Engine acts like it hits a rev limiter at 5k rpm?!?

[BuickNick]

So I got my stang back on the road, everything runs mint except....
When it hits about 5k the engine bogs down to about 4600 rpm and then climbs back to 5 k and does the same thing all over again. It does this in neutral or moving under load. I have replaced the tfi module, the distributer pickup, cap/rotor, and no change. The valve springs are matched to the cam, so valve float isn't an issue, plus valve float would not act like this under no load. No power loss until it hits 5k, then it just shuts down.

The only thing I can think of now is the computer is crapping out at 5k rpm, maybe the tables got glitched.

Any other guesses before I dump $$$ down for a new cpu?
and the tach is accurate. The computer appears to have been replaced before, the date on it is 1995, car is a 1994, build date is end of 93....

I don't know if this did it before i rebuilt the engine. I got the car, 2 months later i pulled the engine, and after a year and a half i finally finished the engine.

The car specs are as follows-
94 Mustang GT, Auto trans (i know it's lame, have a t5 in my garage)
5.0 engine, .030 over, balanced, windage tray, 9.0 comp, factory heads, ported, 1.94/1.60 valves, TFS street intake, 1.6 roller rockers, e303 cam, stock fuel injectors, stock unmodified computer.

 

[dastang2]

need to hook a fuel pressure gauge and read what it says at that rpm. i am betting that you need to either upgrade or replace a weak pump.

 

[goodfastncheap]

Assuming you don't have any obvious trouble codes, I guess I'm with dastang here in that my first instinct says to troubleshoot fuel delivery.

 

[BuickNick]

Thanks for the replies.
Fuel pressure is good at idle, haven't checked it revving. If it only did this while moving, I would agree fuel issues were a possibility. But since it does this in neutral with no load, I'm counting out fuel issues.

 

[Roland69]

tach could be off....have a good tach or a good timing light hooked up (one that has tach option) and see if it is running out at 5K. maby it's actually hitting 6K and your stocker is only reading 5K.

 

[dastang2]

load or not, if you got a weak fuel pump or clogged filter, it's going to do it.

 

[BuickNick]

no load = very little fuel requirment. If fuel where the issue it would do it at alot lower rpm under load. it does it same rpm load or not.
tach vs speedo calculates out right on.

 

[goodfastncheap]

You said your tach was OK in your original post - assuming that is correct...and we have indeed ruled out every fuel delivery issue...

If you have 9:1 comp on E7TE heads, in combination you must have milled your heads/decked your block a total of .020". You installed larger valves and 1.6 rockers and e-cam with correct spings also, if I follow your sig. After you measured with a checking spring and checking pushrod, what length pushrods did you end up with? What preload? What is your rocker to retainer clearance?

 

[dastang2]

no load = very little fuel requirment. If fuel where the issue it would do it at alot lower rpm under load. it does it same rpm load or not.
tach vs speedo calculates out right on.

i know from experience that this can happen. it did it on my old 95 and also on inlaws van. it also made the cats glow red hot and exhaust gasses cinge leg hair if you walk in front of exhaust pipes. it happens. don't rule out just yet. just put the gauge on it first, it takes less than 5 minutes.

 

[BuickNick]

new stock length pushrods, rockers are comp cams adjustable, adjusted correctly. I forget the exact spec.

Fuel delivery issues would have to change symptoms between load/no load. Yes it could effect a no load situation, but it would have to effect a load situation at a different rpm. It is always right at 5000 rpm no matter if i am WOT, slowly accelerating at part throttle, revving it up fast or slowly in neutral. And it runs mint, no missing until it hits 5000 rpm, then sounds like it shuts off until the rpm comes down to about 4600-4800 range, the climbs back up to 5k normally.

 

[goodfastncheap]

Pull codes?

Just a theory; something harmonic other than detonation could be picked up by the knock sensor and interpreted as detonation -- the EEC will then pull timing until the sensors voltage signal stops.

Set total timing to something your comfortable with, say 34 degrees, and disconnect your knock sensor then take it up to 5K and see what happens.

 

[Blackstallion8]

yep

mine did the same thing before i got my tune done. I installed a bigger pump and that fixed it.

 

[PJC Racing]

new stock length pushrods, rockers are comp cams adjustable, adjusted correctly. I forget the exact spec.

Fuel delivery issues would have to change symptoms between load/no load. Yes it could effect a no load situation, but it would have to effect a load situation at a different rpm. It is always right at 5000 rpm no matter if i am WOT, slowly accelerating at part throttle, revving it up fast or slowly in neutral. And it runs mint, no missing until it hits 5000 rpm, then sounds like it shuts off until the rpm comes down to about 4600-4800 range, the climbs back up to 5k normally.

Stock pushrod length with new rockers and valves? Did you measure for length? Valve train geometry could be way off. And you didn't say anything about valve springs. You didn't use the old stock ones did you?

 

[goodfastncheap]

After you measured with a checking spring and checking pushrod, what length pushrods did you end up with? What preload? What is your rocker to retainer clearance?

Bad manners to quote yourself probly...but it seems like all of the people who've replied to your post agree that your problem is fuel or valvetrain, not your computer.

If after all your modifications after thoroughly and correctly measuring for properly-sized pushrods, you wound up with STOCK length...hmmm. OK, if that IS the right pushrod length, did you check your FP at 5K? Do a tuneup (with a new fuel filter)? Try my off-the-wall suggestion about the knock sensor?

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I've never heard a 5.0 rev past 5.5k that I can remember. In fact I doubt the one I'm working on now has been past 5k in it's life.

 

[BuickNick]

There is no knock sensor on a 94 5.0.

Tune up is fine, all new parts. Posibally defective new wire set, I have seen that before, so that's on the list to change. No codes, cylinder balance test comes back 100%.
As stated, springs match the cam, yes they are new aftermarket with correct installed hight. I used stock length pushrods, but adjustable roller rockers to compensate for head milling, difference in valve tip length for the aftermarket valves and valve recessing from the valve job...
Free revving (no-load) an engine takes very little horsepower, so very little fuel. An underpowered fuel system will cause problems when the engine is making power under load.
A bad/weak fuel pump or clogged fuel filter that will not deliver enough fuel to free rev to 5K, will certainly not provide enough fuel to run the car to 5k under load. Also most likely it would cause the engine to just not go above 5k and mss fire such. The car hits 5k, and the engine shuts down till it hits around 4600-4800, then runs normal up to 5k, and shuts out again. No missfiring along the way.

 

[dastang2]

you will never find the problem without trying suggestions first. strange things happen at times.