SOLVED

So, the very last problem with my stock 305 TPI, after doing everything but the cam, lifters, pistons and crank, the engine runs like brand new. INTs and BLMs are good engine pops nicely but...

at more than about 3/4 throttle the engine just stumbles and stops putting out power.

Two conditions: 3/4 throttle or more and above 3900 RPM-"ish".

It is consistent. For example, with my 3:42's in third I am going about 75 mph and at about 3600 RPM. I start to apply throttle and right around 3900 RPM-"ish", and 3/4 throttle-"ish" if I push the pedal down a little more its starts to stumble. Let up a hair and it stops, push down a hair again and it stumbles.

I can make it come and go. If I push down further it keeps stumbling though one time I kept bobbing the pedal up and down between 3/4 and full rapidly (pumping the throttle) and it acted like it wanted to clear the stumble but never did.

I came home, checked the TPS voltage (a new BWD). 0.54V to a solid, steady 4.12 V at the top. Solid at every point.

I have no idea. I did get a couple of weird codes though Battery Voltage High (it wasn't) and MAF low (it wasn't). Thinking they are flukes.

 

Ignition coil. Something is not sparking under load.

i had the exact symptoms that u described, it was coil in my case.

 

Crappy MAFs cause a bunch of problems as well..find a fellow tpi car and borrow his maf real quick see if fixes it

 

How do you know these are flukes?

 

New Wells MAF. Reading have been solid between 8 and up.

New AC Delco alternator. New Duracell AGM 75 battery.

New AC Delco Ignition Coil.

If it was the coil, it would miss under heavy throttle at low RPM also, it doesn't.

The high voltage wouldn't cause a misfire and neither would a bad MAF reading under those conditions.

The fact that I can consistently make the stumble start and stop with a slight move of the pedal down or up, and only above 3900 RPM, under load, tells me it is isn't an intermittent.

I can also rev it in neutral and park, up to whatever and no miss.

Something wacky is going on.

 

Agreed something wacky is going on, so we need to think a little wacky too... lol...

High battery voltage will drive the ECM to start tailoring the injector pulse widths to something different in the injector offset table. A value that may result in a consistent lean-out at that rpm and load condition.

What about your engine grounds? A bad engine ground can easily result in lower spark energy, again resulting in a consistent lean out at that rpm and load condition.

Do you have any scan data you could send me? Ultm8z@yahoo.com

 

I'm good at thinking wacky. Comes natural for me.

Thanks, I will get some. My scan app has been buggy.

I was thinking ground too.

In fact I am planning a major grounding campaign. Just going to slap a bunch of #12 THHN black all over the place with self-tapping screws. Block to chassis, head to chassis, manifolds to chassis, ECM to chassis, chassis to battery.

 

Can never have too many grounds IMO.

My bad experience with grounds includes forgetting to hook up an engine ground wire and the electrical system subsequently using the transmission shifter cable as the main engine ground (and me not figuring out why the internal cable sheath kept melting and seizing up the cable because the cable was getting hot!)…. Lol. Went through 3 cables before I figured it out (the last one was wrapped in almost 1/2” thick insulation from me thinking it was the exhaust heat).

As of now, I have grounds from engine to chassis, engine to battery, and chassis to battery. I also have a ground going from one cylinder head to the other (since main engine ground is only connected to one head). Believe it or not, I also have a small braided ground strap going from the distributor housing to the back of the cylinder head. Since I have coil-in-cap, I wanted my coil ground wire to have a good path back to ground as well.

Lastly, I hooked up a ground strap from both the NB and WB O2 sensor bodies to the back of the cylinder heads as well. Basically took a braided ground strap and used a hose clamp to secure it to the sensor bodies. I didn't trust the header tubes, or the header bolts lubed with anti-sieze, and the somewhat rusty O2 sensor bungs to be a good ground path. With this mod, I actually had to slightly retune the VE tables (there must have been a significant enough voltage drop through the headers (prior to the ground strap addition) that the ECM was getting a faulty signal from the NB O2 sensor).


Have you tried Tunerpro?

 

Wacky minds think alike.

I was going to mention I am inclined to do the distributor housing, the O2 sensor housing and the ECM housing.

Now I will for sure. My manifolds are old and weathered. I do not trust the bolts to carry a quality ground through. Same with the distributor. If the ground wire is shaky, then it has to go through the housing to the cam or the clamp bolt, etc.

I may just tap the ICM ground wire (ckt 453 blk/red in my case) and drop it right onto the chassis nearby...

 

Yeah, the ignition output from the coil is not DC voltage, so simply hooking up an ohmmetrr to verify the ground won't take into account any inductive impedance that a voltage at frequency would encounter.

The higher the rpm the higher the frequency. Maybe somehow you're reaching "critical mass" at 4000 rpm? Impedance gets too high for sufficient spark energy to reach the plugs. Something you may not see in neutral since "sufficient" would be defined by the load condition.

 

So I swapped out my ECM (Delphi 16198259 APYY 3529) with a backup 87 Auto TPI 305 Delco 1227165 (Delco ABUS 8534).

I broke my rule and also added an ECM case ground wire to the chassis at the same time so I'm not sure exactly what solved it.

High RPM missing is gone. Revs solid to redline.

Powertrain runs great.

Thanks for all the help everyone.

 

Great news. If you were really curious, remove the ground wire and see if the problem returns.

 

I am but I'd like to leave well enough alone for a couple weeks. Once the car is proven, then I'll lift that wire...