High RPM miss - Ford Mustang Forums : Corral.net Mustang Forum
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post #1 of 14 Old 09-11-2019, 06:41 PM Thread Starter
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High RPM miss

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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post #2 of 14 Old 09-11-2019, 08:29 PM Thread Starter
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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.

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post #3 of 14 Old 09-11-2019, 09:29 PM
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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?

89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.
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post #4 of 14 Old Yesterday, 07:32 AM
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check the injector harness ground on the back of the drivers side head.
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post #5 of 14 Old Yesterday, 08:43 AM Thread Starter
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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.
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post #6 of 14 Old Yesterday, 09:49 AM
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an ohm test will be useless

measure the voltage at the ecm while the car is running

compare it to bank 1, that you say is working
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89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.
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post #7 of 14 Old Yesterday, 02:30 PM
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No code or CEL when you unplugged the MAF?

'92 GT--YSI 2.95/8, Bennet Racing A4 347, TEA ported TW Heads, TFS-R Intake, TFS Stage 2 Cam, 65 TB, 72's, 3.55, C4, 275/60 MT DR's-Gone but not forgotten
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post #8 of 14 Old Yesterday, 04:07 PM Thread Starter
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[QUOTE=BigDumbRedneck;18585932]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.
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post #9 of 14 Old Yesterday, 05:32 PM
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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?

89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.
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post #10 of 14 Old Yesterday, 09:02 PM Thread Starter
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Quote:
Originally Posted by indy2000 View Post
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?
Thanks, I'll check tomorrow when I get home from work and let you know.

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post #11 of 14 Old Today, 05:53 PM Thread Starter
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Ok at idle the o2 are still bouncing between the numbers I posted above and when I hit the throttle the voltage drops for a second to .002 or .003 it's hard to tell. Both side do the same thing and at the same frequency.
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post #12 of 14 Old Today, 07:29 PM
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that is lean

they should go to close to 1 volt and stay there for a few seconds

look to fuel pressure

89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.
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post #13 of 14 Old Today, 09:57 PM Thread Starter
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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.
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post #14 of 14 Old Today, 10:06 PM
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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

89 GT gr-40, KB2200(10psi), tweecer R/T,LC-1, 22gal cell, w/tailpipes.
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