Engine Occasional misfires normal for a HCI?

AnthonyA1234

Active Member
Aug 17, 2020
465
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38
Florida
Even after doing a full resto on my engine, I still get occasional misfires. The car will misfire when it is both hot and cold. I’d say about half the time I start driving the car when it’s cold once I get up to speed in 2nd gear the car bogs down, I’ll hear multiple misfires (pops from the engine) and then after that it regains power and runs fine. The other scenario it happens is very rarely during rev match downshifts when I blip the throttle I’ll hear a pop from the engine. Also sometimes when I do full throttle pulls I’ll hear a pop but the car doesn’t bog.

The car has edelbrock performer heads, cobra upper lower intake, and a x303 cam.

Could something be wrong or is she dying for a tune?
 
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There is something wrong, it should not misfire.
The stock computer should be able to easily handle what you are throwing at it.
Most likely you have some sort of ignition issue.
 
I agree with others. It shouldn't misfire. Did it always misfire after the new build?

You really gotta start at the basics.

Fuel pressure
compression test
vacuum leaks? (smoke test)
spark plug gap and condition?
Condition of all ignition components
Perform cylinder balance test as part of ECU code reading process
 
Do you still have the original distributor and coil?
Unfortunately no, when I bought the car it had the MSD dizzy and coil and I replaced them with the same parts during the tune up.
I agree with others. It shouldn't misfire. Did it always misfire after the new build?

You really gotta start at the basics.

Fuel pressure
compression test
vacuum leaks? (smoke test)
spark plug gap and condition?
Condition of all ignition components
Perform cylinder balance test as part of ECU code reading process
Yea it’s always misfired when it’s cold like I said it’ll pop a few times and then run perfect after that. However I don’t remember it misfiring when the car is hot as I mentioned before like it is now. Then again I am driving the car much more now after the rebuild so that could be why I notice it.

Fuel pressure is at 38 psi, vac line off and plugged, the car has new 24lbs light blue FoMoCo injectors, new fuel pump, and new pro-m 24lbs calibrated maf.

Last time I did a smoke test the only place I had smoke was the lower intake gasket which has since been replaced. I’ll do another smoke test tomorrow and see if anything new pops up.

I suspect it could be the plugs as a shop I had a very poor experience with installed them for me. I’ll pull them out and replace them and properly gap them. What is the best spark plug for my car and gap?

All ignition components are brand new.

Last time I did a cylinder balance test everything came out good but I’ll do another tomorrow.
 
Dump the plugs and get Autolite 3924. I run these in the same heads.

If you are running a stock fuel pressure regulator then the stock fuel pressure is 38-39 psi. with the vacuum line unhooked and plugged. With ithe vacuum line connected it should be around 30 psi. Seems to me your fuel pressure regulator is fubar. I would get an adjustable one if it were mine but it’s your call.
 
Dump the plugs and get Autolite 3924. I run these in the same heads.

If you are running a stock fuel pressure regulator then the stock fuel pressure is 38-39 psi. with the vacuum line unhooked and plugged. With ithe vacuum line connected it should be around 30 psi. Seems to me your fuel pressure regulator is fubar. I would get an adjustable one if it were mine but it’s your call.
Ok I’ll get those plugs, what gap should I run?

Yea that’s where my fuel pressure is at, I installed a new kirban adjustable fpr and set it there.
 
If you're still having issues, I would run through this checklist from top to bottom. It pretty much goes through everything so you can't help but find the issue:

 
Dump the plugs and get Autolite 3924. I run these in the same heads.

If you are running a stock fuel pressure regulator then the stock fuel pressure is 38-39 psi. with the vacuum line unhooked and plugged. With ithe vacuum line connected it should be around 30 psi. Seems to me your fuel pressure regulator is fubar. I would get an adjustable one if it were mine but it’s your call.
Is that the Autolite racing 3924 or just the normal Autolite 3924?

The two part numbers are AR3924 and 3924.
 
Is that the Autolite racing 3924 or just the normal Autolite 3924?

The two part numbers are AR3924 and 3924.
The only difference is that one is resistor type and one is not. If you don't want excess noise through the radio and electrical system then always go with the resistor type. In this case, the AR.
 
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So I went and bought the plugs and it turns out both the 3924 and the racing 3924 have resistors so I just ended up getting the normal 3924s that I see everyone else in the forums gets to play it safe. They have the little metal seal separate and I believe the larger diameter half of it goes towards the plug and the smaller diameter piece that protrudes more goes towards the head but in online images I see pictures where it’s that way and some where it’s the other. Could anyone verify this for me?
 

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I just run the 3924's and that is the way I install the metal rings. I have a lot of stereo in the car with zero noise. I have also replaced all of the grounds and charging cables in my car so that is most likely why I do not have the issue.

I cannot speak on the spark plug gap as my combo is different than yours (more inches, port work on the heads and intakea, and probably shift at a higher rpm) so listen to Mike.