360 stroked to 408. Running issues.

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Jared Adamache

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Hey guys,

I recently had boughten a 1972 demon 340 coupe. It has the blue print 408 stroker.
Keith black forged pistons
Hughes 4248S cam
Scat cast crank
Melling High volume oil pump
Aluminum edelbrock performer heads
Ceramic TTI headers with 3” TTI exhaust
Edelbrock air gap intake
Holley sniper efi.
It ran absolutely perfect when I bought it, insane throttle response tons of power. Now the issues are it’s having trouble meeting the target air fuel ratio on the controller. (Efi issues I know) and then when idleing the engine shakes bad. I have tried having the idle anywhere from 800-1000 to try and correct it and it still shakes bad. But settles down when cruising. Any one have any advice. Definatly scrapping the Holley sniper and getting a Holley 750 dbl as I can’t seem to figure out the afr issues with it. Holley tech seems to think it’s wiring but the thing is absolutely clean and have checked all wiring multiple times.
 
Jared Adamache said:
Hey guys,

I recently had boughten a 1972 demon 340 coupe. It has the blue print 408 stroker.
Keith black forged pistons
Hughes 4248S cam
Scat cast crank
Melling High volume oil pump
Aluminum edelbrock performer heads
Ceramic TTI headers with 3” TTI exhaust
Edelbrock air gap intake
Holley sniper efi.
It ran absolutely perfect when I bought it, insane throttle response tons of power. Now the issues are it’s having trouble meeting the target air fuel ratio on the controller. (Efi issues I know) and then when idleing the engine shakes bad. I have tried having the idle anywhere from 800-1000 to try and correct it and it still shakes bad. But settles down when cruising. Any one have any advice. Definatly scrapping the Holley sniper and getting a Holley 750 dbl as I can’t seem to figure out the afr issues with it. Holley tech seems to think it’s wiring but the thing is absolutely clean and have checked all wiring multiple times.
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Is it rich or lean on the air/fuel ratio? Does it shake badly cold, warm, or both? Have you made sure the grounds are good, verified there are not bald wires that could be grounding and that the connections are good and tight? Just a few questions that could help us answer your questions.
 
RammerJammer75 said:
Is it rich or lean on the air/fuel ratio? Does it shake badly cold, warm, or both? Have you made sure the grounds are good, verified there are not bald wires that could be grounding and that the connections are good and tight? Just a few questions that could help us answer your questions.
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Yes sir checked it all. The air fuel will be on target at idle, but when gearing down to come to a complete stop it’ll spike to like 35, then down to 7,8,9. Wiring is all sound.
RammerJammer75 said:
Is it rich or lean on the air/fuel ratio? Does it shake badly cold, warm, or both? Have you made sure the grounds are good, verified there are not bald wires that could be grounding and that the connections are good and tight? Just a few questions that could help us answer your questions.
Click to expand...

All wiring is sound. it runs and idles fine. It’s when I’m starting to slow down to stop it struggles to stay running. The afr will jump to 35 and stay there for a bit and then down to like 7,8,9. Fuel system is all good as well.
 
Jared Adamache said:
Yes sir checked it all. The air fuel will be on target at idle, but when gearing down to come to a complete stop it’ll spike to like 35, then down to 7,8,9. Wiring is all sound.


All wiring is sound. it runs and idles fine. It’s when I’m starting to slow down to stop it struggles to stay running. The afr will jump to 35 and stay there for a bit and then down to like 7,8,9. Fuel system is all good as well.
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Is it a manual car? If so, the reason the afr is lean on decel is you're off the accelerator, the tps shows the engine is at idle and therefore it only receives idle fuel amounts, even though the rpm isn't at idle. If it's an auto and the engine isn't at idle on decel that could be the reason as well. A carburetor won't fix that issue. If the afr is at 9:1, that's pig rich and needs to be fixed immediately. Have you checked the plugs to see if they're fuel fouled?
 
RammerJammer75 said:
Is it a manual car? If so, the reason the afr is lean on decel is you're off the accelerator, the tps shows the engine is at idle and therefore it only receives idle fuel amounts, even though the rpm isn't at idle. If it's an auto and the engine isn't at idle on decel that could be the reason as well. A carburetor won't fix that issue. If the afr is at 9:1, that's pig rich and needs to be fixed immediately. Have you checked the plugs to see if they're fuel fouled?
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yes plugs were fouled and just changed them yesterday and it still ran like ****. Previous plugs were gapped at .027 I gapped new ones to .040. I just drove it to storage though so maybe it wasn’t enough time for it to relearn. Transmission is a Manuel valve body, no kick down. I wasn’t checking the afr when I first got it because it ran amazing. 0 issues then one day it just started to run like garbage
 
As long as the afr is that rich it will continue to foul plugs. I know they have a leaning phase, so you may want to reset it and try to do a relearn. It could be an o2 issue. I'm sure the Holley has a DTC monitor that you can view any DTCs.
 
The holley shouldn't make it shake really bad like your saying. That sounds like an ignition issue to me. Its misfiring and not burning the fuel, the ECU sees that and tries to lean it out.

What is your IAC sitting at while at idle?
 
1994redram said:
The holley shouldn't make it shake really bad like your saying. That sounds like an ignition issue to me. Its misfiring and not burning the fuel, the ECU sees that and tries to lean it out.

What is your IAC sitting at while at idle?
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anywhere from 0-10 when it’s warm and learning
 
I run mine at ~10% when warmed up. I had my 408 running great this summer. I tried to start it a few weeks back at 40 degrees ambient and the car would fire up and immediately die. I ended needing to mess with the idle screw to keep the IAC happy in cold temps.

What are you using for your ignition setup?
 
Msd

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Start with the basics, check the cap & rotor, ohm out the plug wires, try changing the coil with one that you know is good. From there, get the operations manual for the EFI & start there from square one.
 
Hi,

Need to know the exact idle AFR and cruise. The AFR at "closed loop" should swing back and forth from 13.5 to 15.0 approx. What is the AFR at Wide Open Throttle (WOT) "open loop"? Should be in the 12.6 AFR plus or minus a tad.

Here is what I recommend. 1. inspect the AFR sensor in your exhaust collector and look for ANY issues. In particular cracked welds or frayed wireds or even burnt harness from exhaust. ANY intrusion of fresh air will skew the input to the ECU. 2. Remove the sensor and look for any "stuff" on the probe. It must be clean! 3. Reset the ECU to "relearn". This will wipe the ecu of any learned timing an fuel correctied tables and start fresh with a new learn. Just like a PC sometimes the software may incur an error and corrupt a file within. 4. Check all the grounds!!! I even added a additional ground from the efi throttle body stud on the mani to the block to assure sufficient grounding. Grounds often go unchecked when things get intermittent or otherwise off.

Try this and report back.

Marion
 
A vacuum leak will lean it out and it will try and compensate by adding more fuel giving you the rich condition
 
Biff said:
A vacuum leak will lean it out and it will try and compensate by adding more fuel giving you the rich condition
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This is exactly what came to my mind but of course it was just a wild swing...
With that EFI everybody either loves the hell out of it when it's running good or this... Not a fan...
 
If you are going to take the EFI off and put a carburetor on it and you're not planning on drag racing it for a living do yourself a favor and just put an Edelbrock on it and make it even simpler...
 
As stated a vacuum leak will reap havoc on EFI (and carbs are not to happy with them either).

Remove and clean the O2 Sensor. Use low pressure dry compressed air or nitrogen. They can soot up in extreme rich conditions. No not use chemical cleaner.
 
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pull each wire off cap and check too see if shaking goes away /could be intake valve not sealing
 
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