GT40P high RPM cut out

Silvertip78 · Jul 29, 2008

I have a:
1991 LX hatchback 5.0L w/ 5 speed manual & 3:08 traction lok
circa 1999 GT40P crate motor w/ about 12K miles on it
Ford Cobra cast intake
FMS E303 cam w/ 1.7 roller rockers w/ stock GT40P valve springs
65 mm FMS throttle body
24# injectors
73mm C&L mass airbody calibrated for 24# injectors
FMS shorty headers for GT40P drop in conversion
To review recent mods I have changed to new OEM coil, new cap & rotor, platinum plugs w/ stock motorcraft gap, new wires and new OEM pressure regulator. I put in a BBK 155 lph fuel pump last summer. Of note it seems quieter than the stock factory 89 lph unit. Also set the timing back to 10~12 degrees. Was about 16~18 initially. No pertinant error codes form a hand held box.

At 4500~5000 RPM it begins to miss / cutout. It is worse under a load. It did all this with the, stock fuel pump, stock mass air body & 24# injectors too.
It tends to idle at about 1000 RPM also.

Any suggestions please??

bz393 · Jul 29, 2008

You need to upgrade your valve springs. Stock springs arent good for anything but the stock cam. I took mine to a reputable ford machine shop and was told that the stock springs are to weak and spring pressure is not suitable for an
e-303. I havent completed this build yet but it includes the famous e-303 and the "p" heads. I had auctually asked the same ? on this fourm I think...

jrichker · Jul 30, 2008

A high speed miss on a warm engine is usually a TFI ignition module problem.

See the "Surging Idle Checklist for help with all your idle/stall problems. The first two posts contain all the updates to the fixes. At last count there were 24 possible causes and fixes] for surging idle/stall problems. I continue to update it as more people post fixes or ask questions.

BK_CAULEY · Jul 30, 2008

nevermind

2000xp8 · Jul 30, 2008

It's more than likely the valve springs, they are not capable of handling an E cam, then on top of it, 1.7 rockers.

Even if for some reason it's not the valve springs this time, you still need them, they will get worn down and then you will have a problem.

Dan02gt · Jul 30, 2008

I got to agree on the valve springs. I've seen the stock springs be fine with a E cam and stock rockers, but with 1.7 RR that's way to much lift. If my math it right you have .529" of lift, and past experiences have told me .500" is the absolute max you can go with stock springs. You could also just go with 1.6RR as I'm not sure your combo needs that much lift anyway. Maybe someone else can help you more on that.

Silvertip78 · Jul 31, 2008

The motor was installed in the summer of 2000 and the car was basically in storage until 2005. After some digging a friend reminded me that I had bought from him and apprently installed the following:M-6513-A50 springs, M-6514-B50 retainers, M-6518-B351 locks. I found the boxes with teh old unused crate motor parts in my basement. See attached.

Also it looks like if I want to go to 1:6:1 Roller ROckers I hav eonly two choices FRPP ( M-6554-B351 FMS Roller Rocker 1:6:1 ( CJ Pony $309.95 ) ( FRPP $ 399.95 ) or Steeda 1.6 Ratio 5/16" Bolt Down Rocker Arms - 79-95 $255.95

Per the post of jricjker it appears that I may have some electrical problem. For example I have an ACCEL ICM module which may be going bad and perhaps some other issues. The miss prtoblem is much worse in a warmed up or hot motor.

Any other input is apprreciated. Sorry for the "false" start on the valve springs info.

2000xp8 · Jul 31, 2008

The TFI or coil i'd say if the problem is much worse when the engine is heat soaked, now that you know the springs are good.

I don't know what an accel icm is, but if that's fancy talk for the TFI, remove it, and get a geniune for TFI module, the rest are all crap, i don't car who makes it.

795.0pacecar · Jul 31, 2008

I had an accel ICM and after about a year it started to crap out and cause the ignition to cut out. It was a big waste of money imo.

Silvertip78 · Aug 17, 2008

Thanks to all for the suggestions and advice.
It appears that it was the ICM i.e Ignition Control Module ( or TFI Thin Film Ignition module ). I initially had problems with the 1st Accel TFI that was in the motor in the inital install. Apparrently in the many trips to Autozone /PepBoys we picked up another and I had it in my basket of parts. I changed it out and still had the same problem.
I did some web searching and found a Motor Craft unit ( DY425 ) at Rock Auto parts for around $90 ( local Ford dealers wanted $166 ).

Car runs great now. I can't wait to get my 3:55 gears in.

jimerson503 · Dec 12, 2009

help me!!!

hey guys got a 94 gt mustang just done a gt40p swap.. and igniton but a street fire col knew wires and cap and rotor when done. my car is a stick but at about 5000 rpm it cuts out and falls on its face im thinking the icm or something but seeing if anyone else is had this problem thanks

Mustang5L5 · Dec 12, 2009

Is it valve float? Did you change the springs?

GT40P high RPM cut out

Silvertip78

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mazing how much gas smell came from that tiny hole

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GT40P high RPM cut out

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