Engine Misfire below 3K RPM

foxbodyclark · May 25, 2025

I'm back...

Misfire issue, looking for advice.

I have a new issue where the car misfires/ feels down on power under 3,000 RPM. As soon as you hit 3k it goes away and runs smooth. Also only does it under load. A gentle acceleration won't make it happen but going up a hill or stepping on it will.

I have a new fuel pump, fuel filter, fuel pressure regulator so shouldn't be those.

I also have new spark plugs, plug wires, distributor cap, and ignition coil.

I've heard some people say O2 sensor, fuel injectors, or MAF sensor.

Any thoughts? Thanks

General karthief · May 25, 2025

Funny thought (I seem to have a lot of those) people will suggest a whole bunch of money wasting parts swapping but they will not tell you how to diagnose an issue, like looking at a low tire and suggesting you need to replace the wheel or hubcap
that said, first is check codes, next (I would) check the plugs for difference in color/condition, plug wires need to be checked, I don't care if they are new, look at the distributor cap, if you spray the inside of the cap with electronic/carb/brske cleaner it will reveal cracks by showing up on the outside top.
A few simple things to check for free. Also never 'assume ' a part is good because it's new.

Noobz347 · May 25, 2025

Just to verify...

No aftermarket ignition boxes etc.?

foxbodyclark · May 26, 2025

Noobz347 said:
Just to verify...

No aftermarket ignition boxes etc.?

No igniton boxes. The ignition Coil is an MSD and I changed it to an Autozone special but it didn't make a difference, Im going to put the MSD back on.

foxbodyclark · May 26, 2025

General karthief said:
Funny thought (I seem to have a lot of those) people will suggest a whole bunch of money wasting parts swapping but they will not tell you how to diagnose an issue, like looking at a low tire and suggesting you need to replace the wheel or hubcap
that said, first is check codes, next (I would) check the plugs for difference in color/condition, plug wires need to be checked, I don't care if they are new, look at the distributor cap, if you spray the inside of the cap with electronic/carb/brske cleaner it will reveal cracks by showing up on the outside top.
A few simple things to check for free. Also never 'assume ' a part is good because it's new.

Thanks, I'll dump the codes tomorrow and double-check the plugs and wires.

Noobz347 · May 26, 2025

Also... After it get dark...

Run the engine at idle with the hood open and check for any signs of lightning. Check in and around the coil too. I've seen brand new units [arc] between the metal frame and coil pack.

foxbodyclark · May 30, 2025

No codes, no stray arcs when running at night. Took off the valve covers and found a broken valve spring lock on cyl. 6 exhaust valve. That would definitely cause an issue. I'm going to replace that tomorrow and see how it runs

AeroCoupe · May 30, 2025

How many miles are on the springs and are they the right spring for the cam? May want to change them all is what I’m getting at.

foxbodyclark · May 30, 2025

AeroCoupe said:
How many miles are on the springs and are they the right spring for the cam? May want to change them all is what I’m getting at.

Gt40P heads, remanufactured. They came with new stock springs. And I have a B cam. Ive heard of people changing the valve springs on these heads but that's also $.
I had a really hard time getting the pushrod length right on the heads so there were many start ups and test drives with BAD valvetrain setup. I'm honestly not surprised something broke.

AeroCoupe · May 30, 2025

Do you know what springs were out on them? Just asking as if the springs do not have enough open or close pressure or get into a bind condition on max lift it’s no bueno.

foxbodyclark · May 30, 2025

AeroCoupe said:
Do you know what springs were out on them? Just asking as if the springs do not have enough open or close pressure or get into a bind condition on max lift it’s no bueno.

Not exactly sure what you're asking? The exhaust valve spring on cyl 6 is what has the broken lock. Here's a picture of the head. And the broken part

Noobz347 · May 30, 2025

foxbodyclark said:
Not exactly sure what you're asking? The exhaust valve spring on cyl 6 is what has the broken lock. Here's a picture of the head. And the broken part

He's asking if you know whether the springs were matched to the cam.

What can happen is that a cam can provide more lift (more movement) than the spring will allow. This causes 'binding' and parts start to break.

If you're running an aftermarket cam, this can happen. Not so much on an OEM cam.

foxbodyclark · May 30, 2025

Gotcha, yeah the springs are just the stock ones. I looked back where I bought the heads, not even new springs. My cam is fairly mild. The previous owner told me there is a cam for sure but we don't know what it is. Guessing B cam.
My E7 heads ran the stock springs with this cam and didn't have issues. I know, new head = different geometry, but it's not like I have a block breaking setup... So maybe they springs are ok?

General karthief · May 30, 2025

Or maybe not?
issues with unknown parts is real more so with valve train geometry.
the hint is the valve lock leaving the party, do yourself a favor and swap a better cam and matching springs.

Noobz347 · May 30, 2025

I dunno... Sometimes these thing just 'happen'.

I'd put new keepers in it and send it.

General karthief · May 30, 2025

Noobz347 said:
I'd put new keepers in it and send it.

@foxbodyclark
Keep in mind this advice is from 'the janitor'
I've done some 'hope it holds together' crap too...so....

Noobz347 · May 30, 2025

General karthief said:
@foxbodyclark
Keep in mind this advice is from 'the janitor'
I've done some 'hope it holds together' crap too...so....

If I had not had this happen to me before, I would be more skeptical. In my situation, it was a [single] keeper in a fairly new engine where I already [knew] that everything was measured and built to work together.

AeroCoupe · May 30, 2025

if you have no idea what springs are on the heads then you probably should change them. TFS makes a really great kit to do this.

https://www.trickflow.com/parts/tfs-2500100

foxbodyclark · May 31, 2025

It's put back together and runs great silent valvetrain and full power!

General karthief · May 31, 2025

Hopefully you get to keep 'The Lucky Dog' award.
if not welllll.......
Spin The Wheel And Take Your Chances..
glad it worked.

Engine Misfire below 3K RPM

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wonder how much it would cost to ship you a pair

Stangnet Facilities Maint Tech... Er... Janitor

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Stangnet Facilities Maint Tech... Er... Janitor

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lube between the nut and the face. I know my lubes

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lube between the nut and the face. I know my lubes

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Stangnet Facilities Maint Tech... Er... Janitor

Member

wonder how much it would cost to ship you a pair

Stangnet Facilities Maint Tech... Er... Janitor

wonder how much it would cost to ship you a pair

Stangnet Facilities Maint Tech... Er... Janitor

lube between the nut and the face. I know my lubes

Member

wonder how much it would cost to ship you a pair

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