Engine seems to miss in higher RPM range, Any thoughts?


dandaman

GT Owner
Apr 8, 2007
113
Tyler, Tx
My Ford GT has 4900 miles on it and I took it out yesterday and today to stretch its legs. I notice when I took it though the RPM range from 4,000 to 6,000 it seem to have a several misfires. Do to the misfires, there seems to be a loss of power as well. For tonight's run I added an octane booster and added a few gallons of 93 methanol free gas. That did not seem to make a lot of difference. It seems to run fine around normal driving conditions. Engine idles smooth. I would appreciate any thoughts on how to sort this out.
Thank you
 

fjpikul

GT Owner
Mark IV Lifetime
Le Mans 2010 Supporter
Jan 4, 2006
11,475
Belleville, IL
Check the ODB port and look for codes. Fuel filler? DIRTY MAFSensor? Air inlet tube collapse?
 

dandaman

GT Owner
Apr 8, 2007
113
Tyler, Tx
thank you for the reply. I will start there. Any other ideas to look into?
Thanks,
 

GT@50

GT Owner
Mark II Lifetime
Dec 14, 2019
852
Issaquah
Check for codes. Secondary fuel pump not kicking in? Swap relays. Fixed, bad connection, won't run, bad relay. It's in left front under frunk liner.
 
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dandaman

GT Owner
Apr 8, 2007
113
Tyler, Tx
will a bad secondary fuel pump show up in a code? has there been issues with the secondary fuel pump in the past?
 

GT@50

GT Owner
Mark II Lifetime
Dec 14, 2019
852
Issaquah
will a bad secondary fuel pump show up in a code? has there been issues with the secondary fuel pump in the past?
Not sure if it would set a pump code or misfire code but would cause a lean misfire so you'd think something. They have been known to be bad and or have bad connections. Secondary kicks on with boost.
 

GT101

*UK Support*
Supporting Vendor
Jan 20, 2010
380
Colchester, Essex, UK
As GT@50 says I'd check the fuel pump relays.
Also check the intercooler pump is running ok - these can get weak or fail and the higher intake air temps cause real power issues.
You should see a strong flow from the pump swirling in the coolant reservoir
 

dandaman

GT Owner
Apr 8, 2007
113
Tyler, Tx
You guys have given me several things to check out. I will check it out and let you know if I find something.
Check for codes. Secondary fuel pump not kicking in? Swap relays. Fixed, bad connection, won't run, bad relay. It's in left front under frunk liner.
In regards to swapping relays which ones do I swap with the secondary fuel pump relay?
 

RPM217

2005 white/blue stripe
Jun 18, 2010
1,658
Rye Brook, New York
I had a similar issue, Rich (GT GUY) sent me some replacement spark plugs, and the problem was solved
 

GT@50

GT Owner
Mark II Lifetime
Dec 14, 2019
852
Issaquah
Check for rodent nest in airbox too.
There's 2 fuel pump relays, just swap them. I've never been in that box so any relays that look similar, swap to a new position. If the problem moves get a book and pinpoint the relay.
 

fjpikul

GT Owner
Mark IV Lifetime
Le Mans 2010 Supporter
Jan 4, 2006
11,475
Belleville, IL
Do you have a service manual? If I remember correctly, if the fuel pump relay is bad, the headlights do not work correctly. Similar to the wipers not working correctly if the trunk isn't closed all the way.
 

dandaman

GT Owner
Apr 8, 2007
113
Tyler, Tx
I had a similar issue, Rich (GT GUY) sent me some replacement spark plugs, and the problem was solved
Do you have Rich's number?
 

dandaman

GT Owner
Apr 8, 2007
113
Tyler, Tx
What Spark Plugs are best to use with a Pulley and Tune?
 

mmlcobra

GT Owner
May 25, 2013
1,215
Would recommend that you check with the person that wrote the tune.
Please don't drive the car again until you get this resolved.
Could hurt your motor!
Best,
Mark
 
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twobjshelbys

GT Owner
Jul 26, 2010
6,053
Las Vegas, NV
Would recommend that you check with the person that wrote the tune.
Please don't drive the car again until you get this resolved.
Could hurt your motor!
Best,
Mark

This is part of the info that is missing.

Is this a factory configuration or a "pulley/tune" config, or a Whipple config?

Factory tune? Custom tune?

Had this not been an issue in the past and it just started happening (RPMs that high are really kind of rare for extended periods in normal driving, so it might have been there for a while and you just didn't know it.)
 

68Rcodeman

GT Owner
Mark II Lifetime
Apr 5, 2008
800
Spring Hill, FL.
GT Relay box. Fuel pump relays.
 

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Art138

GT Owner
Mark II Lifetime
Jan 4, 2011
624
Weston,FL
I had a similar issue, Rich (GT GUY) sent me some replacement spark plugs, and the problem was solved
I had the same issue and was recommended to change the plugs. Particularly if you are running the OEM plugs with a tune.
 

nota4re

GT Owner
Mark IV Lifetime
Le Mans 2010 Supporter
Feb 15, 2006
4,176
Do you have a service manual? If I remember correctly, if the fuel pump relay is bad, the headlights do not work correctly. Similar to the wipers not working correctly if the trunk isn't closed all the way.

The wipers will not function by plan if the car believes the bonnet may be open - for fear of interference. There is no correlation between fuel pumps and headlights.

OP - It is very likely not a fuel pump or relay as the ECU can/will detect that and generate a fault code. Consider these low odds as places to look. It's important to note what mods are on the car - and if there's a tune, who created it.

With the modest boost increase from a pulley upgrade and a *good* tune, OEM sparkplugs *should* be OK. If you were running a Whipple with higher boost pressures, you may want to try a plug that is one step colder - an NGK TR7IX would be my recommendation. My suggestion at this time would be to replace the sparkplugs with the same OEM NGK TR6IX plugs (assuming you are using the stock SC with or without an aftermarket pulley.) This has a good possibility of resolving the problem and the added benefit is that pulling each plug will give you information about how the car is running and being able to compare cylinders.
 
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dandaman

GT Owner
Apr 8, 2007
113
Tyler, Tx
Thank you, I was able to pick up today some fuel pump relays and I guess I will put them in anyway. I do have a smaller pulley on a stock SC with a tune. I had plan to drain the gas tank and put new gas in it to see if by chance I got some bad gas. I really appreciate everyone's suggestions and help. I will pick up a set of plugs as well and put them in.
Thank you
 

fjpikul

GT Owner
Mark IV Lifetime
Le Mans 2010 Supporter
Jan 4, 2006
11,475
Belleville, IL
Kendall is correct. There is a relay affecting the turn signals and the wipers. Took me a while to find my old post. Has nothing to do with fuel pump relay.