Engine Misfire at high rpm

T5live

Member
Oct 2, 2020
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35
Douglas Ga
Hello I have a question I’ve seen a few threads on some of this but none of the fixes I have worked on my car purchased a 91 Fox 5.0 unknown engine been changed it’s still a 302 not a roller 5.0 stock injection some kind of mild cam. I am in automotive tech by trade I’m with the main diagnostic people in my town I do not mind TickTock it’s not likely it would go over my head so any information would be appreciated thank you in advance
 
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T5live

Member
Oct 2, 2020
15
2
13
35
Douglas Ga
Hello I have a question I’ve seen a few threads on some of this but none of the fixes I have worked on my car purchased a 91 Fox 5.0 unknown engine been changed it’s still a 302 not a roller 5.0 stock injection some kind of mild cam. I am in automotive tech by trade I’m with the main diagnostic people in my town I do not mind TickTock it’s not likely it would go over my head so any information would be appreciated thank you in advance
Misfire at high rpm is the problem
 

General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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Depending on the cam and valve springs, the misfire could be several things, timing chain slop, valve float, msd stuff sometimes does not play well with eec controls, chip could be corrupted.
What mods have been done that a chip is needed?
 

T5live

Member
Oct 2, 2020
15
2
13
35
Douglas Ga
I’m having the exact same problem with a completely different set up I’m a master technician by trade 20+ years experience and I’m a bit stumped but if you would like the information on my car and what’s going on just post on this thread and I’ll be more than happy to share
 

T5live

Member
Oct 2, 2020
15
2
13
35
Douglas Ga
Sorry I didn’t realize that it was just you and I on this thread my car does not have an MSD controller it as a stock distributor stock module MSD wires and It is not a roller motor but it is a 302 with the mustang injection on top mild cam check valve clearance springs and any damage pull valve covers nothing going on there saying several other people having this problem but no one has posted a fix fairly sure it’s not in the ignition system or valve issues any other ideas
 

Mr.Kelley

Active Member
Aug 9, 2019
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San Diego Ca
What RPM range are you calling "High RPM" some engines think 4,000 RPM is too many. If you have tried more than a couple of the easy fixes and the result is the same I would lean toward a valve control issue. Valve float can sound just like a ricer on the rev limiter. Do you know what ECU is running the mustang injection. It could have a lower rpm rev limit. We here can only guess with the information given so far.
 
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T5live

Member
Oct 2, 2020
15
2
13
35
Douglas Ga
What RPM range are you calling "High RPM" some engines think 4,000 RPM is too many. If you have tried more than a couple of the easy fixes and the result is the same I would lean toward a valve control issue. Valve float can sound just like a ricer on the rev limiter. Do you know what ECU is running the mustang injection. It could have a lower rpm rev limit. We here can only guess with the information given so far.
Stock ECU with quarter horse tune it’s not the limiter it was the TFI thinking about Late model restorations TFI for manual transmission.
 

90sickfox

I didn't really have an issue with the stink...
SN Certified Technician
Mar 2, 2015
5,376
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Check the pickup reluctor wheel below the rotor button. Look down inside the distributor. You'll see the pick up. Rust on the vanes can cause high rpm false readings. If the pick up has deteriorated it can also cause issues. I've seen those things turn into powder.
 

Mustang5L5

i'm familiar with penetration
Mod Dude
Feb 18, 2001
33,978
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Massachusetts
Like stated, the cause could be many many things. You'll just need to go through things one by one to troubleshoot.


I had a misfire above 3500RPM that ended up being a crappy aftermarket coil
 
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2000xp8

SN Certified Technician
Aug 8, 2003
7,008
1,095
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I'm with Kartheif, the stock heads with almost any aftermarket cam will float valves.
I know, i've done it...

I'd also lose the quarter horse tune (at least for testing purposes, although i doubt it's useful on stock engine anyway).

If you are buying a TFI, make sure to make your best effort to get a real ford. The rest will fail.