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


dandaman

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
 
Check the ODB port and look for codes. Fuel filler? DIRTY MAFSensor? Air inlet tube collapse?
 
thank you for the reply. I will start there. Any other ideas to look into?
Thanks,
 
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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will a bad secondary fuel pump show up in a code? has there been issues with the secondary fuel pump in the past?
 
dandaman said:
will a bad secondary fuel pump show up in a code? has there been issues with the secondary fuel pump in the past?
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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.
 
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
 
You guys have given me several things to check out. I will check it out and let you know if I find something.
GT@50 said:
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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In regards to swapping relays which ones do I swap with the secondary fuel pump relay?
 
I had a similar issue, Rich (GT GUY) sent me some replacement spark plugs, and the problem was solved
 
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.
 
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.
 
RPM217 said:
I had a similar issue, Rich (GT GUY) sent me some replacement spark plugs, and the problem was solved
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Do you have Rich's number?
 
What Spark Plugs are best to use with a Pulley and Tune?
 
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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mmlcobra said:
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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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.)
 
GT Relay box. Fuel pump relays.
 

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RPM217 said:
I had a similar issue, Rich (GT GUY) sent me some replacement spark plugs, and the problem was solved
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I had the same issue and was recommended to change the plugs. Particularly if you are running the OEM plugs with a tune.
 
fjpikul said:
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.
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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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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
 
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.
 
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