RPM breaking up when engine gets hot, need help

[clayton]

I recently have been having an issue with my 95 gt 331 stroker. The car starts fine, and will run great ( besides bucking at low rpm here and there). Once the engine starts running a little bit hot (on the temp gauge the needle gets in between the M and A in NORMAL), it wants to stall out easily at red lights and is difficult to start again. The worst issue is that while driving, out of no where it will decide to break up and miss real bad around 2k rpm. Under any heavy acceleration it will instantly do this until I get it home and park it. Once it cools down, it starts right up and runs great until it gets hot again, and the missing and breaking up begins all over again. Once again this does not occur until the engine is hot. Everything is aftermarket, no factory parts at all. Recently ive replaced the dizzy rotor, throttle body andTPS sensor ( set at .98V). It has a MSD dizzy and TFI coil. Im not sure how to diagnose the problem. Ive read it could be the TFI coil, the TFI module, the PIP sensor....do i just start replacing all of these until the problem is fixed? Has anyone had any experience with this problem? The car will spin up to 6500rpm and not miss a note until it seems to start getting hot ( not over heating but hot) and then it wont get up past 2k rpm. Any advice is appreciated thankyou

 

[07GtS197]

That’s usually a sign of a failing tfi module. Do you have a spare you can swap on there? If you do replace it, use a lot of thermal paste.

 

[Terrorist 5.0]

Yup, sounds like a TFI module to me too. Out of curiosity, does your tach go crazy too? Or is it still reading properly when you are having these issues?

 

[shovel]

I agree tfi or pip. Remember to replace your noise filter cap, if yours is original it's long dead

 

[Musturd]

tfi is a good possibility . I ended up doing coil near plug to eliminate that high rpm headache. .

 

[clayton]

That’s usually a sign of a failing tfi module. Do you have a spare you can swap on there? If you do replace it, use a lot of thermal paste.

No, I do not have a spare. Ive never replaced one before, actually I had to look it up to see what it was. Hopefully autozone can get one in soon. Is thermal paste something you can get at an auto parts store also?

 

[clayton]

Yup, sounds like a TFI module to me too. Out of curiosity, does your tach go crazy too? Or is it still reading properly when you are having these issues?

The tach reads fine. The only time it goes crazy is whenever the car starts missing and breaking up around 2k+ rpm, which is when the car starts getting hot...besides that, the tach reads accordingly to the engines rpm

 

[clayton]

I agree tfi or pip. Remember to replace your noise filter cap, if yours is original it's long dead

Thanks for the advice. This is going to be a silly question but what is the noise filter cap? I have never had to replace a TFI module before so im unfamiliar with the term. Is this the gray cover that looks like is mounted over the module? Kind of looks like it has fins?

 

[clayton]

tfi is a good possibility . I ended up doing coil near plug to eliminate that high rpm headache. .

Do you recommend i start with just the TFI module replacement and see if that resolves the problem or should I go ahead and order both the module and a new coil while I am at it? I figured if the coil is the problem, it would give me issues all the time. But the rpm disruption and missing doesnt start until the engines running a little on the hot side.

 

[Musturd]

Do you recommend i start with just the TFI module replacement and see if that resolves the problem or should I go ahead and order both the module and a new coil while I am at it? I figured if the coil is the problem, it would give me issues all the time. But the rpm disruption and missing doesnt start until the engines running a little on the hot side.

the coil could be possible too

 

[weendoggy]

Thanks for the advice. This is going to be a silly question but what is the noise filter cap? I have never had to replace a TFI module before so im unfamiliar with the term. Is this the gray cover that looks like is mounted over the module? Kind of looks like it has fins?

The TFI should be remote mounted on the right fender well on a "heat sink" unit. You should also get thermal paste included with the module. If not, get it at the store or any computer shop. I use to carry a spare when I had that system many moons ago, if not for me, for someone who needed one. I've never replaced a noise filter due to engine miss. It's usually there for radio/whine noise. It should be near the coil, usually looks like a small phenolic block with wire. I'd concentrate on the TFI, heat is their enemy.

 

[clayton]

Thwre is only one thing I did leave out. None of this started happening until directly after I installed a blue tooth receptor in the back of my pioneer head unit. Then this rpm misfiring when hot occurred. Im sure its totally un related. Its just wierd.

 

[shovel]

Thanks for the advice. This is going to be a silly question but what is the noise filter cap? I have never had to replace a TFI module before so im unfamiliar with the term. Is this the gray cover that looks like is mounted over the module? Kind of looks like it has fins?

The battery (constant) side of the ignition coil is connected directly to the power supply of the PIP (hall effect sensor inside the distributor) and the TFI module. Every time the coil fires a surge of flyback voltage gets dumped into the primary coil and feeds back into the electrical system. Ultimately the battery takes up that surge because it has very low resistance but the wires, fuse and connectors between the battery and coil do have some resistance which means there's still quite a bit of noise (fluctuating voltage) at the coil end of the circuit. Any voltage differential across a semiconductor resolves as heat whether it's doing a job or not so that noise is heat and heat kills.

So the capacitor lives as close to the coil as possible and provides almost infinite resistance to DC voltage while approaching a short circuit to AC noise. Whatever noise it eats doesn't feed back into the PIP and TFI so they stay cooler and live longer.

On 5.0 SN95's this cap is electrolytic - it's basically a little taquito made of mylar and rubber soaked in gatorade. The gatorade leaks out over time, typically 10-12 years is expected. In a hot environment like the engine bay especially there's no such thing as a 30+ year old noise cap that's still in business.

You can replace it with another OEM one like that or you can get one made for a newer car that looks like a black lego croissant and replace its connector with yours. You just cut the wire on your old dead one, cut the wire on the new one, solder them together with some heat shrink and you're in business. Don't use those "solder and heat shrink in one!" things.

The newer noise filter caps are dry and have effectively unlimited lifetime.



This is the newer style of noise filter cap on almost any "coil pack" vehicle from the 90s-00's. If you need a specific model it's any 99-04 mustang.

Here's a diagram showing where the cap lives in relation to the coil, PIP and TFI module.

 

[Terrorist 5.0]

The tach reads fine. The only time it goes crazy is whenever the car starts missing and breaking up around 2k+ rpm, which is when the car starts getting hot...besides that, the tach reads accordingly to the engines rpm

Ok. So this points me in a bit of a different direction. Your PIP sensor is your crank sensor, which is what your tach displays. I had an issue very similar to yours and I solved it with a new PIP sensor. So if your tach is going haywire when your problem occurs, this is likely your PIP sensor acting up.

Replacing it requires you to disassemble the distributor, but that’s very easy so don’t worry. I’m not saying 100% the PIP sensor is your issue, as I never concretely diagnosed the issue when I had it, rather just took an educated guess using my reasoning above, but I think it is your culprit here.

I wouldn’t recommend replacing the coil. If your coil was bad you wouldn’t be getting weird tach readings.

If you want, a new TFI module is cheap and easy to replace. Try that, and report back.

 

[RAU03MACH]

from the experiences i have had with TFI
they're very touchy and delectate in extreme weather
their resistance is not what it used to be
built very cheap