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High RPM miss

1K views 15 replies 5 participants last post by  obrut 
#1 ·
I have a 1989 mustang 5.0 5 speed that falls on its face at 5000 rpms. Timing set to 12 degrees, new autolite 25s, new coil, new ford wires, new cap rotor, new TFI, new O2 sensors.

Passes engine balance test. Stock except for headers exhaust, air filter no air tub or emission stuff except for charcoal canister. No koeo code, and koer I get 84 for no egr.

I've check the tps and it's fine .90volts at idle. Car starts right up, idles nice at about 7-800 rpm and pulls strong all the way to 5000rpm and then it falls on it face.

Fuel pressure is 32 and 40 and it's at 40 at 5000 rpms when I have problems. I tried unplugging the maf and then starting it and going for a ride and I have the same problem.

I've check and cleaned the grounds and a added two extra grounds from the engine to the body.

I'm running 93 octane, plugs gap is .05 and I cleaned the maf wire.

What else can I check?
 
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#2 ·
Just ran codes again
koeo 34 and 84
Koer 4 then 91 and 34
Engine balance showed 9

So code 91 is O2 sensor, it's a new sensor and I replaced both o2s because I had code 91 before. I cleared the codes and went for a drive after replacing them and no code 91.

Tonight is the first time I ran the codes after driving it for a week. After I ran the codes I shut the car off, pulled the test jumper and unplugged the o2 sensors. I went for another ride and same problem, falls on its face at 5k.
 
#3 ·
code 91 does not mean change the sensor

the code means the 02 is reporting low voltage, and is not switching to high voltage, even if the ecm adds more fuel

think to yourself

how does that voltage get from the 02 sensor all the way to the ecm?
 
#5 ·
Injector harness ground is clean and tight. Cleaned and open pins on the salt and pepper connectors too. Fuel filter is new too.

I'm going to ohm out the wires in the o2 harness today and make sure I don't have high resistance or an open on any of the wires.

I did a bunch of searches and tried a bunch of stuff before making my post checking and testing what I could.

It happens when the car is warm or cold and at wot or part throttle.

If I disconnect the maf then start it and try it does the ecm reference a table for the maf readings? I did this and had the same problem but does it eliminate the maf?

I searched on the PIP but the symptoms don't match my problem at all.

No codes for the BAP sensor but the problem reminds me of a time when I had a bad map sensor on another car.
 
#7 ·
No code or CEL when you unplugged the MAF?
 
#8 ·
No code or CEL when you unplugged the MAF?[/QUOTE

Yes I think the CEL was on when I went for a ride. I pulled the battery to reset it after I went for the ride and hooked the maf back up.

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.
 
#9 ·
LOOK AT THE VOLTAGES AGAIN

This time while at idle, snap the throttle to WOT, what do the voltages do at the snap?

Are the voltages swinging at the same frequency on both sides during the test you did?
 
#13 ·
Fuel pressure looks great and it holds pressure for at least an hour when I shut it off. Maybe I didn't do the o2 test right. I'll try again tomorrow. When I snap the throttle should the o2 go immediately rich, above .6 volts? I think I snapped the throttle while looking at the tach then I looked at my meter.
 
#14 ·
has to go to near 1volt

if not its fuel problem

if your telling me you may not have done the test correctly, then that would be a problem


diagnosis relies on proper tests
 
#15 ·
Have you gotten the motor hot recently? I had the same problem on a 1968 coupe. The recently built n/a 289 was a screamer that would zing to 6000 easily. When I took it on a road trip for the first time the car ran a little on the high side of the operating range the entire trip. It never got HOT enough for me to feel the need to pull over, just a little high. When I got home I did a radiator flush and that fixed the problem of running hot. However it would just start to fall on its face at 5000 and wouldn't crank any harder than 5800. I checked everything over and over, and took it to several different shops, and couldn't find the problem. I finally found a very knowledgeable mechanic that told me that I had taken the temper out of the rings by running it for such a long time with the temp being on the high side. I yanked the motor back out. Did a re-ring and the problem was solved. Ran great till I got rid of it.

Good luck with whatever is going on. I know how frustrating things can be.
 
#16 ·
Thanks for the post, never overheated or got it hot but who knows what the previous owner did. I still haven't gone back to check the o2 volts. I did do a little more reading and I personally think my issue is valve float. I should have time to measure the o2 volts this week just to be sure those are right.
 
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