run midgrade? Low grade? We have 87, 91, and 93 octane here. Ive been running 93.

I do know the car has a clifford alarm to which the PO didnt have the fob and the alarm has been disconnected from the battery. He has no idea why its happening.

The car does have cats, from what I can tell it has at least 2 possibly 4, but they look stock. How do I find out if its a clogged cat?

Fuel pressure was checked and is over 39psi at the engine under all driving conditions. < Told by my mechanic, dont know if they tested the pressure at WOT or not.

Car has 125k miles.

 

[ratio411]

Thanks for the input ratio. I should back it off of premium and run midgrade? Low grade? We have 87, 91, and 93 octane here. Ive been running 93.

I do know the car has a clifford alarm to which the PO didnt have the fob and the alarm has been disconnected from the battery. He has no idea why its happening.

The car does have cats, from what I can tell it has at least 2 possibly 4, but they look stock. How do I find out if its a clogged cat?

Fuel pressure was checked and is over 39psi at the engine under all driving conditions. < Told by my mechanic, dont know if they tested the pressure at WOT or not.

Car has 125k miles.

Fuel pressure can be made just fine with a clogged filter.
You not only need pressure to run up the rpms, but also volume.
You have pressure, but not volume to support the rpms.

You check fuel pressure by 'simulating' full throttle by removing the vacuum from the regulator. That 'simulation' ups the pressure, but does not simulate the increased need for fuel volume.

 

[ratio411]

Change the fuel filter and come back and talk to us.

They are cheap, easy to change, and I find that our cars need them changed about once a year, 10k miles absolute max.


125k miles is nothing for the valvetrain unless it was mercilessly abused all it's life.

 

[Ryleighsdad]

That makes perfect sense, and I have experienced that very thing with a dodge and cummins diesel. Clogged fuel filter would cause a misfire. I was told to change it, but I forgot! First thing in the AM that is on the list.

 

[toyman]

And don't underestimate the importance of the grounds and cables themselves.

 

[Ryleighsdad]

Changed fuel filter, cleaned egr, iac, tps. No change.

 

[madspeed]

try disconnecting the battery for 15 minutes, then reconnect and go out for a drive to let the computer relearn. Might clear out any funk thats in there

 

[Ryleighsdad]

Did that last night. Gonna swap the dizzy when I get home.

 

[ratio411]

Changed fuel filter, cleaned egr, iac, tps. No change.

Well, I hate to hear that, I had convinced myself it was the filter, and that would have been perfect because you needed to change it anyway, and it's dirt cheap.

I hope you get it figured out, and without spending a fortune in time or money to do so.

 

[madspeed]

how about the sock on the fuel pump, could it have some crap sucked up into it?

 

[Ryleighsdad]

I dont think its stock, it whines pretty loud. Its getting good pressure out of the pump, but I guess anything is possible. Changed the distributor and reset timing last night, also sprayed ether all around the engine bay to check for vacuum leaks. No change.

 

[Ryleighsdad]

Turned out the ign coil was breaking down under load. Swapped that an she runs like a scalded dog now! Thanks for your help.

I tried mid grade fuel Ratio, and she detonated like crazy! Had to add a can of octane booster to get it to stop. From here on out she gets premium. lol