Engine '91 5.0 Pop & Sputters Around 4k Rpm

Knowbody

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Sep 4, 2016
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The other day I finally took the car out for a spin around the neighborhood after a 2 month hiatus. In those 2 months I did a semi wire tuck on the drivers side, relocated the battery to the trunk, ran all new 1ga cable, added 1ga grounds, with extra grounds all over. All wire terminals are brass with heat shrink tubing. I wired in a MSD 2-Step module to the existing 6AL ignition, added a fuel pressure gauge on the cowl, and new aftermarket gauges under the radio. Other than that, nothing else was touched or has been altered.

The car starts right up, idles fine, everything is working and I have good voltage everywhere. I take it out for a spin after it's warmed up, and it does fine under normal driving. If you were just driving it to work in the morning you wouldn't know there was any issue. But when I stab the throttle, it starts breaking up at around 4k rpm. It's a sudden power loss, spitting and popping. The fuel pressure gauge doesn't spike or drop more than it should. It's almost like it's hitting a rev limiter, but it still pulls through it. This happens when I'm just sitting in the driveway too, not only under a load. Cold or warm, it doesn't matter.

At first I thought it was something to do with the MSD stuff, so I swapped pills in the two step and it's definitely working fine. I then bypassed the 6AL box altogether and the hesitation is still there. I swapped the MSD ignition coil since I have a new one, and no change. Vacuum is good, no leaks.

So today I swapped in new spark plugs, checked TPS, readjusted the timing, swapped MAF for a spare I have... No changes. I played with the FPR with no change. All the old plugs looked normal, and I pulled the new ones after half an hour of driving and they looked fine.

The gas is 93 octane from 2 months ago, brand new fuel filter, 255 fuel pump. Car is NA 5.0 with H/C/I, long tubes and 3" exhaust, 65mm TB, stock injectors and stock A9L with a 5-spd. Car ran great before I did the wiring cleanup and battery relocation. I'm confident in my work and don't believe any of those things I did are the culprit. This seems like fuel or spark. I know the injectors are maxed out and it needs a tune, but it's about to get boosted with standalone, so I didn't bother. The car ran perfect and clicked off low 12's on drag radials 2 months ago.

Any ideas? Thanks
 
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Knowbody

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I'll check that voltage tomorrow. I just swapped the coil for another I had and didn't think about voltage TO the coil. All the grounds throughout are brand new. Battery to chassis, battery to block, chassis to block, with plenty of extra grounds all over. The relocated wiring behind the fender has a new ground stud, which is also grounded to the block, stripped to bare metal and new connectors on each wire. I went overkill with grounds.

When I mash the pedal the car pulls hard, like it always has, until around 4k. It also does it when free revving, and even increasing the rpms slowly. I know usually when TFI modules go bad, there is a noticeable idle issue and power loss down low in the rpms, but I'm not ruling out the TFI module just yet.
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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Crazy as this sounds (could'n resist) did you run codes? Confidence in workmanship is great, but crap happens, gets shook loose, don't know what you got, how long you had it or if you did the h/c/i. What makes you think your injectors are max'd out? Why do you need a tune?
Just curious, I'm just learn'n so I can impress my bow ty friends.
 

jrichker

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High speed miss on a warm engine

Revised 7 Nov-2016 to add PIP sensor as possible problem and dumping the codes to help determine if it is the TFI or PIP.

The TFI module mounted on the distributor is one of the culprits for a high speed miss on a warm engine. The other suspect is the PIP sensor inside the distributor. If the problem does not occur when the engine is cold, the TFI module or PIP is definitely suspect. Dumping the codes may help determine which one it is.

You may need a special socket to remove the TFI module, but most auto parts stores will have one for $5-$7.

Be sure to use plenty of the heat sink grease on the new TFI and clean the old grease off the distributor.



See Automotive Tools Specialty | Auto Mechanic & Technician Diagnostic, Testing Equipment | Thexton


diagram courtesy of Tmoss & Stang&2Birds




PIP Sensor functionality, testing and replacement:

Revised 17-Jul-2014 to add check for loose PIP mounting screws

The PIP is a Hall Effect magnetic sensor that triggers the TFI and injectors. There is a shutter wheel alternately covers and uncovers a fixed magnet as it rotates. The change in the magnetic field triggers the sensor. A failing PIP sensor will often set code 14 in the computer. They are often heat sensitive, increasing the failure rate as the temperature increases.

Some simple checks to do before replacing the PIP sensor or distributor:
You will need a Multimeter or DVM with good batteries: test or replace them before you get started.. You may also need some extra 16-18 gauge wire to extend the length of the meter’s test leads.
Visual check first: look for chaffed or damaged wiring and loose connector pins in the TFI harness connector.
Check the PIP mounting screws to see if they are loose.
Check the IDM wiring – dark green/yellow wire from the TFI module to pin 4 on the computer. There is a 22K Ohm resistor in the wiring between the TFI and the computer. Use an ohmmeter to measure the wire resistance from the TFI to the computer. You should see 22,000 ohms +/- 10%.
Check the PIP wiring - dark blue from the TFI module to pin 56 on the computer. Use an ohmmeter to measure the wire resistance from the TFI to the computer. You should see 0.2-1.5 ohms.
Check the SPOUT wiring – yellow/lt green from the TFI module to pin 36 on the computer. Use an ohmmeter to measure the wire resistance from the TFI to the computer. You should see 0.2-1.5 ohms.
Check the black/orange wire from the TFI module to pin 16 on the computer. Use an ohmmeter to measure the wire resistance from the TFI to the computer. You should see 0.2-1.5 ohms.
Check the red/green wire; it should have a steady 12-13 volts with the ignition switch on and the engine not running.
Check the red/blue wire; it should have a steady 12-13 volts with the ignition switch in Start and the engine not running. Watch out for the fan blades when you do this test, since the engine will be cranking.
If you do not find any chaffed or broken wires, high resistance connections or loose pins in the wiring harness, replace the PIP sensor or the distributor.

The PIP sensor is mounted in the bottom of the distributor under the shutter wheel. In stock Ford distributors, you have to press the gear off the distributor shaft to get access to it to replace it.

To remove the gear, first you drive out the roll pin that secures the gear to the shaft. Then you get to press the gear off with a hydraulic press or puller. When you go to press the gear back on, it has to be perfectly lined up with the hole in the gear and shaft. I have been told that the hole for the pin is offset slightly from center and may require some extra examination to get it lined up correctly.

Most guys just end up replacing the distributor with a remanufactured unit for about $75 exchange
PIP problems & diagnostic info
Spark with the SPOUT out, but not with the SPOUT in suggests a PIP problem. The PIP signal level needs to be above 6.5 volts to trigger the computer, but only needs to be 5.75 volts to trigger the TFI module. Hence with a weak PIP signal, you could get spark but no injector pulse. You will need an oscilloscope or graphing DVM to measure the output voltage since it is not a straight DC voltage.

See http://www.wellsmfgcorp.com/pdf/counterp_v8_i2_2004.pdf and http://www.wellsmfgcorp.com/pdf/counterp_v8_i3_2004.pdf for verification of this little detail from Wells, a manufacturer of TFI modules and ignition system products.
 
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Knowbody

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Haven't had a chance to mess with the car since my last post. Lots of rain and overtime have kept me from doing anything but thinking about the issue. I'm going to recheck the ECM ground that I relocated, to make sure I didn't overlook something. Thanks for the replies, I'll post back when I've got it sorted out.
 

Knowbody

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Forgot to post what I found. Turns out the MSD box is bad. I'm going to pack it up and ship it to MSD for repair.

When I bypassed the MSD box, it ran perfect and revved clean all the way through to 6k RPM.
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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Good catch, I knew it all the time but I didn't want to show up anybody that is actually smarter than me.
Not really, I don't know nuth'n, or is that anything?
Once again, I don't know.
Just keep us posted and thanks for the update.
 

SF Lex

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Crazy as this sounds (could'n resist) did you run codes? Confidence in workmanship is great, but crap happens, gets shook loose, don't know what you got, how long you had it or if you did the h/c/i. What makes you think your injectors are max'd out? Why do you need a tune?
Just curious, I'm just learn'n so I can impress my bow ty friends.
How much do you think it can run at a shop to get this done ?
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
18,570
6,095
193
polk county florida
How much do you think it can run at a shop to get this done ?
If your ask'n about running the codes there is a sticky on how to do it.
Shop time on diagnostics very, some will charge a flat fee, some a one hr minimum. 50 to 100 bucks. Thats to run codes. That said most 'modern' shops don't know how 20+ year old computers work so find a shop that can handel it although you can do it for free and post you codes here and get help. And likely better advice on how to fix it.
 
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