I've got my 94 miata running with a 89 mustang 5.0 and 5 speed and I've been driving it around a little. I've put about 200 miles on it and so far everything is good. The only thing is it falls on its face at 5000 rpm. I adjusted the factory tach to match the aftermarket tach at 2000 rpm but I wanted to know how far the tach is off at 6000 rpm. Am I hitting the rev limiter when the tach is at 5000?

 

My 88 Mustang does the same thing. Mine feel like low fuel pressure, but I have not put a gauge on it to see.

 

I check fuel pressure and I have 32 and 40 and I have 40 psi at 5000rpm when it falls on its face.
My car has new
Autolite 25s gap is .050
New ford wires
New cap and rotor
New coil
New o2 sensors
New TFI
Two extra engine to chassis grounds
HeGO ground is clean and tight
Ground from the battery all the way to the bell housing
Ten pins cleaned, spread the pins
Timing set to 10 with the spout out
Starts, idles and runs great, pulls hard up to 5000 and then it wont rev. Does it warm or cold and at wot or part throttle.

 

o2 voltage test
pin 43 I get .15 -.8 and its bouncing around between the two numbers
pin 29 I get .07 - .8 and its the same bouncing around

This was with the car fully warmed up and at idle

I also tested free reving it (I never liked having to do this) and it still cuts out at 5000 rpm. I pulled the ecm and removed the cover. The ecm looks great, caps are in good shape, no swelling or leaking and I did not see any burned traces.

 

It's generally believed that the factory ECU has a 6250 rpm rev limiter built in. So there's that, it could have shifted somewhat over the last 30 years. You might check the accuracy of both tachs with a known good. Have you considered valve float? Not sure how to test that, but again, you're dealing with 30 year old technology and hardware here. If it makes you feel any better, I have a 93 Tbird motor that does the same thing, the auto trans is programmed to shift at 5000, sometimes the stutter occurs first, sometimes not. It has Megasquirt with a 6000 rpm limiter. Go figure. Be sure to let us know if you find a definite answer.

 

are you rocking the efi large dist. cap or the early small one. I had it happen to me with the small diameter old school cap and my tbird did the same in the 1/8th mile with the large cap till I removed the rubber weather shield and it all cleared up. Was told it may have been an O3 buildup when the weather shield is left installed??. Worked for me, Good Luck!!

 

I'm using the stock 89 EFI distributor but thanks for the idea anyway.

I pulled the fuel pump the other day just to check the inside of the tank and I notice the rubber isolator on the pump looked like it melted and was all over the pickup screen.


The car has 187k so probably getting close to the time for a new one anyway. So I bought a new 94 miata denso replacement because I don't want a noisy aftermarket pump. I drained and cleaned the gas tank and changed the pump and this didn't fix my problem but I wasn't going to drive any more after seeing how bad the pump looked.

After some more searching and reading I think it might be valve float. The symptoms of valve float are spot on to my problem. My car runs awesome, starts right up hot or cold and is smooth with no bucking or surging. You wouldn't know anything was wrong until you rev it up to 5K.