Svo gurus-- help !

[Rickrey]

I have a high speed miss(above 4500 RPM) that has appeared-- previously, no miss.I had removed my turbo and had it rebuilt. Before rebuild max boost was about 8psi in the "Regular" position and 12psi in the "Premium" position. The rebuilt and upgraded turbo produces 12psi and 15-17 psi.A few days after reinstalling the rebuilt turbo, the car started missing above 4500 RPM. There is also an occasional backfire through the exhaust at high speed. Absolutely no problem at lower RPM-- even at full boost.The miss is like cutting the ignition off and rapidly back on-- not like detonation. The tach reading rapidly drops a couple hundred RPM then recovers.I thought, well, the higher cylinder pressure is quenching the spark or the stock ignition is not adequate. I replaced the coil and plug wires(MSD) dist. cap, rotor and spark plugs-- absolutely no change.I reduced the ignition timing and increased fuel pressure-- no help.I have an air/fuel gauge which indicates good fuel delivery. There is no indication of head gasket failure.I had the car checked on one of the old Ford diagnostic machines. This showed a code for the BAP sensor. This was replaced with a NOS Motorcraft unit-- no help.I changed the fuel filter--no help.I unplugged the knock sensor--no help.Last-- the high speed miss is present also in the "Regular" mode-- decreased boost and ignition timing and under no load/boost, as in gently accelerating downhill.One more clue-- there is no miss ever on a rough road or over bumps-- as in a loose connection.Could this be in the distributor? The shaft seems tight. Any ideas ??Thanks

[Meaty]

It's unclear to me, are you running the ford Thick film module on the distributor? I have had multiple WEIRD misses with a bad TFI module.

[Rickrey]

Yes, it's the stock SVO distributor. I have been told if the distributor module fails, it fails completely -- doesn't run. Is this not true ?

[Laredo]

I have a high speed miss(above 4500 RPM) ...

Is the miss just RPM related or is it also affected by load (i.e. does it miss only under boost)?

... increased fuel pressure-- no help...

You need to check / confirm adequate fuel pressure WHILE under load DURING the misfire condition.

[Rickrey]

It's RPM related. It misses in the Primium mode, the Regular mode (reduced boost and ignition timing) AND under light load/no boost-- all above 4500-5000 RPM.Second part-- my fuel/air gauge shows a consistent rich condition during the missing.It goes "Puuh-- Puuh", not like lean or detonation.

[svono50]

With the rebuilt turbo you are now pushing a lot more air into the motor, even at the same boost levels the CFM has likely increased quite a bit. If the ignition isn't dropping out/stalling after some heat soak, I think you can rule out the TFI and PIP in the dizzy. What spark plug gap are you running? Could be blowing out the flame, which would give a rich condition misfire. I run at 0.030", just a little under OEM specs and also make sure you are using 'cheap' copper plugs, none of the fancy platinum/iridium stuff as it doesn't play well with our ignition system. I would also consider the ignition switch in the column as a potential source for the ignition dropout as you explained it (i.e.- tach drop). I know a rough road doesn't create a problem, but the higher frequency of our engines above 4000rpm could be sending a decent vibration up the steering column to the switch. Keep plugging at it and you will find the root cause.

[gbeaird]

And if you had Motorcraft wires on it, and still have them, put them back on. If not, consider replacing the MSD with Motorcraft wires. The fancy-shmancy 'race' wires have been known to cause issues like you are having. Not likely to correct the issues, since you have the same before and after, but one never knows.

The plugs and gap are important, too.

[Rickrey]

Thanks Ted and Gene-- I did widen the gap a bit with the MSD coil and wires. I'll go back to 0.030.Is there a safe way(no damage to the ECU) to bypass/parallel the ignition switch-- direct from battery to distributor module ?

[svono50]

Thanks Ted and Gene-- I did widen the gap a bit with the MSD coil and wires. I'll go back to 0.030.Is there a safe way(no damage to the ECU) to bypass/parallel the ignition switch-- direct from battery to distributor module ?

Not sure, never looked into it. I hope someone on here would have a quick answer, but I will have to dig a bit to see.

[SVOeric]

my advice, throw away the MSD wires, go back to stock motorcraft.
I've tried $100 taylor, accell, etc, never found anything that worked better than stock motorcraft wires
and they are less than $30 a set.

expensive does not mean better.

I did a dyno session one year, Stock Coil VS MSD coil (which i still run today)
ZERO HP difference.

[Rickrey]

Gentleman-- Let's review and I will add a couple more bits of information. When I was removing the turbo, I pulled the plug wires-- in doing this one of the metal plug connector clips pulled out of the plug wire. So, in putting it all back together I installed a new set of Motorcraft wires and a new set of Autolite plugs. With these new wires and plugs, the high speed miss was present. Again, when ,subsequently, I replaced the coil, plug wires(MSD), distributor cap, rotor and plugs-- this time NGK V plugs-- I got EXACTLY the same miss. No difference whatsoever. Sooo, let's not consider plugs or wires anymore. It's unlikely they are at fault. Also, remember that the high speed miss seems wholly RPM related-- not due to load or boost. One big clue, I think, is the coinciding sudden drop in RPM -- shown on the tach --with each individual miss. I have the suggestion of the ignition switch. Anything else ? Again, could anything in the distributor cause this ?Thanks

[gbeaird]

How old is the gas?

When you reinstalled the updated turbo, you went back with the OEM inlet and outlet hoses?

[Rickrey]

Gas is good/new. Inlet is stock. Outlet -- to intercooler-- is not stock, but good quality and properly sized.

[DRAGPAK]

Have you checked out the vam ? Get a code reader and run the codes on it .

[Rickrey]

When checked three weeks ago, the only code shown was for the MAP/BAP sensor. I will clean the connectors to the VAM and distributor-- see what this does. I know I keep harping on the coinciding sudden RPM drop (shown on tach)-- like cutting the key off. I can't help but think that there is an electronic/electrical source that is common to the miss and sudden tach drop. Anyone know where the tach feed comes from ?