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Discussion Starter · #1 ·
Hey everyone, been a lurker for a while but I need some fresh ideas on this one. Cliffnotes...

03 Cobra
Built motor (9.2-1CR)
ported heads, cams
4.0L Whipple
123mm slot style maf
Full return fuel system, 80lb injectors
Headers, full offroad exhaust
all kinds of other junk that doesn't matter lol.

Car was previously running a 2.3 whipple on a stock motor which let go. I have very little prior history on the car before it showed up at my shop other than that alot of the previous work including the tune was poorly done.

Anyway, this car has a very serious case of spark blowout. Currently, the car is pullied for about 20lbs, running BR7EF plugs, stock coils. On the typical setup, 0.028" gap, the car breaks up bad under load. No load it will pull to redline, and on the highway it runs like a champ. As soon as load hits about 120% it breaks up bad and you can not drive through it. Gap the plugs down to 0.024" and it gets marginally better. I have had the car on the dyno, and have probably hours of logs, and the wierd thing is, no codes, A/F is good (or at least good enough that it should pull), pull the plugs and they look perfect, no vacuum/boost leaks, nothing that just jumps out.

Things I have tried: Swapped plugs, different gaps, etc etc. Load tested all coil circuits and tested pin fit on all connectors. Checked all grounds (PCM, chassis, engine, etc). Ran some auxilary grounds from the battery to the chassis. New alternator and battery. Swapped in different sensors such as maf, tp, ckp.

So, as a last ditch effort, I installed a boost-a-spark (which IMO shouldn't be needed). With the BAS cranked to 50%, the breakup/blowout is about 90% gone. It now only breaks up when it initially hits heavy load, about 3000-4000rpm, but you can now drive through it and it will continue on to 6000rpm (and WOW does it continue on LOL).

So, I still have some sort of an issue here. I have built plenty of these cars, and this one is just driving me nuts. I had access to some OEM equipment so I took a look at the mode 6 data. All cylinders are registering missfires, some worse than others, but all are. NO CODES. Power balance is all over the map under load.

What do you guys think? Possibly a fuel delivery issue that is making the spark issue worse? Maybe these FRPP 80's are just spraying like crap? The car idles quite well considering, short term trims are within 2-3%. Adjusting base pressure makes no difference, trims respond to DC changes, I don't know lol. PCM issue?

Sorry for the long post, but I am at a loss other than just continuing to throw parts at this thing (which I hate doing). Any input would be muchly appreciated.

Thanks,

Mike
 

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What's the voltage at WOT when it's breaking up? The BAS can band-aid a low voltage issue to a point, but may have trouble boosting voltage if it's low to begin with. I know you said you replaced the alternator and battery, but you didn't mention voltage (unless I missed it).
 

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Discussion Starter · #3 ·
Logged voltage (livelink) never drops below 13.4. With the car at idle, voltage to the coils is 13.5-13.8. Load tested the coil circuits which pass. If i get time today, i am going to tie my DVOM directly into one of the coils and road test it to see if I get a voltage drop. Maybe something is drawing at heavy load (fuel pumps?) and I am losing some voltage to the coils. I completely agree though, the BAS is band-aiding a bigger issue. I have never had to rely on a BAS sub 800rwhp. The stock system should be more than enough for the setup this car currently has.

This car has never had a 2-step or wot box, and the engine harness was unmolested when I had it apart other than a splice in the clean tach takeout which is now gone.
 

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Interesting the A/F looks good with so much spark blow out. I would expect it to look lean due to the unburned fuel (and the resulting excess o2). Did you check the A/F again after installing the BAS? Are you running the Hobbs switch on the BAS or full time?
 

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Mine needed a BAS and after market coils at 24psi. The motor just didnt like the stock coil with that much power(750whp on MD).
 

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It has never made sense to me that one car has issues with blowout, and another doesn't.

My first thoughts are to verify grounds and the RF noise connections on the back of the head are all as they should be. A while back, I had an issue that was a pain in the ass to diagnose. The car would idle fine, putt around with easy driving fine, but the second you went WOT, the car would just stop. After PCM swaps, COP swaps, etc. I found that the crank position sensor connection was goofed. It's a weather pack-like connector off the engine harness, and somehow one of the metal "spades" had pushed out of the connector just enough to lose connection during hard acceleration. It was dumb luck that I found it because you could barely tell, but once I saw it I knew that was the problem. So check your crank and cam sensor connections well.

I ended up having to drop to a NGK 8 gapped at .020 to cleanly pull on the dyno. You really sound like you've covered your bases well, I'd try a plug swap after ensuring all grounds and connections are good.
 

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Discussion Starter · #7 ·
^^^Yeah, I agree about the blowout thing. I have ran accross some silly things in my day, and i know it will end up being something completely stupid like a pushed out pin. I have completely gone through the engine harness already though, as well as the main PCM connector, so who knows.

As for RF, the supressors are there and good, I have done a ripple test on the new alternator, all shielded wiring and grounds check out. I spent enough time with FOMOCO to know about the joys of RF on a cop engine.

As for the BAS, it is wired full on, and I have not had it back on the dyno again since. Before, the A/F would lean out to 17-19 during breakup, but if I pedaled through it and kept in throttle just enough to trigger wot fueling but not the breakup, A/F was steady 11.5. Every time we leaned it out, or added fuel, wot A/F would respond as it should. That said, with the bas in now, my LM1 still reads a steady mid 11 A/F, but farts all over the place during breakup as I would expect an LM1 to do.

Here is a few graphs showing what happens. The rest of the pull is about 2/3 throttle just at the boost threshold.

Rectangle Slope Plot Font Line
 
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