Weird miss at higher rpms, need help

[snaxwell]

This is my first time to post on a forum, but I've been in the performance game for a little while. I am about to pull my hair out over this car.

Usually beginning at ~3800rpms, with half throttle or more, the engine will miss and pop out of the exhaust.

Have changed cap, rotor, wires, and coil.
Also swapped distribur and injectors off of a friend's coupe.

Car is 87 GT with h/c/i, 80mm Pro-M, 24# injectors, 75mm tb, AOD trans.

Any ideas would be much appreciated.

 

[bubba-dough]

Have you pulled the codes? Also have you checked the fuel pressure to make sure it isn't a lean pop as the rpms rise.?

 

[TomCat]

Also, pull the plugs? You say HCI so you sure you have the right plugs for the head? Gap? Check voltage at the TPS. I'll asume the MAF is calibrated for th 24#s... and even if so, you might as well clean off the little sensor. Good luck!

 

[87'GTstang]

Make sure the plug gap is not gapped too large, all plug wires are sound or in other cases - no cheapies, check ignition timing (also make sure timing is not ATDC), check ignition components. Popping a lot of times though is improperly/overly advanced timing. As previously said, I have also seen 10 degrees ATDC before - ran decent, but would start bucking and falling on it's face at higher rpms. Check TPS and make sure it is good.

How does it feel at WOT, but below the rpm you say it starts missing?

 

[BadAndy]

fuel filter??

 

[noslow1986]

hey join the club i have the same problem. been 2 weeks i still cant figure it out

 

[TheBocSez]

Valve float?? Proper valve springs for the cam in your HCI set up?

 

[snaxwell]

Fuel pressure is set to 38 psi. Do not have a gauge where I can drive and monitor.
I do have a FAST air/fuel meter, when the miss occurs the meter goes very rich, about 10:1.

Spark plugs are correct for head. Using Autolite 3923's with .035" gap. I normally run nitrous on the car when it is running good.

I have changed and adjusted TPS.

MAF was ordered from Pro-M calibrated for 24#'s. Cleaned the sensor with carburetor cleaner.

Using Motorsports 9mm wires, less than a month old.

Below ~3800 rpm car runs great.

Replaced pump(255lph intank) and filter when problem first started.

 

[Strype]

Are you running a 6al? Unhook if so.

Man I've had the same problem from a 6al, and incorrect fuel pressure. My friend pulled his hair out until he changed his brand new cap and wires out over the same problem...

Must think about this one

 

[mob]

Is it running rich or lean? Does the car kind of bog at a constant speed? Sounds kinda like a bad vacuum leak, try hooking up a vacuum guage, see what you get.

 

[Dmarstroker]

hey join the club i have the same problem. been 2 weeks i still cant figure it out

I have also encountered these problems with my 90 stang, except it sounds like its having an intake backire (or just detonation). I have changed the fuel filter, spark plugs, cap and rotor, wires, coil, vaccum check, fuel pressure check(42 PSI), timing set at 10 degrees. The only thing I could think of is that the voltage on my TPS is off. I pulled some codes also and I think 95 came up the most and believed it was fuel injectors, fuel pressure, fuel lines, and oxygen sensors.

 

[jrichker]

Dump the codes and see what the computer says is wrong…Codes may be present in the computer even if the Check Engine light isn’t on.

Here's the link to dump the computer codes with only a jumper wire or paper clip and the check engine light, or test light or voltmeter. I’ve used it for years, and it works great.

See http://www.troublecodes.net/Ford/
OR
See http://www.mustangworks.com/articles/electronics/eec-iv_codes.html

IF your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.

Codes have different answers if the engine is running from the answers that it has when the engine isn't running. It helps a lot to know if you had the engine running when you ran the test.

Trouble codes are either 2 digit or 3 digit, there are no cars that use both 2 digit codes and 3 digit codes.

For those who are intimidated by all the wires & connections, see http://www.actron.com/product_detail.php?pid=16153 for what a typical hand scanner looks like. Normal retail price is about $30 or so at AutoZone or Wal-Mart.

Or for a nicer scanner see http://www.midwayautosupply.com/detailedproductdescription.asp?3829 – It has a 3 digit LCD display so that you don’t have to count flashes or beeps.. Cost is $35.

Wal-Mart may now have the LCD display scanner for less than $35

 

[thehueypilot]

Reduce the plug gap to .024" to see if the miss goes away. If it does your ignition system needs help.

 

[snaxwell]

Update

Changed plugs to Autolite 3924's (1 step hotter) and opened gap to .060".

Pop is all but gone. It will only happen occasionally under a hard load(can be part throttle or WOT).

Computer has codes for thermactor air and EGR. Neither systems are used on the car.

 

[fastranger347]

on my 347 i use a crane hi-6 with a ps92 coil, motorsport 9mm wires, autolite 3925 plugs gaped at .080, pulls fine to my 6,500 rev limit