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This is a question I have put out on two other forums with no results:

I’m looking for some diagnostic help on my ’95 Mustang GT with a 93,000 mile 5.0 motor with a stock intake system (except for K& N filter), MAC shorty headers, and a Flowmaster cat-back exhaust. In other words, it’s a basically a stock original motor without many modifications. I have also added 3.55 gears and underdrive pulleys but that’s about it.

The car runs great except above 3000 RPM at wide open throttle. About 2000 miles ago I had some intermittent starting problems so the fuel pump, idle air control valve, ignition control module (not the computer), ignition coil, and fuel filter have all been replaced. (Replacing the ignition switch finally fixed the problem….). Since then, the car has been fine (up to about a month ago) but then it started having this wide open throttle issue.

For normal everyday driving the car is fine. If you run it up to 4500 at part throttle it’s fine and it will run continuously (cruise) at 4500 without a problem. It’s when you punch it and try to rev it above 3000 RPM that it has a problem. In other words, it will run up to 4500 fine with a part throttle but if it’s WOT (floored from a take off) it will run up to 3000 RPM and then act like it’s running out of fuel above 3000 -3500 RPM. It will start to die and loose power at 3000-3500 but momentum will carry it up to about 4500 (starts acting up about 3000 but will still rev up to 4500) but by then it’s dying enough to start slowing itself back down. If you hold it WOT it will continue to slow itself back down just like it’s running out of fuel. If you back off to part throttle at any point it’s fine and will run at 3000, 3500, 4000, what ever you want and even start to increase speed again if you press the throttle down slowly, but as soon as you step back down to WOT, it acts like it’s loosing it’s fuel flow and begins to die again.

No check engine light is showing and no codes are coming up. The fuel pressure regulator seems to be operating ok (I have a fuel pressure gauge mounted on the rail and at idle the PSI bumps up with the vacuum line disconnected like it should), I have cleaned the MAF, and the voltages on the TPS checkout ok. With a new fuel pump and fuel filter, new idle air control valve, new ignition control module, new coil, new plugs, Mallory Spiro-Core 8MM wires with only 6-7000 miles on them, a fuel regulator appearing to work, and MAF and TPS that check out ok, I don’t really know where to go from here.

Even though it feels like a fuel or spark issue, I really don’t think it can be because as long as I don't have it at WOT it will run at 4500 RPM all day long. If it's a bad spark or low fuel, how can could it continue to run at 4500 RPM at part throttle with no problem, 4500 RPM is 4500 RPM. Something about WOT triggers something. The best way to describe it is that it’s like a computer signal or vacuum signal at WOT causes some sort of rev limiter be it fuel or spark to kick in. It seems like it has to be a computer control/signal that has changed (is that even possible?) or some sensor is bad but it’s not causing a code to show up. I’ve checked everything I can think of. I just hate to just throw money at new parts (particularly the computer) and hope something fixes it.

Looking for help!!
 
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Mine does the same thing except Its 93k miles and looks to never have had a tune up (just got it) the air pump hose was also disconnected for some reason? stumbles at idle sometimes. Im interested to see what people have to say about your problem though.
 

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How's the cap & rotor? Did you replace it when you put on the 'new' plug wires?

A buddy of mine is having a similar issue. His cap & rotor is shot. He hasn't replaced it yet - went out of town on business. Let me know what you find.
 

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You might want to check your fuel pressure regulator.
 

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4500 rpm and accelerating through 4500 rpm are not the same. the computer has different tables at cruise and wot. you might also check your MAF and see if it is clean I had a thunderbird that would cruise fine but would stumble on acceleration, I found a piece of the paper filament in the MAF sensor pulled it out and ecerything was fine.
 

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Check my previous posts. It may be the Halls Effect Sensor...a.k.a "stator" inside the dizzy. Alot of people have been having this problem over tha past month or so. Before you go wasting alot of money, check that. Pull the cap and rotor. The wheel that the rotor sits on is called an armature. There are two little screws that hold it on. Take it off and clean the rust off with some steel wool or fine sandpaper. If your "stator" is loose you'll have to replace it. It's easier to just replace the dizzy with a stock one depending on your budget. I'm almost certain this is your problem. I had the same problems you are having back in October. I replaced everything you did except the fuel pump. Most of the guys on here told me it was the stator, so I changed the dizzy and the problem is gone. Hope this helps. You can get a stock replacement dizzy at Napa for like 100.00
 

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You won't believe this because I didn't either.....but here is the story:

My kids car, 1994 GT had the same symptoms. This showed up even when we were test driving it, but it was such a good car he bought it anyway. At first it didn't bug him, but after he got used to the car (24 hrs) it was a pain. The car would "go flat" at higher rpms at full throttle.

He said he was going to clean the afm. What the hell, its free I said. So he sprayed CRC electrical parts cleaner all over the afm from both sides (doesnt take much) At the same time we sprayed the connector and put some spark plug grease in there just for grins.

Guess what? The thing runs awesome. 10 minutes of work max, Huge difference. So I say again, why not? Its free.

Jay

PS: Use electrical parts cleaner or afm cleaner, not brakekleen or carb cleaner. You might need to buy a can, so maybe its not exactly free, but you can clean about 1000 afms with 1 can.
 

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He said he was going to clean the afm. What the hell, its free I said. So he sprayed CRC electrical parts cleaner ..

Guess what? The thing runs awesome. 10 minutes of work max, Huge difference. So I say again, why not? Its free.

Jay

:eek::eek: WHOA NELLIE! Thanks. I went right out and did this and now it runs tons better. I had the CRC Electric Parts Cleaner in my cabinet! Whoo hoo...
:eek:
 

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At ford it was only like $60-$80 CDN hall effect sensor.



Yes cleaning the MAF is a big differance, I use red stallion brake cleaner then after it dries I use the electical spray from CTC works great everytime.
That's expensive. The Halls Effect Sensor is only about 20 bucks. The whole dizzy is about 100.00. If you can clean it up and it works great, then good for you. The cheaper the better. Glad she runs good for you again.
 

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Discussion Starter · #15 ·
The distributor at least appears to be ok. The cap and rotor were replaced the same time as the wires, pulled them both and both look ok, clean and no cracks. Stator was tight but pulled it anyhow, no rust, really pretty clean. Distributor seems tight, no wiggle to speak of.

I've cleaned the MAF twice but I've used carb cleaner both times. Would electrical cleaner vs carb cleaner really make a difference? I would think if things were that touchy that the oil off a K&N filter would really be an issue!

I've been reading up on the computer operation on a fordfuelinjection site, and that's why I'm leaning towards the computer. Like dub9 silver gt mentioned, WOT is different than cruise. The car starts out in a preset open loop control until it hits approx 170°F and then it goes into closed loop where the O2 sensors etc help the computer make decisions. At warm cruise and part throttle acceleration the computer is in closed loop mode but at WOT full throttle acceleration it goes back into open loop and runs on the computers preset values. At WOT everything is controlled by the computer which makes its decisions based on what signals it gets from the engine coolant temperature sensor, the throttle position sensor, the vehicle speed sensor, and whatever the preset value are in the computer. The speedo is working fine and the TPS voltages check out fine. The ECT (temp sensor ) appears to be working ok and from what I read they hardly ever go bad. The other thing against it being the ECT would be that if the temp sensor is bad, that would keep the computer from going into closed loop but at WOT the computer is in open loop anyhow so what does it matter if the temp sensor is bad, that just means cruise closed loop never kicks in and fuel mileage and emmisions just suck. At this point I think I only have two choices, replacing the ECT or replacing the computer. The fact that it only has issues at WOT is what really makes me think it's the computer but is there anyway possible that the computer can actually change its pre-programmed settings all by itself? That doesn't seem possible but I dunno at this point!
 

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Discussion Starter · #16 ·
Switched out the computer today. No change, still bogs ....
 

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how are your O2 sensors??.......i know it would regularly throw a CEL but its worth a shot just to take a look at them because of the miles on your car. i've got a similar problem with my Cobra where if i haven't waited until the car warms up before i accelerate then the car bogs like you wouldn't believe. its got a complete MSD ignition (Pro-Billet distributor, blaster coil, 6A box) and i'm pretty sure its the distributor doing that on my car.
 

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Go buy a stock OEM dizzy from Napa, even if the pickup isn't loose it can still be bad. If you replace it and it isn't the problem then clean it up and take it back, you've eliminated the computer, now eliminate the dizzy.
 

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Discussion Starter · #20 ·
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