Engine High rpm miss

[claypoolxx]

Hi. I recently got my constant fuel pump running issue figured out. Was the ECU. ECUexchange did a fantastic job. However, I still have a problem. When the car is warmed up well, I get a miss at 4500 rpm and above. I recently had a friend put on gt40 aluminum heads. Don't remember doing it before, but I rarely drove it and never went above 5000 with the stock heads.

It has new plugs autolite 104s gapped at .045. It had no name 8mm wires. A friend gave me new 9mm ford wires, didn't help. I did my best to try and keep them away from each other and the dipstick tube. Ignition system is stock. It runs fine when just cruising. I ran the codes and got 31, 67, 81,82,84, but I don't think any of those are the issue. Where would you look next?

Car is a 93 with a 5.0. Mild unknown cam, cobra intake, and just put in 24# injectors with a matching MAF.

 

[General karthief]

Cap and rotor?

 

[claypoolxx]

Just looked at it. I didn’t replace it but it looks fairly new. No obvious signs of a problem.

 

[General karthief]

Do a compression test.

 

[claypoolxx]

Compression numbers came back good. Surprisingly low variance. A friend has a new coil (not motorcraft), he'll let me have. I am going to try that since its free, although I thought that coils either work or they don't. But wouldn't an electrical issue make sense wince it gets worse with heat? If not that, where would you look next?

 

[2000xp8]

Is it a miss like bucking or is it a miss like bucking and old garbage cans rattling around?
Stock X springs won't take all that much cam.

 

[claypoolxx]

No rattling. Had trickflow stage 1 springs installed with the new heads.

 

[claypoolxx]

Also, I have a new wallbro 155 fuel pump to put in. However I didn’t think my combo would really need it. Also I would imagine that if it was fuel, it would do it all the time and not just after it’s been driven for a while.

 

[Dan02gt]

My 1st thought would be PIP or TFI module.
I doubt it's the fuel pump, but with about any H/C/I setup you are on the edge of what the stock 88lph pump can handle so upgrading to that 155lph will be a good thing.

 

[General karthief]

Usually a high rpm miss, if it's a cylinder dropping, is felt as a loss of power, now if it's a 'da-da da-da jerky type miss then I'd be looking at the tfi or pip like dan said, also coil possibly.
Is it hard to start cold? Do you have to crank it a long time before it starts?
Swapping the fuel pump is never a bad idea.

 

[claypoolxx]

Took the car out today. Once I got it to act up, I brought it up to 6k rpm WOT, then gradually. I think in the five times I brought it up slowly I only heard one faint hiccup, while WOT it was worse. Would I be correct in thinking that an electric issue would happen either way and maybe this is fuel?

I did make a pit stop by my mechanic and he gave me a motorcraft tfi module to try out. I’m going to replace the fuel filter tomorrow but I literally just filled the gas tank before I noticed this so I have some driving to do before I drop the tank. I’ll keep you all updated when I get to it. I hate it when I don’t see how things turned out.

 

[claypoolxx]

Also, when it does it, it’s more like it loses power, not bouncing off a rev limiter. It has to crank for a couple seconds to start but it’s always done that. Starts the same hot or cold.

 

[Mustang5L5]

I'd suspect ignition issues, as they often don't have a code associated with them.

PIP's and TFI's are known failure points, along with the coil. TBH the aftermarket parts for these that are available now are quite terrible. I'd trust a 30 year old OEM coil over some of the newer stuff. If you can get your hands on spares to try and swap out, I would go this route.

My guess it that it's the coil as i've had a similar issue.


I hate to suggest firing the parts cannon at this, but it's often easier than trying to pull your head out and troubleshoot it. Can you borrow a coil?

 

[claypoolxx]

I did put a new coil in with no change. My mechanic friend gave me a motorcraft TFI module to try, but I haven't done that yet. I was waiting to burn some fuel to do the fuel pump since I have one already, but could do the TFI this weekend. The fuel filter I ordered got lost in the mail so I need to get another. I was thinking more along the lines of a fuel issue since it doesn't really do it if I accelerate up to 6k slowly. I don't mind changing parts if its easy and free. I'll let you know about the fuel filter and TFI this weekend. Thanks

 

[Bullitt347]

Load.
The ignition is breaking down due to the load. Slow acceleration to 6K rpm has a lot less load on the ignition than WOT.

 

[claypoolxx]

So quick update. Went to go switch out the TFI my mechanic had, but it’s for a ford van. It doesn’t have the three prongs. Is there an unbranded motorcraft one that is supposed to be good that’s less than 170? Or would I be better off getting a used one on eBay? I see lots of stories of people getting brand new ones that are bad.

 

[claypoolxx]

Also, if I do wind up buying a tfi module, any reason not to get a new distributor with one? Or maybe a 94-95 distributor with remote tfi. A Cardone distributor with module is $85 from rock auto.

 

[claypoolxx]

One more question if I may. I have a 93 with a motor that was swapped by the previous owner, not sure from what year, but it’s a roller HO motor. I’ve been doing some research on the tfi modules and it seems that a 93 should have a black one and earlier years had a gray one. The one on my car is gray. Would the proper tfi unit be determined by the year of the car or the year of the motor? I’m guessing the wiring in the car would be the determining factor and that I guess I have the wrong one in there. Would it even run at all with the wrong one?

 

[Mustang5L5]

Grey. The TFI doesn't know what motor is in it. Go by the ECU/wiring. The ECU and wiring is for the grey style TFI which is called a "Push Start TFI". The other style is "computer controlled tfi"

New distributors are known for coming with crappy electronics in them, especially the PIP.

 

[claypoolxx]

Got my new “motorcraft” tfi from rockauto today. Looks very similar to the one in the car but not a single ford or motorcraft marking. I used some thermal grease from a friend but it was only enough for a thin layer. Is that something that should be fairly think? Took it for a 15 minute test drive with lots of accelerating and all seems to be well. This weekend I’ll be able to take it on a longer cruise and see what happens. Thanks all.